Symposium EP04—Soft and Stretchable Electronics—From Fundamentals to Applications

This talk will discuss recent work in our group (and through collaborations) to utilize liquid metals as electrical conductors for soft and strethcable electronics. Gallium and its alloys have low viscosity, low toxicity, and negligible volatility. Despite the large surface tension of the metal, it can be patterned into non-spherical 2D and 3D shapes due to the presence of an ultra-thin oxide skin that forms on its surface. Because it is a liquid, the metal is inherently soft and flows in response to stress to retain electrical continuity under extreme deformation. By embedding the metal into elastomer or gel substrates, it is possible to form soft, flexible, and conformal electrical components, stretchable antennas, and ultra-stretchable wires that maintain metallic conductivity up to ~800% strain. In addition to introducing the advantages of these materials for stretchable electronics, this talk will focus on recent advances with (1) patterning the metal and metal composites, (2) intergrating liquid metal into functional devices including energy harvesters and transistors (in collaboration with the Ozturk and Franklin groups, respectively), and (3) encasing the metal within optimized and functional elastomers that are less prone to fail mechanically (and warn the user colorimetrically before failure, done in collaboration with the Craig group). These advances have implications for creating electronics that are softer than skin and with stretchability limited only by the encasing elastomer.

Robust stretchable conductors with advanced capabilities such as stimuli-responsiveness and high power delivery over large strains are needed for advanced soft robotic, physically reconfigurable, and wearable electronic applications. Room temperature liquid metals such as eutectic gallium-indium (EGaIn) have shown potential in next generation stretchable electronic systems, but suffer from problems with containment, fabrication, and metal oxide formation. Here we synthesize phosphonic acid-functionalized reactive EGaIn core-shell particles and polymerize them on elastomer substrates to create polymerized liquid metal networks consisting of >99.9 wt% EGaIn. Under strain the polymerized particles in the network rupture and release their encapsulated liquid metal, rapidly transitioning the network from an electrically resistive to a conductive state (10⁸ fold conductivity increase from 0-100% strain). Once activated, these network composites adopt microstructures which mitigate the deleterious effects of strain on electronic performance. As a result of these microstructures, these networks show many remarkable characteristics including increasing volumetric conductivity with strain to over 20,000 S/cm at >700% elongation, electronic memory of previous strain states, and R/R₀ < 1 between 0-300% strain. The unique electromechanical properties of these activated liquid metal networks present a superior design strategy to provide ultra-stretchable (>300%) conductors with near-zero change in resistance and can find additional use for powerless active strain indication and triggered healing in stretchable conductors.

Gallium is a metal in a liquid state at room temperature that offers unique electromechanical properties when integrated within a soft, elastic carrier material. However, precise patterning and manipulation of the liquid metal are challenging. High surface tension prevents homogeneous spreading of liquid gallium on lyophobic silicone carriers. A thin gallium oxide skin forms immediately at the gallium surface upon exposure to ambient conditions and sets the shape of the material. We overcame these peculiar properties to pattern reliably thin gallium features with µm-scale resolution, high surface density and over centimeters side surface areas.

Our manufacturing process combines soft lithography, self-assembly and physical vapor deposition techniques, all compatible with wafer scale manufacturing. First, a PDMS layer is cast on a photoresist mold lithographically patterned with a matrix of 1.5µm thick, 6µm wide holes distributed across the entire 4in. wafer. The PDMS layer is then demolded and transferred to another carrier for further processing on the patterned side. A 160nm thick gold film is blanket-sputtered on the textured surface, so as to increase the chemical affinity of gallium to the silicone substrate. Next a tilted Ion Beam Etching (IBE) process is used to selectively remove the gold layer only above the patterned recesses. Finally, gallium is evaporated on the substrate, where it alloys with gold and fills all empty spaces in-between the PDMS micro-posts.

This process enables a range of channel designs and geometries leading to soft conductors displaying highly stable electromechanical properties when reversibly and repeatedly stretched to 50% elongation for hundred cycles. This work represents a significant advance towards the miniaturization of gallium-based technology for robust and reproducible, soft and mechanically compliant electronic circuits.

This paper describes shape memory polymer fibers consisting of an elastic shell and a gallium liquid metal core. These fibers are interesting because they have metallic electrical and thermal conductivity, elastic mechanical properties, and excellent shape memory properties due to the ability to change the phase of the core from a low viscosity liquid to a solid metal. It is straightforward to form the fibers by via melt processing followed by injection of the liquid metal into the core. The elastic energy stored in the fiber allows it to relax back to its original geometry upon melting the solid gallium. The ability to change the core of the fiber from liquid to solid allows an enormous modulus change in the range of from 4.5 and to 1618.3 MPa and the ability to have shape-memory effect; that is, the fiber can preserve a deformed shape and then return to its original shape upon heating. The shape memory behavior of these fibers is quantitatively characterized in terms of fixity (i.e. the ability to retain deformed temporal shape) and recovery (i.e. the ability to return to its original geometry). The use of a rigid metallic core provides perfect fixity. Recovery rate time is effectively minimized due to the elastomeric shell with having minimal viscous dissipation as well as rapid conversion of the metallic core to a liquid with a low viscosity. Notably, the use of gallium—with a melting point above room temperature but below body temperature –allows the user to melt and deform local regions of the fiber by hand. These stretchable elastic shape memory fibers may enable new applications in soft robotics, stretchable and wearable sensors, and bioinspired electronic skin.

Room-temperature eutectic liquid metals have become an exciting basis of research into soft materials for sensors, actuators, and robotics. This work relies on the low melting point of the liquid metal, which allows for a material which is both conformable and conductive. While the melting point of metals such as eutectic gallium-indium (EGaIn) and eutectic gallium-indium-tin (Galinstan) are often cited, their mechanical and electrical properties around the melting point are not well-studied. These properties, particularly the mechanical transition between a liquid and solid phase, are crucial to understanding the use of EGaIn and Galinstan as conformable conductive elements in a wide variety of environments. This work focuses on Galinstan, which is reported to have a melting point <-10^oC and a low room-temperature modulus. By measuring the rheology of Galinstan between 50^oC and -20^oC, the range at which Galinstan can truly be called “soft” with respect to common elastomer matrices was determined. Mechanical changes caused by the effects of the freeze/thaw cycle on Galinstan were also observed. The transition between the solid and liquid state were also observed using powder x-ray diffraction to determine if a crystalline solid was formed, and what changes the freeze/thaw cycles may cause in the material. The mechanical properties of Galinstan-in-PDMS dispersions were also measured through freeze/thaw to better understand the use of Galinstan as a conformable conductor in a stretchable, electronic device. Through this work, we were able to add important mechanical performance detail to the description of Galinstan as a conformable conductor, as well as point out important considerations when manufacturing a device intended for hot and cold environments.

Flexible organic electronics have attracted considerable attention over the past decade. Stretchable electronics represent another type of optoelectronic devices that are intrinsically elastic, that is they are foldable, twistable, and stretchable while maintaining performance, integrity and durability. Incorporated into devices, properly designed stretchable materials may result in more robust devices under bending and strain compared to flexible but not stretchable materials. For intrinsically stretchable electronics, it is desirable to have intrinsically stretchable materials, ranging from stretchable conductors, stretchable dielectric to stretchable semiconductors. In this talk, I will present various molecular design concepts for realizing stretchable electronic polymers without compromising electronic properties. Their applications in bioelectronics will also be presented.

Wearable bioelectronic devices rely on oxidoreductase enzymes and have already demonstrated considerable promise for on-body applications ranging from highly selective non-invasive biomarker monitoring to epidermal energy harvesting. Critical to such progress is the judicious design of the enzyme-electronic interface, along with flexible platforms with mechanical properties similar to those of biological tissues. Such devices require special attention to the enzyme-electronic interface and to several considerations related to wearable applications, such as mechanical properties (flexibility and stretchability), operational stability in different biofluids and under changing conditions (e.g., pH, temperature), biofouling, selectivity, and low target concentrations. Keeping these requirements in mind, our group has pioneered a variety of wearable biocatalytic sensors and biofuel cells devices. By leveraging the advantages of biocatalysis, electrochemistry, and flexible electronics, and addressing key challenges, wearable bioelectronic devices could have a tremendous impact on diverse biomedical, fitness and defense fields.

Among various pressure sensors, transduction mechanisms, piezoresistive, capacitive, triboelectric, and piezoelectric effects are generally considered for converting tactile stimuli into electrical signals.^1,2 Owing to the fast response time and higher sensitivity, piezoelectric pressure sensors (PEPS) are the most useful for the detection of full-range human activities. It has been found that pressure sensor based artificial electronic skin (e-skin) can mimic the human skin and detects subtle pressure (1 Pa–1 kPa). A broad range of applications is expected to be implemented such as wearable health care systems, human–machine interfacing devices, artificial intelligence and prosthetic skin. Nevertheless, piezoelectric materials with adequate flexibility, light weight, ease of large-area processing, low cost and environmental safety are attractive but several issues are remaining for next-generation pressure/force sensors and mechanical energy harvesters. Recent advances on PEPS development is primarily focused on varies types of inorganic and semiconducting piezoelectric materials such as, barium titanate (BaTiO₃), zinc oxide (ZnO), zinc sulfide (ZnS) and lead zirconate titanate (PZT).² In spite of their superior electromechanical responses, the brittleness and toxic properties limit their implementation in wide range of biomedical and flexible electronics applications. On the other hand, natural piezoelectric materials are not yet extensively investigated as an e-skin for measuring and quantifying human physiological signal.²
In this context, we have developed human skin interactive self-powered piezoelectric biomaterials based e-skin (Bio-e-skin) which can detect and discriminate acute human physiological signals such as low frequency as subtle as wrist pulse and even intra-body pressures such as intraocular and intracranial pressure, to relatively high frequency dynamic human motions such as movements of synovial joints of wrist, elbow, knee and finger bending. Natural bio-polymers such as, collagen, chitin, gelatin and cellulose were found to possesses superior mechanosensitivity which was sufficient to fabricate self-powered wearable bio-e-skin that can mimic spatiotemporal human perception and monitors real-time human physiological signals in non-invasive rational strategy. In addition, the bio-skins are found to generate superior output power density which is sufficient to operate the commercial consumer electronics. These observations suggest that fabricated bio-e-skins could eventually find a wide range of applications in autonomous epidermal electronics, implantable medical device, surgery, e-healthcare monitoring, in vitro and in vivo diagnostics apart from its broad range applications in the field of self-powered personal portable electronic devices.^3-5
References
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2. F. R. Fan, W. Tang, Z. Lin Wang, Adv. Mater., 2016, 28, 4283–4305.
3. S. K.Ghosh, D.Mandal, Appl. Phys. Lett., 2016, 109, 103701.
4. S. K. Ghosh, D. Mandal, Nano Energy, 2016, 28, 356–365.

Recently emerging classes of battery free, ultrasmall¹, fully implantable devices for optogenetic neuromodulation² eliminate physical tethers associated with bulky head-stages and batteries in alternative wireless technologies and conventional setups by leveraging cellular scale light emitting diodes on flexible injectable filaments as light sources³. These highly miniaturized systems enable untethered, operation for behavioral studies that eliminate motion constraints and enable new experimental paradigms in a range of complex 3D environments and contexts (e.g. social interactions) that cannot be explored with conventional technologies. These devices are, however, purely passive in their design, thereby precluding any form of active control or programmability, resulting in limitations when investigating circuit dynamics where independent operation of multiple light sources with precise active control is needed. Here we present a series of important concepts that enable controlled device operation, independent of position and angle relative to the experimental arena, with advanced wireless power harvesting capabilities and full user-programmability over multiple devices. This level of functionality is demonstrated in integrated platforms that are compatible with noninvasive imaging technologies such as computed tomography and magnetic resonance imaging and have sizes and weights not significantly larger than those of previous, passive systems. The resulting devices qualitatively expand options in brain tissue illumination for optogenetic neuromodulation and multimodal operation, with broad potential applications in neuroscience research, with specific advances in precise dissection of neural circuit function during unconstrained behavioral studies.


1 V. K. Samineni, J. Yoon, K. E. Crawford, Y. R. Jeong, K. C. McKenzie, G. Shin, Z. Xie, S. S. Sundaram, Y. Li, M. Y. Yang, J. Kim, D. Wu, Y. Xue, X. Feng, Y. Huang, A. D. Mickle, A. Banks, J. S. Ha, J. P. Golden, J. A. Rogers, and R. W. I. Gereau, PAIN 158,2017).
2 G. Shin, A. M. Gomez, R. Al-Hasani, Y. R. Jeong, J. Kim, Z. Q. Xie, A. Banks, S. M. Lee, S. Y. Han, C. J. Yoo, J. L. Lee, S. H. Lee, J. Kurniawan, J. Tureb, Z. Z. Guo, J. Yoon, S. I. Park, S. Y. Bang, Y. Nam, M. C. Walicki, V. K. Samineni, A. D. Mickle, K. Lee, S. Y. Heo, J. G. McCall, T. S. Pan, L. Wang, X. Feng, T. I. Kim, J. K. Kim, Y. H. Li, Y. G. Huang, R. W. Gereau, J. S. Ha, M. R. Bruchas, and J. A. Rogers, Neuron 93,2017).
3 T. I. Kim, J. G. McCall, Y. H. Jung, X. Huang, E. R. Siuda, Y. Li, J. Song, Y. M. Song, H. A. Pao, R. H. Kim, C. Lu, S. D. Lee, I. S. Song, G. Shin, R. Al-Hasani, S. Kim, M. P. Tan, Y. Huang, F. G. Omenetto, J. A. Rogers, and M. R. Bruchas, Science 340,2013).

On-skin sensors have attractive much attention as the next-generation wearable devices, because the conformal contact on human skin enables accurate and continuous detection of physiological signals. One of the most important technologies to improve usability of on-skin sensors is a power source to continuously supply electricity to health-monitoring systems. In this talk, we will report on recent progresses of ultraflexible organic photovoltaic cells for applications to wearable sensors. First, ultraflexible organic power sources that can be wrapped around an object have been developed with mechanical and thermal stability in long-term operation. Then, the integration of these power sources with functional electric devices including sensors has been achieved.

Recent studies have demonstrated that in addition to biochemical and genetic interactions, cellular systems also respond to biophysical cues, such as electrical, thermal, and mechanical signals. However, we only have limited tools that can introduce localized physical stimuli and/or sense cellular responses with high spatiotemporal resolution. Inorganic semiconductors display a spectrum of physical properties and offer the possibility of numerous device applications. My group integrates material science with biophysics to study several semiconductor-based biointerfaces. In this talk, I will first pinpoint domains where semiconductor properties can be leveraged for biointerface studies, providing a sample of numbers in semiconductor-based biointerfaces. Next, I will present a few recent studies from our lab and highlight key biophysical mechanisms underlying the non-genetic optical modulation interfaces. In particular, I will present a biology-guided two-step design principle for establishing tight intra-, inter-, and extracellular silicon-based interfaces in which silicon and the biological targets have matched mechanical properties and efficient signal transduction. Finally, I will discuss new materials and biological targets that could catalyze future advances.

Organic electronic devices, apart from consumer applications, are presently paving the path for key applications at the interface between electronics and biology. In such applications, organic polymers are very attractive candidates, due to their distinct properties of mechanical flexibility, self-healing and mixed conduction.
My group investigated the processing conditions leading to high electrical conductivity, long-term stability in aqueous media as well as robust mechanical properties of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonate (PEDOT:PSS) [1-3].We have demonstrated that stretchable PEDOT:PSS films can be achieved by adding a fluorosurfactant to the film processing mixture and by pre-stretching the substrate during film deposition. We have achieved patterning of organic materials on a wide range of substrates, using orthogonal lithography and pattern transfer [4-5]. Recently we have discovered that PEDOT:PSS films can be rapidly healed with water drops after being damaged with a sharp blade [6] or show autonomous self-healing if processed in presence of certain additives.
My talk will deal with processing, characterization and patterning of conducting polymer films and devices for flexible, stretchable and healable electronics. I will particularly focus on the strategies to achieve films with optimized electrical conductivity and mechanical properties, on unconventional micro patterning on flexible and stretchable substrates, on the different routes to achieve films stretchability and self-healing.

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Organic-based thin film transistors (OTFTs) are beneficial for wearable architecture because of their lightweight, flexibility and low cost. One of the critical issues in flexible OTFTs is gate dielectric for high performance flexible elecronics. The hybrid materials consisting of organic and inorganic materials have emerged as a feasible gate dielectric for high-performance flexible electronics. Therefore, it is necessary to develop ultrathin hybrid organic-inorganic dielectrics and correlate their material properties with device performance at different bending strains. In this work, we propose a one-step synthesis method to form a high-k, ultrathin and homogeneous organic-inorganic hybrid dielectric via initiated chemical vapor deposition (iCVD). iCVD is a well-established process to deposit various functional polymer films based on a radical polymerization via a vapor-phase deposition. In this study, trimethylaluminum (TMA) and 2-hydroxyethyl methacrylate (HEMA) were used as the inorganic precursor and monomer, respectively, leading to the formation of an AlO_x-embedded polymer matrix hereafter called a hybrid dielectric. The chemical bonds of the synthesized hybrid dielectric were characterized using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) which clearly demonstrated that the hybrid dielectric was successfully synthesized and deposited via iCVD process. The X-ray diffraction (XRD) spectra shows that the amorphous AlO_x moiety is dispersed in the polymer matrix to achieve high dielectric strength and mechanical flexibility. No apparent grain-like or pinhole-like morphologies are observed from the atomic force microscopy (AFM) images, which indicated that the hybrid dielectrics were homogeneously synthesized without any phase separation. To characterize the electrical properties of the hybrid dielectric with various Al contents, crossbar-type metal-insulator-metal (MIM) devices were fabricated. The dielectric constant was extracted from the capacitance (C_i) measured at a frequency (f) of 10 kHz and film thickness measured via scanning transmission electron spectroscopy (STEM). The dielectric constant (k values) increased from 3 to 7 as the Al concentration in the hybrid dielectric increased, which was attributed to a greater number of dipoles in the polymer matrix. Unlike the higher k values with higher Al concentrations, the electrical breakdown field (E_break) decreased linearly from 6 to 3 MV/cm, and the gate leakage current (J) at 2 MV/cm increased exponentially from 1.0 × 10^-9 to 3.0 × 10^-7 A/cm². The electrical results revealed that there was a trade-off between performance and power consumption. The electrical energy bandgap (E_g) was extracted from the electron energy loss spectroscopy (EELS) with low loss spectra resulting in a wide E_g of 7.0 ~ 7.5 eV. Based on these preliminary experiments, the hybrid dielectric with an Al concentration of around 18% exhibited a decent k value of 6 and J value of lower than 10^-7 A/cm² at 2 MV/cm. The MIM device with the hybrid dielectric exhibited negligible J changes of up to 2.6 % applied strain, where the soft polymer matrix effectively released the applied strain. Finally, the iCVD hybrid dielectrics were applied to the bottom-gated OTFTs with p-type pentacene and n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13). Two typical linear and saturation operating regions emerged, revealing well-behaved transistor characteristics. The mechanical flexibility of the p-type pentacene OTFTs with the hybrid dielectric was investigated at different bending strains. The transfer characteristics of the OTFTs were shown as a function of mechanical bending strain, showing negligible changes while maintaining a low gate leakage current. The results suggest that the hybrid dielectric synthesized via the iCVD process is a promising candidate for high-performance low-power consumption flexible electronics.

Wearable electronics have attracted attention as an emerging technology to realize practical e-textile and smart garments. For future wearable automatic systems, robust data storage media are required to record and process the electrical signals generated under various unpredictable strain conditions. Here, we report an organic field-effect transistor (OFET)-type memory integrated on a thin and flexible metal wire. A new fiber coating technique by using a capillary tube and controlling the solution viscosity allows the formation of a thin and uniform organic ferroelectric film on the wire. The uniform morphology imparts excellent switching stability, long-term retention time and low-voltage operation to the flexible fiber organic memory devices. The fiber-shaped memory achieves reliable data storage even under tough environments when sewn in a stretchable textile fabric, providing the possibility of the practical, wearable fiber memory for recording electronic signals in smart textile applications.

In the recent years, the smart electronic skin (E-skin) has attracted extensive attention due to the human skin provides remarkable sensor networks such as touch, temperature, vibration and pressure sensors, etc. There are many wearable electronic devices integrated with glasses and watches in the consumer electronics with the rapid development. However, all of these devices face an important issue about the stretchability which could sustain complex deformation and conformal contact with irregular surfaces. The stretchable materials are mainly required as a substrate such as organic polymer in order to make the devices stretchable. Apart from these, nanomaterials, such as carbon nanotubes, graphene and metal nanowires have been utilized to realize stretchable electronics. And the special structure designs have also played a very important role in improving stretchability. Wave, island-bridge structure, porous structure and grid structure are widely applied in these novel designs.

Herein, we developed a transparent conductive membrane which is uniform and stable even under elongation through spin-coating AgNWs solution on the stretchable substrate. The motion detection and location is indispensable for human-machine interfaces. However, as most devices work in digital method, there has been limited progress in the resolution enhancement that may lead to an increase in the number of electrodes. Therefore, the stretchable analog devices play an essential role in wearable devices, which owns higher resolution with a few electrodes. we demonstrate these new properties by using PDMS/AgNWs thin film.

A novel stretchable, transparent location sensor has been demonstrated in this work. Through the spin-coating process, the AgNWs are deposited on the surface of the PDMS substrate, which enhances the transparency and stretchability of the device. It can be attached to the human skin with conformal coverage and keep the function under the elongation of 60%. When the sensor is pressed, the positon of the contact point can be acquired by the relative resistances in the analog method, which can enhance the resolution without increasing in the number of electrodes. All of the excellent properties makes the sensor have much potentials in the wearable devices and smart robots.

Three-dimensional (3D) architectures qualitatively extend the functionalities of electronic and optoelectronic devices. Complex functional 3D structures have been fabricated by 3D printing, stress-controlled folding, and mechanically-guided assembly. However, the potential of 3D functional architectures built by previous methods is restrained by their static or human-intervention required dynamic structures. Herein, we demonstrate a soft 3D scaffold integrated with electronic and optoelectronic devices which can be dynamically tuned by responsive polymer substrates. The 2D films are selectively bonded onto the responsive substrates which can respond to heat simulation and actuate the assembly of the 2D films to 3D architectures. The assembled structures are dynamically controlled by the heat. The dynamic assembly is highly controllable by both experimental study and numerical simulation. Soft electronic and optoelectronic devices, i.e., a strain sensor array and a photodetector array, are integrated to the 2D films and also assembled to 3D structures. The strain sensors monitor the motion of the assembly. The photodetectors can measure both intensity and direction of incident light attributing to its 3D architectures. Moreover, the photodetector array has the capability of tracking lights incoming from all directions because its spatial configuration can be dynamically tuned by heat.

Conductive polymers coatings on metal electrodes are an efficient solution to improve neural signal recording and stimulation due to their mixed electronic-ionic conduction and biocompatibility [1]. However, only a few studies have been reported on conductive polymers coatings on metallic wire electrodes for muscle signal recording [2]. In this work, we developed mechanically and electrochemically stable invasive electrodes for muscle signal recording in small animals based on stainless steel multi-stranded wires coated with the conductive polymer PEDOT.
PEDOT doped with LiClO4 was galvanostatically electropolymerized on stainless steel wires using three different solvents: propylene carbonate (organic), acetonitrile (organic) and water (inorganic). The coatings adhesion to the metallic substrate was tested through ultrasonication and the electrochemical stability was evaluated through phosphate buffer solution soaking test and autoclave sterilization.
The solvent played a key role on the adhesion of the PEDOT coating, with organic solvents giving the best mechanical stability. Electrodes prepared with these solvents possessed excellent electrochemical stability and survived sterilization and prolonged soaking without major changes in electrochemical properties.
A solution for high quality invasive muscle signal recording in small animals based on conductive polymers has been demonstrated.

[1] R. Balint, N. J. Cassidy, and S. H. Cartmell, "Conductive polymers: Towards a smart biomaterial for tissue engineering," Acta Biomaterialia, vol. 10, no. 6, pp. 2341-2353, 2014.
[2] S. Kim, L. K. Jang, M. Jang, S. Lee, J. G. Hardy, and J. Y. Lee, "Electrically Conductive Polydopamine–Polypyrrole as High Performance Biomaterials for Cell Stimulation in Vitro and Electrical Signal Recording in Vivo," ACS Applied Materials & Interfaces, vol. 10, no. 39, pp. 33032-33042, 2018.

Compared to other liquids (e.g. water, ionic liquids, organic liquids), liquid metals have outstanding thermal and electrical conductivity, as well as distinct optical properties (i.e. reflectivity). As a result, they are excellent candidates for stretchable and soft metallic components such as antennas, electrodes, and interconnects. Thus, there is great interest in facile methods to create and pattern these liquids into useful components. The large tension and low viscosity of liquid metals presents a challenge: the metal tends tend to adopt a spherical shape as it exits a nozzle. There are a number of strategies to address this challenge that require the metal to directly contact other solid or visco-elastic surfaces. Here we report a way to create wires by directly extruding the metal from a nozzle. To control the shape of the liquid metal, we used electrochemical control of surface oxidation because the formation of the surface oxide significantly lowers the interfacial tension and allows the metal to adopt shapes that overcome Rayleigh-Plateau instabilities (i.e. it does not bead up).
To create these wires, we pump liquid metal into electrolyte (1M NaOH). As it emerges from the nozzle, the liquid metal forms large spherical drops to minimize the surface area. However, this droplet size diminishes with an increase of potential, which indicates a decrease in interfacial tension. At a critical potential, the interfacial tension drops significantly, and the liquid metal forms wires. At larger values of electric potential (>0.8V), the oxide layer gets thicker, causing the wire to deform and turn into unusual shapes. We successfully analyze each distinct regime of EGaIn as a function of flow rate and electric potential to identify conditions that form wires. The ability to deposit or remove the oxide electrochemically enables unprecedented control over interfacial activity (well beyond what is possible using electrocapillarity), which provides a means to reconfigure and manipulate the shape and position of liquid metal. This method requires minimal energy and provides rapid and reversible control of interfacial tension over an enormous range compared to conventional molecular surfactants. By adjusting the electrical potential and flow rate, the configuration of liquid metal can be manipulated on demand, which offers the possibility to create actuators or soft and stretchable analogues of rigid conductors.

E-skin tactile sensors mimicking human mechanoreceptors have been studied intensively and expected to be applied to various future electronics such as haptic device, robot tactile sensor, human-machine interface, prosthetic applications and implantable electronic devices. To imitate the human cutaneous tactile sensors, E-skin is characterized by extreme mechanical flexibility or deformability and excellent recognition of force distribution. The mechanical flexibility of the E-skin has been successfully achieved using ultrathin substrates or stretchable interconnection between the rigid sensor units. Those device structures minimize the stress accumulated on the device units so that E-skin can operate under repeated stretches. The breakthroughs open the possibility of fabricating printed active matrix tactile sensors.
In order for E-skin to sense the spatial distribution of mechanical stimulation, a large number of sensors must be integrated to provide enough spatial resolution. For this purpose, some advances have been made in the fabrication of transistor-operated tactile sensor arrays. Although the transistors could prevent electrical crosstalks between the pixels and give large on-off ratios, the complexity of the transistor-based device is high. Active matrix structure is advantageous for individual pixel operation especially for highly integrated devices such as display or memory. However, in terms of E-skin tactile sensors, a passive device with a simple two-terminal structure would be more desirable to achieve high flexibility or deformability simultaneously with enough spatial resolution. Using a rectifying diode can be another approach to remove the electrical crosstalks. Someya and coworkers fabricated a pressure sensor matrix in which each cell was composed of a resistor-selector combination. They developed a mechanically robust organic diode and integrated a pressure sensitive elastomer resistor on top of the diode layer, which was the first demonstration using the rectifying diode for a flexible tactile sensor. This type of sensor matrix effectively avoided the electrical crosstalks. In terms of E-skin devices, the rectifier-based approach deserves more studies in the directions of simple device structure, high spatial resolution, and high sensitivity in wide range of pressure.
In this study, we propose a simple pressure sensor matrix for E-skin, in which each pixel is separated geometrically from each other and operated by the diode current sensitively changing upon applied external pressure. Therefore, each pixel has the two-terminal one-selector structure which is the pressure sensor itself. We used position-registered conductive microparticles (MPs) to achieve the one-selector device structure. Research on the arrangement and application of assembled MPs has been conducted over the past three decades, but there has been no report showing the feasibility of MPs-based integrated electronic devices. This is because it is difficult to assemble the MPs in a large area with accurate position registration and the concept of using the MPs in the electronic devices has not been developed yet. We have fabricated a highly flexible MP-based pressure sensor matrix that can precisely sense the distribution of external pressure. Since each pixel of the sensor can independently sense the pressure, we use the sensor matrix a flexible electronic scale and as an artificial fingertip to read Braille.

Neurological disorders strike millions of people each year, accounting for an annual economic cost of hundreds of millions of dollars that include not only direct expenses such as hospitalization and prolonged care, but also indirect ones caused by subsequent productivity losses. The most common treatment is oral or intravenous administration of pharmaceutically active compounds. These methods, however, require repeated administration and allow the drug to interact with tissues indiscriminately, causing side effects. Moreover, drug delivery in the brain is often precluded by a variety of physiological and metabolic obstacles such as the blood–brain barrier (BBB). The pursuit of novel administration strategies for efficient delivery of drugs to the central nervous system with greater precision and fewer side effects than conventional methods is an ongoing target of the biomedical community. We will discuss a novel class of devices that are implanted past the BBB and deliver the drug where and when is needed. These devices use a microfluidic channel to carry a drug solution into the brain where they use electrophoresis to deliver the drug without the solvent. They offer excellent spatiotemporal resolution, low-voltage operation, and high ON/OFF ratios. In vivo validation in a rodent model of epilepsy shows that these devices can stop, or even prevent seizures, paving the way for a new treatment option for neurological disorders.

To totally understand brain-function networks, monitoring of brain signals by placing brain-signal sensors at various positions from the cerebral cortex to deep brain regions using advanced technology is important. However, the brains of most primates with advanced brain activities are small and are easily compressed by these sensors. Therefore, the development of a thin, flexible, and lightweight sensor system is indispensable.

In this study, we have developed a brain-implanted multichannel system for monitoring brain activities using (1) thin, flexible, and stretchable integrated circuits and sensors and (2) a highly conducting and flexible material with high stretchability like rubber and an organic integrated circuit technology using this material. Furthermore, we added optical functions on the basis of the technology of thin light-emitting diodes, successfully developing a minimally invasive brain-function monitoring system for optogenetics.

In neuroscience, weakly electric fish are used to study how motor signals influence sensory processing [1-3]. Specifically, understanding how information is encoded in the precise timing of electroreceptor spikes relative to sensory inputs will significantly contribute to behavioral studies. Previously, stimulating the electroreceptors of weakly electric mormyrid fish has been restricted to dipole current sources [4]. In this work, we present a dense flexible electrode array that conforms to the fish’s skin and is seamlessly integrated with a robust user interface. The electrode array consists of 96 square electrodes with a side length of 250 μm and diagonal spacing of 1 mm. The pattern was chosen to match the hexagonal pattern of the fish’s electroreceptors. Gold electrodes were photolithographically patterned on parylene-covered spin-coated PDMS, as shown in multiple previous studies [5,6]. A second parylene layer was deposited and patterned to encapsulate the electrode traces and avoid crosstalk between electrodes. Chemical surface modification was completed to match the surface energy of the fish’s skin for several days. A multiplexer based circuit was used to individually select electrodes. A LabVIEW program allows a user to choose an electrode and synchronize the electrode’s information with neurophysiological data acquisition software. Local field potential was measured in the electrosensory lobe of the hindbrain while individually driving electrodes. Results demonstrate that this system is satisfactory for more complex neuroscience experiments.
Ref: [1] C. C. Bell, Science, 214, 450-453, 1981. [2] A. Kennedy et al., Nat. Neurosci., 17, 416-422, 2012. [3] T. Requarth et al., Neuron, 82, 896-907, 2014. [4] C. C. Bell, J. Neurophysiol, 63, 303-318, 1990. [5] M. Ochoa et al., Biomed Microdevices, 15, 437-443, 2013. [6] K. Meacham et al., Biomed Microdevices, 10, 259-269, 2008.

Neuromorphic electronics is promising to process complex real-world problems such as visual information, speech recognition, and body movement control based on their compactness, fault tolerance, and high-energy efficiency. Artificial synapses are rapidly emerging for neuromorphic electronic devices that emulate biological nervous systems. Emulating learning and memory functions of the brain has been the main focus in neuromorphic electronics, but mimicking the complicated biological sensory and motor nervous systems that operate sequential functions related to proprioception, signal processing, and a motor response is also a challenging issue especially to realize bio-inspired soft robotics and electronics. Herein, we demonstrate flexible and stretchable organic artificial synapses for sensory and motor nervous systems of bio-inspired electronics. Sensory and motor electronic nerves are demonstrated by integrating organic synapses with sensory and motor organs. The sensory organs detect stimuli and fire artificial neural signals which will be transmitted to organic synapses as presynaptic action potentials. The flexible and stretchable organic synapses which are favorable for soft robots generate a post-synaptic response which will stimulate motor neurons and muscles. Thus, we realize i) a hybrid reflex arc system composed of artificial pressure-sensory nerves and biological motor nerves in a detached insect leg, and ii) artificial sensorimotor nervous system composed of an artificial light-sensory receptor and electronic neuromuscular system with an artificial muscle fiber. In addition, we demonstrate that these neuromorphic systems are promising to develop human/machine interface by distinguishing braille characters and conducting wireless optical communication. Our flexible and stretchable organic artificial synapses for sensory and motor nervous systems can be used for human-like soft robots and prosthetics which help people with neurological disabilities.

The adoption of a thin-film substrate is a key aspect for the development of skin-contact imperceptible devices, in the so–called cutaneous or epidermal electronics. Together with reducing the thickness, lowering the mismatch in the mechanical properties of the device, with respect to the skin’s own, plays a major role. These properties reflect in devices inherent bending stiffness, determining their ability to flex to small curvature radius, following the skin relief profile in conformal adhesion. Temporary Tattoo Electrodes (TTEs) were developed to meet these requirements. TTEs are all made by polymers and their overall thickness is < 1-2 µm. They are fabricated by inkjet printing of the conductive polymer PEDOT:PSS on top of commercially available temporary tattoo paper. Recently, ultraconformable TTEs have been adopted in the recording of many electrophysiological signals and compared with standard Ag/AgCl electrodes. Wet Ag/AgCl electrodes are routinely used in electrophysiology (EP) thanks to their high signal quality. On the other hand, these electrodes exhibited many disadvantages which impose severe restrictions in all EP applications. The major issues are related to their limited time stability, due to gel drying, and to their cumbersome nature. With TTEs we showed the recording of electrocardiography (ECG) and electromyography (EMG) demonstrating their outstanding capabilities in interfacing with human body (1,2).
Now, we investigated the recording of electroencephalography (EEG) signals, the most challenging scenario due to the weakest signal and the lowest frequency range in EP (typical 10 - 100µV signal amplitude, frequency components 0.5Hz - 100Hz). EEG monitoring was performed in a clinical environment with the aim to open for new perspectives in EEG. TTEs have been thus tested in different clinical conditions, to cover the most of EEG diagnostics. We firstly validated TTE with the recording of alpha waves, the most known and studied brain rhythm. TTEs were also compared with Ag/AgCl electrodes, through Power Spectral Density (PSD) assessment, showing a good match over the whole signal spectrum. TTEs were further evaluated in auditory evoked potentials (AEP) measurements, in comparison with the standard Ag/AgCl electrodes, exhibiting comparable at all results. Furthermore TTEs’ compatibility with magnetoencephalography (MEG) equipment have been performed. EEG/MEG are frequently used in combination and at the best of our knowledge it is the first time that a dry electrode exhibits complete compatibility with MEG.
The successful evaluations of TTEs in the EEG field paves the way for future development of imperceptible alternatives to standard electrodes in clinical practice.

1. Zucca, A., et al. "Tattoo Conductive Polymer Nanosheets for Skin–Contact Applications." Advanced healthcare materials 4.7 (2015): 983-990.
2. Ferrari, L.M., et al. "Ultraconformable Temporary Tattoo Electrodes for Electrophysiology." Advanced Science 5.3 (2018).

Conductive hydrogel-based strain sensors hold great promise for wearable electronics, point-of-use medical sensors, and soft robotics. However, their sensitivities are generally low and they suffer from signal hysteresis and fluctuation due to the viscoelastic property, which can compromise their sensing performance. In our recent work, we prepared MXene (Ti₃C₂T_x)-based Poly (vinyl alcohol) (PVA) hydrogel (M-hydrogel) via a simple method [1]. The obtained M-hydrogel sensor exhibits high strain sensitivity with remarkable stretchablity, instantaneous self-healing ability, excellent conformability, and adhesiveness to various surfaces including the human skin.

More importantly, the M-hydrogel shows much higher sensitivity under compressive strains than tensile strains. This asymmetrical strain sensitivity coupled with viscous deformation (self-recoverable residual deformation) is for the first time proposed to add new dimensions to the sensing capability of hydrogels: both direction and speed of motions on the hydrogel surface can be detected conveniently. Based on this effect, the M-hydrogel shows superior sensing performance in advanced sensing applications such as recognition of signature and vocal signals. Thus the traditionally disadvantageous viscoelastic property of hydrogels is turned into an advantage for sensing, which reveals new prospects for hydrogel sensors.

[1] Y.-Z. Zhang, K. H. Lee, D. H. Anjum, R. Sougrat, Q. Jiang, H. Kim, H. N. Alshareef, "MXenes Stretch Hydrogel Sensor Performance to New Limits”, Science Advances, 4(6), eaat0098 (2018).

Conformal polymer electrodes can be vapor printed directly onto the surfaces of living organisms without damaging their health and self-sustenance. These polymer electrodes can then be used as contact pads for bioimpedance spectroscopy, which reveals detailed information about the health of the organism. Notably, polymer electrodes can be created directly on living plants, meaning that bioimpedance measurements can be performed on-demand, throughout the growth cycle of a plant. Vapor-printed polymer electrodes, unlike their adhesive thin-film counterparts, do not delaminate from microtextured living surfaces as the organism matures and do not observably attenuate the natural growth pattern and self-sustenance of the plants investigated herein. Deep tissue damage caused by dehydration and UVA exposure can be reliably detected throughout the life cycle of a plant. Long-term plant health monitoring will find strategic use in food farming, crop management, and biohazard signaling.

Requirement of real-time health monitoring boosts the development of wearable electronics. Especially cutting-edge technologies of integrated circuit board have been successfully utilized to obtain flexible devices. Here we report development of 3D ion selective field sensors integrated by 3D printing as ion selective field effect transistors (ISFETs). Separately fabricated FETs and ion selective membranes are both verified before hybridization. Then, the combined ISFETs show high sensitivity to ammonium (NH₄⁺), potassium (K⁺), and calcium (Ca²⁺) ions with linear responses depending on ion concentrations. Moreover, high selectivity of these ISFETs has also been proved during the interference tests, not only by the separate interference ions testing, but also by artificial saliva with multiple interference ion concentrations. Additionally, unique wireless protocols for the 3D printed ISFET has also been implemented. The AC output signal converted from DC input during this signal transmission process is identified as the ion concentration. This new wireless sensing protocols open the potential of the 3D printed ISFETs in the field of wireless health monitoring sensors.

Intrinsically stretchable conductors form a vital component of advanced soft bioelectronics. And novel nanocomposites based on conductive nanomaterials have been used in diverse applications of implantable and wearable bioelectronics. Among many nanomaterials for the composites, silver nanowires (Ag NWs) have been popular. However, achieving highly conductive and soft composites simultaneously is challenging. Furthermore, because bioelectronics is necessarily exposed to biofluids, preventing Ag NW oxidation and Ag ion leaching is a significant challenge. Here we have achieved a highly conductive, biocompatible, and soft nanocomposite by using silver-gold (Ag-Au) core-sheath NWs and polystyrene-butadiene-styrene (SBS) elastomer. We synthesized ultralong Ag NWs encapsulated with a smooth and uniform Au sheath and then mixed them with the polymer. Phase separation of Ag-Au NWs and SBS occurs, which forms microstructures in the composite, reduces Young’s modulus, and increase softness and conductivity of the composite. We used the nanocomposite to fabricate a customized multi-channel soft cardiac mesh for the diseased swine heart. The mesh could be also fabricated as a wearable device which monitored electrophysiological signals and applied electrical and thermal stimulations on the human skin.

Particles made from liquid or low-melting-point alloys can be leveraged to create a new class of functional responsive composite materials. In this talk, I will present two types of metallic particles: eutectic gallium-indium particles and Field’s metal particles. Eutectic gallium-indium particles have been used to achieve printable, soft, and stretchable electronics. More recently, we have demonstrated the role of Field’s metal particles in enhancing the range of stiffness properties in a thermally responsive epoxy, and in creating variable elasticity and switchable anisotropy when embedded in silicone elastomer. These responsive material properties derived from metallic particle additives enable new capabilities for wearables, soft electronics, and soft robotics.

Liquid crystal elastomers (LCEs) are soft, anisotropic materials that exhibit large, reversible shape changes in response to external stimuli. The director (average alignment) of an LCE can be localized into pixels via photoalignment. By rationally designing the director profile, out-of-plane shape transformations are possible when the material is activated. In this talk, single-wall carbon nanotube – LCE nanocomposites are presented, and their distinctive electromechanical actuation explored. When a DC electric field is applied through the material thickness, the composite with contract along the alignment direction. By localizing the orientation of the LCE and SWNT via photoalignment, complex 3-D shapes can be electrically triggered. Additionally, a brief explanation of the novel electrostrictive mechanism will be presented.

A vision for many potential applications of flexible and stretchable electronics has led to new and diverse research approaches in the previous decade, such as intrinsically stretchable organic electronics and strain relief geometries in otherwise non-compliant traditional materials. A third approach is to engineer materials to respond and adapt to strain by repairing themselves through self-healing mechanisms. In this work, we utilize the stimulus-responsive characteristics of a colloidal room temperature eutectic gallium-indium alloy (EGaIn) to achieve electronic self-healing triggered by strain and mechanical damage. Unlike organic conjugated systems, EGaIn possesses the high conductivity of a metal and while its fluidic properties allow it to flow and adapt to induced strain. We utilize EGaIn’s intrinsic ability to grow a native passivating viscoelastic oxide skin to produce liquid metal core-shell nanoparticles, serving as small reservoirs of the fluidic metal core, which upon rupture provide the potential to reconnect damaged electronic components and restore electrical connectivity. We report on our ability to modify the mechanical properties of the core-shell nanoparticles by correlating changes in stiffness, shell modulus, and ultimately the rupture force required to induce healing with various chemical treatments of the oxide shell (Farrell et al., Langmuir 2018). Flat-punch nanoindentation techniques with in situ electrical characterization were developed to experimentally characterize single particle stiffnesses and the critical buckling loads associated with particle rupture which would lead to electrical restoration. The modulus of the oxide shell was estimated using Reissner’s elastic thin plate theory, with estimated moduli between 0.37-1.38 GPa depending on functionalization. A fifty-fold increase in critical buckling load was measured across the various particle systems, providing a large range of engineering control and allowing application specific mechanical properties for implementation in self-healing coatings, stretchable conductors, and push-to-connect low temperature solders. In light of the standalone nature of the particle system, we anticipate their addition to existing electronics in a variety of ways, such as chemical binding through ligand anchoring, spray coating, or drop casting; imparting healing functionality and mechanical robustness to traditional electronic materials.

The multi-disciplinary efforts in the last few decades have enabled mechanically softer and skin-like electronic devices^1–5. These exciting progress opens multiple doors of opportunity to utilize innovative materials and devices intimately with epidermal surfaces, wearable technologies and robotics. In this talk, I will discuss some of the new material platforms we are developing for more robust human-machine interactions, such as self-healing stretchable optoelectronics devices. At the same time, I will also present our efforts in developing new system architectures to scale the density and number of such devices for rapid environment perception. Such system architectures can be broadly applied in electronic skins, and has broad potential for continued applications in wearables, brain-machine interfaces, prosthetics and robotics.

References
1. Heikenfeld, J. et al. Wearable sensors: Modalities, challenges, and prospects. Lab Chip 18, 217–248 (2018).
2. Wang, S. et al. Skin electronics from scalable fabrication of an intrinsically stretchable transistor array. Nature 555, 83–88 (2018).
3. Tee, B. C. K. et al. A skin-inspired organic digital mechanoreceptor. Science (80-. ). 350, 313–316 (2015).
4. Gao, W. et al. Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis. Nature 529, 509–514 (2016).
5. Tan, Y. J., Wu, J., Li, H. & Tee, B. C. K. Self-Healing Electronic Materials for a Smart and Sustainable Future. ACS Appl. Mater. Interfaces 10, 15331–15345 (2018).

Recently emerged electronic skins with applications in on-body sensing and human−machine interfaces call for the development of high-performance skin-like electrodes. In this presentation, a family of recently developed stretchable, transparent, and breathable epidermal electrode will be introduced, which is composed of a nanofibers scaffold and a silver nanowires network.
Through techniques such as vacuum filtration or electro-spinning/electro-spraying, the silver nanowires are embedded into a scaffold made of polymer nanofibers. Optical transmittance of 84.9% at 550 nm wavelength is achieved at a significantly low sheet resistance of 8.2 Ωsq⁻¹. The resistance of the epidermal electrode only slightly increases by less than 0.1% after being bent for 3000 cycles at the maximum curvature of 300 m⁻¹ and by less than 1.5% after being dipped in saline solution for 2500 cycles. Attributed to silver nanowires with hierarchical dimensions, great robustness against tensile strain is achieved. The reinforcement from the nanofiber-based scaffold as a backbone maintains the connections among the Ag nanowires by undertaking most of the loaded stress. The epidermal can still maintain its conductivity when experience a strain of over 300%.
The epidermal electrode can form tight and conformal bonding with genuine skin through Van Dal Walls force. Because of the conformal contact as well as the high conductivity, the epidermal electrode shows a contact impedance 50% lower than commercial gel-based electrode. As a result, the epidermal electrode exhibits apparently higher noise resistance again body motions when measuring physiological signals such as ECG, making it promising to be used in ambulatory on-body monitoring for healthcare applications. Besides, the epidermal electrode allows the evaporation of perspiration, making it suitable for long-time use.

The most important function in soft robotics is to travel through harsh spaces and receive the real-time sensing signals wirelessly based on today’s smart technologies. However, to date, the reported soft robotics has the limitation of low degrees of freedom of movement because tethered systems should be connected to soft actuators to initiate the actuation. Although few research groups have demonstrated an untethered soft actuator, the reported untethered soft actuators only mimicked the movement of people or animals without any environmental-sensing capability. The key requirements for designing human-like soft robots with somatosensory systems are to operate the soft actuators without supporting equipment and to have wireless environmental-sensing capability. Fabricating fully wirelessly-operated soft actuators with environmental-sensing capability has advantages of improving degrees of freedom of movement and adapting them to various applications like biomedical fields. For the Internet of Things (IoT)-based smart wireless environment, providing wireless environmental-sensing systems has highlighted the need for using the electronic sensors and integrating them to a soft actuator body. However, the conventional electronic sensors are rigid, the integration of the electronic sensors to the soft actuator body has challenges for poor adhesion between electronic circuits and the soft actuator body and low reliability and stability of electronic sensors due to their rigid properties. Thus, improving the reliability of electronic sensors is essential for the advances in soft robotic fields. Herein, we report an unconventional approach for demonstrating fully wireless soft robotics with wireless tactile-sensing systems. The electrothermally-actuated phase transition soft actuator is composed of the entirely soft and flexible body frame. A liquid-metal particles are embedded in the soft actuator matrix so that the rate of initiating the actuation of the soft actuators increases. Soft actuators we designed are integrated to a pressure sensor and temperature sensor with improved stability and reliability due to conformal actuating properties, causing the actuation strain to distribute throughout the entire soft body actuated. However, it still has a problem of generating of electron-hole pairs (EHPs) in pressure sensors as the temperature of flexible heaters increases. The generation of EHPs results in the increase in the output current. Thermally excited charge carriers can affect the sensing signals of pressure sensors. Evading the generation of EHPs is an important feature for the sensory system to receive pressure sensing information exclusively. To that end, we designed the configuration of pressure sensor and temperature sensor above the heater film. By integrating them in parallel above the soft body frame, only tactile sensing signals can be received with calibration steps using sensing signals of the temperature sensor. It provides the pressure sensors are independent of the increasing temperature of heaters. Our soft actuators can be actuated wirelessly with remote temperature control systems interconnected to heaters. And, they can also receive real-time sensing feedback wirelessly by connecting the wireless pressure sensory module to electronic sensors. Our demonstration of wireless actuation of soft actuators with wireless environmental-sensing capability suggested that the soft actuator has an important feature for providing the capability of the use in the harsh spaces.

Achieving implantable devices that reside in human body with chronic stability has long been a goal of the bioelectronics community. A major challenge in achieving long-term stability lies in the difficulty in establishing reliable interconnection of the implantable devices to the outside world. For example, implantable neural devices in the periphery undergo significant strain due to repeated muscle flexion and extension. Furthermore, device encapsulation can be significantly weakened under such strain, causing infiltration of physiological fluids into devices that leads to chronic failure. Therefore, efforts toward developing implantable electronics based on high barrier and strain tolerant materials are needed.

Here, we present a liquid crystal elastomer (LCE)-based, soft, and stretchable electronics architecture for their potential as a platform for chronic implantable bioelectronics. The unique shape-morphing capability of LCE is leveraged to fabricate electronic devices in strain tolerant three-dimensional (3D) shapes. Specifically, we employ LCEs as substrates for electronic devices that are flat during processing but then morph to 3D shapes for use. A variety of thin-film electronic components, including conducting traces and metal-insulator-metal (MIM) capacitors, is fabricated on surface-aligned LCE substrates by using photolithographic microfabrication processes. These devices are shown to morph their shapes from 2D to 3D as programmed, upon the release of the LCE films from the carrier wafer. The 3D helical cable conductors and capacitors created by this method demonstrate strain tolerance up to 100% with less than a 5% change in resistance or capacitance. We will discuss the mechanical and electrical properties of these cables with various encapsulation coatings including parylene-C, Al₂O₃/parylene-C bilayer, and LCE. Our preliminary results suggest that the LCE-based soft electronics technology proposed herein may open up a new avenue for developing chronic implantable bioelectronics and neural interfaces.

The application of silicones and thermoplastic polyurethanes (TPUs) in the number of electronic and flexible device applications has multiplied through the years with its excellent composition control and the ability to prepare various forms primarily through the injection molded and thermo-formed processing shapes. This means that their thermo-mechanical properties can be tuned towards more mechanically compatible properties to the human body including elastomeric properties. However, in a number of prosthesis devices including artificial body parts, sensors, implants, etc. 3D printing is emerging to be an alternative for additive manufacturing with its rapid-prototyping advantage and the ability to incorporated new polymer and nanocomposite in processing methods such as fused deposition modeling (FDM), stereolithographic apparatus (SLA) and viscous solution printing (VSP). It should be a viable method for the limited and specific production of flexible electronic devices that are fit even for personalized healthcare This talk will highlight our work on 3D Printed Silicones, 3D printed TPU, etc. and 3D Printed nylons, to form basic material compositions flexible <span style="background-color:rgb(246, 213, 217)">electronic nanocomposite materials containing graphene, carbon nanotubes, carbon black,e etc. </span> Structure-property relationships correlated with the formation of improved modulus and flexural strength with increasing amounts of electronic nanofiller components.

Photothermal materials absorbing near infrared light have received strong attention for optical filters, sensors, energy harvesters, and photothermal theragnosis agents. In particular, NIR absorbing ionic dyes containing multi aromatic units have high transparency and optical selectivity, so they are challenging materials as a transparent photothermal material. However, the researches on these materials are rather limited due to the multi-step synthetic process as well as difficulty in the characterization of molecules. Herein, we report synthesis and application potential of NIR absorbing ionic dyes as a photo-actuating electrode. NIR absorbing ionic dyes were synthesized through modification of aromatic amines over >80% yields, and characterized by spectroscopic methods. The ionic dyes showed strong absorption of NIR light at ~ 1000 nm region, while they were transparent in visible region (%T > 90) both in solution and film state (Fig. 1(a)). Thin film of the ionic dyes was applied to a photothermal actuator, which showed large temperature rise upon exposure to a NIR light (Fig. 1(b)). The temperature rise, transparency, and photo-actuation of the transparent actuator of the ionic dyes will be presented.

The cracking of electrochromic materials due to aging or reiterative bending is a major problem which noticeably degrades the performances of electrochromic devices. In this research, a novel self-healing electrochromic polymer PPFMA was synthesized, which integrated the electrochromic triphenylamine and self-healing Diels-Alder groups. The PPFMA film showed all superior electrochromic properties, such as stable color changes (yellow to green to blue) with a maximum optical contrast of 42.7 % at 1050 nm and high coloration efficiency of 108.3 cm² C^-1. Meanwhile, the polymer PPFMA also showed excellent self-healing performance, cracks cut on the film surfaces (bleached or colored states) could be self-healed at 110 °C within 4 min, and the self-healing rate was about 80 %. The results indicate that the polymer PPFMA has the mutual independent bifunction of electrochromic and self-healing, it is a promising material for prolonging the service life of electrochromic device.

Recently, studies on the fabrication of stretchable electrodes using AgNW have attracted much attention. Among the many methods, stamping transfer process is most considered as an advanced technology which can overcome the limitations of existing methods such as lithography, and wet coating. In this study, UV curable pressure sensitive adhesives (PSAs) consisting of acrylic adhesives and UV curable oligomers are prepared. The novel UV curable oligomers are synthesized through two steps. The aminolysis of the cyclic carbonate with amine-capped polypropylene glycols (PPGs) afforded multifunctional alcohols in the first step. Next, the reaction between the alcohols and methacrylic isocyanates produced multi-arm PPG methacrylates.
These compounds were mixed with the acrylic adhesive to generate two kinds of UV curable PSAs. The peel strength of the two PSAs on th PC substrate was 1,113 gf/in, and 991 gf/in, respectively. Interestingly, the peel strength of the PSAs was reduced by two orders of magnitude after UV irradiation. The initial tack forces of the PSAs also were dramatically reduced. The reduced adhesion of the PSAs was attributed to the high storage moduli induced by UV treatment. The UV curing of PSAs was confirmed bymonitoring the disappearance of an IR absorption band corresponding to H-C=C stretching by FTIR spectroscopy.
Finally, AgNW and UV curable PSA were mixed and coated to make a film.
The sample was irradiated with UV through a mask to produce a pattern in which only the exposed portion to lost its adhesion. The adhesive portion was transferred to the opposite substrate through the stamping transfer process,
and the sheet resistance was measured as nearly same to that before the transfer. Stretching and bending tests were performed on the fabricated samples, which demonstrated that they are well suited for the production of stretching electrodes.

Flexible tactile sensors that can precisely and rapidly detect touch and provide location are of great importance for e-skin development [1]. Piezoelectric sensors are a great candidate because of their high voltage response, speed and sensitivity [2]. Current implementations use dense taxel arrays to accurately detect touch with high precision [3,4]. However, these solutions suffer from complex designs, slow response rates, expensive detection equipment or large wire arrays required to address individual sensors. The work presented here improves on current work by achieving high spatial resolution with a simple fabrication scheme and a minimal number of sensing elements. The sensing circuitry for the device utilizes a single charge-amplifier and a multiplexer controlled by a microcontroller to achieve high-speed measurements at a low cost.

Yu et al., have shown that multiple sensing modalities can be combined within a single PVDF sensor and utilizing compressible compliant layers, such as foam, can improve device sensitivity [5]. Here we iterate on this design by stacking two layers of PVDF with patterned electrodes to create a dense sensing area encapsulated and sandwiched by PDMS layers. The deformable nature of the PDMS offers increased sensitivity by acting as a compliant layer. PDMS can be used in a release mold to create a flexible skin that can be attached to existing grasping fingers, or the PDMS can be directly bonded to custom finger architectures to improve adhesion to the substrate while being fully integrated into the finger.

PVDF pre-coated on both sides with sputtered Cr/Au (40/250nm) was lithographically patterned and wet-etched utilizing backside alignment techniques to form electrode arrays on a 15x15mm active area. The electrode arrays were electrically connected with heat-seal connectors before being stacked and encapsulated in PDMS. This sensor consists of two PVDF arrays with 16 electrode pairs to create a grid pattern. The electrodes were individually addressed with a multiplexer as seen in Geng et al., to simplify the circuit by using only one charge amplifier while maintaining a high sampling frequency [5].

The sensor was characterized with a robotic indenter by collecting individual sensor readings across a dense grid of indentation locations to train a kernel ridge regressor to predict locations against a test set of randomized locations. The prediction accuracy improves at deeper indentation depths and has a minimized prediction error of 0.78 mm with a standard deviation of 0.61 mm. Compared to a 15x15 array of individual taxels, this design achieves comparable spatial resolution with 14x fewer sensing elements. The sensor can also be implemented as a tri-modal sensor to capacitively sense object proximity, measure pressure by implementing strain gauges along with localization data from the grid. This rich information set can be used to augment robotic grasping applications.

[1] R. Dahiya, IEEE Trans. Robot, 2010, pp. 1-20. [2] R. R. Reston and E. S. Kolesar, IEEE Conf, 1990. [3] B. Choi et al., Diffus. Defect Data Pt.B Solid State Phenom, (2007), pp. 229-234. [4] L. Seminara et al., IEEE Sens. J., (2013), pp. 4022-4029. [5] C. Yu et al., MEMS 2018, 886-888, 2018 [6] Z. Geng et al., IEEE Instrum. Meas. Technol. Conf., (2011), pp. 74-78.

A development of single-crack-activated impedance strain sensors with unprecedented sensitivity is demonstrated first. The gauge factor of the device is beyond 10⁸ in 10^-4 strain range in comparison with the reported highest gauge factor 1.5 × 10⁵ within 60% strain range, and the displacement sensitivity is 1.6 MΩ nm⁻¹. The extremely high sensitivity is attributed to the transition region which has never been studied before. Multiple crack-based sensors, however, cannot work in the transition region due to complicated interaction among cracks, which essentially limits their sensitivity. Additionally, studying a precisely controllable single crack rather than multiple cracks is favorable for excluding other factors such as crack spacing, difference among cracks, and interaction among cracks, simplifying the model and facilitating better understanding of the underlying mechanism of the device. The device can satisfy requirements of mechanical flexibility, durability, and repeatability. In addition, the device developed is capable of measuring displacement in nanometers range or force in tens of nanonewton range, and has the potential to be applied in various fields, such as specific biomolecular recognition.

Strain sensors with high sensitivity and stretchability are highly required for the application of wearable or implantable sensor to detect the human motion. In this study, we have prepared a highly stretchable double network (DN) of soft polyacrylamide and brittle calcium-alginate microfibers containing poly(3,4-ethylenedioxythiphene)-poly(styrenesulfonate) (PEDOT:PSS) by using microfluidic devices. The resistance changes in response to the stretching of the microfiber due to the connection/disconnection of the PEDOT:PSS domain can be observed up to 400% of elongation with the resolution of 0.1%. Furthermore, these changes are fully reversible and repeatable over 10,000 cycles at 300% elongation. On the basis of these mechanical and electronic properties, the DN microfibers are envisioned to use stretchable strain sensor, demonstrating the detection of the human motion including bending of the finger and walking and running in real time. We also demonstrated that the developed sensor can be stably used in outdoor by measuring the growth of the bamboo planted in the garden.

Polymer ceramic nanocomposites exhibit performance characteristics superior to those of the parent materials as they harness the mechanical properties of the polymer and the dielectric properties of the ceramic. However, the rational design of flexible high-k dielectric nanocomposites with high filler loading is still challenging as the increase of the ceramic content deteriorates the mechanical properties of the nanocomposites. In this work, we investigated the fabrication of flexible electronics by dispersing BaTiO₃ (BTO) colloidal nanocrystals with various sizes (10-20 nm) into the styrene butadiene styrene (SBS) matrix. The resulting SBS-BTO nanocomposite films contain up to 50% (weight) BaTiO₃ nanocubes fillers and possess high energy density values along with excellent mechanical properties. It has been observed that the nanocomposite films can easily be peeled off from any type of substrate which is an indication of good flexibility and strength of these free standing films. We observed a linear increase in the dielectric constant with respect to fillers content, ranging from 6.1 to 24.1 for 0% to 40% respectively. The alternate current (AC) conductivity of the polymer remained below 10^-8, which shows the polymer is insulative and can be used in flexible capacitor applications. Also, with scanning electron microscope, we observed that, the BTO cuboidal nanocrystals are uniformly distributed in the polymer matrix that can explain the reason behind of good flexible property of the film. The experimental results show that these nanocomposite thin films exhibit superior properties which make them attractive for implementation in high-performance capacitive storage devices, wearable technology and nanoelectronics.

Transformable photovoltaics (TPVs) have been attracting much attention because of the limitation of the PV’s installation and a continuous power source for the portable and wearable electronic devices. At present, TPVs have been developed based on the perovskite and organic photovoltaics. However, these PVs have problems of the low efficiency and instability. In contrast, crystalline silicon (c-Si) PVs exhibit high efficiency and stability, but they are not able to be transformable because they tend to be fragile when an external force is applied. Therefore, a new strategy for the TPVs technology based on the crystalline silicon is required to realize the high efficiency and stable c-Si TPVs. In this research, we developed the c-Si TPVs through the interdigitated back contact (IBC) structured c-Si module and introduction of a stretchable electrode. Because all electrodes (positive and negative contact) of the IBC PVs are positioned on the rear side, we can easily fabricate the TPVs module using the stretchable electrode. The stretchable electrode is consisting of the combination of the carbon materials and elastic polymer. The stretchable carbon filler/ polymer composite is found to effectively retain its electrical conductivity, even when under the high strain of ≈200%. Due to the outstanding electrical conductivity and highly stretchable property of the carbon filler/ polymer composites, the fabricated IBC PVs can be transformable without the efficiency degradation. Thus, we expect that the proposed c-Si TPVs are possible to overcome the PV’s installation limitation and integrate with flexible or wearable electronic devices for the continuous power source.

Intrinsically stretchable transistors are a critical building block for an array of stretchable electronic applications. They also represent a model system where conductors, semiconductors, and insulators must all be stretchable and effectively integrated. Here, we present an intrinsically stretchable organic field effect transistor that incorporates Ag nanowires (NWs) as the source, drain, and gate electrode with a stretchable PDMS gate dielectric and stretchable DPP-4T polymer semiconductor. The Ag NWs provide a significant improvement in electrical conductivity compared to other candidate stretchable electrode materials such as carbon nanotubes and PEDOT:PSS. However, there are several challenges associated with roughness and patterning that can limit their application. Here, we demonstrate an effective processing strategy of embedding the Ag NWs in a PDMS elastomer allowing for high stretchability and high conductivity. This high conductivity enables the electrical leads and electrodes to be composed of the same material. The NWs are patterned using direct electrohydrodynamic jet printing, and through drop casting followed by laser ablation. While the NWs are embedded in the elastomer, we show that the surface of the NW network is able to effectively contact the semiconductor for low contact resistance transistor performance. Through this processing strategy we are able to demonstrate a stretchable transistor with a saturated field effect mobility that remains largely unchanged when under an applied strain of 40%, and remains functional under cyclic loading. As part of this work, the morphology of each active component is studied in detail along with detailed device characterization.

Although stretchable polymer-based devices with promising electrical performance have been produced through the polymer blend strategy, the interplay between the blend film microstructure and macroscopic device performance under deformation has yet to be unambiguously articulated. Here, we will discuss the formation of robust semiconducting networks in blended films from a thermodynamic perspective. Thermodynamic behavior along with the linear absorption and photoluminescence measurements predict the competition between polymer phase separation and semiconductor crystallization processes during film formation. Semiconducting films comprised of different pi-conjugated semiconductors were prepared and shown to have similar mechanical and electronic properties to films comprised of a model P3HT and PDMS blend. These results suggest that a film’s microstructure and therefore robustness can be refined by controlling the phase separation and crystallization behavior during film solidification. Fine-tuning a film’s electrical, mechanical, and optical properties during fabrication will allow for an advanced next-generation of optoelectronic devices.

Aerosol jet printing (AJP) has been proven to be capable of printing microelectronic devices and relevant circuits with relatively high precision, repeatability, and scalability using a wide range of materials because it can handle inks viscosities in the range of 1-1000 Cp. In this work, fully-printed thin film transistors (TFTs) have been fabricated on flexible substrates. Metallic, semiconducting, and dielectric materials such as silver, single walled carbon nanotubes (SWCNTs), and xdi-dcs, have been printed successively with high quality using AJP. In this fabrication process, CNT network is printed in the channel region of TFTs as a semiconducting CNT solution. Highly uniform CNT network film is achieved by performing a multiple layer-by-layer deposition method. The printing of thin layer of xdi-dcs as gate-dielectric is realized by diluting with n-butanol solvent and plasma treatment for better surface wetting during printing. Fully printed CNT-TFTs showed a very stable performance with on/off current ratio as high as ~10⁵, mobility around 5 cm²V^-1s^-1, negligible hysteresis, and good uniformity. More importantly, this device can be operated using bias voltages as small as 5V, which is much smaller than the previously reported values because of the improvement in both CNT network and dielectric layer (thinner). CNT-TFTs were used as gas sensors to detect NO₂ and NH₃ and their performance was compared with two-electrode CNT gas sensors. Our printing method has potential to further reduce the bias voltages for operation. This improvement is key for the application of printed microelectronics and CNT-TFTs based circuits on flexible substrates because of the relatively lower voltage operation and power dissipation.

Skin patch based stretchable sensors for Physiological monitoring have drawn great attention due to their significant finding in many human-machine interaction applications. As of now, stretchable serpentine structures supported by biocompatible elastomeric substrates have been widely examined for conformable and strain relief stretchable electrophysiological sensors. However, the substrates are not breathable, which is crucial for next-generation wearable electronics. Additionally, the fabrication of these sensors mostly relies on conventional lithographic techniques followed by etching processes that are complex, expensive and entail multiple steps, which greatly impede the realization of low-cost and scalable manufacture electronics. The development of flexible structures, stretchable materials and novel processing techniques, can enable a viable solution for these electronics.
In this, we present a novel low-cost and rapid “cut and transfer” method to fabricate electrophysiological sensor using a benchtop programmable mechanical cutter plotter. Furthermore, the sweat dependent sensor performances have been avoided by introducing a water permeable patch possessing filamentary serpentine patterns that are stretchable, sticky, and conformable. Moreover, the sensors maintain the sensing performances irrespective to the deformations that are imposed. Overall, the fabrication method offers the chance of developing a new format of stretchable and flexible electronics.

In this work, we report the effect of interface anomalies such as disorder-induced interface states, Schottky barrier formation/dissolution for a resistive switch or memristor. Distribution of bulk defects with applied bias governs switching and also attributes to formation/dissolution of interfacial oxide. The present work considerably contributes to further understand the conduction mechanisms of a wide range of resistive switches.
Al/ZnO/Al (AZA) device depicts forming-free bipolar resistive switching with memristive behavior [1]. Bipolar against Unipolar resistive switching [2] is attributed to formation/dissolution of Schottky barrier. To further understand the effect of various other interface anomalies, we performed CV characterization of device in HRS and LRS states for different frequencies. As we can observe from Fig. 1(a), we may observe higher capacitance at a lower frequency which denote the presence of trap charges in bulk as at lower frequencies trap charges also make significant contribution to total capacitance. Further, we can also observe from CV characteristics of device for device in HRS and LRS at 1 MHz in Fig. 1(b) that for device in LRS and near SET voltage, the thickness of the oxide barrier reduces to minimum which leads to set device into LRS and hence total capacitance increases to maximum. As we vary the voltage and it comes near RESET voltage, the thickness of the oxide barrier will increase to its maximum and device will set into HRS and hence total capacitance will decrease to minimum. Fig. 2 (a) shows the I-V characteristics of AZA device. The device AZA shows excellent repeatibility for 250 set/reset cycles (Fig. 2(b)).
We believe our work could play a crucial role in further understanding of conduction mechanism of resistive switching devices in the future.
Acknowledgement: Authors are thankful to Sophisticated Instrument Centre (SIC) of IIT Indore for DIBS facility. Mangal Das and Gaurav Siddarth would like to thank Ministry of Electronics and Information Technology (MeitY), Government of India, for providing fellowship under Visvesvaraya PhD Scheme for Electronics and Information Technology (IT). Amitesh Kumar would like to thank Council of Scientific and Industrial Research (CSIR) for providing fellowship. Prof. Shaibal Mukherjee is thankful to MeitY for the Young Faculty Research Fellowship (YFRF) under the Visvesvaraya PhD Scheme for Electronics and IT.

In this report, we study factors that dominate resistive switching (RS) in yttria (Y₂O₃) based memristive devices. Our experiment consists of two significant steps. First, we have studied the effect of crystallinity to find out the conducive state for RS in yttria film.
Subsequently, in the second step, the effect of metal/semiconductor interface on the RS behavior of yttria has been investigated via changing bottom electrodes (BE) in Al/Y₂O₃/BE-type device structure. We have fabricated five different types of devices; these are N1, N2, N3, P3, A3 by varying deposition temperature and BE material. Deposition temperature for N1, N2 and rest of the devices (N3, P3, A3) are 100, 200, and 300 ^oC respectively. Bottom electrodes, which are used in devices, are n-Si (N1, N2, N3), p-Si (P3) and Al (A3).
Dual ion beam system (DIBS) system is deployed to deposit yttria (~100 nm) on low-resistive Si substrate (BE) for devices N1 (100 °C), N2 (200 °C), N3 (300 °C), and P3 (300 °C). For device A3 (300 °C), a 250 nm of the SiO₂ layer is deposited upon p-Si. Further, a 150 nm layer of Al (BE) is sputtered on SiO₂. A 100 nm thick yttria layer (switching layer) is deposited on all substrates, using commercially-available sintered yttria target.
In the First phase of our study, To understand the effect of crystalline properties on the RS of yttria film in all these devices (N1, N2, and N3) X-ray diffraction (XRD) is performed. N1 and N2 films show high intensity (highly crystalline) peak near 2θ = 28.8° (m111) corresponding to monoclinic phase along with another low-intensity peak around 59.4° (c 631) corresponding to cubic phase. To study the current-voltage (I-V) characteristics, triangular voltage waveforms with peak voltages of -5 to 5 V is applied to devices N1, N2, and N3. The absence of resistive switching in N1 and N2 can be attributed to higher crystallinity (as compared to N3) of yttria thin films which are counterproductive for the realization of reliable resistive switching devices. Contrary to highly ordered and crystalline structure (N1, N2), amorphous films (N3) do not have long-range order and contain defects which may be a more favorable condition for RS.
In the second phase of our study, three devices N3, P3, and A3 are used to further understand the effect of the interface on the behavior of yttria-based RRAM. Devices P3 and A3 are fabricated under the same condition as N3 but with different BE. N3, P3, and A3 have n-Si, p-Si and Al as their bottom electrode, respectively. It is important to note that N3 has one Schottky interface between Al and Y₂O₃, while A3 has two Schottky interfaces between Al and Y₂O₃. P3 shows diode-like characteristics after forming, which turns out to be useless for RS. Diode-like characteristics of P3 may be attributed to p-n heterojunction between p-Si and n-yttria. Device A3, peak voltages of -2 to 2 V shows bipolar switching which may be originated due to the occurrence of Schottky behavior at both interfaces. Comparison between N3 (Unipolar) and A3 (Bipolar) shows that the transformation from unipolar to bipolar RS can be realized via moving from single Schottky interface (N3) to both-sided Schottky interfaces (A3) structure. It can be observed that by merely changing BE from n-Si (N3) to p-Si (P3) or Al (A3), the shape and slope of I-V characteristics change significantly.
Endurance measurements do not show much degradation even after ~29000 switching cycles for the A3 memory cell. The resistances for HRS and LRS states are measured using a read voltage of 0.1 V, to confirm the non-volatility of resistance states. Our device (A3) shows excellent retention as there is no significant switching degradation even after a duration of 1×10⁵ s.

This talk will present multidisciplinary work from material composites and robotics. We have created new types of actuators, sensors, displays, and additive manufacturing techniques for soft robotics. For example, we now use stretchable optical waveguides as sensors for high accuracy, repeatability, and material compatibility with soft actuators. For displaying information, we have created stretchable, elastomeric light emitting displays as well as texture morphing skins for soft robots. We have also created new materials for optical stereolithography of soft robotics. All of these technologies use light as the medium for fabrication, sensing, or actuation. I will describe these processes, what is the present state of the art, and future opportunities for science in the space of additive manufacturing of elastomeric robots.

We will propose the first demonstrations of using triboelectric effect to realize various actively sensing and responsive capabilities in soft robots. Robots that can move, feel, and respond like organisms will bring revolutionary impact to today’s technologies. Soft robots with organism-like bodies have shown great potential in vast robot-human and robot-environment applications. Developing skin-like sensory devices allows them to naturally sense and interact with environment. It would be better if the capabilities to sense can be active like real skin. However, challenges in complicated structures, incompatible moduli, poor stretchability and sensitivity, large driving-voltage, and power dissipation hinder applicability of conventional technologies.
In this present, for the first time, various actively perceivable and responsive soft robots are enabled by self-powered active triboelectric robotic skins that simultaneously possess perfect stretchability and excellent sensitivity in low-pressure regime. The robots’ skins enable to actively sense proximity, contact, and pressure to external stimuli via self-generating electricity. The driving-energy of its sensing ability comes from natural triboelectrification effect.
Various kinds of actively perceiving soft robots will be demonstrated to use triboelectric effect to perform different actively sensing and responding tasks. For a conscious gripper, it can actively be aware of different actions in moving an object including approaching, grabbing, lifting, lowering, and even the accident of dropping off the objects. A perceivable robot-finger can check a baby’s diaper condition. A conscious robotic crawler enable to perceive its muscle motions during undulating gaits and detect very subtle human physiological signals, showing their potential in palpation. Such robots with large-area skins have been demonstrated for actively multiplexing sensing uses. Moreover, the actively responding signals can directly drive optoelectronic components for intuitive communication and be further processed for more sophisticated uses such as answering with sound, light, phrases, and so on. We believe the presented robotic skins that are self-powered, highly-sensitive, highly-stretchable can meet a wide range of applications where soft interfaces are needed. And, the first achievements in the actively perceiving and responsive soft robots can push the boundaries of artificial intelligences, soft robotics, as well as their vast related applications.

[Ref.]
[1] Ying-Chih Lai, et. al, "Actively perceiving and responsive soft robots enabled by self-powered, highly extensible, and highly sensitive triboelectric proximity- and pressure-sensing skins." Advanced Materials, 2018, doi.org/10.1002/adma.201801114
[2] Ying-Chih Lai, et. al, "Electric Eel-Skin-Inspired Mechanically Durable and Super-Stretchable Nanogenerator for Deformable Power Source and Fully Autonomous Conformable Electronic-Skin Applications." Advanced Materials, 2016, 28, 10024-10032.
[3]https://www.youtube.com/watch?v=ZdlqWJgi6ys

Nature inspired a broad spectrum of bio-mimetic systems – from soft actuators to perceptive electronic skins – capable of sensing and adapting to their complex erratic environments. Yet, they are missing a feature of nature’s designs: biodegradability. Soft electronic and robotic devices that degrade at the end of their life cycle reduce electronic waste and are paramount for a sustainable future. At the same time, medical and bioelectronics technologies have to address hygiene requirements. We introduce materials and methods including tough yet biodegradable hydrogels for soft systems that facilitate a broad range of applications, from transient wearable electronics to metabolizable soft robots. These embodiments are reversibly stretchable, are able to heal and are resistant to dehydration. Our forms of soft electronics and robots – built from resilient biogels with tunable mechanical properties – are designed for prolonged operation in ambient conditions without fatigue, but fully degrade after use through biological triggers. Electronic skins merged with imperceptible foil technologies provide sensory feedback such as pressure, strain, temperature and humidity sensing in combination with untethered data processing and communication through a recyclable on-board computation unit. Such advances in the synthesis of biodegradable, mechanically tough and stable iono-and hydrogels may bring bionic soft systems a step closer to nature.

Soft robots have been designed and developed to fulfil the demands of better deformability and adaptability to changing environment [1-2]. These soft robots could be made of various stimuli responsive materials that can be actuated by magnetic field [3], light [4], temperature [5], electric fields [6], chemicals [7], pressure [8], etc. In contrast to other actuation mechanisms, magnetic fields are appealing for numerous application scenarios (e.g. environmental, biological, medical), where the benefits stem from their long-range penetration, easy accessibility, and controllability [2]. Very recently, there are already impressive demonstrations of magnetically triggered millimetre- and centimetre- scaled soft robots performing multimodal locomotion [9] and complex 3D actuations [10]. However, their thick and bulky bodies [9-10] challenge themselves to reveal better performances for specific implantations which require more, for instance, high actuation speed, and reversible large-scale actuation amplitude by a rather low magnetic field.

Here, we present an ultrathin (7-100 μm) and lightweight (1.2-2.4 g/cm³) soft robot that can be actuated in a tiny magnetic field of 0.2 mT reaching full actuation amplitude with a reaction time of 10 ms only. By programming the foils into different geometries, these soft robots are readily used for multifunctional nature-mimicking motion with a magnetic coil or a permanent magnet, such as a quick fly gripping and releasing, complex fast human cross-clapping mimicking, etc.

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The goal of this project is to create a class of electronic materials that can measure signals and interface with the nervous system for two-way communication with biological systems. The project is exploring three classes of materials. (1) Semiconducting polymers with properties inspired by biological tissue. The goal of organic bioelectronics is to detect and treat disease by using signal transducers based on organic conductors and semiconductors in wearable and implantable devices. Except for the carbon framework of these otherwise versatile materials, they have essentially no properties in common with biological tissue: electronic polymers are typically stiff and brittle, and do not degrade under physiological conditions. Such properties can be realized in a single-component polymer by incorporating biocompatible subunits. We have synthesized a new type of stretchable, biodegradable polymeric semiconductor whose electronic performance is unaffected by the biodegradable components. Such materials have applications in wearable and implantable sensors. (2) Metallic nanoislands on single-layer graphene for cellular biomechanics and wearable sensors. We have used these materials to measure the forces produced by the contractions of cardiomyocytes using a piezoresistive mechanism. Separately, we have developed orthogonal methods of stimulating myoblast cells electrically while measuring the contractions optically (a modality we nicknamed as “piezoplasmonic”). We have also used these sensors to measure the swallowing activity of head-and-neck cancer patients who have received radiation therapy and are at risk of dysphagia arising from fibrosis of the swallowing muscles. The combination of strain sensing, surface electromyography, and machine learning can be used to measure the degree of dysphagia. (3) We have developed ionically conductive organogels for haptic feedback. Medical haptic technology has myriad potential applications, from robotic surgery and surgical training, to tactile therapy for premature infants and patients with neurological impairment.

The recent rise of the so-called “tattoo electronics” has pushed further the innovation on flexible and ultra-conformable electronic devices that can be transferred onto the skin, being this unprecedented possibility of obtaining a seamless skin/electronics interface particularly interesting for biomedical applications. In fact, these systems can be specifically engineered in order to adhere to the skin and they could include several kinds of electronic devices, such as sensors, light-emitting diodes, photodetectors or electrodes for biopotentials acquisition. In this work, we have developed a very simple approach for the realization of ultra-conformable electronic systems, such as electrodes and sensors that can be employed for the recording of biopotentials and/or different physiological parameters.
The proposed approach is based on the employment of a nanometric film of Parylene C. The process starts from a plastic substrate on which we spin coat a sacrificial layer which can be dissolved in either polar or non-polar solvents. After that, a submicrometer film of parylene C is deposited, such nano-film acting as the actual substrate where electrodes for biopotential acquisition and OTFT-based sensors can be fabricated using standard techniques (such as, for example, photolithography, micro-contact printing, and inkjet printing). Once the fabrication process is completed, the sacrificial layer can be dissolved allowing the electrodes/sensors to be easily transferred onto a different substrate, as for instance onto the skin.
In this work we will report about the employment of such tattoo electrodes for the recording of both electrocardiographic (ECG) and electromyographic (EMG) signals, which can outperform standard commercial pre-gelled electrodes in terms of signal-to-noise-ratio and movement artifacts suppression.
Moreover, a particular kind of organic transistor sensor called organic charge-modulated field-effect transistor (OCMFET) has been fabricated onto such nanometric thin films and coupled to a solution processable and printable piezoelectric/pyroelectric polymer, namely PVDF-TrFE, in order to obtain ultra-thin multimodal force and temperature sensors. Such devices can detect very small pressure (below 300 Pa) in both the static and the dynamic regime, being in fact able to detect stimuli at a frequency up to 500 Hz. Moreover, thanks to the pyroelectric properties of PVDF-TrFE, temperature variations ranging from 10 °C up to 45 °C can be also detected. Interestingly enough, we will show that with the proposed approach it is also possible to employ the same device and the very same active layer for the fabrication of multimodal tactile sensing systems. The highly flexibility of the developed structure and the simple fabrication process make this solution a very interesting candidate for the development of ultra-sensitive, multimodal, and mechanically compliant electronic systems that can be employed in many different scenarios, ranging from biomedical applications to robotics and prosthetics.

For the satisfactory fulfilment of the two primary human senses (i.e., vision and hearing), every contemporary media platform employs an independent speaker and display, as display panel technology currently disallows tunable audio-frequency vibration. However, mechanochromic systems that feature amenability towards color tuning based on structural distortion have an inherent potential for the generation of mechanical vibration; all the same, most systems are constrained to demonstrations in low-frequency regimes (< 1 Hz) as color change events necessitate large strains, which has until now precluded the possibility for simultaneous high-frequency vibrations. It is well known that overcoming sluggish strain response rates that are characteristic of stretchable viscoelastic material systems is extremely challenging. Hence, most mechanochromic systems have thus far only aimed at satisfying visual perception and have overlooked the possibility of a merger of sound and display modules, despite their disparate inherent mechanical vibration modes. Here, we target both senses, especially under single input control by actuating a single mechanochromic platform built from an organogel skin composed of close-packed photonic lattices with an organogel matrix. Exploiting a dielectric elastomer actuator, the skin features large areal strains at low frequencies of actuation, which allows the reversible tuning of the photonic stop-band. Notably, the skin remains incompressible and exhibits negligible strain when actuated at higher frequencies (e.g., audio frequencies), thereby making it amenable for sound modulation in this regime. Remarkably, a pseudo-synesthetic event with color and sound can be orchestrated with a single actuation mode; that is the simultaneous generation of the large strain (for color) with the high-frequency vibration (for sound) by taking advantage of the soft and highly responsive photonic organogel skin. This strategy will be widely helpful to eliminate the need for the independent construction of modules for color and sound, as required in all current medial platforms.

The evolution of flexible electronics has enabled the appearance of revolutionary concepts like electronic skins (e-skins) [1-3], wearable electronics and smart implants, among others. E-skins have the potential to seamlessly integrate with the human body owing to their mechanically imperceptible and compliant nature. Due to these characteristics, they provide a much more natural way of interacting with electronic devices by eliminating the need for bulky or stiff parts interfering with human perception. Furthermore, conventional flexible interactive electronics typically use tactile stimuli like pressure or temperature [4,5]; limited to direct contact detection. To progress beyond tactile approaches, we have recently demonstrated magnetosensitive skins [7-12] as a touchless alternative for interactivity. These skins can utilize a plethora of magnetic stimuli like permanent magnets [7-10], the geomagnetic field [11] and even submicrotesla fields [12], to reconstruct and digitize spatial interactions.

Here, we present the technologies at the core of these interactive magnetosensitive skins and showcase some of the applications stemming from them [10-12]. These technologies utilize different configurations of magnetoresistive (MR) and planar Hall-effect sensor elements, based on metallic thin films fabricated on 6-µm-thick polymeric foils. The combination of this broad spectrum of sensors allows multiple magnetic range detection possibilities for these magnetosensitive skins. Besides, by introducing geometrical conditioning like barber poles [13] or measurement schemes like zero-offset anomalous Hall magnetometry [14,15], the output sensitivity and offset can be tuned to increase the overall performance.

We anticipate that this novel kind of magnetic skins could be used to track and digitize fine motion in an ultra-thin and lightweight format. This feat could ease the integration of usually rigid magnetic detection systems into on-skin, textile-based or Internet of Things (IoT) applications. A successful implementation could lead to a new class of virtual or augmented reality systems and interactive input devices which extract information from their surroundings through magnetic signals.

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[12] P. Granell, G. S. Cañón Bermúdez et al., npj Flexible Electronics, in press.
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[15] T. Kosub et al., Nat. Commun. 8, 13985 (2017).

Flexible pressure sensors with high sensitivity, broad working range, and low fabrication cost are highly desired for wearable electronics and human-machine interaction. However, the trade-off between fabrication cost and structural qualities (e.g. regularity, tunability, etc.) of pressure-sensing microstructures greatly limits their large-area application. Here, we present a large-area compliant and low-cost strategy to fabricate high-performance pressure sensors via the alliance of mesh-molded microstructures and printed electrodes. Firstly, robust, periodical and size-tunable conductive microstructures are fabricated via a mesh-molding method, which is scalable and cost-efficient. On the other hand, electrodes with side-by-side configuration are prepared through printing techniques. The side-by-side electrodes show higher sensitivity and much broader working range compared to conventional bottom-top electrodes and interlocked microstructures. After simple assembly of the scalable microstructures and printed electrodes, flexible pressure sensors with high sensitivity (23.87 kPa^-1), low detection limit (7.4 Pa), ultra-broad working range (7.4~1,000,000 Pa), fast response/recovery (25/20 ms), excellent reliability (over 10,000 cycles), and good tunability are obtained. All these capabilities of our pressure sensors provide a solid platform for applications in monitoring various human physiological activities (e.g. pronouncing, artery pulse, finger touching, grasping, etc.) and resolving spatial distribution and magnitude of applied pressure as an e-skin. Remarkably, for the first time, we demonstrate a smart insole with a high level of integration for plantar pressure mapping and foot temperature monitoring simultaneously based on printing techniques, which can greatly extend its application range compared with reported smart insoles with only pressure-sensing function. The desirable comprehensive performance of our pressure sensors, along with their significant advantages of scalability and low-cost, makes them attractive for the future development of wearable medical devices and human-machine interfaces.

Healthcare is important for modern societies because of the aging population and the strong desire to improve our quality of life.Especially, real-time measurements and preventative managements of information in the body become more urgent, resulting in the emergence of electronic sensors that can be integrated in the body. Previous implantable sensing devices have mainly concentrated on monitoring bioelectric signals such as electroencephalography, electrocardiography, and electromyography. However other bio-signal parameters such as strain, stress, and pressure are also of considerable importance in both medical fields and trauma- or sports-related biomechanics. For example, the strain that occurs on a ligament should be measured in real-time to prevent sprain or rupture of the ligament during hard activities or to control the strain in the ligament precisely during various ligament reconstruction surgeries. Such capabilities are increasingly important in modern society where population of old people and sports people are rapidly increasing. Nevertheless, thus far, few studies on monitoring such biomechanical signals have been reported because it is inefficient to apply the existing 2D sensor systems to complex structures in the body such as the 1D fibrous structure of a ligament. Furthermore, the need for high sensitivity and wireless read-out remains challenging for implantable strain-sensing systems.
We present a suturable and seamless fiber strain sensing system for sport-biomedical and biomedical applications. We use highly conductive and stretchable fibers for the fabrication of a highly sensitive and wireless fiber strain sensing system. The conductive fiber is fabricated by absorbing and reducing a large amount of metal nanoparticles inside of the stretchable fibers. With the help of such conductive fibers, an RLC resonant antenna circuit can be fabricated without any soldering point, where a sensitive fiber capacitive strain sensor acts as the sensing element. Because all the organs in the body have their inherent stretching ranges, the sensitivity and stretchability of the capacitive strain sensor can be modulated and optimized according to the application.

The unique design and integration technology of wearable electronics have enabled the advent of the most advanced forms of electronics, and the so-called “epidermal electronic system” (EES). With state-of-the-art performance inorganic semiconductor devices, serpentine type interconnects have modified rigid and bulky electronics into highly stretchable EES. This technology to integrate conventional semiconductor electronics with any arbitrary platform granted the capability to manufacture cutting-edge devices on robots, textiles, and human skin. As such, wireless communication technology has been playing a key role in the internet of things (IoT) applications, in which radio-frequency (RF) electronics are embedded.
This talk will discuss the improvement of stretchable RF electronics working beyond the gigahertz (GHz) region by tuning the conventional design of active/passive components in a microwave/RF circuit. As a fundamental component, the transmission line in an RF circuit has been re-designed with a unique twisted-pair structure to overcome its limit in terms of electrical properties on unusual substrates. The twisted-pair transmission line showed excellent RF performance up to 40 GHz and RF filters using what the twisted-pair transmission line demonstrated. However, the quasi-differential structure of the twisted-pair transmission line possess difficulties with fabrication and modeling of integrated RF circuit. Here, we introduce a linear structure design of a stretchable RF transmission line using a microstrip transmission line structure, which is easy to parameterize RF characteristics and fabricate while maintaining applicable electrical and mechanical performance during stretching or bending. The novel design still provides relatively low RF insertion loss (less than -5 dB) and high return loss up to 40 GHz. To further study its potential application in RF circuits, we demonstrated a stretchable AlGaN/GaN HEMT by combining the microstrip transmission line with miniaturized thin-film AlGaN/GaN HEMT.
Promising results show the feasibility of stretchable RF electronics for wirelessly connected EES.
Acknowledgment: The work was supported by AFOSR PECASE.

Fabrication of complex circuitry for ultra-conformable and imperceptible devices is a key challenge for tomorrow-wearable technologies. As a matter of facts, the only way today to perform complex computational operations, such as data processing and transmission, is by using standard rigid silicone-based systems. The mismatch in mechanical properties between conformable substrates and rigid components induces specific reliability issues, especially for wearable systems, as they are subjected to wearing and washing.
In this work, we focus on different solutions to create thin and ultra-conformable electronic circuit, using a customized stack of thin metallic foil on a polymer film. We consider and study the aforementioned aspects of imperceptibility (wearability) through conformability, and robustness through specifically designed bending and washing tests. This study investigates the optimization of electrical connection between ultra-soft substrates and rigid components, and the influence of the stack composition in multilayer electronic systems on the overall reliability of the device.
The combination of optimized stack layout and compliant interconnections allow the fabrication of robust and ultra-conformable, imperceptible devices.

With the emergence of unusual format electronics such as flexible and wearable devices, an effort has been made to integrate devices with various functions in smart clothing and human body for providing enhanced convenience for the users. However, it is difficult to accomplish such emerging electronics with conventional rigid inorganic materials. Two-dimensional (2D) materials such as graphene and transition metal chalcogenides have superb electronic properties that make them a promising host for device applications and they have a good mechanical property, offering a great opportunity to flexible and wearable electronics that should maintain a stable operation under a high strain. In this talk, I will present various flexible and wearable electronic applications for including touch and self-powered communication devices, and biosensors.

The extraneous thermal regulation of the human body has a wide array of applications ranging from assisting with medical conditions such Multiple Sclerosis (MS) to being an essential component in the design of protective suits used for maneuvering extreme environments. Out of the plethora of thermoregulatory garment designs that are available commercially or have been proposed in the literature, liquid-cooled garments provide one of the highest heat removal capacities along with sustained and nearly-environment independent performance. In addition to the thermal regulation of the human body, these liquid-cooled garments show potential to meet the rising demand to dissipate the heat produced by wearable electronics and high power robotics while maintaining the sought after flexibility. However, in these thermoregulatory liquid cooled garments the tubing material often used is rigid PVC which has a very low thermal conductivity, of around 0.2 Wm^-1K^-1; a value far too low to effectively deal with the amount of heat that will inevitably be generated by future wearable electronics. A low thermal conductivity contributes to magnifying the total thermal resistance which results in an excessive amount of tubing being networked around the human body or electronic device to effectively regulate its temperature. This excessive amount of tubing in turn has to be carried around by the wearer.
In this work, we explore how tubing made of soft, thermal conductive elastomers composites can improve on the performance of these liquid-cooled jackets (LCJ). In doing so, we study the effect of filler material on the tube-wall thermal resistance in combination with the interfacial thermal resistance. The interfacial resistance depends on mechanical properties of the materials in contact, thus, a microscale composition of the filler particles and matrix elastomer that provides a balance between thermal and mechanical properties of the composite must be achieved for optimal performance of the cooling device. To explore this relationship, we develop a closed-form thermomechanical model that predicts thermal performance of a composite silicone LCJ as a function of metallic filler content. To validate the model, we fabricate a set of liquid-cooled jackets out of silicone-aluminum composites. We relate the cooling ability of these devices with composition as well as thermal and mechanical properties of the composite materials. Based on these results, we develop LCJ architectures with material properties tuned via composition adjustment at each device level to achieve optimal device performance for personal (skin substrate) and robotic (metal substrate) cooling.

Rapid, on-site assessment is highly desirable in the fields of both medical treatment and novel robotics. To achieve this goal, our research aims to develop low-cost, flexible, large-area sensor devices for different health and environmental applications. In this presentation, we discuss case studies using touch and pressure sensors for two different point-of-use applications:
I) Autism spectrum disorder (ASD) motor skills characterization. There is no objective metric for evaluating motor skill training progress in autistic children, and current assessments rely on qualitative surveys. A common method used is a finger tapping test that requires videotaping and unreliable, time-consuming manual analysis, with large room for errors, even with computer vision analysis algorithms. We have fabricated an instrumented glove with touch sensors on textile for straightforward finger tapping patterns characterization. The results provide immediate objective feedback not only on the tapping counts, but on the temporal data (such as tapping duration and variation in duration) as well, which was not collected before. For the index finger tapping test, children with ASD perform less counts per minute compare to typically developing (TD) children. In addition, children with ASD tap their finger for an average longer duration, with larger variation between tap durations. In a 4-fingers tapping test children with ASD tend to have more irregular patterns and skip fingers compared to TD children. This glove could find future use for characterizing motor skills of people suffering from Parkinson’s disease, epilepsy seizures, and other neurological motor disorders.
II) Robotic sensors for simultaneous pressure and organophosphate (OP) pesticide detection. There is an urgent need of sensor technologies to monitor hazardous materials for security and environmental applications, in particular, the occurrence of OP pesticide residues in agricultural products that poses a serious concern in the food and agriculture industries. Hence, rapid on-site detection of OPs through remote robotic sampling is highly desired to avoid placing people at exposure to the OPs risks. To handle sample collection, the robotic manipulator requires tactile feedback, in order to ensure no damage will be done to either the robot or the other object in contact due to excessive force. To provide tactile feedback, pressure sensors based on capacitive mechanism were chosen. The sensitivity of the capacitive pressure sensors was tuned by adjusting the dielectric compressibility. Particularly, the sensitivity was increased by choosing softer materials, i.e. elastomers with low elastic modulus and porous structure that further lower elastic moduli of the foam dielectrics compared to solid films of the same material. We have combined low-cost chemical and pressure sensors together on disposable gloves, and demonstrated successive simultaneous tactile sensing and OP pesticide detection in a point-of-use platform that is scalable and economical.

There is currently a great deal of interest in stretchable electronics for widespread applications such as wearable devices, smart sensors, healthcare devices, electronic skins, and soft robotics. The stretchable electronic devices that consist of soft and deformable components can stably maintain their functions under the complex physical deformations such as bending, twisting, folding, and stretching. To achieve independent and reliable stretchable electronics, a key challenge is the development of stretchable energy-storage devices that could power such devices. Among them, Li-ion batteries (LIBs) is a representative power source for the portable electronics owing to their advantages such as high working voltages, stable cycle life, and high energy density. However, LIBs are based on a volatile and flammable non-aqueous electrolyte, which make them a high risk of explosion owing to the internal short circuit during extremely mechanical deformations. For this reason, typical LIBs are not suitable for energy storage-devices system in stretchable electronic devices. Compared with LIBs, aqueous rechargeable batteries hold great potential for a power supply in stretchable electronics because of the aqueous electrolyte having inherent safety features and high ionic conductivity.
Herein, we successfully replaced rigid components of typical batteries including, electrodes¹ and a separator membrane², with stretchable materials using straightforward and scalable fabrication strategies. First, we fabricated a conductive polymer composite composed of bio-inspired Jabuticaba-like hybrid carbon/polymer (HCP) composite as a stretchable current collector, which effectively maintained its electrical percolation network even under 200% uniaxial strain. To further understand the behavior of carbon fillers in the polymer matrix under strain. And then, a poly(styrene-b-butadiene-b-styrene) (SBS) block copolymer-based stretchable separator membrane was fabricated by the nonsolvent-induced phase separation (NIPS). The diversity of mechanical properties and porous structures can be obtained by using different polymer concentration and tuning the affinity among major components of NIPS. The stretchable separator membrane showed a high stretchable feature (~270% uniaxial strain) and porous structure (~61% porosity). Using as-prepared the stretchable electrode and the stretchable separator membrane, we assembled stretchable aqueous Li-ion batteries as a power source for use in stretchable electronic devices. As a result, our all stretchable aqueous batteries manifested good cycling performance and stable capacity retention even under 100% uniaxial strain, without failing the battery performance.

1) Song et. al, Advanced Energy Materials, 2018, 8, 1702478
2) Song et. al, Advanced Energy Materials, 2018, 8, 1801025

To expand the application area, unconventional forms of displays have been actively emerging beyond the flexible displays. The wearable display is conceded as one of the promising forms of such unusual emerging displays due to its remarkable functions such as real-time communication and superior portability. The wearable cloth-typed displays are particularly imperative in the commercial fields because of their notable contribution to offering a high degree of freedom in terms of designs and functions. There have been successful demonstrations of the so-called ‘on-cloth’ displays, where textiles (i.e. woven fabrics) were instead used as substrates of the devices. Without technical issues in defining a two-dimensional pixel matrix and making their interconnections, the reported systems of the on-cloth displays could work as fully-operating display panels. On the other hand, current technology for the ‘in-cloth’ displays that comprise emitting fibers is limited to a level of the lightings rather than the information displays, as there has been a hurdle to form a two-dimensional interconnection network with one-dimensionally configured fibers, and hence no route to perform the matrix addressing for driving designated pixels. In this regard, here we present a highly feasible approach to achieve the in-cloth class of matrix-addressable displays by weaving assembly with perpendicularly arranged organic light-emitting diode (OLED) fibers and conducting fibers. To create the OLED fibers that behave as scan lines of the matrix addressing, periodically patterned phosphorescent OLED pixels are thermally deposited on polyethylene terephthalate (PET) fibers coated with indium tin oxide (ITO) common electrodes, where complete passivation of the deposited organic constituents and subsequent formation of contact pads connected to the top electrodes of each OLED pixels are implemented to prevent the mechanical damage to the pixels as well as the water/oxygen permeation. The contact pads of OLED pixels on the OLED fibers can be electrically linked to the other perpendicular conducting fibers of data addressing lines by simply tailoring cross spots layout of woven fibers, which allows the successful operation of the resulting woven OLED fabric displays by the matrix addressing scheme. Details of fabrication strategy to enhance the reliability of matrix-addressable OLED fabric displays can provide technically compelling solution options for developing practically applicable in-cloth displays.

The shortwave infrared spectral region (SWIR: 1-3 um) is particularly powerful for health monitoring and medical diagnostics, because biological tissues show low absorbance and minimal SWIR auto-fluorescence, enabling greater penetration depth and improved resolution in comparison to visible light. However, current SWIR photodetection technologies are largely based on epitaxially grown inorganic semiconductors which are costly, require complex processing and impose cooling requirements incompatible with wearable electronics. Organic semiconductors offer numerous advantages including large-area and conformal coverage, temperature insensitivity, biocompatibility, and low-cost integration for enabling ubiquitous SWIR optoelectronics. This talk will discuss organic SWIR devices and discuss the main bottlenecks associated with charge recombination and trapping, which are more challenging to address in narrow bandgap photodetectors in comparison to devices employing wider bandgap materials that operate in the visible.

As progress is made towards overcoming challenges associated with losses due to recombination and increasing noise at progressively narrow bandgaps, the performance of organic SWIR photodetectors is rising with detectivity exceeding 10^¹¹ Jones, comparable to commercial germanium photodiodes. The organic photodetectors are easily integrated within a wide range of portable systems spanning wearable physiological monitors to SWIR spectroscopic imagers that enable compositional analysis for food, water quality monitoring, and medical and biological studies. There are exciting opportunities for low-cost organic SWIR technologies to be as widely deployable and serve as an empowering tool for users to discover information in the SWIR to inspire new use cases and applications.

In the past decade, strain engineering has been used to markedly manipulate characteristics of functional materials such as increasing tunability. However, the ways to apply strain reversibly into materials such as hydrostatic pressure, strong magnetic, electric and lattice mismatch are difficult to achieve in general environment and our daily life. In this study, In order to surmount this obstacle, we adopt flexible muscovite mica substrate to fabricate epitaxial (Ba_0.5Sr_0.5) TiO₃ (BSTO) thin films with high and tunable dielectric constant via van der Waals epitaxy. The combination of X-ray diffraction and high-resolution transmission electron microscopy was conducted to reveal the heteroepitaxy of the BSTO/muscovite system. Due to the mechanical flexibility of muscovite sheet, the tunability of dielectric constant were highlighted by the capacitance-voltage measurement under various bending measurement which is ranging from 5~15 mm radius of curvature including tensile and compressive strain. In the bending measurement, the dielectric constant of BSTO thin films with different thicknesses was altered nonlinearly and reversibly from -77 % to 36 % compared to the unbent state. Such a system composed of flexible BSTO/muscovite heteroepitaxy delivers a new path way to apply mechanical strain on thin film system with tunable dielectric feature.

Softness describes the ability of being easily shaped, conformed or folded. Stiff electronic materials may be engineered in thin, sub-micron thick, structures with 3D geometries to form electronic circuits with new form factors and reversible deformability. To increase further the circuits’ softness, a new class of materials, namely hydrogels, is emerging as a soft carrier for electronic devices. The versatility of hydrogel chemistry combined with their biomimetic properties inspires numerous applications in soft robotics, wearable and implantable electronics. However, their long-term integration within a functional electronic thin film assembly remains a challenge. In air, hydrogels dry progressively, leading to massive shrinkage of the organic network. Kept in water, they can swell up to more than ten times their initial volume thereby resulting in important mechanical stress at the interface with the electronic structure. In both cases, this significant volume change is not compatible with the cohesion of a multilayer system and has irreversible impact on the electronic properties.
This talk will report on our recent efforts to mitigate this mechanical and structural mismatch. We integrate a low-swelling type of hydrogel, the poly(2-hydroxyethyl methacrylate) [PHEMA] with thin-film electronics on polymer substrate. Because of a good balance between its hydrophilic and hydrophobic properties, we imagined a hydrogel-elastomer micrometric bilayer with a stable interface that can sustain multiple swelling-drying cycles. We will discuss the dynamic modifications of the mechanical properties of the bilayer from its initial dry state (stiff mechanical properties) to its soft and swollen state at equilibrium in wet environment.
Combined with thin-film electronics technologies, this mechanically-adaptive hydrogel-elastomer assembly offers an exciting avenue for soft electronic circuits and neural devices.

Developments in 3D-printing of flexible and integrated electronic interconnects will enable advances in the design of soft robots and other emerging flexible electronics technologies. Printing flexible interconnects with high conductivity (>10^5 S/m) faces challenges like post-process sintering steps, resolution limitations, viscosity requirements, and substrate incompatibility that limit production speed, scalability, and cost-effectiveness. One way to overcome these challenges is to print architected composite materials with microstructures engineered for high electrical conductivity at low particle loading. We demonstrate a novel approach to accomplishing this by employing a structure-assembly technique called acoustic focusing, in which microparticles suspended in a fluid are manipulated with pressure fields generated within a microfluidic printing channel coupled to piezoelectric actuators. This technique produces composites with embedded structures that have high electrical conductivity at low particle loading in the ink (as low as 0.5% by volume). These composites are 3D-printable, have good strain tolerance, and can be modulated on-the-fly during printing to have either anisotropic conductive, nearly-isotropic conductive, or insulating behavior.
Electrically conductive filler particles (here, carbon fibers and silver-coated fibers) are aligned and compacted into dense bundles 10-200 µm in diameter, which serve as electrically conductive networks integrated within the host material. Assembly control parameters (i.e. focusing driving frequency and amplitude) and ink properties (filler fiber length and volume fraction) are modulated to adjust the composite microstructure, and thereby material properties. We demonstrate that we can form composites with anisotropic electrical conductivity (on the order of 10^5 S/m in the printing direction within assembled bundles which are insulated from each other in the perpendicular directions), nearly-isotropic conductivity, or fully insulating behavior all using the same precursor ink. Furthermore, assembly control parameters can be modulated mid-print, so that one can switch between printing anisotropic conductive, isotropic conductive, and insulating material within the same printed line of material.
Microstructural characterization of the resulting composites reveals that the anisotropic conductive composites consist of “wires” of compacted particles insulated from each other within the material, enabling each structure to be individually addressed. These materials have sufficient conductivity to carry currents of >100 mA without heat accumulation. Furthermore, we assemble these networks in elastomeric photopolymers to form flexible conductive composites that can withstand deformation to bending strains of >20% with only small changes in conductivity.

Stretchable conductive adhesives are one of the key elements in medical and wearable devices. Most interconnections of components in electronic devices have been made using Sn/Pb based solders or epoxy-based conductive adhesives. However, Sn/Pb based solders and epoxy-based conductive adhesives are not flexible and stretchable, thus leading to limitation their uses in various flexible and stretchable devices. Herein, we have demonstrated the silicone based conductive adhesive (SCA) that exhibits remarkably low resistivity of 1.55x10^-4 Ω cm. It maintains the electrical resistivity even when it is stretched by 2.2-fold increase in length. In addition, it shows the resistivity change of less than 3% after 3000 stretching cycles. The SCA shows excellent adhesion to various substrates such as polydimethylsiloxane (PDMS), thus enabling it to be used on various stretchable electronic devices. In order to demonstrate its versatility, the SCA has been applied to LED device as a stretchable interconnection material and electrode. Furthermore, the SCA has been used as an electrocardiogram electrode. These results indicate that the SCA with excellent adhesion to various substrates has great potential as stretchable interconnection materials and electrodes in stretchable and wearable electronics.

Since the first discovery of semiconducting organic materials, tremendous effort has been made to improve their electrical properties. Their mobility exceeds that of amorphous silicon (>1 cm² V^-1 s^-1), which makes them potential alternatives to inorganic electronic materials. Thanks to this significant improvement of electrical properties, research interest has now moved from mobility-oriented study to low-cost soft electronics. Along with the need for stretchable semiconductors for soft electronics, the development of intrinsically stretchable polymer semiconductors has gained much interest. However, the brittleness of conjugated polymers is a direct result of their semi-crystalline nature and rigid backbone structure. Furthermore, electrical performance and mechanical compliance of semiconducting polymers are usually inversely correlated. Thus, it is both important and challenging to develop intrinsically stretchable polymer semiconductors without sacrificing mobility.
This presentation will focus specifically on using flexible conjugation breakers for stretchable polymer semiconductors. Non-conjugated spacers have been mainly utilized to enhance the solution processability of polymer semiconductors.[1-3] Reduced backbone rigidity arising from conjugation breakers can improve solution processability of semiconducting polymers. Several studies reported that these conjugation breakers can also tune mechanical properties of polymeric semiconductors.[2-3] Specifically, some non-conjugated spacers resulted in the improvement of ductility of semiconducting polymers; however, they accompanied significant compromise of electrical performance.
Herein, we investigate the effect of flexible non-conjugated spacers on electrical and mechanical of semiconducting polymers.[4] Various conjugation breakers with different flexibility are synthesized and incorporated into diketopyrrolopyrrole (DPP)-based semiconductors. We show that more flexible spacers resulted in greater stretchability of semiconducting polymers without any noticeable decrease in mobility. Specifically, a dodecyl spacer exhibits both high crack onset strain of 100% and high mobility of >1.00 cm² V^-1 s^-1. Furthermore, the polymer semiconductor containing the dodecyl spacer maintained much higher level of mobility than the fully conjugated counterpart under strain. Finally, fully stretchable transistors are demonstrated as a potential application of stretchable semiconducting polymers for wearable electronics. Such results show great potential for exploring conjugation-break spacers as a method of optimizing the performance of polymer semiconductors for soft electronics.

[1] G. -J. N. Wang, et al., Macromolecules, 51, 4976 (2018)
[2] S. Savagatrup, et al., Macromol. Rapid Commun., 37, 1623 (2018)
[3] Y. Zhao, et al., Adv. Funct. Mater., 28, 1705584 (2018)
[4] J. Mun, et al., Adv. Funct. Mater., 28, 1804222 (2018)

There is a growing interest in integration of sensors and electronic circuits on irregularly shaped surfaces. These 2.5D circuits can be dynamically deformable (e.g. elastic) and adopt many shapes when no other forces than gravity act on it. They can also take a predetermined fixed shape without the action of external forces. In the latter case they can either still be deformable under external forces (e.g. a smart soft lens or shoe insole) or they can be rigid and non-deformable under moderate external forces (e.g. a car dashboard with integrated electronics). In any case, for the fixed shape circuits it is desirable that the production process allows for a precise control of the components positions on the 2.5D circuit surface. In order to achieve this, the circuit can be produced on a pre-shaped 3D carrier followed by assembly of the components on the 3D circuit. This approach is e.g. used in the 3D-MID technology. However, production of circuits and assembly of components on 3D carriers is time consuming, expensive and non-standard in industrial circuit production, where all processing steps, even for flexible circuits, are carried out on flat 2D substrates. In order to comply with these standard production technology the only way is to produce the circuit on a 2D flat carrier and as a final step deform it (e.g. by a thermoforming step) from flat to its final 2.5D shape. In this case, methods should be found to precisely predict and control the component position on the 2.5D surface, starting from their positions on the 2D surface, and taking into account the effects of different parameters in the thermoforming process. In this contribution, we will describe our approach to fabricate 3D thermoformed circuits and ways to control component positions. We have found that structuring the thermoplastic polymer in which the circuit is embedded before thermoforming, provides possibilities for this components position control. We have demonstrated this approach in smart lens applications where a thin-film circuit with assembled components (chip, antenna) is deformed from flat to a spherical shape.

Recent research reveals that transient CO₂ laser heating can convert various polymer films into porous graphene (LIG) with continuous structures under ambient atmospheres. The process is rapid and automatic, and the motorized laser beam can write the computer-designed layouts of porous graphene on polymer substrates with precisely controlled patterns. Moreover, LIG has low sheet resistance (~ 10 Ohm per square) and large surface areas (~ 150 m²/g), which is therefore a promising conducting material for a broad range of electronic and energy devices. In this talk, I will introduce our recent research in exploring soft electronic and energy devices with unusual attributes using laser-induced porous graphene. First, we have successfully made porous, multifunctional on-skin sensors, consisting of LIG as device components and PDMS sponges as substrates. The device examples include electrophysiological sensors, hydration sensors, temperature sensors, and joule-heating elements. The porous geometries of the devices could facilitate perspiration transport and evaporation, and minimize discomfort and inflammation risks, thereby improving their long-term feasibility. Second, we have achieved the construction of various 3D hierarchical structures of LIG using mechanically-guided, 3D assembly approach. The well-designed, 3D hierarchical structures of LIG exhibit outstanding electromechanical properties. Mechanical loading, such as bending, stretching and compressing, has negligible effects on their electrical performances. The device examples include highly-stretchable LED arrays and 3D supercapacitors with solid-state electrolytes.

Electroactive and semiconducting polymer materials attract attention because of their potential applications in e.g. sensors, actuators or energy harvesting, which are crucial in development of the wearable electronic devices. In order to take advantage of such materials it is necessary to control the charge generation and charge carrier transport through their volume. The crystal phases play key roles here: crystallinity and crystal sizes as well as polymorphism and crystal orientation have crucial influence on electric properties of electronic devices. Control over the materials performance can be achieved by controlling crystallization from the length scales characteristic of crystal unit cells up to microdomain morphology.^1,2
In this work we showcase the crystallization in hybrid blends based on two semicrystalline polymers: poly(3-hexylthiophene) (P3HT): the p-type semiconductor, and poly(vinylidene fluoride) (PVDF) with remarkable piezo- and ferroelectricity. Despite dissimilar in terms of the architecture of their main chains, these polymers have an important thing in common: crystallinity-driven electrical properties. Typically, PVDF and P3HT are used in the form of films or fibers being active parts of the devices. Polymorphism and orientation of crystals in such films can be controlled during their fabrication. In the case of the solution-based processing of pure polymers, the crystallinity can be controlled by tuning polymer-solvent interactions, aggregation of macromolecules in solution, and solvent evaporation rate.² In the case of melt-processing, the crystallinity depends mainly on the cooling regime whereas the orientation of crystals is controlled by machine-induced shearing forces or mechanical deformation after the processing.³
Blending of either PVDF or P3HT with other materials typically leads to heterogeneous systems, where the crystallinity is additionally driven by interfacial phenomena like heterogeneous nucleation or epitaxy. These together with the aforementioned effects are particularly important in nucleating the preferred polymorphs. In PVDF the ferro- or piezoelectricity are observed only for polar crystal polymorphs, i.e. the crystal forms where the unit cells are non-centrosymmetric, as in the case of Form I or Form III resulting from e.g. nucleation by e.g. silver nanoparticles. In addition, nanoparticles with high aspect ratios such as nanoplatelets of organoclays have an ability to “direct” the diffusion of the polymer chains towards the crystal growth zones, which allows formation of the oriented PVDF crystals.⁴ Formation of the oriented crystals of P3HT can also be driven by anisotropic nanoparticles, such as needle-like nanocrystals of perylene diimides or graphene nanoribbons.⁵ In the case of P3HT, however, the formation of oriented crystals results from the specific interactions between the nanofibers and the polymer.⁶
The orientation of the polymer crystals can be further enhanced by thermally stimulated diffusion of polymer macromolecules to the crystal growth zones, which can be achieved by e.g. local laser heating. For this purpose we have developed Laser-Assisted Zone Crystallization technique (LAZEC) enabling solution crystallization of the polymers and other organic materials under controlled thermal conditions. Application of the LAZEC in crystallization of the blends with finely tuned composition enables a large-scale formation of continuous films with controlled polymorphism and spatial orientation of polymer crystals.

The work was supported by National Sci. Centre Poland (NCN) through the grants UMO-2016/22/E/ST5/00472 and UMO-2017/25/B/ST5/02869

References:

1) Zhang G. et al; Energy & Environmental Science 11, 2018, 2046
2) Zhao K. et al; ACS Applied Materials & Interfaces 8, 2016, 19649
3) Martin J. et al; Materials Horizons 4, 2017, 408
4) Kiersnowski A. et al; Langmuir, accept. 2018
5) ElGemayel M. et al; Nanoscale 6, 2014, 6301
6) Chlebosz D. et al; Dyes and Pigments 140, 2017, 491

Iontronics is an interdisciplinary research topic which bridges ionics and electronics. In a hybrid iontronic system, the spatial and temporal distribution of ion species can either transmit signal or modulate electron's behavior through electrical double layer (EDL), leading to novel functions which are impossible to realize within solid-state electronics. Most ionic conductors, such as polymer electrolytes, hydrogels and ionogels, are fully transparent over visible spectrum, intrinsically deformable and biocompatible. Such remarkable optical and mechanical properties make them promising to serve as soft electrodes in applications where extreme optical transmittance or biological compatibility is stringently required. The strategy of incorporating ion conductive materials into electronic systems has enabled diverse iontronic devices spanning from artificial muscles, wearable sensors, energy harvesters, luminescent light emitters to biologically matched interfaces.

Emerging touch panels integrated in next generation wearable devices require the electrode material to be both skin-conformable and highly transparent, yet fatigue failure and sharp resistance increment are bottlenecks in stretchable and transparent electronic conductors. In this work, we demonstrated an inkjet printing-assisted iontronic touch panel with epidermal conformability and over 95% device-level light transmittance. Ionic electrolyte was directly “written” onto elastomer substrates as column and row electrodes to form a mutual-capacitance touch sensing array, in which each intersection constituted a picofarad-level capacitor sensitive to finger's approach. Owing to the micrometer-scale patterning resolution provided by drop-on-demand (DOD) inkjet printing technology, a coplanar electrode layout was achieved in replacement of the commonly adopted parallel-plate capacitor configuration. Unprecedented high touch sensitivity over 50% can be achieved and noise signals induced by deformations including vertical compression and lateral elongation can be suppressed. By integrating with a customized readout circuitry, the skin-intimate touch panel could detect proximity, gentle single touch as well as multi-touch inputs, and acted as a robust touch sensing interface with ultrahigh signal fidelity even under static or dynamic deforming conditions.

Polymer semiconductors are promising materials for stretchable electronics owing to the opportunity to tune mechanical behavior. For stretchable applications, the mechanical loading on these films is quite demanding, yet many important mechanical properties of these films related to their ability to operate over large strains has not been characterized in detail. This is due in part to the change in polymer properties when forming ultrathin films, and the difficulty in measuring the mechanical properties of these films. While a number of tools have been developed to probe ultrathin films, they all have limitations that inhibit a complete view of the film behavior. For example, the mechanical behavior of films under in-plane compression has not been captured well, which is a key part of a stretchable device that undergoes large cyclic strain. Here, a novel mechanical testing technique is employed that is able to measure a broad array of mechanical properties of polymer thin films including compressive properties. The proposed approach consists of a polymer thin film laminated on a thin elastomer support that is then mounted in a dynamic mechanical analyzer (DMA). The elastomer provides support for the semiconductor film, but is thin enough that the properties of the polymer of interest are accurately captured. The elastomer support also provides a restoring force under large strain so that the film’s properties under compression can be obtained. By using a conventional DMA, variation in the load frequency, strain rate, and environment temperature can be used to extract detailed thermomechanical properties. In this presentation, we show the array of properties that this method can obtain from stress-strain curves under cyclic loading, to glass transitions, to stress relaxation master curves. Through this testing method, new insights into film behavior that lead to effective stretchable polymer semiconductors will be highlighted.

This presentation will describe our research efforts in developing materials for flexible and intrinsically stretchable electronic devices. A polymer composite comprising surface-embedded silver nanowires with high transparency, high surface conductivity, and low surface roughness is explored as a transparent electrode essential to the fabrication of deformable LEDs. Several electroluminescent materials are employed, including OLED, LEC, perovskite-polymer nanocomposites.

High degrees of mechanical deformability in organic electronics (i.e., stretchability) have enabled a range of applications in energy and healthcare. These applications include large-area organic solar cells (via roll-to-roll solution printing), and wearable and implantable sensors (to allow intimate contact with biological tissues). But, polymers which have a high electronic performance and are mechanically compliant, biosafe, and easily processable remain difficult to obtain. Typically, the highest performing organic electronics are hard and undergo brittle fracture at low strains. Some of the strategies that have been reported to enhance the stretchability of conducting and semiconducting polymers include composites, blending with plasticizers, deposition on elastomers, formation of fibers and gels, and side chain engineering.^1,2 While effective, each of these methods can have drawbacks in terms of biocompatibility, toxicity, ease of processing in solution, and electronic sensitivity to strain. Instead, we have investigated the use of block copolymers to obtain stretchable organic electronics. First, we reported the use of water-soluble poly(styrene sulfonate) block copolymers with poly(polyethylene glycol methyl ether acrylate) (PSS-b-PPEGMEA) as a scaffold for the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT).³ The resulting conducting polyelectrolyte PEDOT:PSS-b-PPEGMEA has a toughness up to 10.1 MJ m^–3 and can withstand elongations up to 128%. Second, we developed stretchable and degradable semiconducting block copolymers.⁴ By combining blocks of semiconducting diketopyrrolopyrrole (DPP) with insulating and flexible aliphatic polyester poly(ε-caprolactone) (PCL) blocks, we obtained materials that can be stretched >100%. Remarkably, the field-effect mobility of these block copolymers remains the same as the pure semiconductor.

1. Root, S. E.; Savagatrup, S.; Printz, A. D.; Rodriquez, D.; Lipomi, D. J., Chem. Rev. 2017, 117 (9), 6467.
2. Kayser, L. V.; Lipomi, D. J., Adv. Mater. Progress Report. In press.
3. Kayser, L. V.; Russell, M. D.; Rodriquez, D.; Abuhamdieh, S. N.; Dhong, C.; Khan, S.; Stein, A. N.; Ramírez, J.; Lipomi, D. J., Chem. Mater. 2018, 30 (13), 4459.
4. Sugiyama, F.; Kleinschmidt, A. T.; Kayser, L. V.; Alkhadra, M. A.; Wan, J. M. H.; Chiang, A. S. C.; Rodriquez, D.; Root, S. E.; Savagatrup, S.; Lipomi, D. J., Macromolecules 2018, 51 (15), 5944.

The vast amount of biological mysteries and biomedical challenges faced by human provide a prominent drive for seamlessly merging electronics with biological living systems (e.g. human bodies) to achieve long-term stable functions. Towards this trend, the main bottlenecks are the huge mechanical mismatch between the current form of rigid electronics and the soft biological tissues.

In this talk, I will describe a new form of electronics with skin-like softness and stretchability, which is built upon a new class of intrinsically stretchable polymer materials and a new set of fabrication technology. As the core material basis, intrinsically stretchable polymer semiconductors have been developed through the physical engineering of polymer chain dynamics and crystallization based on the nanoconfinement effect. This fundamentally-new and universally-applicable methodology enables conjugated polymers to possess both high electrical-performance and extraordinary stretchability.[1] Then, proceeding towards building electronics with this new class of polymer materials, the first polymer-applicable fabrication platform has been designed for large-scale intrinsically stretchable transistor arrays.[2] As a whole, these renovations in the material basis and technology foundation have led to the realization of circuit-level functionalities for the processing of biological signals, with unprecedented mechanical deformability and skin conformability. Equipping electronics with human-compatible form-factors has opened a new paradigm for wearable and implantable bio-electronic tools for biological studies, personal healthcare, medical diagnosis and therapeutics.[3]

References
[1] J. Xu^#, S. Wang^#, et al. Highly stretchable polymer semiconductor films through the nanoconfinement effect, Science, 2017, 355, 59-64.
[2] S. Wang, et al. Skin electronics from scalable fabrication of an intrinsically stretchable transistor array, Nature, 2018, 555, 83-88.
[3] S. Wang, J. Y. Oh, J. Xu, H. Tran, Z. Bao. Skin-inspired electronics: an emerging paradigm, Accounts of Chemical Research, 2018, 51, 1033–1045.

Upon designing high performance polymer semiconductors, degree of polymerization or molecular weight is an important polymer property that greatly affect the resulting polymer physical properties. The polymer chain length can directly influence on the morphology, packing and degree of crystallinity based on the polymer conformation and the dominating thermodynamics driving force (enthalpic vs entropic). The packing structures, especially p-p stacking distance, crystalline domain size and distribution have significant impact on not only charge transport but also thin film mechanical properties, such as modulus, ductility and deformability. Until now, it is very difficult to realize a fully conjugated rigid backbone with significant repeat units for study of the relationship between electronic performance, mechanical endurance, and molecular weight in the research community. Here we present the successful synthesis of diketopyrrolopyrrole (DPP)-based semiconducting polymers with controllable molecular weight over a wide range from approximately 20 to 100 kDa, realizing both high molecular weight and high solubility at the same time. The effects of strain on polymer thin film microstructures (alignment of polymer chains, change in crystalline domain size, degree of crystallinity and orientation), moreover, is investigated and a charge carrier mobility over up to 2 cm²V^-1s^-1 is achieved. Thanks to the relatively high molecular weight in our new polymer system, large-area biaxially stretched polymer thin film is firstly developed with superior electrical performance in stretchable devices.

Soft, noninvasive and multifunctional epidermal electronics (a.k.a. electronic tattoos or e-tattoos) have demonstrated many applications in mobile health, athletic training, human-machine interface (HMI) and so on. However, e-tattoos are only practically useful when they are low cost and wireless. Previously, our group has invented a dry and digital manufacturing approach called the “cut-and-paste” method for the rapid prototyping of e-tattoo sensors using a paper/vinyl cutter plotter. The cut-and-pasted e-tattoos are low-cost and can be used to measure a variety of physiological signals such as electrocardiogram (ECG), skin hydration, skin temperature and so on. To make the e-tattoos go wireless, we now report the “cut-solder-paste” process to incorporate integrated circuits (ICs) such as chips for near field communication (NFC) or Bluetooth Low Energy (BLE). The key is to come up with a temporary adhesive that can survive solder-reflow temperature up to 240 degree C. To overcome the limited patterning resolution of mechanical cutter plotters and to recycle the IC chips, we propose a modular concept in which the communication layer, the readout circuit layer, and the sensor/electrode layer are fabricated individually and stacked up as the final step of fabrication. The thickness of a fully assembled multilayer e-tattoo (excluding IC chips) is less than 200 um and the overall stretchability is still beyond 20%. In addition to already mentioned capabilities, such wireless e-tattoos can also track motion, mechano-acoustic heart signals, and oxygen saturation (SpO2). The NFC-enabled e-tattoos can be wirelessly charged so no battery is needed but the sampling rate is limited to 25 Hz and the wireless communication distance is limited to 5 cm. The BLE-enabled e-tattoos require on-tattoo batteries but the sampling rate can be up to 4 kHz and the wireless communication range can be up to 10 m. We demonstrate that the different layers can be disassembled and reassembled multiple times. After disassembly, the communication and readout circuit layers are reusable and can be reassembled with different sensor/electrode layers. The wireless and reconfigurable capabilities and the low-cost, rapid prototyping method together represent exciting advancement towards practically useful e-tattoos.

This talk will summarize recent research activities on 3D printed soft electronics in the Additive Manufacturing laboratory, Simon Fraser University. Design, fabrication and characterization of 3D printed soft sensor systems including ion selective electro-chemical sensors, 3D printed circuit boards for portable electro-chemical sensing applications will be discussed. 3D printing enables custom design of smart 3D form factors that enable electronics to be integrated into unique places. Different electro-chemical sensor platforms such as ion-selective field effect transistor and disposable wireless RF circuits have been developed for the portable electronic applications. It is shown that 3D design integration can significantly accelerate the hybridization with the fabrication process of conventional electronics, and merge its capability into portable sensor applications.

Soft polymer-based sensors as an integral part of textile structures have attracted considerable scientific and commercial interest recently because of their potential use in healthcare applications, security, structural health monitoring, and other applications where flexible, conformable sensors are required. While electronic sensing functionalities can be integrated into textile structures at any one or more of the hierarchical levels of molecules, fibers, yarns, or fabrics, arguably a more practical and inconspicuous means to introduce the desired electrical characteristics is at the fiber level, using processes that are compatible to textiles. Fiber-based sensors have been developed using a variety of materials such as polyethylene glycol (PEG), Kevlar, and polydimethylsiloxane (PDMS) which are doped with conductive particles such as silver, platinum, and carbon particles. A variety of techniques such as fiber extrusion, conductive coating of fibers, and 3D printing have been used to develop these sensors. Here, we report a prototype fiber-based sensor capable of multimodal and multifunctional sensing, formed within a typical woven textile structure. This is achieved by developing bicomponent fibers with ordered electrically insulating and conducting domains, composed of PDMS composites. The multifunctional characteristics of these sensors are successfully demonstrated by measuring their response to tactile, tensile, and shear deformations, as well as by their capability to detect wetness and report biopotential response. While the unobtrusive integration of sensing capabilities offers possibilities to preserve all desirable textile qualities such as comfort, flexibility, and conformability, this scaled-up fiber-based approach demonstrates the potential for scalable and facile manufacturability of practical e-textile products using extrusion-based processing of flexible sensor systems, and can be remarkably effective in advancing the field of electronic textiles (e-textiles).