Symposium S.SM04 : Fundamental Materials, Devices and Fabrication Innovations for Biointegrated and Bioinspired Electronics

There is an increasing demand for specialized pressure sensors in various biomedical applications, especially health monitoring. For example, cardiovascular monitoring, which has implications in early diagnosis of heart failure.^[1] To date, there have been numerous reports on pressure sensor designs in order to meet the demands of specialized pressure sensors. However, in order to address the growing demand for tailored pressure sensors for such a diverse range of applications, it is important to understand how different sensor parameters impact performance. By quantifying these relationships, we aim to determine design parameters that will enable researchers to predict a priori the specific design needed to achieve the targeted performance. Previously, capacitive pressure sensors have been shown to have wide versatility in use, with a high degree of potential tuning possible through manipulating the geometry or material selection of the dielectric layer. One effective approach first reported by our group to tune the performance of capacitive pressure sensors is microstructuring the dielectric layer, which improves its compressibility and, consequently, its sensitivity.^[2,3] As researchers seek to incorporate novel materials into sensors, there is also a need to understand how various intrinsic material properties, such as compressive modulus, impact sensor performance. Compressive modulus is qualitatively known to affect the sensitivity of the pressure sensor.^[4] Quantifying the relationships between intrinsic material properties, dielectric geometry, and sensor performance will enable a design of pressure sensors and selection of materials to meet requirements for specific applications.
Presented here is an improved fabrication method to achieve simple, tunable, consistent, and reproducible pressure sensors by using a pyramid microstructured dielectric layer along with an added lamination layer to improve consistency and reproducibility. The as-produced sensor performance matches predicted trends of parameters that can be explicitly controlled. Further, we develop a simple computational model using intrinsic properties of the elastic dielectric layer and experimentally confirmed its efficacy. We then use our model to predict other properties and a wider range within the tested properties to better understand the effect of material property and microstructure geometry on the sensor performance. This would allow us to anticipate trends and sensor performance. Finally, we demonstrate that we can more directly design sensors for a specific application, such as wrist pulse sensing, using our model and fabrication method.


[1] Z. Liu, Y. Ma, H. Ouyang, B. Shi, N. Li, D. Jiang, F. Xie, D. Qu, Y. Zou, Y. Huang, H. Li, C. Zhao, P. Tan, M. Yu, Y. Fan, H. Zhang, Z. L. Wang, Z. Li, Adv. Funct. Mater. 2018, 1807560, 1807560.
[2] S. C. B. Mannsfeld, B. C. K. Tee, R. M. Stoltenberg, C. V. H. H. Chen, S. Barman, B. V. O. Muir, A. N. Sokolov, C. Reese, Z. Bao, Nat. Mater. 2010, 9, 859.
[3] G. Schwartz, B. C.-K. Tee, J. Mei, A. L. Appleton, D. H. Kim, H. Wang, Z. Bao, Nat. Commun. 2013, 4, 1859.
[4] T. Q. Trung, N. E. Lee, Adv. Mater. 2016, 28, 4338.

In recent years, as the number of patients with Alzheimer’s disease increases due to the advent of early-onset dementia, the social severity of Alzheimer’s disease is on the rise. Accordingly, there are increasing demands for various dementia diagnosis methods because Alzheimer's disease can be prevented from disease development by early detecting and treatment. Among these methods, biosensors are emerging as applications that enable point-of-care testing with low cost and fast response time. The interest in these technologies has mainly focused on field-effect transistor-based biosensor (bio-FET), where the conventional gate and gate insulator are replaced by a reference electrode and the electrolyte, respectively. In addition, the bio-FET uses receptors such as enzyme and aptamer on the sensing surface for detecting targeted bio-molecules. From then on, a lot of researches on the improvement of performance for bio-FET have been reported. However, the bio-FET suffered from low sensitivity and chemical instability. Therefore, oxide semiconductors have been attracted for leading candidates for bio-FET with high chemical resistance against the bio-materials compared to one- and two- dimensional (1D and 2D) materials. Also, oxide semiconductors based bio-FETs have been attracted much attention for high sensitivity due to low off current characteristics. The low off current (< 10¹² A) of oxide FETs guarantee high signal-to-noise ratio in biosensor because they have large changes before and after sensing.
In this study, we suggest a facile approach to fabricate highly sensitive and durable biosensors based on a bio-mimetic structure (i.e. spongia officinalis) of the oxygen plasma-treated porous indium–gallium–zinc oxide (IGZO):polytetrafluoroethylene (PTFE), which is PIP, thin–film layer. The PIP layer could be easily deposited by co-sputtering process and oxygen-plasm etching. Through this fabrication process, we were able to discuss two effects of the technique on the PIP film: 1) It is possible to drastically higher the surface area of the PIP layer utilizing the oxygen-plasma because the PTFE was selectively etched by oxygen-plasma process, which results in a porous of the PIP surface. 2) Improvement of hydrophilicity through the oxygen-plasma process of the PIP layer with free radical species formed by chemical etching. Due to the two effects mentioned above, the PIP layer could more easily be functionalized for surface silanization compared to conventional IGZO layer.
In conclusion, to test the IGZO with the PIP layer for bio-FET, we investigated molecular detection of the pH and the neurotransmitter nitric oxide, which is known to induce in dementia, using an aptamer. As a result, the pH sensor with ultra-high sensitivity (1 pH⁻¹) as well as a small limit of detection (0.001 pH) was demonstrated. Furthermore, specific detection of nitric oxide also was demonstrated, and an extremely low limit of detection (< 10fM) was achieved in 0.1× phosphate-buffered saline solution.

We focus on developing a new class of nanoscale materials and novel strategies for the investigation of biological entities at multiple length scales, from the molecular level to complex cellular networks. Our highly flexible bottom-up nanomaterials synthesis capabilities allow us to form unique hybrid-nanomaterials. Recently, we have demonstrated highly controlled synthesis of 3D out-of-plane single- to few-layer fuzzy graphene (3DFG) on a Si nanowire (SiNW) mesh template. By varying graphene growth conditions, we control the size, density, and electrical properties of the NW templated 3DFG (NT-3DFG). This flexible synthesis inspires formation of complex hybrid-nanomaterials with tailored optical and electrical properties to be used in future applications such as biosensing, and bioelectronics. For example, we have developed a unique bioelectrical platform based on 3DFG and have demonstrated, for the first time, recording of the electrical activity of excitable cells using ultra microelectrodes ranging from 10µm down to 2µm, without the need of any surface coatings. Currently, we target the limits of cell-device interfaces using out-of-plane grown 3DFG, aiming at electrical recordings with subcellular spatial resolution (<5μm) and μsec temporal resolution. In summary, the exceptional synthetic control and flexible assembly of nanomaterials provide powerful tools for fundamental studies and applications in life science and open up the potential to seamlessly merge either nanomaterials-based platforms or unique nanosensor geometries and topologies with cells, fusing nonliving and living systems together.

Humans have been studying and mimicking animals’ visual crypsis using wearable camouflage since the 18^th century. Yet, only recently we have documented and attempted to understand the origins of chemical crypsis, the ability to become imperceptible to olfaction. Specifically this work aims to elucidate the mechanism of chemical crypsis in the first vertebrate shown to exhibit this trait, a snake known as the puff adder (Bitis arietans). Extensive behavioral and ecological studies show that puff adders are well evolved to be chemically imperceptible to predators, prey, and even potential mates without the capability to modulate pheromone molecular weight, produce odorant counteracters, or eliminate skin odorant sources such as bacteria. Therefore we turned to the skin surface structure, the major feature distinguishing the puff adder from its non-cryptic brethren such as the night adder (Causus rhombeatus). The skin’s micron-scale, high aspect ratio, curved features known as ‘fingers’ create an array of wells for odorants to pool, significantly reducing odorant volatility and therefore rendering the puff adder imperceptible to smell. To study and quantify this phenomena independent of snakeskin material, snakeskin surface biomes, and environmental contaminants, we employ various imaging and 3D printing processes to create a variety of detailed and accurate scaled models of both the puff adder skin and night adder skins. Nano-focused computed tomography (nano-CT) is used to create three-dimensional renderings of skins. To maximize accuracy of replicas, nano-CT image segmentation is informed by focused ion beam (FIB) and scanning electron microscopy (SEM) images. These digitally rendered surfaces are printed in urethane methacrylate or other acrylic using 3D printing. 90:1 models are printed using projection stereolithography (SLA) for millimeter resolution while 1:1 models require the use of two photon polymerization for micron scale feature resolution. 2D Fourier transforms of 2D slices examined along the third axis in real space in addition to 3D Fourier tranforms are used to pinpoint key frequencies present in cryptic versus non cryptic snakes. This analysis is used as a means to quantify periodicity and investigate the degree of quasi-ordered orientation as a predictor for surface thermodynamic phenomena. Physical experiments on printed models and topographical analysis of digital renderings are used together to analyze the effects of structure periodicity and surface-area-to-volume ratio on adhesion, wetting, and evaporation.

Highly efficient charge-transport phenomena in biological systems ignited researchers’ interest to investigate the conductivity of biologically relevant macromolecules processed from aqueous solution. Self-assembling peptides and proteins are of particular interest due to their ease of synthesis and tunable side chains. Polypeptides have been designed and synthesized for self-assembly and charge delocalization of organic pi-electron systems. Of these, peptide-π conjugated materials form robust 1D nanostructures with tunable electronic and photonic properties while retaining significant conductivity and mobility. Electronic and photonic properties of π-conjugated oligopeptides can be tuned by incorporation of electron withdrawing and electron accepting chromophores in the peptide chains. Oligothiophenes are also one of the standard organic semiconductor subunits investigated for semiconducting properties of π-conjugated peptides. However, peptide-based charge transporting materials still suffer from relatively low conductivities due to the large insulating peptidic regions, posing a challenge for characterization of electrical transport pathways in biologically relevant materials.
Here, we investigated 1D nanostructures and electrical properties of a peptide-π-peptide oligopeptide self-assembly composed of peptides, HCl, and KOH in aqueous solution. Using supramolecular systems, oligomers with central chromophore groups with non-polar amino acid side chains plus one glutamic acid were self-assembled into macrostructures via pH triggering . In order to decouple the amino acid side chains from the chromophores, 3- carbon spacers were added in between them on both sides. Using KOH to dissolve the oligomers and subsequently exposing them to HCl vapor triggered self-assembly via pH change leading to protonation of glutamic acid, as well as biomineralization of KCl, presumably initiating at the glutamic acid. Together, the self-assembly achieved by formation of ß-sheets, π-orbital overlaps and biomineralization formed highly interconnected dendritic structure given conditions with well-balanced kinetic and thermodynamic contributions. The solution conditions with HCl exposure enabled biomineralization of KCl nanocrystals templated by the glutamic acid in the underlying oligopeptides prior to self-assembly, resulting in a synergy of self-assembled peptides and biomineralized KCl to form supramolecular structures.
Electrical measurements indicated that these interconnected dendritic structures were highly conductive systems with conductivities comparable to that of a metal at around 1800 S/cm for samples with robust dendritic pathways. About 50 mA current was measured for 0.5 V at drain. Varying the gate V, however, had no effect on the current levels indicating a conductive material instead of a semiconducting material. Various experiments showed the conductivity of these systems were derived from proton doping of chromophores in strong acid environment in addition to closely fixated chromophores from the biomineralized KCl leading to easily transferred π-electrons along the interconnected dendritic pathways. Shelf-life tests showed that the conductive pathways degraded about 90% after 6 weeks; the KCl crystals formed around the self-assembled structures further functioned as a protective layer to dedoping of protons as well as preventing disintegration of the closely packed chromophores. Our findings suggest that in supramolecular systems, biomineralization triggered with appropriate amino acids combined with self-assembly can be used as a template to grow highly conductive dendritic macrostructures as well as control nanowire growth in specific directions.

The development of soft and highly flexible electronics enables seamless interfaces with biology, with applications ranging from implantable medical devices to wearable sensors for health monitoring. Piezoelectric polymers have demonstrated superior performance compared to standard ceramic piezoelectric materials however, these polymer-based devices still require metallic electrodes, patterned through photolithography techniques which are time and resource intensive. Here we aim to fill this gap with inkjet printing, an iterative, low-cost, and scalable fabrication method for patterning polymer electrodes. Specifically, we developed an inkjet printing process, with over 500cm² printable area and linewidth resolution of 100 µm, able to print a patterned 5 cm² polymer electrode layer in minutes. The polymer electrodes showed a layer dependent conductivity with a high conductivity of 5.4*10^-3 S/cm at 10 layers. Utilizing this technology, we developed a flexible piezoelectric PVDF-TrFE nanofiber/PDMS composite, with inkjet-printed patterning of a conductive PEDOT:PSS electrodes on either side of the piezoelectric material. Further, we are able to create a flexible sensor array. For a single 1 cm² element, voltage peak-peak output was measured over a range of forces giving a sensitivity of 258 mV/N, which outperforms many PVDF based force sensors in the literature. The 2x2 piezoelectric array is able to detect localized force, with a 12.5x greater output on the pressed element versus the output from adjacent untouched elements, even though they share the same substrate. This result is significantly higher than that seen in Piezoelectric-OSFET arrays for tactile sensing in the literature. For wearable health monitoring applications, we have shown the ability to detect a resting heart rate of 55 bpm from the carotid artery. This study develops a promising scalable method for high-throughput fabrication of polymer-based piezoelectric sensors on a laboratory scale that can be selectively patterned with inkjet technology.

Room-temperature liquid metals, such as nontoxic gallium alloys, show enormous promise to revolutionize stretchable electronics for next-generation soft robotic, e-skin, and wearable technologies. Core–shell particles of liquid metal with various surface-bound ligands can be synthesized and polymerized together to create cross-linked particle networks comprising >99.9% liquid metal by weight. When stretched, particles within these polymerized liquid metal networks (Poly-LMNs) rupture and release their liquid metal payload, resulting in a rapid transformation from insulating to conducting behavior. These networks autonomously form hierarchical structures that mitigate the deleterious effects of strain on electronic performance and give rise to emergent properties. Notable characteristics include nearly constant resistances over large strains, electronic strain memory, and increasing volumetric conductivity with strain to over 20 000 S cm-1 at >700% elongation. Furthermore, these Poly-LMNs exhibit exceptional performance as stretchable heaters, retaining 96% of their areal power across relevant physiological strains. Remarkable electromechanical properties, responsive behaviors, and facile processing make Poly-LMNs ideal for stretchable power delivery, sensing, and circuitry.

Quantifying humidity has long been an unavoidable task in science, industry, and society. Recent developments of nanoscience and technology that deal with ultrasmall droplets have aroused interest in microscopic moisture. Utilization of nanomaterials has been emerging as a promising strategy to miniaturize hygrometers for high-sensitive, ultrasmall-area sensing. However, a lack of high-precision, on-demand position control of sensing nanomaterials makes it difficult to explore spatial distribution of humidity at the micro- and nanoscale. Here, we develop a scanning probe hygrometry (SPH) that enables not only micro/nano-resolution but also scalable spatial mapping of humidity distribution. The SPH is realized with an unprecedented scanning nanowire probe interferometer (NPI) that is produced by direct 3D nanoprinting of a moisture-sensitive polymer on a tapered optical fiber. We observe the interferometric response of the NPI probe in ultrasmall-areas quantitatively depends on humidity, arising from its refractive index change and volumetric swelling. By scanning the NPI probe and reading out the interferometric signals, we demonstrate not only multiscale spatial mapping of humidity distribution with versatile scanning steps from ~ 10² nm to a few mm, but also local humidity in very small spaces. We expect our NPI to provide a new nanoscale metrology that could answer fundamental questions about evaporation-related science and engineering.

Chemical vapor deposited (CVD) graphene is considered a promising material for biomedical applications, namely in vitro and in vivo sensors. When the graphene sensor surface comes in contact with an analyte solution, electrochemical effects caused by a liquid aqueous environment start to take place and interfere with the material stability and detection of an analyte. Here we report our observations about two instances of such reactions. In the first system, fracturing of the graphene layer during anodization is recorded using a Quartz Crystal Microbalance (QCM) method combined with a 3-electrode electrochemical configuration. Positive biasing in a region of potentials that enable an oxygen oxidation reaction caused a layer fracturing and loss of the graphene film integrity. In the second system, origins of gate/leakage currents on an electrolyte-gated Graphene Field Effect Transistors (GFETs) are investigated. A potential-dependent reaction of oxygen is found to be responsible for these currents. Moreover, a surface activation towards this process is observed. As revealed by a complementary method, cyclic voltammetry with a redox probe Fc(MeOH)₂, more defects appear on the graphene transistor surface when extensive leakage currents are present during the GFET operation mode.

As more hospitals close in rural America, healthcare in these areas has been pushed towards community- and home-care situations. To support this shifting market, new point-of-care (POC) diagnostic techniques are needed to fill the void left by the healthcare industry. Thin-film transistor (TFT) -based electronic biosensors are ideally suited for POC applications due to their ability for miniaturization, low operating voltage, and high sensitivity. Yet, one significant hurdle in developing TFT biosensors is that the Debye length in complex fluids, such as serum or whole blood, is sufficiently small that electrical interactions between the analyte and the TFT are completely screened, rendering the device insensitive to changes in analyte concentration. Combining the TFT design with a non-fouling polymer brush layer, poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA), has the potential to both extend the Debye length to support the measurement of analyte binding events and minimize nonspecific binding that could prove detrimental to the small electrical perturbations related to sensing low concentration analytes. In this work, we demonstrate the impact of the field-effect gating structure on the operation of a printed carbon nanotube (CNT) -TFT biosensing platform, harnessing the high sensitivity of the CNT thin film and the Debye length extension (in addition to non-fouling properties) of the POEGMA layer. As a demonstration, we print down capture and detection antibodies for the protein leptin on the POEGMA-coated CNT-TFT and measure leptin concentration in both phosphate buffered saline (PBS) and whole human blood. We demonstrate that, unlike optical techniques, no cumbersome wash procedure is required to enable concentration-dependent measurements. Furthermore, a comparison is made between substrate-gated and solution-gated approaches with the CNT-TFT, revealing the role of solution charge and leakage currents in the successful operation of these biosensors. Also explored is the impact of the solution gate stack for the CNT-TFT, where inclusion of a passivating buffer layer of Al₂O₃ over the entire device structure enhances yield without significantly compromising sensitivity and the addition of a secondary passivation from SU8 directly over the contacts increases measurement stability over longer duration tests. These results mark key progress towards realizing a printed CNT-TFT biosensing platform that can open the way for low-cost, biomedical sensors – a paradigm shift for POC electronic diagnostic systems.

A major challenge of the 21^st century will be to establish meaningful two-way communication between biology and electronics. The study of protonics, devices that mimic electronics but pass protons instead of electrons, seeks to bridge this gap. Protonic conductive materials (PCMs) are essential elements of these devices and we have demonstrated significant improvement in conductivity for chitosan PCMs when deposited as electrospun nanofibers. The observed improvements stem from both enhanced molecular alignment and from chemical doping due to the electrospinning carrier fluid, trifluoroacetic acid (TFA). We deposited electrospun chitosan nanofibers over palladium protodes and then used the helium ion microscope to isolate single nanofibers for detailed study. We observed that single chitosan nanofibers are strongly doped by TFA with x-ray photoelectron spectroscopy demonstrating extensively protonated nitrogen functionality. With the isolated, single chitosan nanofibers we observed that water uptake, fiber/electrode contact area, and doping concentration are critical parameters of protonic device performance and lead to increased conductivity (i.e. low resistivity). The average resistivity of single chitosan nanofibers is 6.2×10⁴ Ω●cm, approximately two orders of magnitude lower than the resistivity of cast chitosan PCMs (cast from acetic acid solutions not TFA). We have observed excellent agreement between theoretical models and experiment results that explore each of the contributions to the improved conductivity. Finally, the fabrication and measurement of ionic field-effect transistor of single chitosan fiber using conductive atomic force microscope will be discussed.¹

1. Lee, W.K., Pietron, J.J., Kidwell, D.A., Robinson, J.T., McGann, C.L., Sheehan, P.E., and Mulvaney S.P., “Enhanced protonic conductivity and IFET behavior in individual proton-doped electrospun chitosan fibers,” J. Mater. Chem. C, 7, 10833 (2019)

Neuromorphic organic electrochemical transistors (OECTs) have been reported to demonstrate different synaptic plasticity features including short and long-term plasticity (STP & LTP) and spike-timing-dependent plasticity (STDP), which allows us to mimic synaptic transmission thus construct the neuromorphic circuits. Apart from the inherent advantages of neuromorphic circuits like low-power operations (~nW-pW) and learning capability, the operating mechanism of OECT has a high resemblance with the actual neurons: the movement of ions, which allows OECT to be a competent candidate for developing neuromorphic device. However, in order to build a comprehensive neuromorphic device, not only the synaptic transmissions, but also the signal propagations within single neuron should also be taken into consideration. In this study, a device mimicking signal transmission in single neuron based on a linear organic electrochemical transistors array featuring the regeneration of action potential at Nodes of Ranvier has been constructed. By utilizing the organic electrochemical transistors based on conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), together with UV-cured poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel and PVC based ion-selective membrane (ISM), we attempt to fabricate a neuromorphic transistors device in mimicking the propagation of neuronal signals from dendrites to axon. The regeneration of action potentials at the Nodes of Ranvier is achieved by using the ion-sensing capability of OECT and the ion-selectivity of the ISM. In our devices, voltage as low as 0.2 V is enough for the operation, comparable with the ~0.1V of an actual action potential. This device can be incorporated with existing neuromorphic circuits and possibly bridges the future human–machine interfaces between biomimicking organic electronics and authentic neuronal systems.

Biopsy is a well-known minimal invasive technique to extract tissues from patients through a needle followed by performing histology to view cellular structures under the microscope. Biopsy studies of cancerous tissue is very common in surgical practice. To identify the cancer tissue, pathologists will usually examine the tissue features such as cell pattern, cytological atypia, mitotic activity, size and shape of cell nuclear. These structural features are highly correlated to the conductivity of the tissues which are composed of tightly packed cells. The cell walls are made of lipid which is a good insulator. When a D.C. current passes through the tissue, the cell wall stores the charges and prevent the current from passing through the cell. However, at high frequency, current can pass through the cell wall because the cell wall behaves as a capacitor. This difference of frequency response is often a characteristic for different tissue with different fluid composition. Currently, biopsy is still at the level for qualitative analysis and the measurements take a relatively long time around 20 minutes. There is a strong demand to develop a fast, localized biopsy tools which can provide quantitative characterizations of the tissue structures. In the current work, we will demonstrate a micro-scaled impedance sensing device developed on the tip of a glass capillary tube of 1mm diameter. Even the size of the electrodes is limited, we employed the organic semiconductor material, PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)), to reduce interface impedance of microelectrodes. By adding this PEDOT:PSS layer on the bare gold electrode, the interface becomes highly capacitive and possess a lower impedance. It is important to point that many of the reported impedance measurement tools uses electrode with large surface area and therefore very inefficient in performing localized measurement. As the proposed needle diameter is only 1mm, the reduction of the electrode surface area will suffer from the effect of mass diffusion and data in the low frequency spectrum will be masked. Our approach to modify the impedance allows us to reveal the impedance response of tissues at low frequency range for more precise equivalent circuit model fitting.

Laser Induced Graphene (LIG) was first described in 2014 and ever since it has been a far-reaching field of research. Research has focused on fundamental understanding of LIG structure and properties, as well as on technological applications in flexible/stretchable electronics, energy, among others^[1,2]. LIG is a porous and conductive carbon material produced by laser-induced pyrolysis of various polymer precursors, including biologically derived ones (e.g. wood, paper, silk)^[3]. An infrared laser, as typically used in commercial CO₂ laser cutter for manufacturing, is used for this purpose. This process allows for fast scribing of circuits onto insulating substrates in ambient condition.
We investigated the LIG formation process on polyimide to get a better understanding of the boundary conditions for the formation of LIG and the tuning of its morphology and properties (especially conductivity). LIG conductors are prepared by varying the various process parameters affecting laser fluence (i.e. energy per scribed area).
In this way, either a flat porous LIG or a dense forest of long (100-200 µm) carbon fibres bundles (with single fibres having a diameter of around 50-100 nm) can be obtained.
Composition, morphology and structure are characterized by optical, scanning and transmission electron microscopy, EDX, and correlative Raman-SEM spectroscopy. A variation of graphene content along the fibre growth direction was found. Variations in structure and morphology were correlated with conductivity.
Due to the huge potential of LIG in stretchable and wearable bioelectronics, latest developments of this research include investigation of biologically derived precursors.
References
[1] J. Lin, Z. Peng, Y. Liu, F. Ruiz-Zepeda, R. Ye, E. L. G. Samuel, M. J. Yacaman, B. I. Yakobson, J. M. Tour, Nature Communications 2014, 5, 5714.
[2] R. Ye, D. K. James, J. M. Tour, Advanced Materials 2019, 31, 1803621.
[3] Y. Chyan, R. Ye, Y. Li, S. P. Singh, C. J. Arnusch, J. M. Tour, ACS Nano 2018, 12, 2176.

A novel device architecture using a coupled media of an electro-active perovskite oxide (PO) and lead free photo active perovskite halide (PH) layer will be used to enhance signal strength through increased electron mobility, and the photoactive nature of the multimorph. Although, the optical and PA approaches have been studied individually in the past, there were no efforts to integrate the two approaches using perovskite layers and a flexible substrate to improve the wearable form factor, reliability and accuracy of the measurements. So far, all these studies conducted with PA system for non-invasive glucose monitoring used class–IV lasers and non-continuous and mostly in-vitro. This unique approach will integrate electro-active PO, and photoactive PH layers for signal conversion from acoustic and photoluminescence to electrical signals respectively. The following work involves the development of an integrated PO (BaTiO₃-PMMA-Graphene) - PH (Tin Iodide active layer) multimorph with a P3HT hole transport layer as the coupled media and development of the device architecture for the wearable device.

Bio-integrated electronic devices can pliably conform on the textured skin surface to continuously monitor the physiologically relevant parameters or biomarkers, with a huge impact on human health from preventative monitoring and early diagnostic confirmation to non-invasive and convenient therapeutic options. The ultimate application of this class of emerging electronics hinges on the indispensable modules of stretchable wireless transmission and power supplies. While near field communication (NFC) allows for wireless powering and communication with a working distance of ~ 3 cm, radio frequency (RF) antennas enable wireless transmission of data and energy in the far-field. Compared to the approaches that exploit stretchable conducting materials such as liquid metal or elastomers with conductive fillers, designing conventional metals in a serpentine or meshed geometry is still of high interest because of their high radiation efficiency. Although this structural design concept has resulted in various stretchable dipole and patch antennas, the narrow bandwidth still limits their applications in wireless communication and energy harvesting because of the frequency detuning from mechanical deformations. While the efforts on wideband antennas could possibly overcome this challenge, the deformation-dependent radiation properties are yet to fully investigated. Here we demonstrate a stretchable wideband dipole antenna that consists of serpentine units for both main and parasitic arms created by exploiting the laser-induced graphene (LIG) pattern and maskless metal coating. Additionally, combining stretchable dipole antenna with a high-efficiency impedance matching network and rectifying circuit leads to a stretchable rectifying antenna (i.e., stretchable rectenna) that can continuously harvest electromagnetic radiation energies from various widely available RF sources (e.g., WI-FI, 4G, and upcoming 5G). As an added component into the clean energy portfolio for future energy supply, the ambient RF energy-harvesting solution could also contribute to integrated energy systems and enable self-powered systems and remote monitoring of the environment.

Graphene is the gold-standard for electrical conductivity along with its high mechanical strength and excellent thermal conductivity. On the other hand, GC has exceptional chemical inertness, good electrical properties, high electrochemically stability (gold-standard for electrochemistry), purely capacitive charge injection, and fast surface electrokinetics coupled with lithography patternability. Therefore, to leverage the unique strength of these ‘gold-standard’ materials in electrode technology, we introduce a new material system that brings the best qualities of these materials in a single format joined through strong covalent bonds. In this preliminary study, we investigate fabrication methodology, transfer on flexible substrate, bonding between the two allotropes of carbon through FTIR (Fourier transform Infrared) spectroscopy, surface morphology through SEM (Scanning electron microscopy) and topography by AFM (atomic force microscopy), and application of metamaterial based microelectrodes for neural signal recording i.e. electrocorticography (ECoG).

Neurotransmitters are often referred to as the body’s chemical messengers. They are the molecules used by the nervous system to transmit messages between neurons. Dopamine and serotonin are electroactive neurotransmitters and can be detected though conventional neural probes. However, lactic acid is a non-electroactive molecule that cannot be detected by these conventional probes. Lactic acid can be found in the brain as a byproduct of energy production and a buildup of lactic acid leads to lactic acidosis. Lactic acidosis can hamper normal mitochondrial functions and has been linked to seizures and can be caused by ischemia or hypoxia. To detect lactic acid our group at SDSU are functionalizing glassy carbon electrodes by immobilizing an enzyme to catalyze lactic acid into a molecule that is electroactive. By immobilizing lactate oxidase into a chitosan matrix, we aim to cause a reaction that will create hydrogen peroxide which is an electroactive molecule with a redox potential of 1.2. In addition to functionalizing glassy carbon with lactate oxidase, we aim to immobilize lactate dehydrogenase which will catalyze lactic acid into NADH and pyruvate and compare both enzyme detection efficacies. We will use electrochemical methods such as fast scan cyclic voltammetry (FSCV), FTIR, and potentiostat reading to characterize, understand the chemical reactions occurring, and qualify these functionalized glassy carbon electrodes. Preliminary characterization results show a slight increase in impedance values once the immobilization matrix has been applied, but despite the increase in impedance the limit of detection has not decreased. Preliminary looks at FTIR data show an increase in functional groups for chitosan on glassy carbon compared to bare glassy carbon showing the chance of adherence.
1. NanoFab.SDSU, Department of Mechanical Engineering, College of Engineering, San Diego State University,
5500 Campanile Drive, CA 92182

Glassy Carbon (GC) microelectrodes have shown to be a promising material in Neuroscience, specifically electrochemistry with a capability of detecting electroactive and non-electroactive species such as Glutamate.
Here we demonstrate immobilization of glutamate oxidase on a probe with a four-electrode array and subsequently using fast scan cyclic voltammetry (FSCV) where we pass a current through each microelectrode to detect the presence of neurotransmitters. Electroactive species such as Dopamine and Serotonin oxidize when they come in contact with the electroactive surface. The detection of neurotransmitters will happen at specific voltages in-vitro. Here, we focus on some recent strategies for Glutamate probes immobilization on the surface of electrochemical transducer such as adsorption, covalent bonding and Glutaraldehyde and GluOx interaction on the electrode surface for specific interaction with its complementary Glutamate target. Using Glutaraldehyde, BSA and Glutamate oxidase we were able to detect as an electrochemical reduction of O2 to H2O2. The immobilization matrix of GluOx on the GC electrode acts as a barrier that allows the electrode to give supporting electrons.
By functionalizing bare glassy carbon electrodes we have shown detection of Glutamate, non-electrode molecule. Through the chemical reaction with an enzyme happening at the surface of the electrode and cyclic voltammetry we were able to show the chemical reduction of non-electroactive of molecules.

Light trapping and the broadband absorption of the solar radiation is of interest to various solar energy harvesting applications. In the current work, we report a new paradigm for light trapping, that is light trapping based on arrays of subwavelength nonimaging light concentrators (NLCs). We numerically show that silicon NLC arrays provide >75% broadband absorption enhancement of the solar radiation compared with that of optimized nanopillar arrays. The paper focuses on free-floating arrays of subwavelength compound parabolic concentrators (henceforth CPC arrays) as a case study. The calculations reveal that CPC arrays function as anti-transmission layers as only few photons transverse the CPC arrays which is in contrast to nanopillar arrays that function as anti-reflection layers. We show that the absorption enhancement in CPC arrays is due to efficient occupation of Mie modes which is motivated by the unique CPC geometry, and we demonstrate light trapping at the Yablonovitch limit. Finally, we examine the performance of a photovoltaic cell based on CPC arrays with respect to base doping levels and surface recombination. We show that the short-circuit current density of the CPC-based cell is >75% higher than the short-circuit current density of a photovoltaic cell based on optimized nanopillar arrays. We believe that light trapping based on NLC arrays paves the way to various applications such as ultra-thin photovoltaic cells.

In nature, fibre-like organisms such as Geobacter sulfurreducens [1], Shewenalla oneidensis [2] and Cable Bacteria [3] demonstrate surprising electrical transport properties and could be considered as ‘living electrical (nano)wires’. Cable bacteria, a group of recently discovered filamentous, electroactive, multicellular bacteria, have developed a unique energy metabolism and parallel fibre structures demonstrating electron transport for conduction lengths up to 1 cm and with fibre conductivities exceeding 10 S/cm [4]. Conduction measurements carried out in high vacuum excluded the possibility of ionic conduction, but the fundamental charge transport mechanisms remain unknown. The observed electron transport in cable bacteria over distances in the order of centimeters is unprecedented in the biological world.

Cable bacteria as ‘champion living electrical wires’ are of fundamental interest to better understand the underlying biological processes, but are also potentially interesting as alternative organic electronic materials for the emerging field of bioelectronics as e.g. biocompatible electrical connections and circuits, conductive composite materials, (nano-) sensors, transistors,... In order to investigate the intrinsic electrical properties and underlying transport mechanisms, our approach is to study them with a range of solid state electrical characterisation techniques and prototype electrical circuits/devices [4,5]. These activities revealed an unique electrical network architecture [5] and intrinsic electrical properties very similar to for instance organic semiconductors, situating them in the context of electrical functional materials between semiconductors and conductors [4].

While for various so-called biological semiconducting materials (e.g. peptides, DNA,..) the studied semiconducting properties do not have a direct functionality as electron transporting channel in the occurring life processes, for cable bacteria they are expected to be of key importance. The exceptional intrinsic (semi)conducting electrical properties and extracellular electron transport behaviour of cable bacteria highlighted in this work are therefore worthwhile to be further studied in a cross-disciplinary manner at the nexus of (micro-)biology, (materials) physics and electronics and to be further explored in a variety of bioelectronic devices/circuits.

References :
[1] G. Reguera et al., Nature 435, 1098-1101 (2005).
[2] M.Y. El-Naggar et al., Proc. Nat. Acad. Sci. U.S.A. 107, 18127–18131 (2010).
[3] C. Pfeffer et al., Nature 491, 218-221 (2012).
[4] F. J. R. Meysman, Nat. Commun. 10, doi:10.1038/s41467-019-12115-7 (2019).
[5] R. Cornelissen et al., Front. Microbiol. 9:3044 (2018).

Organic Electrochemical Transistors (OECTs) have been investigated as biosensors and logic circuitry due to their biocompatibility and large transconductance [1]. Minute perturbations in gate potentials enable significant changes in channel conductance, allowing OECTs to transduce weak potential changes in biological systems into large current modulations. They can additionally be used in sensing of metabolites enabled by enzymatic reactions [2]. While simple in design, the operation of OECTs still requires independent power and synchronized control of the gate and source-drain voltages, hindering the simplification of supporting control circuits. Furthermore, performing a chemical reaction within the electrolyte of the OECT can be challenging due to effects arising from electrochemical side reactions with both the gate and channel materials.
We demonstrate the development of an OECT that transduces a chemical signal (e.g. oxidation reaction) into an electrical signal (e.g. change of the conductivity of the channel material). To achieve this, the chemical reaction is isolated on a floating-gate, which acts as the chemical-to-electrical transducer for the electrochemical redox-reaction in the OECT. This approach eliminates the need for controlling the gate potential externally, simplifying device operation. We utilize a redox-active polymer on the floating-gate that is sensitive to hydrogen peroxide, intentionally produced by Oxidase enzymatic reactions, to generate a concomitant potential change on the gate of the OECT that modulates the conductivity of the channel material. Investigating different redox-active polymers, we elucidate several design criteria for the floating-gate and OECT channel to optimize the current modulation and amplification of these chemically gated OECTs. Adopting this approach, we achieve large changes in OECT source-drain current of up to three orders of magnitude and demonstrate its versatility by detecting a diverse range of biomolecules spanning glucose to lactate or alcohol. This approach can be further generalized to other Oxidase enzymatic reactions that produce hydrogen peroxide. By harvesting the chemical potential energy of abundantly available biomolecules, the control circuitry and operation of next generation OECT biosensors can be simplified by solely supplying a single source of power.
[1] D. Khodagholy et al., “High transconductance organic electrochemical transistors,” Nature Communications, vol. 4, pp. 1–6, 2013.
[2] A. M. Pappa et al., “Direct metabolite detection with an n-type accumulation mode organic electrochemical transistor,” Science Advances, vol. 4, no. 6, 2018.

One in four people are affected globally by mental and neurological disorders during their lives. There is a stigma towards mental disorders and therefore, most of such issues remain undetected. Electroencephalography (EEG) is noninvasive recording of brain signals. Numerous studies suggest the correlation between electroencephalography (EEG) signals, and mental state. Conventional EEG systems are bulky and stationary which make them unstable for continues ambulatory measurements. Further, accurate EEG measurements, used for diagnosis, requires medical experts and is done in medical centers, which makes it time consuming and costly. In order to overcome these challenges and make the mental health monitoring accessible and low cost, wearable sensors are highly desirable. Here, we report a novel long-term wearable, dry, miniaturized self-adhesive sensor (DSAS). It can firmly adhere to hairy scalp without the use of any adhesive, gel or mechanical support and be used for high quality EEG recording with low motion artifacts. DSAS is a reusable and soft sensor that can be simply attached and detached. It consists of the arrays of low-density tulip-like microstructures made of PDMS-conductive-filler composite (graphene and carbon nanotube) that deploy between the hairs and stick to the scalp. Each tulip-like microstructure consists of a long stem (400-450 µm) and a micro-suction cup head (200-300 µm diameter). DSAS adhere to any surface including hairy skin by accumulative work of adhesion force, generated by forming negative pressure in each micro-structure. Negative pressure is generated when microstructures in DSAS are pressed against a surface and the air is pushed out of micro-suction heads. DSAS is made by a novel scalable fabrication process and comprehensive theoretical studies were performed for the optimization of the fabrication process. Our results suggest that each tulip-like microstructure produces 12 mN adhesion force. The cups’ shell wall in the tulip-like microstructure head is 15 µm thick and ultra-soft which allows forming conformal contact to the rough surfaces such as skin and prevents leaking the air into the interface between sensor and surface after attaching it to skin. An array of just 100 per cm² microstructures in DSAS can hold 120 gm of weight for extended amount of time and maintains the adhesive properties up to 15 attach-detach cycles.

New form of materials can often enable new device and system applications. In this talk, I will highlight our group's recent work under this motivation in two synergistic areas. Firstly, by stacking individual layers of polymer, metal, and low-impedance coating reliably in a same nanomeshed pattern, the final multilayer multifunctional nanomeshes achieved system-level performance from all individual layers, in addition to nanomesh advantages. Compelling demonstrations from this multifunctional nanomesh approach include high-performance transparent and flexible neuroelectrode arrays, which has been recently validated in vivo. The second part of my talk will introduce our recent concept on semiconductor nanomeshes. By making a silicon film into homogeneous nanomeshes, we achieve high mobility semiconductor that is intrinsically stretchable to conventional microelectronic layouts. Together, our work demonstrates that nanomeshing is a unique way of transforming microelectronics for emerging applications.

Microfluidic devices can be used as powerful analytical tools for biology, ranging from drug discovery to medical diagnostics, requiring only small sample volumes for testing. Such devices enable the scaling down of reaction volumes so that a small amount of material is required and the time for testing is reduced. The ability to perform a suite of diagnostic tests on a single device, with minimal reagents, is a major goal of ‘lab-on-a-chip’ research. For more than 20 years, microfluidic technology has promised to deliver such devices and thereby revolutionize biological and chemical research.

However, the impact of this technology has yet to be fully realized, with plenty of room for innovation^[1]. Currently, the microfluidic channels in most devices play a passive role and are simply used to direct flow. The ability to “functionalize” the channels would impart greater use of these channels, by incorporating functional materials directly within the channels that can partake in, guide or facilitate the reactions taking place. Integrating functional materials could also serve to introduce new sensing functionalities within the channels themselves. At the same time, rapid prototyping of microfluidic devices is beneficial in a research environment, yet the high cost, slow turnaround and wasteful nature of the current techniques severely impede the development process. Finally, the lack of portability of the associated detection mechanisms, such as the complex optical microscopes typically employed, makes the ‘lab-on-a-chip’ aspect a moot point. The requirement of complex microscopes and equipment to drive and detect changes within microfluidic channels limits the range of applications of such devices.

Here, we demonstrate for the first time, the use of Aerosol Jet Printing (AJP) to produce bespoke molds for microfluidic devices, as well as functionalized microfluidic channels. We show that such an advanced microscale additive manufacturing method can be used to rapidly design cost-efficient and customized microfluidic devices, and add “functional” coatings while simultaneously embedding sensors and active elements on the same platform for electrical detection. Such functionalized microfluidic biosensors could become a transformative tool in biological and bio-medical research, with potential impact in areas such as drug screening, point-of-care diagnostics, and pharmacology. By using an AJP, these sensors can be localized to specific parts of the microfluidic device for constant monitoring and feedback using integrated electrical detection methods, without the need for complex equipment.

[1] G. M. Whitesides, “The origins and the future of microfluidics.,” Nature, vol. 442, no. 7101, pp. 368–73, 2006.

Soft polymers that are embedded with nano- and microscale droplets of liquid metal (LM) can be tailored to exhibit a broad range of electrical, thermal, mechanical, and dynamic shape morphing properties. In contrast to lithographically-fabricated soft microfluidic architectures, these LM-embedded soft polymer (LMSP) composites are statistically homogenous and exhibit effective medium properties at the mesoscale. Depending on the choice of polymer matrix, LMSPs can be engineered to achieve a wide variety of properties – from thermally conductive rubber for heat management and thermoelectric energy harvesting in wearable electronics to shape memory materials that dynamically response to electrical stimulation. Eutectic Ga-In (EGaIn) and Ga-In-Sn (Galinstan) alloys are used as the liquid metal due to their high electrical conductivity, low viscosity, non-toxicity, and the self-passivating formation of an oxide skin (Ga₂O₃) that enables emulsification and wetting to non-metallic materials. Because they are liquid-phase at room temperature, these alloys have virtually no influence on the mechanics of the surrounding elastomer medium. This allows the resulting composite to exhibit a unique and extraordinary combination of features not seen in other heterogeneous material systems. In this talk, I will review recent experimental and theoretical studies of this unique class of soft material architectures, with specific emphasis on LM-filled liquid crystal elastomers and soft polymers for stretchable electronics and energy harvesting. I’ll also highlight several applications in which LMSPs can have a potentially transformative impact, especially in the domains of wearable computing, physical human-machine interaction, and bioinspired soft robotics.

The discovery of light sensitive proteins led to the development of optogenetics and allowed the precise manipulation of neuronal and non-neuronal cells[1, 2]. This technique uses particular wavelengths of light to precisely manipulate cells that have been engineered to express this class of proteins. It has enabled the study of information processing in the brain and recent studies are looking into clinical translational applications of the technique[3]. Despite the advantages offered by optegenetics, it has the drawback of being invasive as light does not penetrate skin past a few millimeters. It is not possible to target deep tissue within the body using this technique and this limits the application in the context of diseases such as Parkinson's[5] or heart malfunction[4]. We are developing devices that can non-invasively manipulate cells using low-intensity ultrasound, in a technique termed "sonogenetics". This technique uses a combination of engineering cells to be more sensitive to a mechanical stimulus as well as developing ultrasound transducers capable of stimulating these cells. We demonstrate the development of miniature transducers made from non-hysteretic single crystal lithium niobate to be used for affecting behavior of freely moving, awake mice. In addition, we offer mathematical insights to the mechanism of action of ultrasound on neurons. Our analysis of membrane deflection enables us to predict cellular activity based on the applied ultrasound stimulus. By extension, we use this analytical framework to inform the development of transducers and stimulation parameters for achieving non-invasive stimulation of cells.

References
[1] Wignand WD Mühlhäuser et al. "Optogenetics-Bringing light into the darkness of mammalian signal transduction". In: Biochimica et Biophysica Acta (BBA)-Molecular Cell Research 1864.2 (2017), pp. 280-292.
[2] Karl Deisseroth. "Optogenetics: 10 years of microbial opsins in neuroscience". In: Nature neuro-science 18.9 (2015), p. 1213.
[3] Patrick M Boyle, Thomas V Karathanos, and Natalia A Trayanova. "Beauty is a light in the heart": the transformative potential of optogenetics for clinical applications in cardiovascular medicine". In: Trends in cardiovascular medicine 25.2 (2015), pp. 73-81.
[4] Paul S Larson. "Deep brain stimulation for movement disorders". In: Neurotherapeutics 11.3 (2014), pp. 465-474.
[5] Brian D McCauley and Antony F Chu. "Leadless cardiac pacemakers: the next evolution in pacemaker technology". In: Rhode Island Medical Journal 100.11 (2017), pp. 31-34.

While human organs and tissues are mostly soft; conventional electronics are hard. Seamlessly merging electronics with human is of imminent importance in addressing grand societal challenges in health and joy of living. However, the main challenge lies in the huge mechanical mismatch between the current form of rigid electronics and the soft curvy nature of biology. Here, I will present a new form of electronics, namely “rubbery electronics and bioelectronics”, with skin-like softness and stretchability, which is constructed all based upon elastic rubbery electronic materials. These rubbery electronic materials are structured in the format of composites, which can be scalably manufactured from common and commercial available materials without dedicated and complicated synthesis. Specifically, we build nanofibril organic semiconductor and metallic nanowires percolated in the elastomeric polymer matrix in a composite format for the rubbery semiconductors and conductors, respectively. Employing these rubbery electronic materials, we have achieved fully rubber format devices, including transistors and sensors, logic gates, active matrices, elastic sensory skin systems, and biointegrated devices, etc. I will showcase a few examples including artificial skins, biomedical implants, and wearable applications.

One of an important target application of biointegrated electronics is wearable sensors that enable accurate and continuous detection of physiological signals. These devices require conformable and biocompatible sensors and power source to continuously supply electricity to health-monitoring systems. In this talk, we will report on recent progresses of ultraflexible organic photovoltaic cells as power sources and wearable sensors. First, the progresses of power conversion efficiency (PCE) and mechanical/long-term stability of ultraflexible organic photovoltaics have been achieved using new material and process engineering. The certified PCE of 12.3% for the ultra-flexible organic photovoltaic cell has been achieved with novel ternary blend active layer [1]. Second, long-term continuous monitoring using organic electrochemical transistors with nonvolatile gel electrolyte has been achieved with a high mechanical stability and high signal-to-noise ratio (24dB) [2]. Then, the integration of the ultra-flexible organic photovoltaic cells as power sources with OECT-based sensors has been demonstrated [3].
[1] W. Huang et al., Joule, Accepted.
[2] H. Lee et al., Adv. Funct. Mater. doi.org/10.1002/adfm.201906982.
[3] S. Park et al., Nature 551, 516 (2018).

While individual epidermal sensors present a good compromise between comfortability, affordability, and functionality, the assembly of soft electronics adds a performance sacrifice and a cost overhead to the sensor system manufacturing process – especially when interfacing these with wireless communication and data processing. The more functionalities, the more rigid components involved. Therefore, it is a big challenge to compromise between soft individual components and rigid integrated circuits. Even though some reports have shown promising fundamental soft electronics solutions, such as transistor and diode, etc., the epidermal sensor system is still trapped in an early-stage analog level due to the sophisticated Von Neumann hardware architecture.

A novel far-field wireless epidermal pressure sensor system is presented here. The sensor which changes in the antenna electrical parameter is mapped to the changes in the properties of the electromagnetic wave propagation. This mechanical – electrical conversion manifests itself as a controlled sensing method from the signal response of the antenna device. Signal processing is demodulated from the reader side, making the epidermal pressure sensor a passive parasitic component in the antenna. This allows the sensor to directly leverage and benefit from concurrent advancements in wearable antenna research and conductive elastomer development while seamlessly extending to the pervasive low-cost wireless sensing applications.

Besides, an example of this approach will be presented in detail, including the design of the PDMS-based on-body sensor antenna and the signal response of Sub 1 GHz microstructure-based epidermal pressure sensor, etc. All key processing parts share the same on-body packaging, therefore enabling a chip-scale technology that can be designed in different elastic packaging forms. This unique sensor concept can lead to new applications and lead the pervasive epidermal healthcare sensing to a new level.

Dynamic covalent thermosets show superior properties, including self-healability, recyclability, malleability/reprocessability, thanks to the molecular level bond exchange reactions. These properties promise bright future of dynamic covalent thermosets in broad applications. We here report our work on dynamic covalent thermoset-based electronic systems that are rehealable, recyclable and reconfigurable. A flexible multifunctional electronic device that integrates electrocardiograph (ECG), temperature, motion, and acoustic sensing capabilities was demonstrated. It was exhibited that the rehealed and recycled devices showed electronic performance comparable to the original ones. Bond exchange reactions in the polymer network effectively relaxes stresses induced by mechanical deformation, which made it possible to reconfigure the flexible device into different configurations that are suited for different applications. Such rehealable, recyclable and reconfigurable electronics provides an approach to address sustainability and environment issues associated with mass production of electronics. It can find broad applications in prosthetics, health care, and human-computer interface and other areas that are hard to be addressed by conventional approaches.

There is an increasing interest in the development of memristive or artificial synaptic devices that emulate the neuronal activities for neuromorphic computing applications. While there have already been many reports on artificial synaptic transistors implemented on rigid substrates, the use of flexible devices could potentially enable an even broader range of applications. In this talk, we report high performance synaptic thin-film transistors (TFTs) built on ultrathin flexible substrate using high carrier mobility semiconducting carbon nanotubes (CNTs). The synaptic characteristics, including spike amplitude-dependent plasticity, long-term/short-term plasticity (LTP/STP), spike-number dependent plasticity (SNDP), spike-timing dependent plasticity (STDP) have all been systematically characterized on the flexible synaptic CNT TFT. Moreover, we have demonstrated an individual pixel circuit that may be used to construct a flexible neurological electronic skin in the future. The circuit comprises a flexible ferroelectret nanogenerator (FENG) serving as the sensory mechanoreceptor that generates action potentials to be processed by the artificial synapse. In such a system, the flexible FENG sensor converts the tactile input (the magnitude and frequency of force) into presynaptic action potential pulses, which are then passed to the gate of the synaptic transistor to induce change in its drain current (post-synaptic current), mimicking the modulation of synaptic weight in biological synapses. The single pixel circuit closely emulates the behavior of actual human skin and it allows for instantaneous detection of force stimuli and offers biological synapse-like behavior to relay the stimuli signal to the next stage. It could be further integrated into an array to achieve a neurological electronic skin with synaptic behavior and memory capabilities, which could potentially be used to interface with skeletal muscle fibers for applications in neuroprosthetic devices.

Conductive carbon-based nanomaterials, such as graphene, carbon nanotubes (CNTs), and reduced graphene oxide (rGO), are flexible, corrosion-resistant, and exhibit high electrochemical capacitance, which are ideal properties for probing excitable neural tissue. The realization of a high-fidelity, multi-modal electrochemical biosensor, which is capable of simultaneously monitoring the electrical and chemical signaling between neurons in a single platform, has been a focus of much research over the past 15 years. To enable such a technology, rGO has been extensively explored, because it can be handled using scalable, solutions-processing techniques starting from aqueous dispersions of its precursor structure, graphene oxide (GO). The oxygen moieties of dried GO films must be reduced in order to restore the film conductivity and a number of methods have been developed to achieve this goal, both in bulk and on an individual film basis. Exposure to hydrazine in vapor form, as well as high temperature annealing, typically at temperatures exceeding 1000 °C, are two of the most common methods to form rGO dried films. However, these methods pose toxicity and safety concerns, and are not compatible with flexible polymeric substrates that typically compose many bioelectronic devices. To address these issues, we have developed a novel, safe, low-temperature, and scalable method of direct GO film reduction using L-ascorbic acid, or Vitamin C, as the reducing agent. Room temperature aqueous mixtures of GO and Vitamin C were loaded into a spray-coating apparatus and deposited onto pre-heated substrates. The influence of Vitamin C concentration and substrate heating time on film conductivity were studied. Using 50 mM Vitamin C and a holding time of 15 minutes at 150 °C, we achieved our lowest sheet resistance value of 354.70 ± 20.52 kΩ sq^-1. On average, rGO films were 2.20 ± 0.19 nm in thickness, and thus had conductivities as high as 12.81 ± 1.75 S cm^–1. Using a rapid prototyping method, 3 mm diameter disc-shaped electrodes were made on glass substrates encapsulated in polyimide (PI). These PI electrodes allowed for direct comparison with size-matched glassy carbon (GC) and gold (Au) working electrodes in vitro. Electrochemical capacitances as high as 3.05 mF cm^–2 were achieved by spray-coating GO-Vitamin C onto thin film Au, which is an order of magnitude greater than the capacitances of either bare GC (0.29 mF cm^–2) or bare Au (0.11 mF cm^–2). Electrochemical impedance was also observed to decrease as the volume of spray-coated rGO increased, particularly in the frequency range from 0.1–100 Hz. The 10 Hz impedance of rGO electrodes was as low as 0.23 ± 0.15 kΩ, for example, compared to bare Au, which had impedance of 2.78 kΩ at the same frequency. At 1 kHz, the impedances of rGO/Au and Au were 0.13 ± 0.06 kΩ and 0.15 kΩ, respectively. To demonstrate the feasibility of using GO-Vitamin mixtures to realize high-fidelity electrochemical biosensors, the electrocatalytic activity of a handful of biologically relevant molecules was studied, including the direct voltammetric detection of dopamine, ascorbic acid, uric acid, and serotonin. Our novel approach to the production of rGO electrodes offers a safer alternative to chemical reduction using hydrazine or other strong acids, as well as a scalable method which is fully compatible with the soft, polymeric substrates used in state-of-the-art bioelectronic interfaces. Thus, this method enables the development of highly sensitive, biocompatible, and flexible conductive films for applications in fields of biosensors and bioelectronics.

Soft, noninvasive and multifunctional epidermal electronics (a.k.a. electronic tattoos or e-tattoos) have demonstrated many exciting 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 named the “cut-and-paste” method for the rapid prototyping of e-tattoo sensors using a paper/vinyl cutter plotter [1]. This method has been demonstrated to work for thin film metals [1, 2], various polymer sheets [1, 3], ceramics [4], as well as 2D materials such as graphene [5, 6]. The cut-and-pasted e-tattoos are low cost and can be used to measure a variety of physiological signals such as electrocardiogram (ECG), seismocardiogram (SCG), electrooculogram (EOG), skin hydration, skin temperature, respiratory rate and so on [1-3, 5, 6]. To make the e-tattoos go wireless, we now report the “cut-solder-paste” process to incorporate integrated circuits (ICs) for signal readout and processing, near field communication (NFC) [7], as well as Bluetooth transmission [8]. To overcome the limited patterning resolution of the cutter plotter and to recycle the tattoo layers with ICs, we propose a modular concept in which the wireless charging layer (NFC layer), the wireless communication layer (Bluetooth layer), the readout circuit layer, and the sensor/electrode layer are fabricated individually and stacked up at the final step of fabrication. The thickness of a fully assembled multilayer e-tattoo (excluding IC chips) is less than 300 um and the overall stretchability is still beyond 20%. In addition to already mentioned capabilities, such e-tattoos can also wireless track motion, mechano-acoustic heart signals, and oxygen saturation (SpO2) [3, 7, 8]. 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 Bluetooth-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. Combining the NFC layer and the Bluetooth layer in one e-tattoo, we demonstrate that such wireless e-tattoo can also be wirelessly charged on-the-go via stretchable fabric feeding coils [8], enabling long-term, ambulatory and continuous sensing. Moreover, we propose that different layers can be disassembled and reassembled multiple times. After disassembly, the electrode layer should be disposed but the other layers can be reassembled into new e-tattoos. The low-cost, rapid prototyping method together with the wireless and reconfigurable capabilities represent exciting advancement towards practically useful wireless e-tattoos.


1. Yang, S., et al., "Cut-and-Paste" Manufacture of Multiparametric Epidermal Sensor Systems. Advanced Materials, 2015. 27(41): p. 6423–6430.
2. Wang, Y., et al., Low-cost, μm-thick, tape-free electronic tattoo sensors with minimized motion and sweat artifacts. npj Flexible Electronics, 2018. 2(1): p. 6.
3. Ha, T., et al., A chest-laminated ultrathin and stretchable e-tattoo for the measurement of electrocardiogram, seismocardiogram, and cardiac time intervals. Advanced Science, 2019: p. 1900290.
4. Yang, S., E. Ng, and N. Lu, Indium Tin Oxide (ITO) Serpentine Ribbons on Soft Substrates Stretched beyond 100%. Extreme Mechanics Letters, 2015. 2: p. 37-45.
5. Ameri, S.K., et al., Imperceptible Electrooculography Graphene Sensor System for Human-Robot Interface npj 2D Materials and Applications, 2018. 2: p. 19.
6. Ameri, S.K., et al., Graphene Electronic Tattoo Sensors. Acs Nano, 2017. 11(8): p. 7634-7641.
7. Jeong, H., et al., Modular and Reconfigurable NFC-Enabled Wireless Electronic Tattoos for Biometric Sensing. Advanced Materials Technologies, 2019: p. 1900117.
8. Jeong, H., Huang, Yi., et al., Wireless E-Tattoos Chargeable On-The-Go. To be submitted, 2019.

Protein nanowires harvested from microbes are a revolutionary green electronic material. They have unique properties for novel electronic devices with improved performance, for a broad range of applications from energy harvesting to neuromorphic computing and sensing. (1) Thin-film protein nanowire devices were shown to continuously harvest electricity from ambient humidity (Nature 578, 550-554 (2020)), enabled by the unique structural (e.g., nanopores at wire-wire interface) and chemical (e.g., a high density of hydroscopic groups) properties. This leads to a continuous energy harvesting mechanism from the ubiquitous and enormous ambient humidity, and hence a promising strategy to powering self-sustained systems. (2) The protein nanowires are cleverly designed and recruited by the microbe to facilitate charge exchange with environments (e.g., through redox metallization). Thus, protein nanowires can be assembled into bio-dielectrics to construct memristor devices, in which the protein nanowires catalytically reduce the energy barrier involved in memristive switching and enable ultralow operational voltage in the biological regime of <100 mV (Nature Commun. 11, 1861 (2020)). Neuromorphic components (e.g., artificial neuron, synapse) made from the protein nanowire memristors take a further step in bio-emulation from functional emulation to parameter matching, creating opportunities for future ultralow-power electronics and bioelectronic interfaces. (3) Protein nanowires have ultra-small diameters (3 nm) and an ultra-high density of surface functional groups (e.g., 10 per nm) that are advantageous for sensing applications. Electronic sensors made from protein nanowires were shown to have enhanced selectivity and sensitivity (Nano Res. 13, 1479-1484 (2020)) for ammonia detection compared to inorganic nanowire devices. These studies, combined with the advantages of renewability, biocompatibility, and eco-friendliness in protein nanowires, have provided the starting point for future green electronics based on synthetic protein nanowires.

Pathogen invasion not only release toxins to attack mammalian cells, but also colonize and disturb the balance of healthy microbiome. Therefore, it is desirable to develop antibacterial methods that target specific bacteria. Here we show a strategy to selectively immobilize bacteria on surface to allow its eradication by electricity. We designed a flexible PDMS chip that has been imprinted with bacteria and with integrated electrodes. The chip could selectively capture the target bacteria to the surface by shape complementarity. Upon applying a low voltage, we observed a fast killing of bacteria in real time. The work suggests a new approach to design intelligent antimicrobial devices that are suitable for use on microbiota sites.

The ability to directly print biomedical devices on the body could benefit patient monitoring, wound treatment, and even allow for the possibility of human augmentation. In reality, this concept requires the 3D printer to adapt to the various translations, rotations, and deformations of the biological surface. Conventional 3D printing technologies typically rely on open-loop, calibrate-then-print operation procedures. An alternative approach is adaptive 3D printing, which is a closed-loop method that combines real-time feedback control and direct ink writing of functional materials in order to fabricate devices on moving freeform surfaces. Here we demonstrate that the changes of states in the 3D printing workspace in terms of the geometries and motions of target surfaces can be perceived by an integrated robotic system aided by computer vision. This allow us to directly 3D print a wireless antenna based on a novel silver ink a free-moving human hand, to power a skin-mounted LED. Using this same approach, cell-laden hydrogels were also printed on live mice, creating a model for future studies of wound-healing diseases. Moreover, we developed an in situ 3D printing system that estimates the motion and deformation of the target surface to adapt the toolpath in real time. With this printing system, a hydrogel-based sensor was printed on a porcine lung under respiration-induced deformation. The sensor was compliant to the tissue surface and provided continuous spatial mapping of deformation via electrical impedance tomography. This adaptive 3D printing approach may enhance robot-assisted medical treatments with additive manufacturing capabilities, enabling advanced medical treatments, and autonomous and direct printing of wearable electronics and biological materials on and inside the human body.

With the emergence of the internet of things (IoT) era, visual IoT platforms have attracted significant interest, which can offer sensing, collecting, and processing of optical information in hyperconnected society. Flexible displays are a potential candidate for bilateral visual communication, as they can be easily affixed anywhere, such as on the surfaces of human skin, clothes, automobiles and buildings. III-V Inorganic LEDs have superior characteristics, such as long-term stability, high efficiency, and strong brightness compared to OLED. However, due to the brittle property of inorganic materials, III-V LED limits its applications for the flexible electronics. This seminar introduces the flexible vertical GaAs/GaN microLED on plastic substrates using anisotropic conductive film (ACF), resulting high optical power density, The superb properties of the flexible inorganic LED enable the dramatic extension of flexible displays toward not only wearable devices of light source but also full color flexible micoLED displays for consumer TV.
MicroLED stimulation of specific neural populations of the brain is one of the facile and reliable methods used in neuroscience for deduction of functional movement, complex behavior and even long-range connectivity. Recent advanced biomedical tools now employ flexible optoelectronic devices combined with optogenetic mouse models to activate small functional regions using blue-light driven channelrhodopsin. Here we introduce flexible vertical light-emitting diodes (VLEDs) for 2D perturbation of specific functional areas of mouse cortex, capable of stimulating motor neurons deep below layer III from the brain surface. Selective operation of pulsed red light from f-VLEDs induces mouse body movements and synchronized electromyogram (EMG) signals. These results show that the III-V based flexible LED can be used as the future flexible implantable biomedical applications such as skin research and phototherapeutic tool.

References (Prof. Lee’s corresponding authors in micro LED)
1. "Bio-Integrated Flexible Inorganic LED", Nanobiosensors in Disease Diagnosis, 1 , 5, 2012
2. "Water-resistant Flexible GaN LED on a Liquid Crystal Polymer Substrate for Implantable Biomedical Applications", Nano Energy, 1 , 145, 2012
3. "Self-powered Fully-Flexible Light Emitting Systems enabled by Flexible Energy Harvester", Energy Environ. Sci., 7(12), 4035, 2014
4. "Self-powered Flexible Electronic Systems" Nano Energy, 14, 111, 2015
5. "Optogenetic Control of Body Movements via Flexible Vertical Light-Emitting Diodes on Brain Surface", Nano Energy, 44, 447, 2018
6. "Wireless Powered Wearable Micro Light-Emitting Diodes", Nano Energy, 55, 454, 2019
7. "Monolithic Flexible Vertical GaN Light-Emitting Diodes for Transparent Wireless Brain Optical Stimulator", Adv. Mater. 30, 1800649, 2018
8. "Trichogenic Photostimulation Using Monolithic Flexible Vertical AlGaInP Light-Emitting Diodes", ACS Nano, 12, 9587, 2018
9. "Self-powered Flexible Electronics beyond Thermal Limits", Nano Energy, 56, 531, 2019
10."Optogenetic Mapping of Functional Connectivity in Freely Moving Mice via iWEBS" ACS Nano, 10, 2791, 2016
11. "Micro Light-Emitting Diodes for Display and Biomedical Applications", Adv. Funct. Mater, 29, 1808075, 2019
12. "Achieving High Resolution Pressure Mapping via Flexible GaN/ZnO Nanowire LEDs Array by Piezo-phototronic Effect", Nano Energy, 58, 633, 2019

Current neural electrodes which usually employ metallic materials are known to trigger foreign body reaction which leads to material degradation and decline of electrical performance over long time period. Hydrogels as a novel class of materials for neural electrodes come up as a promising alternative because they can be designed to mimic some of the properties of soft tissues. Electrical functionality could be added to the hydrogels by formation of conductive polymers on the hydrogel struts. Porosity of the hydrogels could be used as reservoir of drugs where the release kinetics are controlled by the interactions between drug molecules and hydrogel building blocks.

Here we present electrically conductive hydrogels based on chemically crosslinked 4-arm star-shaped poly(ethylene glycol) (starPEG) and heparin. The conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is incorporated in the hydrogel using interfacial polymerization method. Negatively charged heparin increases electrical conductivity of the hydrogel more than 3 orders of magnitude from 6.75x10^-3 S/m for bulk hydrogels without heparin to 58.5 S/m for hydrogels containing both heparin and PEDOT. The increase in conductivity is due to the ability of heparin to dope the conductive polymer. Macropores are formed by maintaining the polymerization reaction at -15°C as a result of ice crystals formation which act as porogens. Typical pore sizes are 670.6 μm². The conductivity of macroporous hydrogels is measured to be 8.5 S/m. The macroporous hydrogels achieve superior toughness in comparison to bulk hydrogels as observed from compression test. The compression modulus of the conductive porous hydrogels is less than 10 kPa which demonstrates potential applications for interfacing with soft tissue.

Furthermore, we investigate the capability of the composite hydrogels to bind Nerve Growth Factor β (NGF-β) as heparin is known to bind with various types of growth factors. In our ongoing work we are investigating if conductivity in the hydrogel can be used to release the growth factor from the hydrogel in a controlled manner. Optimized condition of uptake and release of NGF-β can then be employed to observe morphological change of PC12 cells.

In conclusion, we will discuss a system which may allow interfacing with electroactive tissue by adopting controlled electrical and biochemical cues.

Brain-inspired computing paradigms have led to substantial advances in automation of visual and linguistic tasks by emulating the distributed information processing of biological systems. The similarity between artificial neural networks (ANNs) and biological systems has inspired ANN implementation in biomedical interfaces including prosthetics and brain-machine interfaces. While promising, these implementations rely on software to run ANN algorithms. Ultimately, it is desirable to build hardware ANNs that can directly interface with living tissue which adapt based on biofeedback. The first essential step towards biologically integrated neuromorphic systems is to achieve synaptic conditioning based on biochemical signaling activity.

Direct communication between artificial and biological neurons is of particular interest because of the vast utility that can be achieved by interacting with the nervous system. In this work, we bridge the gap between artificial and biological neurons by directly coupling an organic neuromorphic device with dopaminergic neuron-like cells to constitute a biohybrid synapse with neurotransmitter mediated plasticity. Voltage pulses at the pre-synaptic electrode drive the oxidation of the excitatory neurotransmitter dopamine, leading to both a short-term potentiation of the channel from ionic drift as well as a long-term potentiation due to the redox reaction with dopamine at the interface.

We elucidate the working principle of the biohybrid synapse by characterizing the electrochemical response of the organic neuromorphic device to dopamine signaling both in solution and in vitro. Furthermore, we use focused-ion beam and scanneing electron microscopy to visualize the interface between the living cells and organic postsynaptic electrode. We utilize microfluidic flow across the channel to mimic the dopamine recycling machinery of biological synapses to demonstrate both long-term conditioning and recovery of the synaptic weight. Finally, we report direct translation of dopamine secreted by PC12 neuron-like cells to long-term modulation of the biohybrid synapse memory state, paving the way towards combining neuromorphic systems with biological neural networks.

In recent years, there has been tremendous interest in developing new technologies enabling the seamless integration of electronics with soft biological tissues. This has required significant innovation in materials and fabrication strategies to develop bioelectronic interfaces which are soft, biocompatible, and highly conformable to the tissue of interest. Many of the most successful technological advances in this realm so far have leveraged traditional metal and silicon materials along with thin-film microfabrication strategies to create highly flexible and conformable bioelectronic interfaces. While these thin-film technologies have been shown to achieve exquisite tissue conformability, one of their primary limitations to achieving clinical translation and widespread use is the high cost and low scalability of the materials and fabrication methods. Here, we present a novel process for rapid, low-cost, and highly scalable manufacturing of flexible bioelectronic interfaces by leveraging the electronic, physical, and chemical properties of Ti₃C₂ MXene. In this process, a laser-patterned cellulose-polyester blend substrate is infused with a water-based ink of Ti₃C₂ MXene to generate a bulk conductive composite which is subsequently encapsulated in thin layers of flexible silicone elastomer. This method is highly customizable, and allows manufacturing of electrode arrays of different scales and geometries for recording a wide range of bioelectronic signals, including scalp electroencephalography (EEG), electromyography (EMG), electrocardiography (EKG), and electrocorticography (ECoG). We have thoroughly characterized the electrochemical and mechanical properties of the MXene-textile electrodes at varying diameters, and validated their impedance and recording properties in vivo in a range of biosensing and stimulation applications. Enabled by the high conductivity and capacitance of Ti₃C₂ MXene, as well as the high effective surface area of the porous MXene interface, these electrodes show remarkably low electrochemical impedance both in saline (1 kHz |Z| 205.6 ± 11.1 Ω, 3mm MXene vs. 287.3 ± 7.8 Ω, 2.3mm Pt) and on human skin (1 kHz |Z| 6.2 ± 4.1 kΩ, 3mm MXene vs. 3.62 ± 0.6 kΩ, 1cm gelled commercial electrodes, Natus Inc.). Particularly interesting for epidermal sensing applications, our mm-scale dry MXene electrodes achieve comparable electrode-skin interface impedance and recorded signal quality compared to cm-scale gelled electrodes, which enables recording with higher spatial resolution as well as improving subject comfort and long-term recording capabilities by avoiding the need to use conductive gels. We demonstrated this advantage through EEG recording on a healthy human subject, where our mm-scale electrodes produced signals which were indistinguishable in signal-to-noise ratio, spectral content, or magnitude from a standard cm-scale gelled Ag/AgCl EEG electrode placed next to the MXene array. The MXene electrodes also exhibit extremely high cathodal charge storage capacities (901.5 ± 38.4 mC cm^-2, 500μm MXene vs. 105.2 mC cm^-2, 200μm PEDOT:PSS on Pt) and charge injection capacities (2.4 ± 0.5 mC cm^-2, 500μm MXene vs. 1.4 mC cm^-2, 200μm PEDOT:PSS on Pt), which make them a promising candidate for delivering electrical stimulation. Finally, we have demonstrated the compatibility of the MXene electrodes with both MRI and CT imaging, due to the low magnetic susceptibility and low density of the electrode materials. The work presented here is a platform technology, enabling rapid and low-cost manufacturing of bioelectronic interfaces by leveraging the unique properties and solution-processability of Ti₃C₂ MXene.

Growing demand in advanced portable and wearable electronics raises requirements for the composed materials to be stretchable, conducting or even healable, for which functional polymers are ideal choices. Flexible devices (triboelectric nanogenerators and solar cells) using organic materials for effectively energy harvesting will be firstly presented (1-3). An integration with flexible energy storage components is followed to enable a self–powered system which can simultaneously harvest solar/mechanical energy and store electricity (2-4). These work pave a new direction for functional polymers in the field of energy harvesting, storage and mechanosensing for potential applications in areas such as soft robotics, biomedical devices, and wearable electronics.

1. R. Liu, S. T. Lee and B. Sun 13.8% Efficiency hybrid Si/organic heterojunction solar cells with MoO₃ film as antireflection and inversion induced layer. Adv. Mater. 2014, 26, 6007.
2. R. Liu, J. Wang, T. Sun, et al. Silicon nanowire/polymer hybrid solar cell-supercapacitor: a self-charging power unit with a total efficiency of 10.5%. Nano Lett. 2017, 17, 4240.
3. R. Liu, X. Kuang, J. Deng, et al. Shape memory polymers for body motion energy harvesting and self powered mechanosensing. Adv. Mater. 2018, 30, 1705195.
4. R. Liu, Y. Liu, H. Zou, et al. Integrated solar capacitors for energy conversion and storage. Nano Res. 2017, 10, 1545.

Recent advances in new materials, electronics, and assembly techniques have allowed optoelectronic systems to interface with biology and contribute significantly to the progress in basic biological research as well as clinical medicine. In this talk, I will introduce a novel class of flexible, multimodal optoelectronic systems that combines high-performance nanoscale electrodes with microscale optical components for simultaneous electrophysiological recording under optogenetic modulation. We envision this unique technology will open up new windows to understanding important biological processes by enabling mapping the dynamics of perturbed cell populations and correlating cellular responses to behavior.

Although recent efforts in device designs and fabrication strategies have resulted in meaningful progresses to the goal of novel high-performance imaging devices, significant challenges still exist in developing a miniaturized and lightweight camera that enables the wide field-of-view (FoV) imaging. This is mainly due to bulky and heavy multiple lenses employed in the conventional wide angle camera. In this work, inspired by structural and functional features of the aquatic vision, we have developed a novel wide FoV camera by integrating a tailored monocentric lens and a hemispherical silicon nanorod photodetector array. This bioinspired camera offers the wide FoV, a miniaturized module size, minimal optical aberrations, the deep depth-of-field, and the enhanced light sensitivity in one simple integrated device. Theoretical analyses in conjunction with imaging demonstrations have corroborated the validity of the proposed concept. The fish-eye-inspired camera is expected to provides new opportunities for the advanced mobile electronics.

Tissue-wide electrophysiology with single-cell and single-spike spatiotemporal resolution is critical for biological studies and biomedical applications. In this talk, I will discuss the creation of cyborg organisms: the three-dimensional (3D) assembly of soft, stretchable mesh nanoelectronics across the entire living organism by the cell-cell attraction forces from 2D-to-3D tissue reconfiguration during organogenesis. We demonstrate that stretchable mesh nanoelectronics can migrate with and grow into the initial 2D cell layers to form the 3D organ with minimal impact on tissue growth and differentiation. The intimate contact between the dispersed nanoelectronics and cells enables us to chronically and systematically observe the evolution, propagation and synchronization of the bursting dynamics in different organisms through their entire organogenesis and maturation. I will also discuss the potential applications of these cyborg organisms in biology and biomedicine.

Microscale electrodes, on the order of 10-100 µm, are rapidly becoming critical tools for neuroscience and brain-machine interfaces (BMIs) for their high channel counts and spatial resolution, yet the mechanical details of how probes at this scale insert into brain tissue are largely unknown. Here, we discuss quantitative measurements of the force and compression mechanics together with real-time microscopy for insertion of a systematic series of microelectrode probes as a function of diameter (7.5–100 µm and rectangular Neuropixels) and tip geometry (flat, angled, and electrochemically sharpened), within phantoms, ex vivo and in vivo. Results from each model system elucidated the role of tip geometry, surface forces, and mechanical scaling with diameter. Surprisingly, real-time microscopy revealed that at small enough lengthscales (<25 µm), blood vessel rupture and bleeding during implantation could be entirely avoided. This appears to occur via vessel displacement, avoiding capture on the probe surface which led to elongation and tearing for larger probes. We propose a new, three-zone model to account for the probe size dependence of bleeding, giving a new conceptual framework for how blood vessels fail and provide guidance for probe design.

Although there are numerous studies on either hard or soft materials, our understanding of the fundamentals at hard/soft interfaces has been limited. As different types of energy (such as electrostatic, mechanical, thermal, and chemical energies) display diverse scaling behaviors and can converge, an appropriate selection of the length scale is critical for promoting new scientific discoveries across these interfaces. Our group integrates material science with biophysics to study several hard/soft interfaces. We synthesize new materials and probe interfacial dynamics, with particular focus at the sub-micrometer and sub-cellular length scales. In this talk, I will focus on the interfaces that enabled non-genetic, freestanding, and semiconductor-based bioelectric modulation. I will also discuss some recent work that exploits the dynamic behaviors of granular materials in polymeric matrices toward bioelectronic and robotic applications. I will end the talk by proposing several new scientific and engineering approaches to improving our fundamental understanding of the (bio)chemical processes at soft/hard interfaces and to exploring new applications of these interfacial (bio)chemical processes.

Constructing electronic systems on ultrathin polymer films is superior to render the system more flexible, conformable, and imperceptible. These characteristics are particularly beneficial for epidermal and implantable electronics. Generally, the system is processed with rigid supporting substrates during fabrication, followed by delamination and transfer to the targeted working areas. The challenge associated with an efficient and innocuous delamination operation is one of the major hurdles toward high-performance ultrathin flexible electronics at large scale. Here, a facile, rapid, damage-free approach is reported for detachment of wafer-scale ultrathin electronic foils from Si wafers by capillary-assisted electrochemical delamination (CAED) with a 100% success rate. Anodic etching and capillary action drive an electrolyte solution to penetrate and split the polymer/Si interface, leading to complete peel-off of the electronic foil. Such a process incurs neither mechanical damage nor chemical contamination; therefore, the delaminated electronic systems remain intact. Via this approach, carbon nanotube (CNT)-based CMOS technology has been realized on a 2.5 μm-thick plastic foil with high performance. The performances of both the p- and n-type CNT thin film transistors (TFTs) are excellent and symmetric on plastic foil with a low operation voltage of 2 V: width-normalized transconductances (gm/W) as high as 4.69 μS/μm and 5.45 μS/μm, width-normalized on-state currents reaching 5.85 μA/μm and 6.05 μA/μm, and mobilities up to 80.26 cm²V⁻¹s⁻¹ and 97.09 cm²V⁻¹s⁻¹, respectively, together with a current on/off ratio of approximately 10⁵. Based on CAED, we also developed a wafer-scale manufacturing process for degradable electronic platforms with an average yield of the transferred TFTs up to 96.6%. Great uniformity was achieved in the transferred TFTs and ICs on each chip and among different chips. Finally, real-time sunlight exposure and temperature monitoring and degradation of the system under artificial rain were successfully demonstrated in a simulated ecosystem outdoors with distributed sensor nodes consisting of TFTs, UV and temperature sensors.

References:
1. High-Performance Carbon Nanotube Complementary Electronics and Integrated Sensor Systems on Ultrathin Plastic Foil, ACS Nano, 12, 2773(2018).
2. Wafer-Scale Fabrication of Ultrathin Flexible Electronic Systems via Capillary-Assisted Electrochemical Delamination, Advanced Materials, 30, 1805408 (2018).
3. Wafer-Scale High-Yield Manufacturing of Degradable Electronics for Environmental Monitoring, Advanced Functional Materials, DOI: 10.1002/adfm.201905518 (2019).

The brain carries out complex cognitive and computing tasks by optimizing energy efficiency, information processing, communication, and learning in massively parallel, dense networks of highly interconnected neurons. To cope with its everchanging surroundings, the brain is able to grow neurons, synapses, and connections—owing to its plastic nature. At the molecular and cellular levels, synaptic plasticity and neuronal excitation are the main mechanisms underlying these processes. Therefore, the ability of next-generation computing devices, robots, and machines to autonomously sense, process, learn, and act in complex and dynamic environments while consuming very little power will require approaches to computing and sensing that are inherently brain-like. Reproducing these features using traditional electronic circuit elements is virtually impossible, requiring the design and fabrication of new hardware elements that can adapt to incoming signals and remember processed information. These elements should be scalable, biomimetic, and preferably ionic to achieve energy consumption levels approaching those in the brain. While a major effort is being invested in developing inorganic materials that could emulate synaptic and neural functionalities, I believe that an overlooked, yet high-reward, pathway to success is through the development of biomolecular materials with the composition, structure, and switching mechanisms of actual biological synapses and neurons. Here, I describe two-terminal, biomolecular memcapacitors and memristors, consisting of highly insulating 5 nm-thick lipid bilayers assembled between two water droplets in oil. These devices exhibit memcapacitance that is nonlinearly dependent on the applied voltage as well as hysteresis in the charge due to voltage-driven, reversible changes in the area and thickness of the bilayer membrane. This is the first demonstration of a memcapacitor in which capacitive memory results from geometrical changes in a lipid bilayer membrane. We also show that the incorporation of voltage-activated alamethicin and monazomycin peptides in these devices results in variable ionic conductance across the membrane and memristive behavior. We discuss how these devices exhibit learning through synaptic plasticity, and how to implement them in online learning applications. These results serve as a foundation for a new class of low-cost, low-power, soft mem-elements based on lipid interfaces and other biomolecules for applications in neuromorphic computing which could have major implications on the robotics and computing fields.

Biodegradable systems are critically important in regenerative medicine since complete systems must be replaced by new tissues and organs. Biodegradable or transient electronics provide the electrical functionalities for typical biodegradable regenerative materials that are generally used in tissue engineering like scaffolds and metal joints. The wireless electrical stimulator made of completely biodegradable materials proposed here can accelerate the regeneration of injured nerve and dissolve away. Passive electrode and capacitors were constructed using biodegradable metal Mg and dielectric SiO2 on a flexible PLGA substrate. Doped Si nanomembranes were used as diodes to rectify the stimulating pulse into monophasic form. Animal demonstrations showed the functional recovery of transected peripheral nerves by monitoring the EMG, muscle mass and force for up to three months.

Electrical stimulation (ES) is a widely used therapeutic treatment strategy. It showed significantly positive results in treating a variety of diseases, biological disorders, and neurological problems. Today, the emergence of wearable devices is rapidly reshaping the development of medical devices, pushing them from conventional bulky and rigid silicon electronics to flexible and primarily polymer-based systems. Among many types of functions, nanogenerators are developed as a unique device for converting biomechanical energy into electrical pulses. In addition to applying it directly as a power source, this pulsed electricity can be applied directly as a ES signal for therapeutic treatment. In our recent research, we successfully implemented such an electromechanical system for skin wound healing and vagus nerve stimulation for obesity control. An electrical stimulation bandage was developed by integrating a flexible nanogenerator and a pair of dressing electrodes on a flexible substrate. Rat studies demonstrated rapid closure of a full-thickness rectangular skin wound within 3 days as compared to 12 days of usual contraction-based healing processes in rodents. From in vitro studies, the accelerated skin wound healing was attributed to the electric field-facilitated fibroblast migration, proliferation and transdifferentiation. In another work, an implanted vagus nerve stimulation system was developed. The device comprises a flexible and biocompatible nanogenerator that is attached on the surface of stomach. It generates biphasic electric pulses in responsive to the peristalsis of stomach. The electric signals generated by this device stimulates the vagal afferent fibers to reduce food intake and achieve weight control. This strategy is successfully demonstrated on rat models. Within 100 days, the average body weight is controlled at 350 g, 38% less than the control groups. Both results bring a new concept in electrical therapeutic technology that is battery-free, self-activated and directly responsive to body activities.

Harnessing the diversity of sensing platforms for applications in complex biological and chemical environments has been a challenge in the field of biosensing for many years. Electrochemical, optical, and magnetic techniques have been proposed, most with limitations ranging from biocompatibility, sensitivity, and signal transduction through opaque media, to longevity under non-laboratory conditions. Through advances in core/shell nanobiotechnology, we have developed an implantable diagnostic device for the measurement of biomarkers using a magnetic particle assay. This assay behaves as a platform technology tunable for biochemical monitoring in health, manufacturing, security, agriculture, and veterinary care. Our current applications focus on medical diagnostics in subclinical, chronic, and transient cardiac and cancer disease models with low sampled concentrations or temporary spikes in biomarker presence frequently missed by serial sampling. In situ diagnostics offer continuous sentineling of critical biomarkers, providing a deeper understanding of the local biology in heterogeneous systems. This monitoring provides valuable timescale data to clinicians giving them insight into their patients’ lab results for early intervention and data collection throughout treatment. Nanoparticle science, NMR technology, and modern biomaterials are combined in a multidisciplinary approach in this study to improve the diagnostic paradigm in addressing biochemical sensing.

Layered core/shell particles have been designed to detect the presence of specific molecules. Magnetic contrast is made possible by careful selection of the core material while biological functionality is provided by tunable surface ligands (antibodies, complementary DNA, aptamers, etc.) for specific affinity toward multivalent targets. Reversible (current concentration), irreversible (dosimetric, cumulative exposure), and switch (on/off) systems have been tuned by varying the affinity properties of the functionalization layers. We have shown proof-of-concept of a system composed of nanostructured materials easily implantable in vivo for both local and systemic chemical monitoring. Colloidal suspensions are kept at a constant particle concentration within a depot, protected from their environment by a semi-permeable membrane. Target diffusion across the membrane induces a switch from a dispersed to an assembled state with a corresponding change in magnetic properties allowing for robust contrast. Devices are deployed within the subcutaneous space or locally at the site of disease and immediately begin tracking their milieu. Magnetic relaxation measurements taken by Magnetic Resonance Imaging (MRI) and single-sided NMR systems are compared to demonstrate the feasibility of implementation in clinic-based and point-of-care, resource-limited settings.

Depot usable lifetime and nanostructure degradation are key parameters in multi-month applications. Recent work in tuning surface chemistry and bioconjugation strategies has enhanced the performance of the switch-based system by an order of magnitude over prior studies, increasing the expected lifetime to over 29 weeks in elevated temperature experiments. Progress in biomaterial capsule development has also shown promise in the deployment of a controlled degradation system with the added benefit of minimally invasive implantation compared to previous surgical placement methods. Overall this implant has the potential to broaden diagnostics in personalized medicine by addressing the hurdles of longevity and robust sensor signal stability in complex environments.

One of an important target application of biointegrated electronics is wearable sensors that enable accurate and continuous detection of physiological signals. These devices require conformable and biocompatible sensors and power source to continuously supply electricity to health-monitoring systems. In this talk, we will report on recent progresses of ultraflexible organic photovoltaic cells as power sources and wearable sensors. First, the progresses of power conversion efficiency (PCE) and mechanical/long-term stability of ultraflexible organic photovoltaics have been achieved using new material and process engineering. The certified PCE of 12.3% for the ultra-flexible organic photovoltaic cell has been achieved with novel ternary blend active layer [1]. Second, long-term continuous monitoring using organic electrochemical transistors with nonvolatile gel electrolyte has been achieved with a high mechanical stability and high signal-to-noise ratio (24dB) [2]. Then, the integration of the ultra-flexible organic photovoltaic cells as power sources with OECT-based sensors has been demonstrated [3].
[1] W. Huang et al., Joule, Accepted.
[2] H. Lee et al., Adv. Funct. Mater. doi.org/10.1002/adfm.201906982.
[3] S. Park et al., Nature 551, 516 (2018).

Although recent efforts in device designs and fabrication strategies have resulted in meaningful progresses to the goal of novel high-performance imaging devices, significant challenges still exist in developing a miniaturized and lightweight camera that enables the wide field-of-view (FoV) imaging. This is mainly due to bulky and heavy multiple lenses employed in the conventional wide angle camera. In this work, inspired by structural and functional features of the aquatic vision, we have developed a novel wide FoV camera by integrating a tailored monocentric lens and a hemispherical silicon nanorod photodetector array. This bioinspired camera offers the wide FoV, a miniaturized module size, minimal optical aberrations, the deep depth-of-field, and the enhanced light sensitivity in one simple integrated device. Theoretical analyses in conjunction with imaging demonstrations have corroborated the validity of the proposed concept. The fish-eye-inspired camera is expected to provides new opportunities for the advanced mobile electronics.

We focus on developing a new class of nanoscale materials and novel strategies for the investigation of biological entities at multiple length scales, from the molecular level to complex cellular networks. Our highly flexible bottom-up nanomaterials synthesis capabilities allow us to form unique hybrid-nanomaterials. Recently, we have demonstrated highly controlled synthesis of 3D out-of-plane single- to few-layer fuzzy graphene (3DFG) on a Si nanowire (SiNW) mesh template. By varying graphene growth conditions, we control the size, density, and electrical properties of the NW templated 3DFG (NT-3DFG). This flexible synthesis inspires formation of complex hybrid-nanomaterials with tailored optical and electrical properties to be used in future applications such as biosensing, and bioelectronics. For example, we have developed a unique bioelectrical platform based on 3DFG and have demonstrated, for the first time, recording of the electrical activity of excitable cells using ultra microelectrodes ranging from 10µm down to 2µm, without the need of any surface coatings. Currently, we target the limits of cell-device interfaces using out-of-plane grown 3DFG, aiming at electrical recordings with subcellular spatial resolution (<5μm) and μsec temporal resolution. In summary, the exceptional synthetic control and flexible assembly of nanomaterials provide powerful tools for fundamental studies and applications in life science and open up the potential to seamlessly merge either nanomaterials-based platforms or unique nanosensor geometries and topologies with cells, fusing nonliving and living systems together.

While human organs and tissues are mostly soft; conventional electronics are hard. Seamlessly merging electronics with human is of imminent importance in addressing grand societal challenges in health and joy of living. However, the main challenge lies in the huge mechanical mismatch between the current form of rigid electronics and the soft curvy nature of biology. Here, I will present a new form of electronics, namely “rubbery electronics and bioelectronics”, with skin-like softness and stretchability, which is constructed all based upon elastic rubbery electronic materials. These rubbery electronic materials are structured in the format of composites, which can be scalably manufactured from common and commercial available materials without dedicated and complicated synthesis. Specifically, we build nanofibril organic semiconductor and metallic nanowires percolated in the elastomeric polymer matrix in a composite format for the rubbery semiconductors and conductors, respectively. Employing these rubbery electronic materials, we have achieved fully rubber format devices, including transistors and sensors, logic gates, active matrices, elastic sensory skin systems, and biointegrated devices, etc. I will showcase a few examples including artificial skins, biomedical implants, and wearable applications.

Soft, noninvasive and multifunctional epidermal electronics (a.k.a. electronic tattoos or e-tattoos) have demonstrated many exciting 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 named the “cut-and-paste” method for the rapid prototyping of e-tattoo sensors using a paper/vinyl cutter plotter [1]. This method has been demonstrated to work for thin film metals [1, 2], various polymer sheets [1, 3], ceramics [4], as well as 2D materials such as graphene [5, 6]. The cut-and-pasted e-tattoos are low cost and can be used to measure a variety of physiological signals such as electrocardiogram (ECG), seismocardiogram (SCG), electrooculogram (EOG), skin hydration, skin temperature, respiratory rate and so on [1-3, 5, 6]. To make the e-tattoos go wireless, we now report the “cut-solder-paste” process to incorporate integrated circuits (ICs) for signal readout and processing, near field communication (NFC) [7], as well as Bluetooth transmission [8]. To overcome the limited patterning resolution of the cutter plotter and to recycle the tattoo layers with ICs, we propose a modular concept in which the wireless charging layer (NFC layer), the wireless communication layer (Bluetooth layer), the readout circuit layer, and the sensor/electrode layer are fabricated individually and stacked up at the final step of fabrication. The thickness of a fully assembled multilayer e-tattoo (excluding IC chips) is less than 300 um and the overall stretchability is still beyond 20%. In addition to already mentioned capabilities, such e-tattoos can also wireless track motion, mechano-acoustic heart signals, and oxygen saturation (SpO2) [3, 7, 8]. 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 Bluetooth-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. Combining the NFC layer and the Bluetooth layer in one e-tattoo, we demonstrate that such wireless e-tattoo can also be wirelessly charged on-the-go via stretchable fabric feeding coils [8], enabling long-term, ambulatory and continuous sensing. Moreover, we propose that different layers can be disassembled and reassembled multiple times. After disassembly, the electrode layer should be disposed but the other layers can be reassembled into new e-tattoos. The low-cost, rapid prototyping method together with the wireless and reconfigurable capabilities represent exciting advancement towards practically useful wireless e-tattoos.


1. Yang, S., et al., "Cut-and-Paste" Manufacture of Multiparametric Epidermal Sensor Systems. Advanced Materials, 2015. 27(41): p. 6423–6430.
2. Wang, Y., et al., Low-cost, μm-thick, tape-free electronic tattoo sensors with minimized motion and sweat artifacts. npj Flexible Electronics, 2018. 2(1): p. 6.
3. Ha, T., et al., A chest-laminated ultrathin and stretchable e-tattoo for the measurement of electrocardiogram, seismocardiogram, and cardiac time intervals. Advanced Science, 2019: p. 1900290.
4. Yang, S., E. Ng, and N. Lu, Indium Tin Oxide (ITO) Serpentine Ribbons on Soft Substrates Stretched beyond 100%. Extreme Mechanics Letters, 2015. 2: p. 37-45.
5. Ameri, S.K., et al., Imperceptible Electrooculography Graphene Sensor System for Human-Robot Interface npj 2D Materials and Applications, 2018. 2: p. 19.
6. Ameri, S.K., et al., Graphene Electronic Tattoo Sensors. Acs Nano, 2017. 11(8): p. 7634-7641.
7. Jeong, H., et al., Modular and Reconfigurable NFC-Enabled Wireless Electronic Tattoos for Biometric Sensing. Advanced Materials Technologies, 2019: p. 1900117.
8. Jeong, H., Huang, Yi., et al., Wireless E-Tattoos Chargeable On-The-Go. To be submitted, 2019.

The brain carries out complex cognitive and computing tasks by optimizing energy efficiency, information processing, communication, and learning in massively parallel, dense networks of highly interconnected neurons. To cope with its everchanging surroundings, the brain is able to grow neurons, synapses, and connections—owing to its plastic nature. At the molecular and cellular levels, synaptic plasticity and neuronal excitation are the main mechanisms underlying these processes. Therefore, the ability of next-generation computing devices, robots, and machines to autonomously sense, process, learn, and act in complex and dynamic environments while consuming very little power will require approaches to computing and sensing that are inherently brain-like. Reproducing these features using traditional electronic circuit elements is virtually impossible, requiring the design and fabrication of new hardware elements that can adapt to incoming signals and remember processed information. These elements should be scalable, biomimetic, and preferably ionic to achieve energy consumption levels approaching those in the brain. While a major effort is being invested in developing inorganic materials that could emulate synaptic and neural functionalities, I believe that an overlooked, yet high-reward, pathway to success is through the development of biomolecular materials with the composition, structure, and switching mechanisms of actual biological synapses and neurons. Here, I describe two-terminal, biomolecular memcapacitors and memristors, consisting of highly insulating 5 nm-thick lipid bilayers assembled between two water droplets in oil. These devices exhibit memcapacitance that is nonlinearly dependent on the applied voltage as well as hysteresis in the charge due to voltage-driven, reversible changes in the area and thickness of the bilayer membrane. This is the first demonstration of a memcapacitor in which capacitive memory results from geometrical changes in a lipid bilayer membrane. We also show that the incorporation of voltage-activated alamethicin and monazomycin peptides in these devices results in variable ionic conductance across the membrane and memristive behavior. We discuss how these devices exhibit learning through synaptic plasticity, and how to implement them in online learning applications. These results serve as a foundation for a new class of low-cost, low-power, soft mem-elements based on lipid interfaces and other biomolecules for applications in neuromorphic computing which could have major implications on the robotics and computing fields.

Electronic devices are typically manufactured in planar layouts but many emerging applications, from optoelectronics to wearables, require three-dimensional curvy structures. The fabrication of such structures has though proved challenging due, in particular, to the lack of an effective manufacturing technology. This presentation will show our development of conformal additive stamp (CAS) printing technology and its employment to reliably manufacture 3D curvy electronics. CAS printing employs a pneumatically inflated elastomeric balloon as a conformal stamping medium to pick up pre-fabricated electronic devices and print them onto curvy surfaces. To illustrate the capabilities of the approach, we will demonstarte various devices with curvy shapes: silicon pellets, photodetector arrays, electrically small antennas, hemispherical solar cells, and smart contact lenses. We also show that CAS printing can be used to print onto arbitrary 3D surfaces.

New form of materials can often enable new device and system applications. In this talk, I will highlight our group's recent work under this motivation in two synergistic areas. Firstly, by stacking individual layers of polymer, metal, and low-impedance coating reliably in a same nanomeshed pattern, the final multilayer multifunctional nanomeshes achieved system-level performance from all individual layers, in addition to nanomesh advantages. Compelling demonstrations from this multifunctional nanomesh approach include high-performance transparent and flexible neuroelectrode arrays, which has been recently validated in vivo. The second part of my talk will introduce our recent concept on semiconductor nanomeshes. By making a silicon film into homogeneous nanomeshes, we achieve high mobility semiconductor that is intrinsically stretchable to conventional microelectronic layouts. Together, our work demonstrates that nanomeshing is a unique way of transforming microelectronics for emerging applications.

Microscale electrodes, on the order of 10-100 µm, are rapidly becoming critical tools for neuroscience and brain-machine interfaces (BMIs) for their high channel counts and spatial resolution, yet the mechanical details of how probes at this scale insert into brain tissue are largely unknown. Here, we discuss quantitative measurements of the force and compression mechanics together with real-time microscopy for insertion of a systematic series of microelectrode probes as a function of diameter (7.5–100 µm and rectangular Neuropixels) and tip geometry (flat, angled, and electrochemically sharpened), within phantoms, ex vivo and in vivo. Results from each model system elucidated the role of tip geometry, surface forces, and mechanical scaling with diameter. Surprisingly, real-time microscopy revealed that at small enough lengthscales (<25 µm), blood vessel rupture and bleeding during implantation could be entirely avoided. This appears to occur via vessel displacement, avoiding capture on the probe surface which led to elongation and tearing for larger probes. We propose a new, three-zone model to account for the probe size dependence of bleeding, giving a new conceptual framework for how blood vessels fail and provide guidance for probe design.

Dynamic covalent thermosets show superior properties, including self-healability, recyclability, malleability/reprocessability, thanks to the molecular level bond exchange reactions. These properties promise bright future of dynamic covalent thermosets in broad applications. We here report our work on dynamic covalent thermoset-based electronic systems that are rehealable, recyclable and reconfigurable. A flexible multifunctional electronic device that integrates electrocardiograph (ECG), temperature, motion, and acoustic sensing capabilities was demonstrated. It was exhibited that the rehealed and recycled devices showed electronic performance comparable to the original ones. Bond exchange reactions in the polymer network effectively relaxes stresses induced by mechanical deformation, which made it possible to reconfigure the flexible device into different configurations that are suited for different applications. Such rehealable, recyclable and reconfigurable electronics provides an approach to address sustainability and environment issues associated with mass production of electronics. It can find broad applications in prosthetics, health care, and human-computer interface and other areas that are hard to be addressed by conventional approaches.

The ability to directly print biomedical devices on the body could benefit patient monitoring, wound treatment, and even allow for the possibility of human augmentation. In reality, this concept requires the 3D printer to adapt to the various translations, rotations, and deformations of the biological surface. Conventional 3D printing technologies typically rely on open-loop, calibrate-then-print operation procedures. An alternative approach is adaptive 3D printing, which is a closed-loop method that combines real-time feedback control and direct ink writing of functional materials in order to fabricate devices on moving freeform surfaces. Here we demonstrate that the changes of states in the 3D printing workspace in terms of the geometries and motions of target surfaces can be perceived by an integrated robotic system aided by computer vision. This allow us to directly 3D print a wireless antenna based on a novel silver ink a free-moving human hand, to power a skin-mounted LED. Using this same approach, cell-laden hydrogels were also printed on live mice, creating a model for future studies of wound-healing diseases. Moreover, we developed an in situ 3D printing system that estimates the motion and deformation of the target surface to adapt the toolpath in real time. With this printing system, a hydrogel-based sensor was printed on a porcine lung under respiration-induced deformation. The sensor was compliant to the tissue surface and provided continuous spatial mapping of deformation via electrical impedance tomography. This adaptive 3D printing approach may enhance robot-assisted medical treatments with additive manufacturing capabilities, enabling advanced medical treatments, and autonomous and direct printing of wearable electronics and biological materials on and inside the human body.

Although there are numerous studies on either hard or soft materials, our understanding of the fundamentals at hard/soft interfaces has been limited. As different types of energy (such as electrostatic, mechanical, thermal, and chemical energies) display diverse scaling behaviors and can converge, an appropriate selection of the length scale is critical for promoting new scientific discoveries across these interfaces. Our group integrates material science with biophysics to study several hard/soft interfaces. We synthesize new materials and probe interfacial dynamics, with particular focus at the sub-micrometer and sub-cellular length scales. In this talk, I will focus on the interfaces that enabled non-genetic, freestanding, and semiconductor-based bioelectric modulation. I will also discuss some recent work that exploits the dynamic behaviors of granular materials in polymeric matrices toward bioelectronic and robotic applications. I will end the talk by proposing several new scientific and engineering approaches to improving our fundamental understanding of the (bio)chemical processes at soft/hard interfaces and to exploring new applications of these interfacial (bio)chemical processes.

Soft polymers that are embedded with nano- and microscale droplets of liquid metal (LM) can be tailored to exhibit a broad range of electrical, thermal, mechanical, and dynamic shape morphing properties. In contrast to lithographically-fabricated soft microfluidic architectures, these LM-embedded soft polymer (LMSP) composites are statistically homogenous and exhibit effective medium properties at the mesoscale. Depending on the choice of polymer matrix, LMSPs can be engineered to achieve a wide variety of properties – from thermally conductive rubber for heat management and thermoelectric energy harvesting in wearable electronics to shape memory materials that dynamically response to electrical stimulation. Eutectic Ga-In (EGaIn) and Ga-In-Sn (Galinstan) alloys are used as the liquid metal due to their high electrical conductivity, low viscosity, non-toxicity, and the self-passivating formation of an oxide skin (Ga₂O₃) that enables emulsification and wetting to non-metallic materials. Because they are liquid-phase at room temperature, these alloys have virtually no influence on the mechanics of the surrounding elastomer medium. This allows the resulting composite to exhibit a unique and extraordinary combination of features not seen in other heterogeneous material systems. In this talk, I will review recent experimental and theoretical studies of this unique class of soft material architectures, with specific emphasis on LM-filled liquid crystal elastomers and soft polymers for stretchable electronics and energy harvesting. I’ll also highlight several applications in which LMSPs can have a potentially transformative impact, especially in the domains of wearable computing, physical human-machine interaction, and bioinspired soft robotics.

Electrical stimulation (ES) is a widely used therapeutic treatment strategy. It showed significantly positive results in treating a variety of diseases, biological disorders, and neurological problems. Today, the emergence of wearable devices is rapidly reshaping the development of medical devices, pushing them from conventional bulky and rigid silicon electronics to flexible and primarily polymer-based systems. Among many types of functions, nanogenerators are developed as a unique device for converting biomechanical energy into electrical pulses. In addition to applying it directly as a power source, this pulsed electricity can be applied directly as a ES signal for therapeutic treatment. In our recent research, we successfully implemented such an electromechanical system for skin wound healing and vagus nerve stimulation for obesity control. An electrical stimulation bandage was developed by integrating a flexible nanogenerator and a pair of dressing electrodes on a flexible substrate. Rat studies demonstrated rapid closure of a full-thickness rectangular skin wound within 3 days as compared to 12 days of usual contraction-based healing processes in rodents. From in vitro studies, the accelerated skin wound healing was attributed to the electric field-facilitated fibroblast migration, proliferation and transdifferentiation. In another work, an implanted vagus nerve stimulation system was developed. The device comprises a flexible and biocompatible nanogenerator that is attached on the surface of stomach. It generates biphasic electric pulses in responsive to the peristalsis of stomach. The electric signals generated by this device stimulates the vagal afferent fibers to reduce food intake and achieve weight control. This strategy is successfully demonstrated on rat models. Within 100 days, the average body weight is controlled at 350 g, 38% less than the control groups. Both results bring a new concept in electrical therapeutic technology that is battery-free, self-activated and directly responsive to body activities.

Tissue-wide electrophysiology with single-cell and single-spike spatiotemporal resolution is critical for biological studies and biomedical applications. In this talk, I will discuss the creation of cyborg organisms: the three-dimensional (3D) assembly of soft, stretchable mesh nanoelectronics across the entire living organism by the cell-cell attraction forces from 2D-to-3D tissue reconfiguration during organogenesis. We demonstrate that stretchable mesh nanoelectronics can migrate with and grow into the initial 2D cell layers to form the 3D organ with minimal impact on tissue growth and differentiation. The intimate contact between the dispersed nanoelectronics and cells enables us to chronically and systematically observe the evolution, propagation and synchronization of the bursting dynamics in different organisms through their entire organogenesis and maturation. I will also discuss the potential applications of these cyborg organisms in biology and biomedicine.

Protein nanowires harvested from microbes are a revolutionary green electronic material. They have unique properties for novel electronic devices with improved performance, for a broad range of applications from energy harvesting to neuromorphic computing and sensing. (1) Thin-film protein nanowire devices were shown to continuously harvest electricity from ambient humidity (Nature 578, 550-554 (2020)), enabled by the unique structural (e.g., nanopores at wire-wire interface) and chemical (e.g., a high density of hydroscopic groups) properties. This leads to a continuous energy harvesting mechanism from the ubiquitous and enormous ambient humidity, and hence a promising strategy to powering self-sustained systems. (2) The protein nanowires are cleverly designed and recruited by the microbe to facilitate charge exchange with environments (e.g., through redox metallization). Thus, protein nanowires can be assembled into bio-dielectrics to construct memristor devices, in which the protein nanowires catalytically reduce the energy barrier involved in memristive switching and enable ultralow operational voltage in the biological regime of <100 mV (Nature Commun. 11, 1861 (2020)). Neuromorphic components (e.g., artificial neuron, synapse) made from the protein nanowire memristors take a further step in bio-emulation from functional emulation to parameter matching, creating opportunities for future ultralow-power electronics and bioelectronic interfaces. (3) Protein nanowires have ultra-small diameters (3 nm) and an ultra-high density of surface functional groups (e.g., 10 per nm) that are advantageous for sensing applications. Electronic sensors made from protein nanowires were shown to have enhanced selectivity and sensitivity (Nano Res. 13, 1479-1484 (2020)) for ammonia detection compared to inorganic nanowire devices. These studies, combined with the advantages of renewability, biocompatibility, and eco-friendliness in protein nanowires, have provided the starting point for future green electronics based on synthetic protein nanowires.

Room-temperature liquid metals, such as nontoxic gallium alloys, show enormous promise to revolutionize stretchable electronics for next-generation soft robotic, e-skin, and wearable technologies. Core–shell particles of liquid metal with various surface-bound ligands can be synthesized and polymerized together to create cross-linked particle networks comprising >99.9% liquid metal by weight. When stretched, particles within these polymerized liquid metal networks (Poly-LMNs) rupture and release their liquid metal payload, resulting in a rapid transformation from insulating to conducting behavior. These networks autonomously form hierarchical structures that mitigate the deleterious effects of strain on electronic performance and give rise to emergent properties. Notable characteristics include nearly constant resistances over large strains, electronic strain memory, and increasing volumetric conductivity with strain to over 20 000 S cm-1 at >700% elongation. Furthermore, these Poly-LMNs exhibit exceptional performance as stretchable heaters, retaining 96% of their areal power across relevant physiological strains. Remarkable electromechanical properties, responsive behaviors, and facile processing make Poly-LMNs ideal for stretchable power delivery, sensing, and circuitry.

Recent advances in new materials, electronics, and assembly techniques have allowed optoelectronic systems to interface with biology and contribute significantly to the progress in basic biological research as well as clinical medicine. In this talk, I will introduce a novel class of flexible, multimodal optoelectronic systems that combines high-performance nanoscale electrodes with microscale optical components for simultaneous electrophysiological recording under optogenetic modulation. We envision this unique technology will open up new windows to understanding important biological processes by enabling mapping the dynamics of perturbed cell populations and correlating cellular responses to behavior.