Symposium S.SM03—Flexible, Stretchable Biointegrated Materials, Devices and Related Mechanics

In this talk, I will present our recent work on developing basic materials and devices necessary for integrated electronic systems, such as transistors, diodes, sensors and displays.

Devices interfaced with the spinal cord are critically important for investigations and treatment of spinal cord injury (SCI) as well as diseases such as spinal muscular atrophy and amyotrophic lateral sclerosis. Traditional intraspinal probes, which are capable of recording and stimulating spinal cord neurons have been used to study and develop protheses for the spinal cord. However, these rigid intraspinal devices generally cause immune responses that lead to glial scar formation and neuronal cell depletion at the tissue/probe interface, thus making chronic applications difficult. Here we report a minimally-invasive and chronically-stable intraspinal cord interface based on tissue-like mesh electronics. Mesh probes containing recording and stimulation electrodes are injected into the mouse spinal cord spanning both the dorsal and ventral regions without removal of vertebrae. Implanted probes do not adversely affect motor behavior and show a similar recovery times as mice undergoing sham surgery but without implants. Significantly, histology analyses show that mesh probes form a seamless interface with surrounding spinal cord tissue and do not elicit glial scar formation or neuronal cell depletion near the probe surface. The seamless mesh implants yield stable in vivo chronic electrophysiology recordings of spinal cord neurons for at least 6 weeks, and have been used to measure neural spike correlations between the motor cortex and spinal cord using optogenetic methods for well-defined stimulation in the motor cortex. Notably, implanted mesh probes have been used to evoke stable movement of mouse limbs via electrical stimulation of spinal cord motor neurons. This ultra-flexible, tissue-like spinal cord interface provides advantages complementary to other spinal cord implants that can enable opportunities for in vivo studies and therapies relevant to SCI and spinal cord diseases.

Wearable acoustic sensors have become an important and efficient platform for human-machine interactions (HMI) and body acoustics monitoring. To enable next generation applications, such sensors should be flexible, ultrathin, self-powered and highly sensitive. In contrast to conventional microphone technologies, piezoelectric acoustic sensors have greater flexibility and sensitivity^1,2. In the past, the effect of Young’s modulus and thickness of substrates on piezoelectric performance has been investigated³. However, the sensitivity of such flexible piezoelectric sensors is still limited to meet the next generation voice-activated human machine interactions or wearable body acoustics monitoring.

Here, we have developed an ultrathin piezoelectric acoustic sensor with 1.5 µm thick parylene substrates having multiple millimeter sized square holes. The holes on the substrates were created by using green laser. The sensor fabrication was done by sandwiching polyvinylidene fluoride (PVDF) nanofiber between two Au-coated substrates. The overall device is ~8 µm thick and can generate a peak voltage of ~11.2 V when a sound wave of 110 dB at 250 Hz is applied. The sensitivity of our sensor is ~1770 mV/Pa which is ~9 times higher compared to the sensors having similar thickness of substrates without holes. To show the practical applicability as voice sensors, human voice uttering the words were measured and compared with the sensor having planar 1.5 µm thick parylene substrates.

1. Persano, L. et al. Nat. Commun. 4, 1633 (2013).
2. Lang, C., Fang, J., Shao, H., Ding, X. & Lin, T. Nat. Commun. 7, 11108 (2016).
3. Lee, B.-S. et al. ACS Appl. Mater. Interfaces 6, 3520–3527 (2014).

Nature inspires a wide range of bio-mimetic systems ranging from soft robotic actuators to perceptive electronic skins that enhance and support our life. The growing demand on assistive, medical and bioelectronic technologies however raises concerns on the ecological footprint of this emerging platforms, as they are often designed for a defined, limited operational lifetime. Introducing a key feature essential to nature - biodegradability - will enable soft electronic and robotic devices that reduce (electronic) waste and are paramount for a sustainable future.

We here introduce materials and methods such as tough yet biodegradable biogels for soft systems that facilitate a broad range of applications, from transient wearable electronics to metabolizable soft robots. These embodiments are highly stretchable, are able to heal and are resistant to dehydration. Our forms of soft electronics and robots are built from resilient biogels with tunable, extreme mechanical properties that uniquely combine performance and durability with degradability. They are engineered for long-term operation in ambient conditions without fatigue, but fully degrade after use through biological triggers. Electronic skins that measure pressure, strain, temperature and humidity serve as human-friendly on-skin interfaces or equip robotic systems with sensory feedback. Such advances in the synthesis of biodegradable, mechanically tough and stable gels that do not compromise in performance when compared to their non-degradable counterparts may bring bionic soft systems a step closer to nature and enable human-friendly technologies with reduced ecological footprint.

Extracelluar vesicles (EVs) (30-1000nm) have garnered much attention lately due to their role in cancer metastasis. EVs act as mediators in cell-cell communication and have the ability to transfer genetic information between donor and recipient cells. Tumor cell-derived EVs (TDEs) have recently been reported to play an active role in tumorigenesis and metastasis owing to their ability to transmit oncogenes. One hypothesis is that TDEs contribute to metastasis by inducing the epithelial-to-mesenchymal transition (EMT), characterized by the loss of barrier function in barrier tissue-forming cells.

This study aims to elucidate the mechanisms of TDE egress and uptake by investigating the process by which TDEs “infect” normal cells inducing EMT prior to metastasis. Current strategies for investigating the spatial and temporal aspects of EV uptake, often optical, lack the ability to obtain quantitative data in real time. We have shown that organic electronic devices (OEDs) based on the conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) can be interfaced with biological systems of varying complexity allowing for quantitative real-time monitoring of biological interactions. The optical transparency of our PEDOT:PSS-based OEDs provides the unique advantage of dual transduction increasing the credibility of our platform and enriching the biological information obtained. Moreover, their compatibility with microfabrication methods allows for high throughput studies.

We have shown that transistor-based OEDs are highly sensitive to the integrity and interactions with barrier tissue-forming cells. As proof-of-principle, we show the chemically-induced disruption of these tissue barriers, and their subsequent recovery, in Madin-Darby Canine Kidney (MDCK) epithelial cells grown on our OEDs. In ongoing studies, monolayers of MCF-10A human mammary epithelial cells will be exposed to TDEs extracted from the human breast cancer cell line MDA-MB-231 to electrically monitor the phenotypic changes elicited by TDEs. This data wil be further substantiated by monitoring the expression of immunofluorescently labelled barrier-forming tight junction proteins at set intervals. These combinatorial optical and electrical measurements of TDE interactions with cell monolayers provide invaluable information for developing strategies for inhibiting TDE interactions by e.g. blocking specific surface markers, thereby preventing TDE-induced EMT.

A lot of people around the world have suffered from alopecia, which can lead to aesthetic issues, low selfesteem, and social anxiety. Therapeutic technology including thermal, electrical, pharmacological, and photo stimulation have been suggested to treat hair loss. Among them, laser stimulation to hair is a strong treatment, activating the proliferation of hair follicles without side effects. Particularly, periodically irradiating red light with a wavelength of 650nm in the hairless area can help efficient stimulation of hair follicles under the skin, because this light can penetrate deeper into the skin tissue than blue and green light with short wavelengths. However, this laser stimulation treatment has difficulty in high power consumption, large size, and spatiotemporal restriction. Recently, flexible micro-light-emitting diodes (μLEDs) have been a great attention for future biomedical devices, such as biosensors, pulse oximetry (SpO2) sensors, and optogenetic stimulators due to its high performance and outstanding stability. However, flexible μLEDs still have significant drawbacks in application to wearable phototherapeutic devices, such as high heat, low optical efficiency, and high energy consumption. Herein, we report high-performance 30 × 30 AlGaInP fVLEDs fabricated with transfer-free monolithic method for a wearable phototherapeutic device. Ultrathin flexible μLEDs were fabricated by vertically interconnecting top and bottom electrodes. The red f-VLED arrays were conformally attached on a human wrist with high irradiance of ∼30 mW mm−2 and low forward voltage of ∼2.8 V. The red light with 650 nm wavelength deeply penetrate the skin and stimulate the hair follicles, which is located 2 mm under the skin. The thermal stability of the monolithic f-VLED was theoretically investigated by using finite element method (FEM) simulation and experimentally optimized by operating f-VLEDs on the skin. Mechanical stability of monolithic f-VLEDs was confirmed by harsh periodic bending/unbending motions, which is necessary for skin-attachable phototherapeutic devices. Finally, photostimulation in hairless region was performed to promote hair growth in a living depilated mouse via f-VLED array with thermal stability. Hair regrowth in the mice via f-VLED was verified by analyzing immunofluorescent and histological effects.

Recently, photobiomodulation using near-infrared (NIR) light has been widely investigated for non-invasive healthcare applications including skin and hair therapy, and ocular therapy for diabetic retinopathy, methanol toxicity, and light damaged retina [1-3]. NIR light activates the photoreceptor of mitochondria and increases the therapeutic effect by accelerating the production of growth factors and the cell proliferation. Here, we successfully developed a NIR light emitting contact lens for the treatment of diabetic retinopathy possibly with greatly improved patient compliance. A NIR light emitting diode (LED) connected with an application specific integrated-circuit (ASIC) chip and a wireless power and communication system was fabricated on a PET film, which was embedded by photo-crosslinking in a silicone hydrogel contact lens. The LED contact lens could be on-off controlled wirelessly even in the presence of water without any significant deformation. The retinal vascular hyper-permeability induced by diabetic retinopathy in rabbits was reduced to the statistically significant level by simply wearing the NIR light emitting contact lens. Histological analysis also confirmed the safety and feasibility of LED contact lenses for treating diabetic retinopathy. This platform technology for LED contact lens might be harnessed for various applications to photomedicines, and augmented and virtual reality display applications.

[1] G. H. Lee, H. Moon, H. Kim, G. H. Lee, W. Kwon, S. Yoo, D. Myung, S. H. Yun, Zhenan Bao, S. K. Hahn, Nat. Rev. Mater. 2020, 1-17.
[2] Eells JT, Henry MM, Summerfelt P, Wong-Riley MT, Buchmann EV, Kane M, Whelan NT, Whelan HT. Proc. Natl. Acad. Sci. USA. 2003,100, 3439–3444.
[3] Qu C, Cao W, Fan Y, Lin Y. Adv. Exp. Med. Biol. 2010, 664, 365–374.

Rapid, on-site assessment is highly desirable in the medical diagnosis and treatment. To achieve this goal, our research aims to develop low-cost, flexible, large-area sensor devices for different health applications. In this presentation, we discuss case studies using flexible sensors for two different applications:
1) Motor skills characterization. There is no objective metric for evaluating motor skill training progress, and current assessments rely on qualitative surveys. We have fabricated an instrumented glove with pressue and motion sensors for motor characterization. This glove is useful for characterizing motor skills of people suffering from spasticity, a neuromuscular disorder that causes muscle stiffness/resistance and jerky movement. Analyses of force versus velocity show movement-dependent muscle resistance in a patient with spasticity. Through the flexible sensor systems, the shift from subjective scores to objective measurement will promote better diagnosis and dramatically improve the accuracy in tracking patient response to therapy.
2) Wound healing characterization. Wound healing is a complex process involving diverse changes in multiple cell types where the application of electric fields has been shown to accelerate wound closure. We show on an in vivo electrical stimulation model in which flexible electrodes were applied to an animal model for monitoring inflammation in a wound. The wound site was subjected to electric fields using flexible electrode arrays, and immune cells were then harvested for flow cytometry analysis to study molecular mechanisms, to identify a signaling pathway that is activated during the healing process.

Flexible sensor arrays have been developed for a broad spectrum of electronic skin (e-skin) applications such as human-machine interfaces, functional prostheses, and health monitoring devices. In many applications, high sensitivity of pressure and temperature are necessary for e-skins. Such multi-functionality, especially bio-modal sensing, of e-skins often requires complex fabrication and integration processes. We present a simple and effective design strategy of a bimodal e-skin sensor array that is capable of visual indication of temperature sensing by changing color in a range of 26 – 40 ^oC, as well as an ability to sense finger tap as a pressure sensor. We fabricated a 4 × 4 flexible array of soft, small-diameter capacitive bimodal sensors. The sensors consisted of a dielectric layer of elastomer composite containing thermochromic liquid crystal (TLC), which were sandwiched by transparent electrodes made of ion conductive, compliant, and adhesive polyampholyte (PAm) hydrogels. They were able to sense gentle touching of ~20 kPa and to perform reliably over a thousand repeated cycles of pressing up to 300 kPa. As functioning bimodally, these sensors also changed color in response to temperature, where the quantification of the temperature was achieved from calibration of hue-temperature curves of the elastomer composite at the precision of 0.1 °C. The e-skin array has demonstrated a promising potential for applications in medical devices and biorobots due to its biocompatibility and stretchability.

In vitro models of biological systems are essential for our understanding of biological systems. In many cases where animal models have failed to translate to useful data for human diseases, physiologically relevant in vitro models can bridge the gap. This is particularly true for modelling the effect of microbiome interactions on the gut-brain axis. An in vitro model of the gut-brain axis provides the opportunity to deepen our understanding at a molecular level, of the intriguing findings emerging from studies on germ-free mice and clinical studies on humans, about the interplay between the microbiome, the gut and the brain. However, many difficulties exist in interfacing complex, 3D models with sensing technology adapted for monitoring function of cells within these models. The introduction of polymeric electroactive materials into the heart of the tissues being modeled, simultaneously provides the opportunity to host the tissues in question, and monitor their growth and differentiation. In this presentation I will discuss our recent progress in developing 3D sponge-like electrodes that host a multi-cellular models with continuous monitoring and show how we are getting towards our goal of a complete platform to model microbiome-gut-brain interactions.

The human body consists of complex and dynamic systems that network throughout the body. The neck is an essential crossroads of several critical systems under soft tissue in a spatially confined area. These unique characteristics of the neck allow access to a rich set of mechanical and acoustic signals of key dynamic systems – all observable at a single location. In our recent work, we demonstrate the application of a novel wireless flexible sensor that can capture these mechano-acoustic signals effectively. The sensor design combines low modulus, elastic mechanics with mechanically isolated islands of conventional rigid electronic components along with added reinforcements. Together, the sensor can conform to intricate and irregular mounting surfaces and maintain contact during the natural motion of the neck without causing skin irritation and mechanical failure of the internal electronics.

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.

Flexible skin-like sensors are the essential component for future human health monitoring and therapeutic treatment. However, the price of flexible devices is still too high for wide applications. The development of ultra-fast and low-cost roll-to-roll (R2R) printing technology of flexible devices is critical for the actual application of flexible devices. In current printing technologies, organic additives are often mixed into the functional components to offer the flexibility of the printed flexible sensors. However, the time-consuming drying process of those additives is the major hinder for the low cost and fast R2R printing of flexible sensors.
We present a new ultra-fast R2R printing technology by taking advantage of the "induction induced absorption" behavior of dry powders in strong electric field. Dry graphene, CNT, silver nanowire, and many kinds of functional powders can be printed. The printing speed can reach as high as 50 cm2/s. The specially printed and naturally formed 3D structure offered high flexibility and sensitivity, allowing our skin-like sensors able to obtain extremely high sensitivity. Different kinds of materials can be integrated into one device. Different kinds of substrates with various surface condition can also be printed, including flat surface and complex 3D structures (e.g. textiles). The printing parameters were studied and optimized, the printing mechanism was simulated, and the printed devices were tested. Those devices are capable of detecting stain, stress, temperature, and even the frequency of sound. This technology opens a new door for fast and low-cost high-performance flexible sensor processing.

With the upcoming of internet of things (IoT) era, visual IOT platforms for hyperconnected visual communication attract a lot of attention and flexible inorganic based micro light emitting diode (micro LED) is one of the most promising candidates for visual communication applications. Inorganic micro LEDs have been considered as a strong candidates to replace the conventional light sources such as organic light emitting diodes (OLEDs), liquid crystal displays (LCDs) for display applications, because of their great electrical/optical properties, long lifetime, fast response time and high stability in severe environments. Despite of several previous researches about the micro LEDs, flexible vertical structured GaN micro LEDs (f-VLEDs) on plastic substrates have not yet been demonstrated due to the difficulty of vertical electrical interconnection with high power efficiency and thermal stability.
Herein, we demonstrated a high-performance flexible 30 x 30 GaN f-VLEDs array through a simple monolithic process. Inorganic-based laser lift-off (ILLO) process separated GaN LED chips from the sapphire substrates which is the mother substrate of GaN epilayer. The separated micro LED chips were electrically isolated by biocompatible polymer layers and electrically interconnected vertically via silver nanowires (AgNWs). These Flexible and transparent GaN VLEDs devices were conformally attached to a human fingernail emitting bright blue light with high optical power of 30 mWmm-2. The lifetime of f-VLEDs was investigated experimentally via high accelerated stress test (HAST) and measured theoretically via finite element method (FEM) simulation. In addition, periodic bending test was employed to verify excellent mechanical stability of f-VLEDs. The wireless power supply system successfully transmitted electrical power to the f-VLEDs on human skin. Finally, the monolithic GaN f-VLEDs successfully inserted into a living mouse brain and emitted bright blue light without severe histological damage in mouse brain.

Direct measurement and stimulation of ionic, biomolecular, cellular, and tissue-scale activity is a staple of bioelectronic diagnosis and/or therapy. Such bi-directional interfacing can be enhanced by a unique set of properties imparted by organic electronic materials. These materials, based on conjugated polymers, can be adapted for use in biological settings and show significant molecular-level interaction with their local environment, readily swell, and provide soft, seamless mechanical matching with tissue. At the same time, their swelling and mixed conduction allow for enhanced ionic-electronic coupling for transduction of biosignals. Such properties stress the importance of bulk transport processes and serve to enable new capabilities in bioelectronics, including new device concepts and form factors. I will demonstrate how such materials properties relax design constraints, allowing for flexible amplification systems for electrophysiological recordings. I will also discuss the unique form factors as applied to electroactive scaffolds and bioactive composites to module tissue state and/or cell fate. New materials design will continue to fill critical need gaps for challenging problems in bio-electronic interfacing.

Skin is the largest organ of the human body, serving as physical and hygroscopic barriers to protect the inner body and control transepidermal water exchange and also containing signal sensors to perceive various environmental stimuli such as pressure, deformation, and temperature. These functions are linked with salient features of the skin: highly deformable polymer networks to resist physical damages; hygroscopic compositions to retain water; a variety of sensory neurons powered by biological sources to transduce stimuli into controlled ion movements and propagation of action potentials. To recapitulate these features for wearable electronics and healthcare applications, significant efforts have been made on engineering electronic counterparts of the human skin (i.e., electronic skin), leading to various design strategies involving electronic components, elastomeric substances or a combination of thereof. Despite their remarkable functionalities and applications that have been demonstrated, existing electronic skins differ from the native skin in terms of charge carriers (electrons versus ions in native skin), the content and permeability of water, resulting in limited biocompatibility and suboptimal human-machine interfaces especially for the long-term usage.
While the gap between real and electronic skins persists, recent progress of iontronics highlights the use of hydrated and ionic materials like hydrogels for making ionic devices (i.e., pressure sensors and touchpad) capable of transducing mechanical stimuli into electrical signals such as of capacitance or resistance. The existing ionic devices usually convert the applied touch, pressure, deformation or temperature into a change in their resistance or capacitance, the electrical readout of which requires continuous power supply. Although stretchable batteries can be integrated with those ionic devices, the requirement of external or on-chip power supplies still, to some extent, compromises long-term usage. Further development is needed toward a multifunctional self-powered ionic device. First, the devices should equip with self-generated power to mitigate the need of external power supply for long-term usage. Second, they need to enable the sensing of more than one stimulus like the human skin. In addition, the devices must be physically stable (i.e., mitigating water loss) during usage and biocompatible when interfacing with the human body.
Herein, a novel artificial ionic skin (AIskin) has been demonstrated to recapitulate the salient features of the human skin. AIskin exhibits excellent skin-like mechanical deformability and ambient stability. It can sustain more than 400% strains without rupture and maintain its function after storage and 800 cycles of deformation. Different from the previous ionic devices consisting of homogenous hydrogels and/or elastomer, our design utilizes a heterogenous hydrogel of bilayer structure, which can realize controlled ion movements responsive to the mechanical deformation and humidity. This design resembles the conventional electronic diodes structurally and the sensory neurons in human skin in terms of stimuli sensing function. The AIskin converts both mechanical stimuli and humidity into four types of electrical signals: resistance, capacitance, open circuit voltage (OCV) and short circuit current (SCC), among which the latter two signals are self-generated without external power supply. Its multimodal sensation maintains in a wide range of relative humidity (13%~85%). It has been demonstrated for wearable strain-humidity sensing, human-machine interaction and walking energy harvesting. These features of our AIskin can potentially blur the boundary between humans and devices, and promise broad applications ranging from wearable and implantable devices, soft robotics to wound dressing capable of monitoring the local body motion and moisture of the wound bed.

The rapid advancement of electronic devices and fabrication technologies has further promoted the field of wearables and smart textiles. However, most of the current efforts in textile electronics focus on a single modality and cover a small area. In this work, we introduce a new platform of modular, conformable (i.e., flexible and stretchable) distributed sensor networks that can be embedded into digitally-knit textiles. This platform can be customized for various forms, sizes and functions using standard, accessible and low-cost manufacturing techniques. Here, we have developed a tailored, electronic textile conformable suit (E-TeCS) to perform large-scale, multi-modal physiological (temperature, heart rate, and respiration) sensing in vivo. The modular approach results in a conformable suit integrated with a large assortment of conformable electronic components for large-scale physiological sensing. The E-TeCS can detect skin temperature with an accuracy of 0.1 °C and a precision of 0.01 °C, as well as heart-rate and respiration with a precision of 0.0012 m/s² through mechano-acoustic inertial sensing. The knit textile electronics can be stretched up to 30 % under 1000 cycles of stretching without significant degradation in mechanical and electrical performance.

Advanced materials engineering and processing technologies provide means to realize a range of thin, flexible and stretchable biosensors for human body-integrated healthcare systems, opening up a new prospect in wearable technology. The mechanical flexibility and stretchability allow the devices to intimately integrate with biological systems such as the skins and organs at their length scale. The embedded semiconducting nanomaterials provide the functionalities that can monitor the clinically useful bio-signals with sufficient spatial and temporal controls. This presentation will introduce a novel technology so called as sticker-like electronics (or sticktronics) in which integrated functional circuits are defined on a temporary sticker that can be attached to anywhere of interests including the skin and organs for their wearable and implantable biomedical applications. Mechanically soft, compliant skin sensor patches and tissue-injectable silicon nano-needles will be discussed to show the representative applications in wearable healthcare monitoring and intratissue drug delivery. The novel design concepts by combining highly networked nanomaterials or nature-inspired layouts offer a route to enhance important mechanical properties, allowing for reliable contact to the skin and tissues even under dynamic motions. These strategies also provide long-term longevity of such devices against potential fracture and delamination by improving the overall crack resistance, contact adhesion, and normal/shear strength. Discussions about the results of detailed experimental and theoretical studies will be followed to reveal the essential attributes of the materials, mechanics, fabrication processes, and system configurations. Pilot demonstrations on patients with swallowing disorders for their efficient rehabilitation will demonstrate an example utility of the devices in current clinical settings.

Clinical prevalence of electrolyte imbalances is frequently seen in kids, beginner athletes, elderly and critically ill patients, and occur in the progression of diseases such as severe cardiovascular events like myocardial infarction, acute or chronic renal failures, etc. Sweat, due to the ease of non-invasive collection and the on-demand of convenient generation, shows great promise for wearable sensing, which is ideal for real-time continuous indicating body’s deeper biomolecular state and personal health condition. Recent advances in soft bioelectronics, especially epidermal sweat sensors, serve as the foundations for novel epidermal-based healthcare systems that overcome critical limitations of conventional personalized healthcare monitoring. However, a bottleneck of advanced sweat sensing is achieving accurate and stable measurement performance, comfortability, and high-throughput fabrication of sweat sensor components, including soft integration, epidermal packaging, and data analysis. Some reports have shown the possible methods of disposable sweat sensor components for fitness application, which still stands in an early research exploration with poor epidermal compatibility and low-efficiency sensing performance.

In this work, we introduce a hybrid stretchable epidermal sweat sensor, comprised of a miniaturized thermistor, signal processor and analog front-end, etc. for integrating into a soft system in package patch module. The sensor allows sweat sensing by using a group of gold-based fine bus electrode with a micro-chamber for real-time measurement of sweat electrolyte parameters. Analytical and experimental study optimized the system to establish a trade-off between the comfort of the user, the capacity for long-term use, and the reliable epidermis sensing data. The hybrid stretchable circuit technology, which is interconnected by a meander shaped conductive line, allows a good compromise between rigid chip components and bio-compatible elastomer packaging. The electrode pattern demonstrates the minimum bio-electricity interference by body organs and fluid. The soft mechanics of the sweat sensor patch allow multi-day wear in the presence of outdoor activity.

This work overcomes current engineering challenges in fabricating a bio-stretchable soft electronics system comprising multiple functional electronic components. As such, by using the existing micro-electrical fabrication process, the design of the sensor is enabling a low-cost technology in soft electronics manufacture. Overall, by enabling a comprehensive sweat analysis, the presented sensor is a promising monitor for advancing sweat testing in the point-of-care medical and fitness applications.

The application of self-healing materials to flexible and stretchable devices that can conform to biological components and deform with soft bodies in a mechanical invisible manner have been emerging. This is driven by the need to prolong their lifetime through spontaneous recovery upon physical damages. Despite these astounding abilities, these materials are often soft and lack intrinsic toughness, attributed to the high mobility of polymer chains to form entanglements and reassociations across the damaged region for self-healing purposes. Consequently, these materials usually fail at high tensile stresses (>1 MPa) making it susceptible to damages upon high loads. In this work, we address this dichotomy of achieving high self-healing efficiency and toughness simultaneously by introducing multiple hydrogen bonds into thermoplastic polyurethane to increase its intrinsic toughness, absorbing energy without fractures. With the introduction of multiple strong, reversible and complementary non-covalent hydrogen bonds, self-healing is achieved as these dynamic bonds are able to reassociate after breaking effectively. We further found that introducing particular solvents during the healing process, leads to a drastic improvement to the healing efficiency. This can be attributed to the solvent’s role as a plasticizer and a de-crosslinking agent that allows greater chain mobility. The healed polymer displayed exceptional mechanical properties of elongation at break of 1800%, a tensile strength of 5.30 MPa and toughness of 37.8 MJ m^-3 within 12 hours. Proving to be one of the first healed polymers that realized both high tensile strengths and elongations concurrently. Furthermore, these hydrogen bond moieties are polar in nature, contributing to the high dielectric permittivity of the film. As such, when applied as a dielectric elastomer actuation (DEA) layer, the DEAs delivers higher actuator performance and achieves significant recovery of its actuator performance in terms of area strains after self-healing.

In this presentation we report the correlation between polymer structure and charge transport in solution-processed indium oxide (In₂O₃):polymer blend films by using several polymers having different amine nitrogen atom atomic content (N%). These amine containing polymers control the electron mobilities of the semiconducting oxide films by tuning the carrier/trap concentrations via a combination of electron transfer/doping and charge trap/scattering center formation, effects which depend on the polymer species and loading contents in the metal oxide matrix. All polymer addition enables the formation of more ductile semiconductor metal oxide films, as measured in a flexible thin-film transistor architecture. However, while PVP addition reduces the field-effect mobility vs. the pristine (no polymer added) In₂O₃ matrix, for all other N-containing polymers there is always a certain polymer weight addition to the In2O3 precursor formulation which increases the mobility of the In2O3 matrix. Equally interesting, increasing N% in the polymer from 0% in polyvinyl phenol (PVP) to 34% in polyethyleneimine (PEI), the electron doping capacity gradually increases, enabling to increase the polymer content in the formulation at which the mobility is maximized from 0 wt.% to 1.0 wt.%, thus further enhancing blend ductability. Note, the positive effect of the N-containing polymer remains effective even with a polymer (PVP-NH2) having a N% as low as 3.9%, Finally, we discovered correlations between the electron donating capacity of the polymer, charge transport, polymer chemical structure and thermal stability, as well as elemental composition in the final In₂O₃-polymer blend film.

Modern electronics is rapidly evolving away from rigid devices to soft, flexible and stretchable devices. This evolution is being driven largely by the growing demand for wearable and human health monitoring applications, which require the development of conformal electronics with added functionality. Much of the effort in this area has relied on printing or transferring structures onto elastomeric substrates. However, the processing involved can often be cumbersome, and the structures themselves tend to suffer from poor fatigue characteristics. Here, I will describe a customised aerosol-jet printing (AJP) process to fabricate fully freestanding stretchable multi-layered devices. The AJP technique has distinct advantages in terms of fast-prototyping, wide ink compatibility with minimum printed feature size down to 10 µm. This versatile method facilitates the incorporation of multiple materials for added functionality that can serve a variety of applications. The ability to print multiple layers of different materials means that multiple conductive wires could be stacked up into one freestanding wire, separated by a dielectric polymer to prevent shorting. This feature makes single-wire circuitry possible by reducing the level of design complexity for practical stretching applications. For example, such freestanding structures can be used as stretchable interconnects, strain and humidity sensors, or even for printed thermoelectric energy harvesters. Desired structures consisting comprising both conductive and functional layers can be printed layer by layer on a substrate with a sacrificial film, that is subsequently dissolved to release the printed structures, thus yielding freestanding self-supported stretchable interconnects and/or functional devices. Devices made from this approach showed good electrical stability and stretchability, and are thus well-suited for applications in flexible/stretchable electronics, such as in conformal tactile, strain and humidity sensing applications, as well as in energy harvesting.

The emergence of devices that combine rubbery elasticity with electronic, optoelectronic, and mechanically transductive properties offers exciting new opportunities for applications, but brings significant materials challenges. This presentation will describe our latest results in the materials efforts in developing highly compliant conductors, semiconductors, and dielectric polymers, as well as in the demonstration of integrated functional devices. Specific examples include a touch responsive sensors, a bistable electroactive polymer for tactile display, dielectric elastomer actuators, and compliant electrode for neural detection and stimulation.

Research on wearable electronics with flexible and stretchable forms has been grown remarkably by adopting the advent of various nanomaterials and their hybrid. This talk presents multifunctional wearable electronic devices for humans based on RLC circuits, where R (resistance) responds to molecular binding of biomarkers in the body fluid while L (inductive) and C (capacitance) change in accordance with structural changes of capacitance materials induced by varying pressure. These devices are based on diverse nanomaterials and their hybrid structures in order to exhibit high transparency and superb mechanical stretchability, with wireless sensing capability. Furthermore, in-vivo tests demonstrate their reliable operations.

Contact lenses, which make continuous contact with the ocular surface and tear fluids, have been utilized recently as a wearable platform for the healthcare device. However, most of the sensors used in contact lenses have been fabricated on lens-shaped plastic substrates with low oxygen permeability using opaque electronic materials, which means that the safety of the devices is not guaranteed. To solve difficulties associated with wearable healthcare devices, the substrate materials of a wearable platform in the entire system must be convenient and harmless for people to wear, and the individual components that make up the system, such as sensors and interconnects, should be stretchable and transparent.
Here, we fabricated a graphene field-effect transistor (FET)-based biosensor on a soft contact lens and skin-attachable thermal therapeutic device using stretchable and transparent electrodes for monitoring and therapy of dry eye syndrome (DES). The smart contact lens that makes up this system is the first point-of-care device to quantitatively diagnosis MMP-9, which is a biomarker for DES. By incorporating both diagnostic and therapeutic devices and the wireless communications into an integrated system, real-time biomarker monitoring signals from the biosensors are transmitted to the smartphone, and the hyperthermia command is issued to the therapeutic device simultaneously. Furthermore, in-vivo and in-vitro test using a live rabbit exhibited the reliable operation of the system. This integrated system can provide ultimate solutions for personal health management when it is combined with advanced technologies such as artificial intelligence and virtual reality.

Every year approximately 9.44 million of adults, and more than 500,000 children, are diagnosed with swallowing disorders (a.k.a. oropharyngeal dysphagia) in the US. Swallowing is a fundamental human need and an act of pleasure; yet, oropharyngeal dysphagia is a common symptom in many diseases (e.g., head and neck cancer, stroke, dementia, Parkinson’s disease, cerebral palsy). Without proper treatment, dysphagia can lead to devastating consequences such as malnutrition, dehydration, respiratory compromise (due to aspiration of food/liquid into the lungs), or even death. Successful rehabilitation of oropharyngeal dysphagia requires frequent performances of head/neck exercises that primarily rely on expensive biofeedback equipment (e.g., oral manometers, electromyographic devices, and endoscopy), often only available in large medical centers. Such conditions require patients to make frequent visits to the clinic for a treatment session, which is often not feasible or economically viable for many patients. This directly affects treatment compliance and outcomes which highlights the need to develop a portable and inexpensive remote monitoring system for the rehabilitation and the tele-rehabilitation of dysphagia. Alternative strategies involve the use of portable monitoring devices to obtain biofeedback signals during swallow exercises in a form that allows real-time data collection or cloud storage for post-data processing. However, much of currently available systems suffer from rigid or semi-flexible platforms that are not well suited to dynamically interface with the curvilinear surface of the submental area (under the chin), particularly within the context of head/neck movement, thereby often resulting in poor data acquisition and discomfort to patients. This talk will outline materials and optimal mechanical design for a non-invasive, skin-mountable sensor patch that can fit conformably on the surface of the submental area and can be attached and detached multiple times without causing skin irritation. The sensor patch can remotely provide high-fidelity recording of surface electromyography signals from the submental muscles, as well as, mechanical strain waveforms from thyroid notch movement during a swallow. The sensor patch has been successfully demonstrated on a patient with Parkinson’s disease suffering from dysphagia and on a healthy control subject to show the feasibility and effectiveness of this system for the remote monitoring of patients with swallowing disorders.

Organic mixed ion-electron conductors (MIEC), included in electrodes and devices, are now being heavily explored as the signal translation interface at the biology-technology interface, targeting biology, medicine and a vast array of internet-of-things applications. Commonly, the MIECs are based on polymers including conjugated conducting pathways for electronic charges and ion transport-promoting side-groups or polyelectrolytes. These MIECs operate as polarizable electrodes and the coupled charge depletion-ion exchange defines the mode of operation for many bioelectronic devices. The fundamentals of MIECs are reported along with a few targeted device concepts targeting both electrode, sensor and actuator concepts for various applications.

Recently, smart contact lens technology has been rapidly growing as a multi-purpose wearable device such as shielding electromagnetic waves, sensing glucose, display, and protecting eye dehydration in addition to its original vision-aid ability. Among many candidate materials for electrodes, graphene, a two-dimensional honeycomb lattice structure of the carbon atoms, has garnered much attention due to the excellent mechanical flexibility and electrical conductivity. Above all, it has superior optical transmittance, proving itself as an excellent alternative for the metal electrodes for a vision device such as a contact lens.
In order to adapt graphene as electrodes for soft smart contact lenses, one of the issues to address is developing a method for the scale-up, cost-effective, and precise graphene patterning on a surface of contact lens. Up to date, the graphene is mostly pre-patterned on a 2D substrate and then transferred onto a target 3D substrate, and such sequence inevitably causes not only distortion of the pattern but also low-yield production. In order to eliminate the distortion of the pattern and increase the yield of the production, transferring the bulk sheet of graphene on the surface of the contact lenses, followed by patterning the graphene directly on the 3D surface would be one of the ideal methods. However, to the best of our knowledge, there has been no report on a direct, nanoscale, and cost-effective patterning method of graphene on 3D surfaces.
Herein in this work, we present Self-Aligning Nanojet (SA-N) as a distortion-free and cost-effective direct-patterning-method of graphene on a soft smart contact lens. The SA-N is enabled at the lowest regime of flow rate in near-field electrospinning, which based upon surface current-induced electrostatic force without relying on hydrodynamic force. Therefore, kinematics of the SA-N is dominated by the electric force, by which the direction of the jet always self-aligns normal to the surface of the substrate rather than being pulled down by the gravity. As a result, by controlling the stage to follow the profile of the 3D surface, the precise patterning of nanofiber on multi-dimensional surfaces, such as the surface of the contact lens, is realized by the SA-N. Using the SA-N, nanofiber can be precisely patterned on top of pre-transferred-graphene as a bulk sheet on the surface of the contact lens. The patterned nanofiber serves as a masking layer during the plasma etching process, providing distortion-free patterned graphene that is ready to be used as electrodes. The proposed process is also much cost-effective compared to the typical graphene patterning method, such as photolithography or e-beam lithography.
In the presentation, we will present a study of the governing mechanism of the SA-N and show a demonstration of the direct writing of functional nanofiber on a 3D surface of the soft contact lens. Moreover, mechanical and electrical characterization of the precisely patterned nanoscale graphene will be analyzed. We believe that the SA-N can provide a promising fabrication-strategy for a wide range of functional materials, including nanoparticles and polymers, for the smart contact lens as well as other 3D structures.

Flexible and stretchable sensors with abilities to detect a light touch, pressure and even shear have been investigated. Most of these sensors are either resistive or capacitive in nature. In case of capacitive sensors, the sensitivity primarily depends on the ease of deformation of the dielectric. To enhance sensitivity to the applied force, a lot of implementations have air cavities in the dielectric material in order to obtain a lower effective Young’s modulus. This, in turn brings about a greater change in capacitance for a given applied pressure. The greater the fraction of air in the total volume of the dielectric, the softer it is. However, the architecture of the dielectric can cause this to differ due to the Poisson effect. Upon applying a force along the perpendicular direction of a unit elemental structure in the dielectric, the structure expands in the longitudinal axis. Depending on the architecture, this effect can lead to a change in the effective Young’s modulus. In addition to the Young’s modulus, the deformation of the dielectric can also couple the electric fields differently and can even be designed to enhance the coupling. Under these circumstances, the capacitance increase due to an applied pressure will not only be due to the decrease in dielectric thickness but also due to the change in the effective dielectric constant.

Typically, the Young’s modulus of a patterned dielectric with air cavities is expected to decrease with a reduction in the volumetric ratio of solid dielectric material to air cavities. In this work, we investigated capacitive pressure sensors with various dielectric architectures keeping the ratio of solid dielectric to air cavities volume constant and therefore an expected similar Young’s modulus magnitude. However, within these architectures, we have demonstrated that it is possible to engineer the Young’s modulus to vary by over 50% and the capacitive sensitivity to vary by over 100%. This research therefore paves the path to designing sensors to suit very specific applications, including being able to have a higher elastic modulus and yet bring about a larger change in capacitance with the same pressure applied. We have also demonstrated the ability to engineer a non-linear Young’s Modulus - sought after by artificial skin researchers in robotics and prosthetics, since human skin has a non-linear elastic response. This work discusses the critical parameters that are to be focused on when designing the dielectric for a capacitive sensor and aims to aid in the development of custom design that fulfils desired specifications.

Graphene Oxide Liquid Crystal (GOLC) is a newly emerging graphene based material, which exhibits nematic type colloidal discotic liquid crystallinity with the orientational ordering of graphene oxide flakes in good solvents, including water. Since our first discovery of GOLC in aqueous dispersion, this interesting mesophase has been utilized over world-wide for many different application fields, such as liquid crystalline graphene fiber spinning, highly ordered graphene membrane/film production, prototype liquid crystal display and so on. Interestingly, GOLC also allow us a valuable opportunity for the highly ordered molecular scale assembly of functional nanoscale structures. This presentation will introduce our current status of GOLC research particularly focusing on the nanoscale assembly of functional nanostructures. Besides, relevant research works associated to the nanoscale assembly and chemical modification of various nanoscale graphene based materials will be presented particularly aiming at energy and catalytic applications

Recent advances in electronics enable powerful biomedical devices that have significantly reduced therapeutic risks by monitoring vital signals and providing means of treatment. Smallfootprint deformable gas sensors that can be deployed on the skin surface to detect and distinguish multiple compounds in real-time are integral to accurate monitoring of health conditions. Low-dimensional nanomaterials or mixed metal oxides are state-of-the-art gas sensing materials that change their resistance upon binding of various important gases that require monitoring. However, it is challenging to integrate within a microscale footprint the numerous nanomaterials required to deconvolute signals from complex gaseous mixtures, as is necessary for deployment. The core innovation will be our ability to use laser writing to synthesize and pattern numerous low-dimensional nanomaterials and mixed metal oxides directly on laser-induced graphene gas sensing platform in order to facilitate the deconvolution of complex gas responses in mixtures. When combined with the stretchable antennas for wireless communication and rectennas for ambient RF energy harvesting, wearable gas sensors for health monitoring or toxic gas detection open new opportunities in epidermal electronic devices to enhance the operator and mitigate exposure.

The use of 3D printing to create devices from elastomeric and polymeric materials has appended the design functionality for new materials including uses in biomedical devices enabling rapid development. While 3D printed polymers can be further classified into thermoplastics, thermosets, and elastomers based on their thermo-mechanical properties. The processability and functionality of flexible elastomers make it a challenge to employ 3D printing methods for additive manufacturing. The transition to a final phase or cross-linked structure results in new properties in combination with the processing method. This is more evident with the choices of 3D printing methodologies (FDM, SLA, SLS, VSP) which can make use of blended or formulated compositions. We have demonstrated the 3D printing of biomedical grade thermoplastic polyurethanes (TPU), silicones, and rubberized epoxies. However, 4D printing allows the design of new materials and applications based on integrating the chemistry of conversion with the printing mode. In this talk, we demonstrate the fabrication of concept objects and elastomeric actuators based on the use of biomedical grade melts and extruded viscous solutions of nanocomposite elastomers as elastomeric actuating materials and shape memory devices.

Recently, the wearable devices combined with the internet of things (IoT) have become a vital part of our daily life. The trend of the wearable electronics is moving from small electronics to directly embedded ones. In this respect, fiber and fabric-based electronics have many advantages such as high wearability, ventilation through pores, and high durability by weave structures.
Smart fibers and fabrics, which can generate electricity from human motions and monitor the motions of human body, can act as wearable energy harvesters and sensors. As such, they will be used as important components of IoT (Internet of Things) technology. Here, we present a smart fiber and an e-textile with piezoelectric poly[(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] for wearable energy harvester applications. We used dip coating and hot pressing to deposit P(VDF-TrFE) on fibers and fabrics. We found that our prototypes had high flexibility and high interfacial adhesion strengths between P(VDF-TrFE) and electrodes, indicative of high stability. We demonstrated that our prototypes could generate piezoelectric output voltage and current from human mechanical energies with low frequencies. We envision that our fiber and fabric-based piezoelectric devices are promising as energy harvesters for supplying energy source to low power wearable devices.

Herein, we demonstrate a new concept of flexible piezoelectric acoustic sensors (f-PAS) using mechanical sequential multi-resonance in membrane. The flexible piezoelectric film of f-PAS fabricated by sol-gel method and inorganic laser lift-off (LLO) exhibits high sensitivity to nanometer scale vibration and multiple frequency resonances in human voice frequency range. The self-powered f-PAS mimicked the mechanism of basilar membrane in the human cochlear that has multiple response to different frequencies depending on the width of membrane. The f-PAS exhibits maximum -42dB of relative sensitivity, which is four to eight times higher than the conventional condenser sensor without any amplification circuit. The f-PAS acquired abundant voice information from the multi-channel electrodes and read the frequency domain characteristics. The standard records dataset (TIDIGITs) were utilized to excite the piezoelectric membrane of the f-PAS, which converts the original analog speech sound to the electrical signal. The voltage-time data was subsequently analyzed by a Fast Fourier Transform (FFT) and a Short-Time Fourier Transform (STFT) to convert to the frequency component. The machine learning algorithm was designed by using the most highest and second highest sensitivity data among multi-channel outputs, exhibiting outstanding speaker recognition rate of 97.5% with error rate reduction of 75%, compared to the reference condense-type MEMS microphone.

Elastic composite-based piezoelectric energy harvesting technology is highly desired to enable a wide range of device applications, including self-powered wearable electronics, robotic skins, and biomedical devices. Recently developed piezoelectric composites are based on inorganic piezoelectric fillers and polymeric soft matrix to take the advantages of both components. However, there are still limitations such as the weak stress transfer to piezoelectric elements and the poor dispersion of fillers in matrix. In this talk, a highly-enhanced piezocomposite energy harvesters (PCEH) is representatively developed using a three-dimensional (3D) interconnected electroceramic skeleton by mimicking and reproducing the sea porifera architecture (although the speaker will introduce other research examples and achievements). This new mechanically reinforced PCEH is demonstrated to resolve the problems of previous reported conventional piezocomposites, and in turn induces stronger piezoelectric energy harvesting responses. The generated voltage, current density and instantaneous power density of the biomimetic PCEH device reach up to few tens times higher power output than that of conventional randomly-dispersed particle-based PCEH. This work broadens the further developments of high-output elastic piezocomposite energy harvesting and sensor application with biomimetic architecture.

Soft pressure sensors may find a wide range of applications in soft robotics, biomedical devices, and smart wearables. In this paper, an inkjet-printed resistive pressure sensor that offers high sensitivity and can be fabricated using a very simple process is reported. The device is composed of a conductive silver nanoparticle (AgNP) layer directly printed onto a polydimethylsiloxane substrate and encapsulated by a VHB tape. The pressure is measured through change in electrical resistance caused by pressure-induced strain in the printed AgNP thin film. The influence of substrate stiffness and thickness on the sensitivity and achieved sensors with an optimized configuration that exhibit highly repeatable response with a sensitivity of up to 0.48 kPa⁻¹ is systematically studied. It is further demonstrated that such a printed soft sensor patch is capable of measuring arterial pulse waveforms or detecting acoustic vibrations under various sound pressure levels. With its simple and low-cost fabrication process and high sensitivity, the inkjet-printed resistive pressure sensor is promising for future biomedical and smart wearable device applications.

Protein-based hydrogels can be utilized for extracellular matrices (ECM) by providing biocompatible and dynamic local microenvironment. Through cell-ECM interactions, these external mechanical signals are transduced to the cell’s interior, triggering biochemical signaling cascades that determine a wide variety of cellular behaviors, ranging from cell adhesion, cell spreading to cell differentiation. However, only a limited number of such hydrogels have been engineered allowing for investigating cell response to the change of hydrogel stiffness. Moreover, the stiffness of these hydrogels can only be switched uni-directionally and irreversibly. Here we report an entirely protein-based hydrogel that allows for a cyclic, reversible modulation of hydrogel mechanics in a relatively large range of Young’s modulus, and used it to investigate the cell spreading behaviors of human lung fibroblasts (HLF) in response to the cyclic change of the hydrogel Young’s modulus.
This protein-based dynamic hydrogel as an ECM, was fabricated by using mutually exclusive protein (MEP, GL5CC/I27) as building blocks. MEP is a specially designed, redox-responsive protein folding switch. In the oxidized state, the host domain GL5CC is folded; in the reduced state, the GL5CC unfolds and the effective chain length of the MEP changes from 2.6 to 26 nm, leading to a change in the polymer chain length between two crosslinking points. Building upon this approach, we engineered the dynamic protein hydrogel based on TNfn3-GB1-resilin-(GB1-GL5CC/I27-resilin)_2. TNfn3 contains a RGD motif for cell adhesion and resilin provides covalent crosslinking residues. We found that this dynamic hydrogel could be reversibly tuned between a stiff and soft state via a redox reaction. Moreover, this protein hydrogel supports cell adhesion and proliferation. In response to a cyclic change of the hydrogel’s Young’s modulus between ~6 kPa and 20 kPa, HLF dynamically change their morphology in a fully reversible fashion, demonstrating the great potential utility of such dynamic protein hydrogels in mechanochemical studies.

The recently developed laser induced graphene (LIG) process allows scribing electrically conductive circuits directly on a polyimide film. Its versatility is enabling a variety of designs, suitable for a broad range of electro-mechanical applications [1]. Silicone rubber layers incorporating LIG patterns were proposed to produce strain responsive devices such as strain gauges [2]. Nevertheless, the sensors had an overall thickness in the mm range, which makes challenging their use for thin, wearable, biointegrated devices. Here conformability plays a main role in determining a seamless interface with skin [4]. In this work we report the results on new LIG-based composites with low thickness and enhanced conformability to skin. The fabricated materials were used to develop various stretchable electrical devices.
Various silicone as well as polyurethane (PU) materials were investigated as the elastic carriers for LIG patterns. The latter were produced either in the form of a porous flat structure or as bundles of graphene fibers, by fine tuning of laser rastering parameters. The research included thin films with/without acrylic adhesives of various compositions and types of finish (including spin coating, blown film extrusion, etc.) The influence of laser rastering direction on the final functional properties of the composites was also investigated. One of the main challenges addressed in this work was an optimization of the transfer process of an electrically integer LIG circuit onto the investigated film. That was done in order to preserve low sheet resistance (100-1000 Ω/sq) and to achieve replicability among specimens. Developing a proper electrical connection suitable for flexible applications was another part of the work. A comprehensive electro-mechanical testing of composites has been carried out.
PDMS-based LIG devices have shown fully recoverable linear elastic behaviour and linear electrical resistance response up to 25% strain, as well as maximum stretchability up to 70%. These results allowed us to develop highly responsive thin strain sensors. A thin and soft (~30 µm, E ~ 15 MPa) wearable respiratory sensor based on biofriendly and medically approved flexible PU film served as another example for the proposed future applications.
The spectrum of patterned LIG applications can be further enhanced by designing compact energy efficient power storage devices such as microsupercapacitors (MSCs) [5] which could be suitable for sports and medical applications, among others. To this aim we worked on creating thin flat electrodes with high surface area for electrochemical applications. These could be integrated into the last generation of bio- and environmentally friendly electrochemical micro supercapacitors (MSCs), based on a low-cost and skin-friendly aqueous electrolyte. Since the surface area of the electrode material plays a crucial role in the charge storage processes, LIG material in form of a scribed powder was investigated by means of BET technique in order to confirm its remarkably high surface area of ~400 m²/g. Thin and soft LIG electrodes integrating electrical contacts were tested for MSC application. The obtained results of electrochemical measurements (cyclic voltammetry, galvanostatic charge/discharge curves) have shown high potential for further applications of the developed devices in the form of miniaturized power suppliers.
[1] R. Ye, D. K. James, J. M. Tour, Adv. Mater. 2019, 31, 1803621.
[2] R. Rahimi, M. Ochoa, W. Yu, B. Ziaie, ACS Appl. Mater. Interfaces 2015, 7, 4463.
[3] M. Parmeggiani, P. Zaccagnini, S. Stassi, M. Fontana, S. Bianco, C. Nicosia, C. F. Pirri, A. Lamberti, ACS Appl. Mater. Interfaces 2019, DOI 10.1021/acsami.9b10408.
[4] L. M. Ferrari, S. Sudha, S. Tarantino, R. Esposti, F. Bolzoni, P. Cavallari, C. Cipriani, V. Mattoli, F. Greco, Adv. Sci. 2018, 5, 1700771.
[5] W. Song, J. Zhu, B. Gan, S. Zhao, H. Wang, C. Li, J. Wang, Small 2018, 14, 1702249.

Polymeric ionic liquids (PILs) are promising candidates for soft electronics. They are amorphous polymers and with ionic functionalities on the backbone that are paired with a counterion that leads to a low glass transition temperature (T_g). Here, we present our work to design redox-active polymers that allow charge transport while maintaining the behavior of PILs. Developing soft redox polymers is challenging because of their relatively high T_g. There have been efforts to decrease T_g of redox-active polymers by increasing the length of the aliphatic chains in between tethered charge-conducting groups on the side chains and the backbone. However, this structure can harm charge transport properties because of the spatial separation of the redox active groups. In this work, we have synthesized the PIL with redox-active phenothiazine-bis(trifluoromethanesulfonyl)imide (TFSI) ionic liquid side chains placed very close to the backbone. By placing the side chains closer to each other, enhanced charge transport was successfully demonstrated upon oxidation and with increasing amounts of the TFSI anion. At the same time, the T_g was significantly lowered by addition of a short ethylene glycol units on the phenothiazine. In this presentation, we will demonstrate that both enhanced electrical and mechanical properties could be attained with our novel polymeric design for future applications.

Protein-based hydrogels have emerged as promising biomedical materials thanks to the precisely controllable structures, mono-dispersity and biocompatibility of genetically engineered proteins. However, many protein-based hydrogels lack the ability to controllably change their biological, physical or mechanical properties in response to temperature variations, which limits their practical applications in clinical injection, pain relief, hyperthermia treatment, and drug delivery. Here we report a method to fabricate protein-based hydrogels with regulable thermo-responsive properties. By fusing an elastin-like polypeptide chain (Vn) to an elastomeric poly-(GB1-resilin) protein (YR), we synthesized a series of block co-polymers, Vn-YRs. With Vn blocks that exhibit a lower critical solution temperature (LCST) behavior, Vn-YRs become di-block amphiphiles and form micelles above the transition temperature (T_t) of Vn blocks in aqueous media. When Tyr-rich YR-blocks are cross-linked into a hydrogel network through a photo-induced di-Tyr reaction, the non-Tyr-containing Vn blocks are dispositioned into the side chains with one end attached to the network. These Vn side chains do not pronouncedly alter the mechanical properties of YR-based hydrogel networks at temperature below their T_t. By contrast, above T_t, Vn side chains undergo phase transitions within the hydrogel networks, similar to their micellization in solution. Such temperature-triggered in situ phase transition of the Vn side chains is reversible and bestows reversible modulus reinforcement and de-swelling to the hydrogel materials. Furthermore, the amplitude of such property changes can be tuned by factors including side chain length and composition. The results demonstrate that in situ phase behaviors of side chains can regulate thermo-responsiveness of protein-based hydrogels. We anticipate that this reported method can be applied to various biomedical applications, as well as inspire the potential development of other stimuli-responsive hydrogels.

The higher demanding for the clarity and flexibility limit the current-leading materials such as indium tin oxide (ITO) further development in optoelectronics. Polymer substrate embedded with metal mesh has recently drawn increased attention for various flexible and transparent display devices because of their high electrical conductivity, optical transparency and robustness. However, for some commercial ITO/PET substrates applied in devices are still not satisfied the performance requirement.
Our study has investigated a low-temperature, particle-free silver ink formed multi-layer micro/nano structured transparent conductor to achieve a higher light management with a lower loss of electrical properties. Ag microgrid embedded into a nanostructured polyethylene terephthalate (PET) substrate shows a high transparency, low haze as well as a low sheet resistance. An anti-reflection nanostructured fabrication is also applied to the back side of PET to further suppress the reflection, thus mostly enhanced transparency. This work focused on minimizing the optical transmittance loss and shadowing losses: (i) Small size feature for linewidth to 3 um, (ii) improve the T-Rs trade-off, making T > 90% with a Rs less than 4 ohm/sq, (iii) reducing the cost-effectiveness of fabrication method. Mechanical reliability is also demonstrated by bending and folding test comparing to ITO/PET. The sheet resistance only increased 5% under 500 bending cycles and still effectively conductive after 10-times completely folding. The mechanism of highly flexible even foldable properties resulting from lowering stress within metal grid is also discussed.

2D materials are at the focus of scientific attention in recent years in view of their several unique and interesting physical properties. The initial research on 2D materials was centred on their novel synthesis and property characterizations, and now the applications of these materials are being sought in different fields. Since clean and renewable energy research has acquired immense prominence lately, applications of 2D materials for energy harvesting are being keenly pursued at this time; mechanical energy harvesting being currently a rapidly developing field with several opportunities to harvest different forms of mechanical energy such as flexing, vibration, friction etc. using device systems known as nanogenerators (NGs) and triboelectric nanogenerators (TENGs) which are based on different principles. While some very interesting research using 2D materials has already appeared in the literature, many innovative design possibilities are still open for evaluations on the use of these materials for enhanced mechanical energy harvesting performance.^1,2
Thus, in this work, we have explored the high mechanical energy harvesting possibility of the uniquely puckered structured elastically and electronically interesting 2D material, namely black phosphorous (BP). Nanogenerator device was fabricated by homogenously and densely embedding BP Nanosheets in Polydimethysillioxane (PDMS) matrix forming a composite film at different concentrations. The BP-PDMS based Nanogenerator displays an exceptionally high voltage output of maximum peak-to-peak voltage output of the order of ±350 V (maximum current density of 12.8 mA m^-2 under an applied impact force of 40 N, at a frequency of 20 to 25 Hz. In terms of output power performance, the NG exhibits a power density of 1400 mW m^-2 and a volume power density of 2 kWm^-3, with active material (BP) contribution of 0.35 W g^-1. By using Piezoresponse force microscopy (PFM), we have calculated the piezoelectric coefficient value (d₃₃) which is 20 pm/V. DFT calculations suggest the presence of strain-induced polarization via deformation-induced redistribution of intra-layer electron charge density. Demonstration of three real life applications namely wheel motion, tap water flow, and bending/flexing movements demonstrate that BP-PDMS NG is indeed a high powering capability device.³
References:
1. YAZYEV, O. V. & CHEN, Y. P. 2014. Polycrystalline graphene and other two-dimensional materials. Nature Nanotechnology, 9, 755.
2. WANG, Q. H., KALANTAR-ZADEH, K., KIS, A., COLEMAN, J. N. & STRANO, M. S. 2012. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature Nanotechnology, 7, 699.
3. SINGH, S. K., MUDULI, S., DHAKRAS, D., PANDEY, R., BABAR, R., SINGH, A., KABRA, D., KABIR, M., BOOMISHANKAR, R. & OGALE, S. 2019. High power mechanical energy harvester based on exfoliated black phosphorous–polymer composite and its multiple applications. Sustainable Energy & Fuels, 3, 1943-1950.

There is a challenge to prepare the hydrogel that hold a high stretchability and conductivity in below zero temperature. In this work, we demonstrate a supramolecular ionic liquid hydrogel, wherein quadruple hydrogen bonding ureidopyrimidinone (UPy) units cross-linked polyethylene glycol (PEG) polyurethane was employed as networks with aqueous solution of imidazolium ionic liquid as free phase. Owing to the hydrophilic PEG segments, the hydrophobic UPy moieties, and the green molten imidazolium ionic liquid, this hydrogel exhibited high water content (~600%) , swelling ratios (~400%), elongation (~750%), low resistivity (~0.51 kΩ●cm), excellent transparency (~98%), especially, the most striking characteristic could be maintain outstanding mechanical and thermal sensitivity at -20°C. The supramolecular ionic liquid hydrogel was utilized to fabricate E-skin for intelligent robot to realize pressure feedback and thermal recognition. This line of research not only demonstrates the anti-freezing stimuli-responsive hydrogel are a promising candidate for E-skins used in harsh condition, but also enrich the design of supramolecular polymers-based hydrogel sensors for future artificial intelligence applications. (This paper is funded by the International Exchange Program of Harbin Engineering University for Innovation-oriented Talents Cultivation.)

Ultra-high elongation siloxane nanocomposites that rely on mechanically interlocked polymer chains that result from high degrees of entanglement and knot formation. In a first step, living anionic ring opening polymerization techniques are employed in the synthesis, yielding monodisperse heterobifunctional polydimethylsiloxanes with a hydrido-, omega vinyl-functionality.. Composites are formed in a second step in which a step-growth polymerization is conducted in the presence of surface modified silica nanoparticles yielding a regularly ethylene interrupted polydimethylsiloxane structure. High elongations, as great as 6000%, and self-healing properties appear to be dependent on the degree of polymerization of the linear step-growth polymers. The polymer chemistry, physical properties and practical examples of high-elongation and self-healing behavior will be presented.

In order to monitor a person’s health condition and diagnose disease, it is important to continuously monitor the vital physical signals or metabolites in body fluids. Especially, eyes provide various physiological signals, thus being a promising site for non-invasive wearable sensing. Therefore, the contact lens provides a unique wearable sensing platform for monitoring various signals from the wearer’s eye. However, in the conventional smart contact lenses, the components were opaque, rigid, and brittle, so they could obstruct the wearer’s vision and damage the eyes. Moreover, to supply power and receive signals from the smart contact lens, a bulky and expensive external equipment was needed, which might restrict the wearer’s movements.
In this talk, we fabricated a transparent and stretchable soft, smart contact lens for quantitative monitoring using a smartphone. One of the diseases that can be diagnosed through eyes is glaucoma, and the only known controllable risk factor of glaucoma is intraocular pressure (IOP). For monitoring the IOP, this smart lens is composed of a strain sensor, a strain-concentrating layer, a transparent and stretchable antenna, and an integrated circuit chip, and each component is electrically connected with three-dimensional structures of stretchable metals. This smart lens can operate without battery through NFC communications with a mobile phone. The in-vivo test using live rabbits and the human pilot experiment demonstrated the real-time monitoring of IOP using the fabricated smart contact lens without blocking the wearer’s view. Another biomarker that can be diagnosed from the eye is glucose concentration, which is a crucial factor to the diabetics. The correlation between blood glucose and tear glucose concentration was investigated by using a smart contact lens integrated with the electrochemical glucose sensor.

Polymer nanocomposite-based piezoresistive sensors have broad applications in physiology monitoring, soft robotics, and human-machine interfaces. In this work, we developed a bio-inspired, blood-vessel-like thermoplastic polyurethane (TPU), graphene nanoplatelets (GnPs), multi-walled carbon nanotubes (MWCNTs) based fibers capable of detecting strains, high pressures, gas/liquid flow, and chemicals. Two-dimensional GnPs contributed to the formation of pores and the one-dimensional MWCNTs contributed to the electrical sensitivities of the compressible piezoresistive middle-layer, which was sandwiched by inner and outer TPU support layers. Continues fibers were manufactured via dry-jet wet spinning with a 3D laser sintered triaxial spinneret. Due to the pores generated during solvent exchange and each layer’s unique thermal stability, a hollow core is observed after undergoing hot isostatic pressing (HIP). Performances of the hollow-core multi-phase sensor were tested for gas/liquid flows at varied pressure and geometry orientations. The smallest detectable pressure is observed at ~ 200 mmHg, similar to pressures in the human arteries. A mapping of the pressure distribution in various geometries is constructed using multiple sensors and the results were confirmed by mechanical analysis. The sensor also shows a linear piezoresistive response for strain smaller than 80%, and is stable for over 1000 cycles at 10% strain. The pores of the piezoresistive-layer also enhance the sensitivity towards different types of solvents and gas vapors as they were guided through the sensor. Mechanical and thermal properties of the sensors were characterized using dynamic mechanical analyzer (DMA), differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA), demonstrating improvements of mechanical durability and thermal stability during the sensing cycles. This blood vessel-based design showed a high potential to be used in fluid pressure monitoring, 3D organ prototyping, and medical/biological educations.

Loss of sight is one of the most severe problems that degrade the quality of human life, rather than any other disorders. Various methods for restoring the vision have been studied. Among them, electronic prosthetic devices for the visually-impaired person allows the individual to recognize the phase and movement of an object. It is based on the principle of converting an external light signal into an electric signal that is transmitted to the human optic nerve, which allows a person to perceive the electric signal as light. This retinal prosthesis has undergone considerable development over the last few decades; however, there are several challenges to be solved. First, the resolution is too low to visualize the information from the outside to the human eye. This problem is mainly originated from the high impedance of the electrodes. Second, the mechanical modulus of the material within the device is a mismatch to the modulus of the retina tissues, followed by severe inflammation and permanent damage to the retina. Finally, since this device is planned to be implanted on the human eye, the reliable operation is important.
Here, we presented the high-resolution, flexible retinal prosthesis based on three-dimensional (3D) soft stimulation electrodes for blind people. The high-resolution phototransistor array with a stimulation electrode allows the patient to perceive the image clearly. 3D structure of the electrode reduces the impedance of each pixel so that the single-pixel can be integrated in high density without any electrical breakdown. Since we formed 3D electrodes using soft, self-healable materials, this stimulation electrode hardly damages the retina surface and easily restore their electrical characteristics under various mechanical deformations, suggesting the reliable operation of the implanted retinal prosthesis. Furthermore, in-vitro and in-vivo animal experiments using mouse and rabbit provided enough biocompatibility for the human trial. This high-resolution, flexible retinal prosthetic device provides a substantial chance to step-up for next-generation implantable electronic devices.

The therapeutic potential of mesenchymal stem cells (MSCs) in regenerative medicine is governed by multiple factors. Recruitment of MSCs to the damaged tissue, where they differentiate into the local components and release the bioactive substances, is a crucial step to initiate and support tissue regeneration.^[1] Stimulating MSC migration via biochemical approaches or genetic modification may not yet be fully feasible for therapeutic applications in terms of controllability and safety. Therefore, regulating MSC migration with pure physical cues presents attractiveness. In this study, MSC migration responding to thermal and mechanical stimuli was investigated using programmable shape-memory polymer actuator (SMPA) sheets. Changing the temperature repetitively between 10 and 37 °C, the SMPA sheets are capable of reversibly changing between two different pre-defined shapes like an artificial muscle.^[2] After the attachment of MSCs on the SMPA sheets, cell movement was recorded using a time-lapse microscope. Compared to non-actuating control sheets, the MSCs cultured on the programmed actuating sheets presented a higher migration velocity (0.45 ± 0.17 vs. 0.58 ± 0.17 µm/min). SMPA sheets can be applied as a platform to study the combinatorial and synergetic effects of thermal and mechanical stimuli as well as their interplay, which helps to understand the orchestration of microenvironmental complexity for regulating stem cell migration. The use of physical cues to increase the homing capacity of MSCs presents a promising approach to improve their therapeutic potential in regenerative medicine.

References:
[1] X. Fu, G. Liu, A. Halim, Y. Ju, Q. Luo, A.G. Song, Mesenchymal Stem Cell Migration and Tissue Repair, Cells, 8 (2019) DOI: 10.3390/cells8080784.
[2] Z. Deng, W. Wang, X. Xu, O.E.C. Gould, K. Kratz, N. Ma, A. Lendlein, Polymeric sheet actuators with programmable bioinstructivity, Proc Natl Acad Sci U S A, (2020) DOI: 10.1073/pnas.1910668117.

With the growing market for wearable and implantable electronics, strain sensors with exceptional performance in sensitivity, stretchability, biocompatibility, and tough adhesion between human/ machine interfaces have become essential. Herein, a novel graphene-ions based strain sensor, whose structure contains a carefully selected graphene foam (GF) and polyacrylamide/calcium-alginate (PAM/CA) double network hydrogel coupled with chitosan, can meet the all above requirements to the maximum extent. Taking the advantage of the excellent electrical properties of graphene and super mechanical properties of double network hydrogel, this strain sensor can measure an exceptionally wide range of strain up to 500%, which allows us to monitor and distinguish almost all human body motions, with a gauge factor of ~1800, which is up to 3 orders of magnitude larger than the 0.1–10 value reported previously. The device also shows high stability and excellent durability for 1000 cycles. Most importantly, the chitosan-bridge and skin-like elastic modulus of 8–90 kPa achieve conformal adhesion between the skin and sensor, with the adhesion energy of ~200 Jm^-2. Moreover, this structured sensor is highly biocompatible and can be implanted in vivo, and has further potential in clinical applications.

Advances in the fabrication and characterization of polymeric nanomaterials has greatly advanced the miniaturization of soft actuators, creating materials capable of replicating the functional physical behavior previously limited to the macroscale. Techniques such as emulsion polymerization and electrospinning have enabled the realization of nano and microscale objects whose shape change can be controlled by ensemble programming methods to realize one-way shape-memory.¹ Previously, we reported that a universal atomic force microscopy (AFM) test platform enables the implementation and quantification of a thermomechanically induced one-way shape memory effect for individual polymeric micro- and nanosystems.² Here, we demonstrate how a reversible shape-memory polymer actuation can be generated in a single micro/nano object, where the shape change during actuation of an individual fiber can be dictated by programming using AFM.
In this work, electrospinning was used to prepare poly(ε-caprolactone) micro/nano fibers, which were fixed and crosslinked on a structured silicon wafer. Using AFM, the programming as well as the observation of recovery and reversible displacement of the fiber were performed by vertical three point bending. A plateau tip was utilized to improve the stability of the fiber contact and working distance, enabling larger deformations and greater rbSMPA performance. Programming strains of 39 ± 1% or 46 ± 1% were realized for fibers with diameters of 1 ± 0.2 µm and 300 ± 50 nm, which were deformed at 80 °C and fixed at 10 °C. Values for the reversible elongation of εrev = 3.4 ± 0.1% and 10.5 ± 0.1% were obtained for a single micro and nanofiber in cyclic testing between the temperatures 10 and 60 °C. The reversible actuation of the nanofiber was successfully characterized for 10 cycles. An actuation induced pulling force of 0.2 ± 0.01 nN and pushing force of 0.5 ± 0.1 nN were measured on the tip. The demonstration and characterization of individual shape-memory nano and microfibers represents an important step in the creation of miniaturized robotic devices capable of performing complex physical functions at the length scale of cells and proteins.

References:
1.Zhang, Q. C.; Rudolph, T.; Benitez, A. J.; Gould, O. E. C.; Behl, M.; Kratz, K.; Lendlein, A. Temperature-controlled reversible pore size change of electrospun fibrous shape-memory polymer actuator based meshes. Smart Mater Struct 2019, 28 (5), 055037.
2.Fang, L.; Gould, O. E. C.; Lysyakova, L.; Jiang, Y.; Sauter, T.; Frank, O.; Becker, T.; Schossig, M.; Kratz, K.; Lendlein, A. Implementing and Quantifying the Shape-Memory Effect of Single Polymeric Micro/Nanowires with an Atomic Force Microscope. ChemPhysChem 2018, 19 (16), 2078-2084.