E-skin Devices Research Articles

Electronic Skin: Opportunities and Challenges in Convergence with Machine Learning.

Recent advancements in soft electronic skin (e-skin) have led to the development of human-like devices that reproduce the skin's functions and physical attributes. These devices are being explored for applications in robotic prostheses as well as for collecting biopotentials for disease diagnosis and treatment, as exemplified by biomedical e-skins. More recently, machine learning (ML) has been utilized to enhance device control accuracy and data processing efficiency. The convergence of e-skin technologies with ML is promoting their translation into clinical practice, especially in healthcare. This review highlights the latest developments in ML-reinforced e-skin devices for robotic prostheses and biomedical instrumentations. We first describe technological breakthroughs in state-of-the-art e-skin devices, emphasizing technologies that achieve skin-like properties. We then introduce ML methods adopted for control optimization and pattern recognition, followed by practical applications that converge the two technologies. Lastly, we briefly discuss the challenges this interdisciplinary research encounters in its clinical and industrial transition.

A skin-integrated device for neck posture monitoring and correction

Cervical spondylosis is a common disease that is often caused by long-term abnormal cervical curvature due to activities such as reading books and using computers or smartphones. This paper explores building an untethered and skin-integrated device in an e-skin form factor to monitor and haptically correct neck posture. The proposed design features a multilayered structure that integrates all flexible electronic circuits and components into a compact skin space while being untethered and skin conformal. An accelerometer in the e-skin attaches to the neck for posture sensing, while four vibration actuators closely touch the neck skin to provide localized vibrotactile stimuli that encode four-direction correction cues of neck flexion pm alpha and lateral bending pm beta. To ensure the reliability of posture sensing and vibrotactile rendering during neck movement, it is necessary to prevent the e-skin device from shifting position. Thus, a hollow structure-based method is implemented for stably attaching the e-skin to the neck skin. Experiments validated the e-skin device’s sensing precision, skin-conformal compliance, stickiness, stability and effectiveness during the motion of neck postures, including its discrimination of localized four-direction vibrotactile cues. A user study verified the device’s performance for sensing and correcting different abnormal neck postures during activities such as using smartphones, reading books, and processing computer files. The proposed e-skin device may create opportunities for more convenient cervical spondylosis prevention and rehabilitation.

A 3D cross-linked hierarchical hydrogel E-skin with sensing of touch position and pressure

Artificial hydrogel electronic skin (E-skin) sensing systems have received considerable attention due to their ability in providing tactile perception. Recent advancements in hydrogel E-skin have allowed for simultaneous sensing of touch position and pressure through construction of sensor arrays. However, the fabrication process is complex, and the sensing resolution is limited by the size and number of array cells. Here, we report a multi-modal sensory E-skin based on a polyvinyl alcohol (PVA) hydrogel embedded with carbon nano-materials, which can recognize touch position and pressure simultaneously without constructing arrays. Combining the advantages of high electron transfer efficiency of 1D nanotube and 2D graphene nanosheet, the 3D hierarchical cross-linked polydopamine (PDA) modified reduced graphene oxide (rGO)/carboxylic multiwalled carbon nanotube (MWCNT-COOH) composites were proposed as a conductive filler, which was added by nano-doping to hydrogel substrate to gain an elastic modulus and stretchability of the hydrogel similar to that of human skin. Meanwhile, a surface-capacitive touch (SCT) system and piezoresistive technology were adopted to sense touch position and pressure due to the stability of conductive networks and the piezoresistive properties of carbon nanomaterials. Our E-skin device demonstrates superior sensing performance and provides a new direction for designing novel E-skin sensor devices.

Innovations in Flexible Electronic Skin: Material, Structural and Applications

Flexible electronic skin (e-skin) has emerged as a promising technology for advanced sensing capabilities in applications such as robotics, prosthetics, and human-machine interfaces. The properties of e-skin devices hinge on the selection of appropriate materials and structures, such as sensitivity, mechanical flexibility, and biocompatibility. This article provides an overview of the current state of e-skin research, focusing on the materials and structures used to create e-skin devices. Various materials were discussed in this paper, including conductive polymers, carbon nanotubes, graphene, bacterial cellulose, metal-organic frameworks, ionogels, and self-healing materials, highlighting their unique properties and potential applications in e-skin designs. Additionally, the structures and architectures of e-skin devices were examined, covering aspects such as multilayer designs, hybrid structures, and hierarchical configurations. This comprehensive review offers valuable insights into the development and optimization of e-skin materials and structures, paving the way for the creation of innovative, high-performance e-skin devices for various applications.

Flexible large e-skin array based on patterned laser-induced graphene for tactile perception

The emergence of flexible multifunctional electronic-skin (e-skin) has attracted worldwide interests in artificial intelligence, wearable devices, and health-monitoring systems. However, the drawbacks of the existing production process have presented significant challenges to the high-yield patterning of large-area array e-skins. Therefore, a honeycomb-patterned e-skin with laser-induced graphene (LIG) was fabricated by one-step carbonization of the polyimide film. Moreover, it exhibits strain sensitivity for both tensile and pressure stimulations in wide ranges. Based on the excellent mechanical properties and high stain sensitivity, this LIG e-skin is well demonstrated with perception capabilities of large-range human activities and subtle tactile. Specially designed to a matrixed mode, the e-skin device effectively senses the point-touch and sliding tactile on the human skin surface. Machine learning has further verified the feasibility of identifying areas of point-touch on the surface of the e-skin. Consequently, the LIG fabrication could facilitate carbon-based e-skins with patterning of the large-area array. Furthermore, this e-skin indicates excellent potential in universal tactile for robotics, artificial intelligence, human-machine interface, and personal medical equipment.

Super Tough and Spontaneous Water-Assisted Autonomous Self-Healing Elastomer for Underwater Wearable Electronics.

Self‐healing soft electronic material composition is crucial to sustain the device long‐term durability. The fabrication of self‐healing soft electronics exposed to high moisture environment is a significant challenge that has yet to be fully achieved. This paper presents the novel concept of a water‐assisted room‐temperature autonomous self‐healing mechanism based on synergistically dynamic covalent Schiff‐based imine bonds with hydrogen bonds. The supramolecular water‐assisted self‐healing polymer (WASHP) films possess rapid self‐healing kinetic behavior and high stretchability due to a reversible dissociation–association process. In comparison with the pristine room‐temperature self‐healing polymer, the WASHP demonstrates favorable mechanical performance at room temperature and a short self‐healing time of 1 h; furthermore, it achieves a tensile strain of 9050%, self‐healing efficiency of 95%, and toughness of 144.2 MJ m−3. As a proof of concept, a versatile WASHP‐based light‐emitting touch‐responsive device (WASHP‐LETD) and perovskite quantum dot (PeQD)‐based white LED backlight are designed. The WASHP‐LETD has favorable mechanical deformation performance under pressure, bending, and strain, whereas the WASHP‐PeQDs exhibit outstanding long‐term stability even over a period exceeding one year in a boiling water environment. This paper provides a mechanically robust approach for producing eco‐friendly, economical, and waterproof e‐skin device components.

Functional Materials and Devices for XR (VR/AR/MR) Applications

Functional Materials and Devices for XR (VR/AR/MR) Applications

Review: Sensors for Biosignal/Health Monitoring in Electronic Skin.

Skin is the largest sensory organ and receives information from external stimuli. Human body signals have been monitored using wearable devices, which are gradually being replaced by electronic skin (E-skin). We assessed the basic technologies from two points of view: sensing mechanism and material. Firstly, E-skins were fabricated using a tactile sensor. Secondly, E-skin sensors were composed of an active component performing actual functions and a flexible component that served as a substrate. Based on the above fabrication processes, the technologies that need more development were introduced. All of these techniques, which achieve high performance in different ways, are covered briefly in this paper. We expect that patients’ quality of life can be improved by the application of E-skin devices, which represent an applied advanced technology for real-time bio- and health signal monitoring. The advanced E-skins are convenient and suitable to be applied in the fields of medicine, military and environmental monitoring.

3D Antidrying Antifreezing Artificial Skin Device with Self-Healing and Touch Sensing Capability.

Hydrogels are attractive, active materials for various e-skin devices based on their unique functionalities such as flexibility and biocompatibility. Still, e-skin devices are generally limited to simple structures, and the realization of optimal-shaped 3D e-skin devices for target applications is an intriguing issue of interest. Furthermore, hydrogels intrinsically suffer from drying and freezing issues in operational capability for practical applications. Herein, 3D artificial skin devices are demonstrated with highly improved device stability. The devices are fabricated in a target-oriented 3D structure by extrusion-based 3D printing, spontaneously heal mechanical damage, and enable stable device operation over time and under freezing conditions. Based on the material design to improve drying and freezing resistance, an organohydrogel, prepared by solvent displacement of hydrogel with ethylene glycol for 3 h, exhibits excellent drying resistance over 1000 h and improved freezing resistance by showing no phase transition down to -60°C while maintaining its self-healing functionality. Based on the improved drying and freezing resistance, artificial skin devices in target-oriented optimal 3D structures are presented, which enable accurate positioning of touchpoints even on a complicated 3D structure stably over time and excellent operation at temperatures below 0°C without losing their flexibility.

Piezoresistive electronic skin based on diverse bionic microstructure

With the introduction of micro nano structure, the performance of electronic skin (E-skin), as a flexible pressure sensor, has been significantly improved. However, most of the pre templates for constructing micro nano structure are fabricated by photolithography, which endows them with the limitation of surface microstructure and the difficulty of forming complex multi-level structures. Distinguished from the artificial micro-structured silicon template, there are abundant microstructure distributed on the surface of natural plant leaves for us to choose. The diversity of plant leaves provides more possibility for us to explore the internal relationship between the microstructure distributed on electrode surface and the performance of E-skin device. Herein, four kinds of plant leaves are selected as templates to fabricate E-skins with different bionic microstructures. The various sensing performances of four kinds of devices are evaluated through the real-time monitoring of current signal, and the device based on nerium oleander template exhibits the most outstanding performance (sensing range: 0–0.8 kPa, sensitivity: 8.5 kPa−1, detection limit: 0.001 kPa). Furthermore, quantitative indicators (radius/height and density) for the surface microstructure of E-skin are proposed to explain the performance differences between these devices. The device with fine (small values of radius/height) and high-density microstructure trends to exhibit high sensitivity and low detection limit as well as good durability. Additionally, the activating phenomenon of the E-skin during repeated compression was observed and the underlying mechanism was discussed. Finally, the E-skin based on nerium oleander template has been successfully employed to detect the subtle distinction of physiological signals in a series of fitness actions.

A self-healable, stretchable, tear-resistant and sticky elastomer enabled by a facile polymer blends strategy

A high-stretchability, high-strength, tear-resistant, self-healing and adhesive elastomer is prepared through a facile and effective physical blending strategy. The elastomer shows potential applications in e-skin devices.

Extremely flexible and mechanically durable planar supercapacitors: High energy density and low-cost power source for E-skin electronics

The development of multifunctional devices on highly flexible and biocompatible substrates has received great attention in the field of wearable and healthcare technologies. To realize such a unique technology typically on a single platform, it is very crucial to adopt highly-flexible and embeddable energy storage devices with adequate efficiencies as power sources. In this direction, we introduce a versatile and scalable approach to fabricate planar and symmetric micro-supercapacitors, which can be mounted on any complicated surface. The micro-supercapacitors developed on highly flexible and ultrathin substrates by the angular spray of graphene-ink showed excellent performance with a maximum areal capacitance of ~8.38 mF/cm 2 and operating flexibility for a bending radius of 1.8 mm. The devices also displayed outstanding mechanical stability for 10,000 bending cycles with a high specific capacitance (~22 F/g) and power density (~1.13 kW/kg) typically at a scan rate of 100 mV/s. Further, the demonstrations on skin-mountable and wrappable characteristics of mSCs emphasized their adaptability as embeddable power sources for various epidermal and wearable devices. • Micro-supercapacitors fabricated using cost-effective spray method. • Device exhibits high energy density and excellent sustainability. • Extremely flexible micro-supercapacitor showed high mechanical durability and stability. • Our device developed on epidermal substrate showed integrability with human skin. • This micro-supercapacitors can greatly support for e-skin device as a power source.

Artificial Tactile Perceptual Neuron with Nociceptive and Pressure Decoding Abilities.

The neural system is a multifunctional perceptual learning system. Our brain can perceive different kinds of information to form senses, including touch, sight, hearing, and so on. Mimicking such perceptual learning systems is critical for neuromorphic platform applications. Here, an artificial tactile perceptual neuron is realized by utilizing electronic skins (E-skin) with oxide neuromorphic transistors, and this artificial tactile perceptual neuron successfully simulates biological tactile afferent nerves. First, the E-skin device is constructed using microstructured polydimethylsiloxane membranes coated with Ag/indium tin oxide (ITO) layers, exhibiting good sensitivities of ∼2.1 kPa-1 and fast response time of tens of milliseconds. Then, the chitosan-based electrolyte-gated ITO neuromorphic transistor is fabricated and exhibits high performance and synaptic responses. Finally, the integrated artificial tactile perceptual neuron demonstrates pressure excitatory postsynaptic current and paired-pulse facilitation. The artificial tactile perceptual neuron is featured with low energy consumption as low as ∼0.7 nJ. Moreover, it can mimic acute and chronic pain and nociceptive characteristics of allodynia and hyperalgesia in biological nociceptors. Interestingly, the artificial tactile perceptual neuron can employ "Morse code" pressure-interpreting scheme. This simple and low-cost approach has excellent potential for applications including but not limited to intelligent humanoid robots and replacement neuroprosthetics.

Flexible and stretchable metal\xa0oxide nanofiber networks for multimodal and monolithically integrated wearable electronics

Fiber-based electronics enabling lightweight and mechanically flexible/stretchable functions are desirable for numerous e-textile/e-skin optoelectronic applications. These wearable devices require low-cost manufacturing, high reliability, multifunctionality and long-term stability. Here, we report the preparation of representative classes of 3D-inorganic nanofiber network (FN) films by a blow-spinning technique, including semiconducting indium-gallium-zinc oxide (IGZO) and copper oxide, as well as conducting indium-tin oxide and copper metal. Specifically, thin-film transistors based on IGZO FN exhibit negligible performance degradation after one thousand bending cycles and exceptional room-temperature gas sensing performance. Owing to their great stretchability, these metal oxide FNs can be laminated/embedded on/into elastomers, yielding multifunctional single-sensing resistors as well as fully monolithically integrated e-skin devices. These can detect and differentiate multiple stimuli including analytes, light, strain, pressure, temperature, humidity, body movement, and respiratory functions. All of these FN-based devices exhibit excellent sensitivity, response time, and detection limits, making them promising candidates for versatile wearable electronics.

Three-Dimensional Self-Healable Touch Sensing Artificial Skin Device.

Human skin is a unique functional material that perfectly covers body parts having various complicated shapes, spontaneously heals mechanical damage, and senses a touch. E-skin devices have been actively researched, focusing on the sensing functionality of skin. However, most e-skin devices still have limitations in their shapes, and it is a challenging issue of interest to realize multiple functionalities in one device as human skin does. Here, new artificial skin devices are demonstrated in application-oriented three-dimensional (3D) shapes, which can sense exact touch location and heal mechanical damage spontaneously. Beyond the conventional film-type e-skin devices, the artificial skin devices are fabricated in optimal three-dimensional structures, via systematic material design and characterization of ion-conductive self-healing hydrogel system and its extrusion-based 3D printing. The ring-shaped and fingertip-shaped artificial skin devices are successfully fabricated to fit perfectly on finger models, and shows large electronic signal contrast, ∼5.4 times increase in current, upon a human finger contact. Furthermore, like human skin, the device provides the exact positional information of an arbitrary touch location on a three-dimensional artificial skin device without complicated device fabrication or data processing.

Stretchable energy storage E-skin supercapacitors and body movement sensors

Electronic skin (e-skin) with natural stimuli is a key factor in prosthetics, robotics, and wearable electronics. At present, most traditional stretchable e-skins fabricated by predesigning complex 3D structures are unsuitable in commercial applications. Energy storage devices with stretchable and arbitrary shapes can widely adapt to wearable electronics. Stretchable supercapacitors and sensors have been used as the primary energy supplies in prosthetics, robotics, and wearable electronics in recent years, because of their superior power density and long calendar life. Here, stretchable energy storage e-skin supercapacitors and sensors were fabricated using two-sublayered silver nanowire (AgNW)/MnO2NW (MNW) hybrid conductive networks fixed into the polydimethylsiloxane (PDMS) layer and sandwiched using AgNW/MNW film electrodes and PVA–KOH solid electrolyte. The obtained e-skin device showed large areal specific capacitance, excellent capacity retention, coulombic efficiency of 2000 cycles, and high energy density. The multifunctional e-skin sensors showed an areal specific capacitance of 371 m F cm−2 at a current density of 1 mA cm−2, extended to 160 %, and had superior sensitivity including stretching and bending by hand. Therefore, the multifunctional e-skin sensors are suitable in robotics, prosthetics, and other stretchable wearable devices.

A Multi-Functional Physiological Hybrid-Sensing E-Skin Integrated Interface for Wearable IoT Applications.

This paper presents a flexible multi-functional physiological sensing system that provides multiple noise-immune readout architectures and hybrid-sensing capability with an analog pre-processing scheme. The proposed multi-functional system is designed to support five physiological detection methodologies of piezo-resistive, pyro-resistive, electro-metric, opto-metric and their hybrid, utilizing an in-house multi-functional e-skin device, in-house flexible electrodes and a LED-photodiode pair. For their functional verification, eight representative physiological detection capabilities were demonstrated using wearable device prototypes. Especially, the hybrid detection method includes an innovative continuous measurement of blood pressure (BP) while most previous wearable devices are not ready for it. Moreover, for effective implementation in the form of the wearable device, post-processing burden of the hybrid method was much reduced by integrating a proposed analog pre-processing scheme, where only simple counting process and calibration remain to estimate the BP. This multi-functional sensor readout circuits and their hybrid-sensing interface are fully integrated into a single readout integrated circuit (ROIC), which is designed to implement three readout paths: two electrometric readout paths and one impedometric readout path. For noise-immune detection of the e-skin sensor, a pseudo-differential front-end with a ripple reduction loop is proposed in the impedometric readout path, and also state-of-the-art body-oriented noise reduction techniques are adopted for the electrometric readout path. The ROIC is fabricated in a CMOS process and in-house e-skin devices and flexible electrodes are also fabricated.

Body Heat Powers Future Electronic Skins

Ruoming Tian recently joined the Mark Wainwright Analytical Centre as an XRD research assistant at the University of New South Wales. She was formerly a Research Scientist at the Toyota Physical and Chemical Research Institute in Japan (2015–2018) and received her PhD from the University of New South Wales in 2014. Her research interests focus on the nano- and micro-materials, flexible hybrid materials, and smart devices for harvesting both high- and low-grade thermal energy. She has published 2 book chapters and 18 peer-reviewed articles and delivered more than 15 talks at international conferences and leading research institutes. Yuqing Liu is currently an Associate Research Fellow and received her PhD (2018) in the Intelligent Polymer Research Institute (IPRI), University of Wollongong, Australia. Her research interest is on the fabrication of wearable energy storage and conversion devices (e.g. thermos-cells, batteries, supercapacitors, etc.), mainly focusing on nanostructured electrode materials, gel electrolyte/electrode interface, and device fabrication via printing techniques. Kunihito Koumoto, Professor Emeritus of Nagoya University, received his BS, MS, and PhD from the University of Tokyo (UT). He served as an Assistant Professor, Lecturer, and Associate Professor at UT (1979–1992) and a full professor at Nagoya University (1992–2015). He has published 495 papers and reviews and 58 book chapters in inorganic materials chemistry. He was awarded Medal with Purple Ribbon by the Emperor of Japan in 2013 and Outstanding Achievement Award by the International Thermoelectric Society in 2018. His current research focuses on inorganic/organic nanohybrid thermoelectrics for energy harvesting. Koumoto can be reached by email at koumoto@apchem.nagoya-u.ac.jp . Jun Chen is currently a Full Professor at IPRI|ACES, Australian Institute for Innovative Materials, University of Wollongong (UOW). He received BE from Zhejiang University of Technology (1995) and PhD from UOW (2003). Professor Chen has published more than 180 papers and has been named a Highly Cited Researcher for 2018 (Clarivate Analytics). His research interests include sustainable energy devices, electro-/bio-interfaces, nano-/micro-materials, 2D/3D printing, and smart wearable electronic devices. Apart from academic achievements, Chen has also been involved in two spin-off companies: AQUAHYDREX as one of the key inventors, and IMAGINE as one of the scientific founders.

Self-powered artificial synapses actuated by triboelectric nanogenerator

Biological synapses were the basic structure for information transmit, which also perform key computation functions in neural circuits. Such functionality inspires artificial intelligence to mimic human brain, which has 1015 synapses and consumed only ∼1–100 fJ per synaptic event during massively parallel information processing. It thus indicates that artificial synapse devices with low power consumption are highly desired to mimic human brain. In this work, we developed a distinctive structure of self-powered synapse transistor to emulate synapse functions, and the voltage was provided by a triboelectric nanogenerator (TENG) without using additional voltage to generate pre-synapse spike. Moreover, TENG could simultaneously function as tactile sensor to realize self-powered tactile synapse with simple device structure. The essential synaptic functions such as excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), dynamic filtering, and short-term plasticity (STP) to long-term plasticity (LTP) were demonstrated in the self-powered synapse transistor devices by touching the TENG. Moreover, the logic gate “AND”, and “OR” were achieved using TENGs as multiple pre-synaptic, and tactile study was also mimicked. The integration of TENG and synapse transistor device has great potentials in the applications of novel e-skin devices for artificial intelligence and human-computer interaction.

Highly sensitive flexible strain sensor based on threadlike spandex substrate coating with conductive nanocomposites for wearable electronic skin

Electronic skin (e-skin) with favorable wearability and high flexibility has increasingly raised attention for potential research prospect in artificial skin, flexible electronics, soft robotics and prosthetic device. A flexible resistive strain sensor with layered structure made up of Graphene nanoplatelets (GNPs) layer and GNPs/polyaniline (PANI) synergetic conductive network based on threadlike spandex substrate is successfully fabricated by a simple layered dipping-drying process. Herein, the fiber-based strain sensors with high sensitivity (the gauge factor calculated at 0%–15% and 15%–30% is ∼6.8 and ∼60.3, respectively), lower detection limit, long-term stability and durability (∼1000 cycles), fine repeatability and low temperature dependence was prepared through designing layer-by-layer structure and synergetic conductive network. And the biocompatible and breathable e-skin device could be prepared by integrating the sensor with semipermeable membrane. The fabricated device can be utilized in detecting small scale human motions, including muscle contraction, relaxation and respiratory movement in different states. Furthermore, a simple facial expression sensing and monitoring application was realized. The spandex-based flexible strain sensor with favorable performance and facile preparation procedure will open potential prospects for wearable e-skin in human motion detecting, physiological sensing and emotional monitoring.