E-skin Systems Research Articles

Recent progress in self-powered multifunctional e-skin for advanced applications.

Electronic skin (e-skin), new generation of flexible wearable electronic devices, has characteristics including flexibility, thinness, biocompatibility with broad application prospects, and a crucial place in future wearable electronics. With the increasing demand for wearable sensor systems, the realization of multifunctional e-skin with low power consumption or even autonomous energy is urgently needed. The latest progress of multifunctional self-powered e-skin for applications in physiological health, human-machine interaction (HMI), virtual reality (VR), and artificial intelligence (AI) is presented here. Various energy conversion effects for the driving energy problem of multifunctional e-skin are summarized. An overview of various types of self-powered e-skins, including single-effect e-skins and multifunctional coupling-effects e-skin systems is provided, where the aspects of material preparation, device assembly, and output signal analysis of the self-powered multifunctional e-skin are described. In the end, the existing problems and prospects in this field are also discussed.

Fiber-Based Electret Nanogenerator with a Semisupported Structure for Wearable Electronics

Fiber-based nanogenerators have great potential applications in wearable electronics such as portable nanodevices, e-skin, and artificial intelligence system. Here, we report a kind of fiber-based electret nanogenerator (FENG) with a semisupported core-shell structure. Owing to its unique structure, the open-circuit voltage and short-circuit current of the FENG reach 40 V and 0.6 μA, respectively, under a short working distance (∼25 μm). No obvious degradation of the output performance under a long-time continuous work (>16 h) and different humidity environments (20-95%) is observed, which demonstrates the FENG's good reliability and stability. Many universal materials, such as cotton rope, conductive sewing thread, and polyvinyl chloride tube, have been successfully used to fabricate FENG. Meanwhile, the FENG-based wearable fabric has been successfully developed to effectively harvest mechanical energy of human motion. The FENG is highly effective, reliable, and stable, promoting the development of fiber-based nanogenerators and their applications in self-powered wearable electronics.

E-Skin: The Dawn of a New Era of On-Body Monitoring Systems.

Real-time “on-body” monitoring of human physiological signals through wearable systems developed on flexible substrates (e-skin) is the next target in human health control and prevention, while an alternative to bulky diagnostic devices routinely used in clinics. The present work summarizes the recent trends in the development of e-skin systems. Firstly, we revised the material development for e-skin systems. Secondly, aspects related to fabrication techniques were presented. Next, the main applications of e-skin systems in monitoring, such as temperature, pulse, and other bio-electric signals related to health status, were analyzed. Finally, aspects regarding the power supply and signal processing were discussed. The special features of e-skin as identified contribute clearly to the developing potential as in situ diagnostic tool for further implementation in clinical practice at patient personal levels.

Visual and instrumental color evaluation of computerized color matching system for color reproduction of maxillofacial prostheses

Statement of problemA commercially available computerized color matching system (e-Skin) has been introduced for maxillofacial prostheses. However, studies that have tested its ability to produce an acceptable color match for skin color are lacking. PurposeThe purpose of this clinical study was to determine the color match acceptability of light and dark skin silicone replicas fabricated with the use of the computerized color matching system. Material and methodsFifteen participants with light skin and 15 participants with dark skin were recruited through personal invitation. These 30 participants (aged around 20 years) voluntarily consented to forearm skin color measurements with a spectrophotometer and a spectrocolorimeter integrated in a computerized color matching system. Silicone skin replicas for each participant were produced from the color formulations provided by the online calculator tool of the system according to the manufacturers’ recommendations. The color difference between initial skin color measurements with the spectrophotometer and skin replica color measurements with the spectrophotometer for all participants was calculated by using both the CIELab (ΔE∗ab) and CIEDE2000 (ΔE00) color difference formulas. To compare the instrumental and visual evaluation of color match, 3 observers (a maxillofacial prosthodontist, a prosthodontist, and a postgraduate student) visually evaluated and rated the color match of each silicone replica on a 5-point scale. Statistical analyses were performed by using the Student t, Mann Whitney U, and chi-square tests (α=.05). ResultsNo significant differences were detected between the light and dark skin groups in terms of the ΔE∗ab and ΔE00 values obtained between the first skin color measurements and their silicone replica color measurements (P=.573, P=.338, respectively). However, a significant difference was found between light and dark skin groups for the ΔL and Δa values obtained from the same measurements (P=.031, P=.017, respectively). L∗, a∗, and b∗ values of light skin were significantly higher than those of their silicone replications (P=.008, P=.033, P=.046, respectively). Color match ratings of observers did not significantly differ when comparing the ratings of light skin replicas to ratings of dark skin replicas. ConclusionsThe e-Skin system provided skin replicas that matched the skin color within clinically acceptable thresholds and with ratings of at least “good” by experienced dental clinicians.

Color Stability and Mechanical Properties of Two Commonly Used Silicone Elastomers with e-Skin and Reality Coloring Systems.

To evaluate the color stability and mechanical properties of two commonly used maxillofacial silicone elastomers after addition of pigments and opacifiers and before and after artificial aging. This study evaluated two maxillofacial silicone elastomers: A-2000 and M511. Two different pigment and opacifier systems (e-Skin and Reality Series) were used with the elastomers. Control groups (no pigment or opacifier) and experimental groups (each with subgroups containing additional pigments and/ or opacifiers) were fabricated for each of the silicone elastomers. A total of 51 specimens were evaluated for color stability, and 100 for mechanical properties. A spectrophotometer was used to assess CIE L*a*b* values before and after aging. CIELAB 50:50% perceptibility threshold (ΔE* = 1.1) and acceptability threshold (ΔE* = 3.0) were used to interpret color changes. A durometer and universal testing machine were used to evaluate the mechanical properties. ANOVA and Fisher least significant difference (LSD) test were performed to determine the statistical significance of the results (P < .05). Significant differences in color measurements (ΔE*) were found for all silicone groups following artificial aging (P < .05). ΔE* values for the mixed pigment/opacifier subgroups of both elastomers were below the perceptibility threshold. Additionally, after aging, the hardness, tear strength, and tensile strength significantly increased for all silicone groups (P < .05), while percent elongation significantly decreased (P < .05). Artificial aging affected the color stability and mechanical properties of the pigmented silicone elastomers with added opacifier. Overall, A-2000 with e-Skin group displayed the most color stability, with its mechanical properties being the least affected by artificial aging.

Reduced Graphene Oxide-Polypyrrole Aerogel-Based Coaxial Heterogeneous Microfiber Enables Ultrasensitive Pressure Monitoring of Living Organisms.

Pressure sensors for living organisms can monitor both the movement behavior of the organism and pressure changes of the organ, and they have vast perspectives for the health management information platform and disease diagnostics/treatment through the micropressure changes of organs. Although pressure sensors have been widely integrated with e-skin or other wearable systems for health monitoring, they have not been approved for comprehensive surveillance and monitoring of living organisms due to their unsatisfied sensing performance. To solve the problem, here, we introduce a novel structural design strategy to manufacture reduced graphene oxide-polypyrrole aerogel-based microfibers with a typical coaxial heterogeneous structure, which significantly enhances the sensitivity, resolution, and stability of the derived pressure microsensors. The as-fabricated pressure microsensors exhibit ultrahigh sensitivities of 12.84, 18.27, and 4.46 kPa-1 in the pressure ranges of 0-20, 20-40, and 40-65 Pa, respectively, high resolution (0.2 Pa), and good stability in 450 cycles. Furthermore, the microsensor is applied to detect the movement behavior and organic micropressure changes for mice and serves as a platform for monitoring micropressure for the integrative diagnosis both in vivo and in vitro of organisms.

Artificial Skin Perception.

Skin is the largest organ, with the functionalities of protection, regulation, and sensation. The emulation of human skin via flexible and stretchable electronics gives rise to electronic skin (e-skin), which has realized artificial sensation and other functions that cannot be achieved by conventional electronics. To date, tremendous progress has been made in data acquisition and transmission for e-skin systems, while the implementation of perception within systems, that is, sensory data processing, is still in its infancy. Integrating the perception functionality into a flexible and stretchable sensing system, namely artificial skin perception, is critical to endow current e-skin systems with higher intelligence. Here, recent progress in the design and fabrication of artificial skin perception devices and systems is summarized, and challenges and prospects are discussed. The strategies for implementing artificial skin perception utilize either conventional silicon-based circuits or novel flexible computing devices such as memristive devices and synaptic transistors, which enable artificial skin to surpass human skin, with a distributed, low-latency, and energy-efficient information-processing ability. In future, artificial skin perception would be a new enabling technology to construct next-generation intelligent electronic devices and systems for advanced applications, such as robotic surgery, rehabilitation, and prosthetics.

Capacitive pressure sensor inlaid a porous dielectric layer of superelastic polydimethylsiloxane in conductive fabrics for detection of human motions

Flexible, sensitive and largescale normal pressure sensors are in high demand of rapid development of healthcare and e-skin systems. In this paper, a new capacitive normal pressure sensor was fabricated based on a dielectric layer of superelastic polydimethylsiloxane (PDMS) with uniformly distributed micro-pores. The micro-pores were formed through vacuum-assisted infiltration of PDMS solution using sugar particles as porogen. Results showed that the PDMS layer with uniform micro-pores without sugar residue was successfully obtained. The developed sensor experimentally displayed a high elasticity, a large sensing pressure range (> 200 kPa), a high sensitivity (0.023 kPa−1), a good hysteresis, a fast response (about 155 ms) and a good structural stability and durability for using more than 1000 cycles. Demonstrations of the fabricated sensor in detecting finger grabbing and plantar pressure were performed, indicating it a high potential in the future use of e-skin or rehabilitation monitoring system.

Biopower-on-Skin: Electricity generation from sweat-eating bacteria for self-powered E-Skins

Abstract Electronic skins or ‘e-skins’ have recently emerged as a novel platform for electronics, taking on more important roles in health diagnostics, therapeutics, and monitoring. Stand-alone and self-sustained e-skins are essential to providing reliable, effective and sometimes life-saving functions. A stable power supply is the most critical factor in developing practical e-skins because their performance depends significantly on power availability. A realistic and accessible power source is urgently needed for the next generation of smart, stand-alone, always-on e-skin systems. This is by no means a simple challenge because intimately integrating technologically advanced e-skins with human skin requires generating power from an extremely complicated, harsh environment. Skin is cool, dry, acidic and has few potential energy sources. In this work, we create the ability to generate an innovative, practical, and longstanding power from human sweat, which is one of the few available energy resources on skin. We use the metabolisms of sweat-eating bacteria that exist on human skin, Staphylococcus epidermidis, Staphylococcus capitis, and Micrococcus luteus. We also test a microorganism, Nitrosomonas europaea, that oxidizes the ammonia in sweat, but is not usually found on the skin. Bioelectrogenesis from all the bacterial species were observed at levels comparable to that of a well-known wild-type electrogenic bacteria, Shewanella oneidensis MR1, which inhabits the natural environment. A biobattery, or a microbial fuel cell, that uses bacteria as a biocatalyst, transforms the chemical energy of sweat into electrical power through bacterial metabolism. The skin-mountable biobattery is pre-inoculated with the sweat-eating bacteria and operates with human sweat, delivered by an integrated battery-free skin-interfaced microfluidic system. Given that the total non-human microbial cells inhabiting in and on our bodies outnumber the human cells by at least a factor of 10, the direct use of the microbial cells to produce power is conceivable for e-skins. The significance of this work stems from the demand for self-sustainable and stand-alone e-skin systems for healthcare, security, fitness, and environmental monitoring applications.

Design and Realization of an Efficient Large-Area Event-Driven E-Skin.

The sense of touch enables us to safely interact and control our contacts with our surroundings. Many technical systems and applications could profit from a similar type of sense. Yet, despite the emergence of e-skin systems covering more extensive areas, large-area realizations of e-skin effectively boosting applications are still rare. Recent advancements have improved the deployability and robustness of e-skin systems laying the basis for their scalability. However, the upscaling of e-skin systems introduces yet another challenge—the challenge of handling a large amount of heterogeneous tactile information with complex spatial relations between sensing points. We targeted this challenge and proposed an event-driven approach for large-area skin systems. While our previous works focused on the implementation and the experimental validation of the approach, this work now provides the consolidated foundations for realizing, designing, and understanding large-area event-driven e-skin systems for effective applications. This work homogenizes the different perspectives on event-driven systems and assesses the applicability of existing event-driven implementations in large-area skin systems. Additionally, we provide novel guidelines for tuning the novelty-threshold of event generators. Overall, this work develops a systematic approach towards realizing a flexible event-driven information handling system on standard computer systems for large-scale e-skin with detailed descriptions on the effective design of event generators and decoders. All designs and guidelines are validated by outlining their impacts on our implementations, and by consolidating various experimental results. The resulting system design for e-skin systems is scalable, efficient, flexible, and capable of handling large amounts of information without customized hardware. The system provides the feasibility of complex large-area tactile applications, for instance in robotics.

E-Skins: Biomimetic Sensing and Encoding for Upper Limb Prostheses

Prosthetic hands, today, have anthropomorphic, multifinger design. A common control method uses pattern recognition of electromyogram signals. However, these prostheses do not capture the human hand's sensory perception, which is critical for prosthesis embodiment and dexterous object manipulation. This problem can be solved by sensorized electronic skin (e-skin) composed of various sensors that transduce sensory percepts such as touch, pressure, temperature, and pain, just as human skin does. This review will present the physiology of the receptors that encode tactile, thermal, nociceptive, and proprioceptive information. The e-skin is designed to mimic these receptors and their responses. We review each sensor subtype, and its design and performance when embedded in the e-skin. Next, we review the spiking response of the receptors, which are then relayed to sensory nerves and encoded by the brain as sensory percepts. The e-skin system is designed to produce neuromorphic or receptorlike spiking activity. Computational models to mimic these sensory nerve signals are presented and then various methods to interface with the nervous system are explored and compared. We conclude the review with the state of the art in e-skin design and deployment in closed-loop applications that demonstrate the benefits of sensory feedback for amputees.

Materials and Design Strategies of Stretchable Electrodes for Electronic Skin and its Applications

The human skin is an essential organ of the human body for sensing various changes in the external environment, including pressure, temperature, and humidity, as well as for interacting with the outside world. Therefore, in the field of soft electronics, the development of the electronic skin (e-skin) that mimics the functions and properties of the human skin is an important research topic. The potential applications of the e-skin technology include wearable electronics, soft robotics, human–machine interfaces (HMIs), and prosthetic devices. Toward such research goals, various technologies are required. Among these, the stretchable electrode technology is a key component for achieving soft, stretchable e-skin and its applications. Therefore, various materials and design strategies for stretchable electrodes have been developed by many research groups. The successful development of stretchable electrodes has resulted in the rapid progress of high-performance stretchable device components such as stretchable sensors and actuators. Moreover, the integration of these device components with stretchable interconnects has yielded an integrated e-skin system, through which interactive human–machine interfacing can be achieved. This paper presents an overview of state-of-the-art technological advances in materials and design strategies for the development of stretchable electrodes. In addition, e-skin systems and their HMI applications based on such stretchable electrodes are briefly reviewed.

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.

Hybrid porous micro structured finger skin inspired self-powered electronic skin system for pressure sensing and sliding detection

Fingertip is the most sensitive region in human body due to the unique fingerprint patterns and interlocked structure between epidermal-dermal, which amplify the tactile stimuli and enhance the sensitivity. Inspired by the complicated anatomical structure, we fabricated a finger skin inspired e-skin system composed of fingerprint inspired triboelectric generator (TENG), epidermal-dermal inspired hybrid porous microstructure (HPMS) pressure sensor and subcutaneous fat inspired fabric based porous supercapacitor (FPSC). The TENG is responsible for detecting sliding direction and speed with the help of the four spiral electrodes, which adopt the frequency rather than the amplitude to detect the signal to avoid interfere from the environment. The HPMS, on the one hand, integrates the advantages from both the microstructure and porous structure to enhance the sensitivity further. On the other hand, the sensitivity of HPMS sensor is tunable by designing the shape and porosity of the HPMS, which is proved by theory, simulation and experiment. The FPSC, which can tolerate some degree of compression, works to supply energy for the pressure sensor. In this way, the sensor system can work independently without external battery. As a proof-of-concept demonstration, this sensor system has been used to detect complex action including pressure and sliding. During this process, the pressure and sliding direction and speed can be detected simultaneously without connecting to external energy source, showing its potential application area in soft robot and wearable devices.

Polyurethane foam coated with a multi-walled carbon nanotube/polyaniline nanocomposite for a skin-like stretchable array of multi-functional sensors

Body-attachable sensors can be applied to electronic skin (e-skin) as well as safety forewarning and health monitoring systems. However, achieving facile fabrication of high-performance, cost-effective sensors with mechanical stability in response to deformation due to body movement is challenging. Herein we report the material design, fabrication and characteristics of a skin-like stretchable array of multi-functional (MF) sensors based on a single sensing material of polyurethane foam coated with multi-walled carbon nanotube/polyaniline nanocomposite, which enables simultaneous detection of body temperature, wrist pulse and ammonia gas. These sensors exhibit high sensitivity, fast response and excellent durability. Furthermore, the fabricated sensor array shows stable performance under biaxial stretching up to 50% and attachment to skin owing to the use of direct-printed Galinstan liquid metal interconnections. This work proposes a facile method for fabrication of high-performance, stretchable MF sensors via appropriate selection of sensor design and functional materials that are applicable to e-skin and health monitoring systems.

Embedded Electronic System Based on Dedicated Hardware DSPs for Electronic Skin Implementation

The effort to develop an electronic skin is highly motivated by many application domains namely robotics, biomedical instrumentations, and replacement prosthetic devices. Several e-skin systems have been proposed recently and have demonstrated the need of an embedded electronic system for tactile data processing either to mimic the human skin or to respond to the application demands. Processing tactile data requires efficient methods to extract meaningful information from raw sensors data.In this framework, our goal is the development of a dedicated embedded electronic system for electronic skin. The embedded electronic system has to acquire the tactile data, process and extract structured information. Machine Learning (ML) represents an effective method for data analysis in many domains: it has recently demonstrated its effectiveness in processing tactile sensors data.This paper presents an embedded electronic system based on dedicated hardware implementation for electronic skin systems. It provides a Tensorial kernel function implementation for machine learning based on Tensorial kernel approach. Results assess the time latency and the hardware complexity for real time functionality. The implementation results highlight the high amount of power consumption needed for the input touch modalities classification task. Conclusions and future perspectives are also presented.

Towards integrating intelligence in electronic skin

Due to its very peculiar features, the development of e-skin can be effectively tackled using a holistic approach. Starting from the definition of system specifications, the mechanical arrangement of the skin itself needs to be designed and fabricated together with the electronic embedded system, to move towards aspects such as tactile data processing algorithms and the communication channel interface.In this paper we present the design, the implementation and the results on the way of the development of an electronic skin (e-skin) system based on arrays of piezopolymer transducers. Focus of the paper is on both the development of innovative approaches for tactile information processing and electronic system embedding into the e-skin structure. In particular, Machine Learning technologies can provide a powerful tool to tackle the pattern-recognition problems involved in the tactile sensing framework and the ability of processing data represented as N-th order tensor is the key aspect of the presented research, which can be seen as an application of an existing method (Signoretto et al., 2011). The experimental session compares two different implementations of the ML-based framework, which differ in the learning paradigm adopted, namely SVM and ELM (K-ELM). The effectiveness of the adopted pattern-recognition technologies in the classification of touch modalities has been confirmed by addressing two different binary classification problems in an experiment involving 70 participants. The computational requirements for the hardware implementation of the proposed algorithm together with an overview of what exists in the existing literature are finally discussed.