Electronic Skin Devices Research Articles

Skin Comfort Sensation with Mechanical Stimulus from Electronic Skin.

The field of electronic skin has received considerable attention due to its extensive potential applications in areas including tactile sensing and health monitoring. With the development of electronic skin devices, electronic skin can be attached to the surface of human skin for long-term health monitoring, which makes comfort an essential factor that cannot be ignored in the design of electronic skin. Therefore, this paper proposes an assessment method for evaluating the comfort of electronic skin based on neurodynamic analysis. The holistic analysis framework encompasses the mechanical model of the skin, the modified Hodgkin-Huxley model for the transduction of stimuli, and the gate control theory for the modulation and perception of pain sensation. The complete process, from mechanical stimulus to the generation of pain perception, is demonstrated. Furthermore, the influence of different factors on pain perception is investigated. Sensation and comfort diagrams are provided to assess the mechanical comfort of electronic skin. The comfort assessment method proposed in this paper provides a theoretical basis when assessing the comfort of electronic skin.

Intrinsically Stretchable Floating Gate Memory Transistors for Data Storage of Electronic Skin Devices.

To propel electronic skin (e-skin) to the next level by integrating artificial intelligence features with advanced sensory capabilities, it is imperative to develop stretchable memory device technology. A stretchable memory device for e-skin must offer, in particular, long-term data storage while ensuring the security of personal information under any type of deformation. However, despite the significance of these needs, technology related to stretchable memory devices remains in its infancy. Here, we report an intrinsically stretchable floating gate (FG) polymer memory transistor. The device features a dual-stimuli (optical and electrical) writing system to prevent easy erasure of recorded data. An FG comprising an intermixture of Ag nanoparticles and elastomer and with proper energy-band alignment between the semiconductor and dielectric facilitated sustainable memory performance, while achieving a high memory on/off ratio (>105) and a long retention time (106 s) with the ability to withstand 50% uniaxial or 30% biaxial strain. In addition, our memory transistor exhibited high mechanical durability over multiple stretching cycles (1000 times), along with excellent environmental stability with respect to factors such as temperature, moisture, air, and delamination. Finally, we fabricated a 7 × 7 active-matrix memory transistor array for personalized storage of e-skin data and successfully demonstrated its functionality.

Latest developments and trends in electronic skin devices

The skin, a vital medium for human-environment communication, stands as an indispensable and pivotal element in the realms of both production and daily life. As the landscape of science and technology undergoes gradual evolution and the demand for seamless human-machine interfaces continues to surge, an escalating need emerges for a counterpart to our biological skin - electronic skins (e-skins). Achieving high-performance sensing capabilities comparable to our skin has consistently posed a formidable challenge. In this article, we systematically outline fundamental strategies enabling e-skins with capabilities including strain sensing, pressure sensing, shear sensing, temperature sensing, humidity sensing, and self-healing. Subsequently, complex e-skin systems and current major applications were briefly introduced. We conclude by envisioning the future trajectory, anticipating continued advancements and transformative innovations shaping the dynamic landscape of e-skin technology. This article provides a profound insight into the current state of e-skins, potentially inspiring scholars to explore new possibilities.

Polyacrylamide/sodium alginate/sodium chloride photochromic hydrogel with high conductivity, anti-freezing property and fast response for information storage and electronic skin

Photochromic hydrogels have promising prospects in areas such as wearable device, information encryption technology, optoelectronic display technology, and electronic skin. However, there are strict requirements for the properties of photochromic hydrogels in practical engineering applications, especially in some extreme application environments. The preparation of photochromic hydrogels with high transparency, high toughness, fast response, colour reversibility, excellent electrical conductivity, and anti-freezing property remains a challenge. In this study, a novel photochromic hydrogel (PAAm/SA/NaCl-Mo7) was prepared by loading ammonium molybdate (Mo7) and sodium chloride (NaCl) into a dual-network hydrogel of polyacrylamide (PAAm) and sodium alginate (SA) using a simple one-pot method. PAAm/SA/NaCl-Mo7 hydrogel has excellent conductivity (175.9 S/cm), water retention capacity and anti-freezing properties, which can work normally at a low temperature of −28.4 °C. In addition, the prepared PAAm/SA/NaCl-Mo7 hydrogel exhibits fast response (<15 s), high transparency (>70 %), good toughness (maximum elongation up to 1500 %), good cyclic compression properties at high compressive strains (60 %), good biocompatibility (78.5 %), stable reversible discolouration and excellent sensing properties, which can be used for photoelectric display, information storage and motion monitoring. This work provides a new inspiration for the development of flexible electronic skin devices.

Self-powered flexible battery pressure sensor based on gelatin

The application of pressure sensors with excellent performance in medicine, flexible electronic skin, artificial intelligence, etc. has received much attention. However, most of the current pressure sensors have either the need for an external power supply or the difficulty of accurately measuring the static continuous signal, which greatly limit the development of sensor technology. We report a gelatin-based battery type pressure sensor in which a gelatin composite hydrogel membrane is introduced as the sensing layer in a zinc ion battery, and the zinc battery itself is designed as a pressure sensor. An isolation layer is also added to the solid zinc ion battery to realize the function of converting mechanical pressure signals to electrical signals more accurately. The gelatin battery pressure sensor designed in this paper has the advantages of being self-powered, high sensitivity (37.9 mV/kPa), wide pressure detection range (up to 524 kPa), good stability, short response/recovery time, and high biological applicability. In addition, the battery-type pressure sensor has the ability to detect static signals and maintain good performance under 10,000 s of external mechanical pressure. The design of this battery-based pressure sensor provides a new strategy for the development of self-powered pressure sensors and flexible electronic skin devices.

Partially insoluble regenerated silk fibroin film induced by UV irradiation for electronic skins

The regenerated silk fibroin (RSF) film has been regarded as an ideal substrate for biocompatible, flexible, and biodegradable electronic skin (e-skin) devices. However, it is still a great challenge to balance the flexibility and solubility of the RSF film by adjusting its secondary structure. Herein, a film prepared with the hydrolyzed RSF was exposed to the 254 nm ultraviolet (UV) light to prepare a crosslinked and partially water-insoluble substrate for a strain-sensing e-skin. The hydrolyzed low-molecular-weight RSF was produced by heating the LiBr-silk fibroin solution at 85 °C for a certain duration. The film cast with the hydrolyzed RSF solution could be thoroughly dissolved in water rapidly. The UV irradiation could induce the crosslinking of the low-molecular-weight RSF to form insoluble substances, thus producing a partially insoluble RSF film. After silver nanowires painting, an e-skin strain sensor was successfully constructed based on the UV-irradiated film. The sensor shows a fast response time (2.01 s), high sensitivity (GF = 1.03 within 0%–40% strain range), and good stability. The device could be tightly attached to human skin with a drop of water. The finger, wrist, elbow, and knee bending could be sensitively detected in real-time. The head nodding and mouth opening could also be sensed by sticking the e-skin at the neck and cheek, respectively. This work may provide a facile way to prepare a stretchable and stickable RSF film, which could serve as an ideal substrate of low-cost, biodegradable, direct-to-skin sensors for wearable applications.

Advanced polymer materials‐based electronic skins for tactile and non‐contact sensing applications

AbstractRecently, polymer materials have been at the forefront of other materials in building high‐performance flexible electronic skin (e‐skin) devices due to conspicuous advantages including excellent mechanical flexibility, good compatibility, and high plasticity. However, most research works just paid considerable attention and effort to the design, construction, and possible application of e‐skins that reproduce the tactile perception of the human skin sensory system. Compared with tactile sensing devices, e‐skins that aim to imitate the non‐contact sensing features in the sensory system of human skin tend to avoid undesired issues such as bacteria spreading and mechanical wear. To further promote the development of e‐skins to the human skin sensory system where tactile perception and non‐contact sensing complement each other, significant progress and advances have been achieved in the field of polymer materials enabled e‐skins for both tactile perception and non‐contact sensing applications. In this review, the latest progress in polymer material‐based e‐skins with regard to tactile, non‐contact sensing capabilities and their practical applications are introduced. The fabrication strategies of polymer materials and their role in building high‐performance e‐skins for tactile and non‐contact sensing are highlighted. Furthermore, we also review the research works that integrated the polymer‐based tactile and non‐contact e‐skins into robots and prostheses, smart gloves, and VR/AR devices and addressed some representative problems to demonstrate their suitability in practical applications in human–machine interactions. Finally, the current challenges in the construction of high‐performance tactile and non‐contact e‐skins are highlighted and promising properties in this direction, by taking advantage of the polymer materials, are outlined.image

Low-Voltage Intrinsically Stretchable Organic Transistor Amplifiers for Ultrasensitive Electrophysiological Signal Detection.

Stretchability is a prerequisite for electronic skin devices. However, state-of-the-art stretchable thin-film transistors do not possess sufficiently low operating voltages and good stability, significantly limiting their use in real-world biomedical applications. Herein, a van der Waals-controlling elastomer/carbon quantum dot interfacial polarization methodology is proposed to form a hybrid polymer dielectric with 620% tensile strain and large-area film uniformity (>A4 paper size). Using the hybrid polymer dielectrics, the prepared intrinsically stretchable organic thin-film transistors demonstrate a low operating voltage below 5V, 100% strain tolerance, and excellent operational stability, as well as a high on-current/off-current ratio of 105 and a steep subthreshold slope of 500mV dec-1 . Based on this device technology, an amplifier with a high gain of 90V V-1 among the highest values of reported stretchable transistors is realized. This amplifier is at the first time applied to detect human electrophysical signals with an output signal amplitude of over 0.2V, which even outperforms other types of the state-of-the-art organic amplifiers for human electrophysiology monitoring. This stretchable device technology sufficiently meets the safety and portability requirements of wearable biomedical applications, opening a new opportunity to e-skin with signal control and amplification capabilities.

Advances in gold-based electronic skins

<p indent="0mm">Due to its good electrical conductivity, mechanical flexibility, chemical inertness, and biocompatibility, gold (Au) has been widely used in the development of electronic skin devices. Various gold materials, such as evaporated Au, AuNPs, and AuNWs, have been developed and utilized in the fabrication of high-performance electronic skin sensors and integrated devices. This work reviews evaporated Au-, AuNPs-, and AuNWs-based electronic skins, with a focus on material design, fabrication methods, and applications, and finally discusses the prospects and challenges for the development of gold-based electronic skins.

Anti-bacterial and multi-functional smart wearable sensor based on organo-hydrogel for diagnosis of the anterior cruciate ligament injuries, and sensing glove for rehabilitation of joints motion

Stretchable multi-functional electronic skin devices capable of interface with parts of human body and/or internal organs to diagnose the anterior cruciate ligament (ACL) injuries and rehabilitation of human joint motion are attractive candidates for next-generation wearables biomedical devices, soft robots, and the Internet of Things (IoT). Such devices need to be of excellent mechanical property, anti-freezing, antibacterial, biocompatible to be comfortable to wear for long term usage and accommodate strains from repeated movement and prevent skin irritation. To address this need, we introduced conductive organo-hydrogels based on silver quantum dots (COH@AgQDs) as multi-functional stretchable sensor with outstanding high-performance diagnostic capabilities. We demonstrated a prototype from the developed sensors integrated into a wirelessly Arduino board and a smartphone was selected to transmit data which offers promising wireless system for rehabilitation after surgeries. Additionally, the developed device has ability to differentiate between ACL injury and healthy knee with different pattern shapes and high sensitivity. Moreover, the flexible sensor can inhibit the growth of bacteria (Escherichia coli (E-coli)) and Staphylococcus aureus (S. aureus) and protect human health for long-term use. It exhibited outstanding anti-freezing property (−53 °C), antibacterial, self-healing, robust mechanical property (strain 420% at stress 96 kPa), and high linearity.

Stretchable Temperature-Responsive Multimodal Neuromorphic Electronic Skin with Spontaneous Synaptic Plasticity Recovery.

Multimodal electronic skin devices capable of detecting multimodal signals provide the possibility for health monitoring. Sensing and memory for temperature and deformation by human skin are of great significance for the perception and monitoring of physiological changes of the human body. Electronic skin is highly expected to have similar functions as human skin. Here, by implementing intrinsically stretchable neuromorphic transistors with mechanoreceptors and thermoreceptors in an array, we have realized stretchable temperature-responsive multimodal neuromorphic electronic skin (STRM-NES) with both sensory and memory functions, in which synaptic plasticity can be modulated by multiple modalities, in situ temperature variations, and stretching deformations. Temperature-responsive functions, spontaneous recovery, and temperature-dependent multitrial learning are proposed. Furthermore, a stretchable temperature neuromorphic array composed of multiple fully functional subcells is demonstrated to identify temperature distributions and variations at different regions and conditions after various strains of skin. The STRM-NES has temperature- and strain-responsive neuromorphic functions, excellent self-healing, and reusable capability, showing similar abilities as human skin to sense, transmit, memory, and recovery from external stimuli. It is expected to facilitate the development of wearable electronics, intelligent robotics, and prosthetic applications.

Fingerpad-Inspired Multimodal Electronic Skin for Material Discrimination and Texture Recognition.

Human skin plays a critical role in a person communicating with his or her environment through diverse activities such as touching or deforming an object. Various electronic skin (E‐skin) devices have been developed that show functional or geometrical superiority to human skin. However, research into stretchable E‐skin that can simultaneously distinguish materials and textures has not been established yet. Here, the first approach to achieving a stretchable multimodal device is reported, that operates on the basis of various electrical properties of piezoelectricity, triboelectricity, and piezoresistivity and that exceeds the capabilities of human tactile perception. The prepared E‐skin is composed of a wrinkle‐patterned silicon elastomer, hybrid nanomaterials of silver nanowires and zinc oxide nanowires, and a thin elastomeric dielectric layer covering the hybrid nanomaterials, where the dielectric layer exhibits high surface roughness mimicking human fingerprints. This versatile device can identify and distinguish not only mechanical stress from a single stimulus such as pressure, tensile strain, or vibration but also that from a combination of multiple stimuli. With simultaneous sensing and analysis of the integrated stimuli, the approach enables material discrimination and texture recognition for a biomimetic prosthesis when the multifunctional E‐skin is applied to a robotic hand.

Experimental demonstration of an arbitrary shape dc electric concentrator

Coordinate transformation (CT) theory has shown great potentials in manipulating both time-varying and static fields for different physics ranging from electromagnetism and acoustics to electrostatic and thermal science. Nevertheless, as inhomogeneous and anisotropic materials are required to be realized for the implementation of CT-based devices, the applicability of this method is restricted due to difficulties in the fabrication process. In this paper, based on transformation electrostatic (TE) methodology, the design principle of an arbitrary shape dc electric concentrator is established which yields the enhancement of static electric fields in a predefined region with only one homogeneous conductivity, named as dc null medium (DNM). It is shown that one constant DNM is sufficient for localizing steady electric current in any arbitrary shape region, which in turn obviates the tedious mathematical calculations that conventional methods suffer from. In other words, the same DNM can be used for different concentrators regardless of their cross-section geometries, which makes the presented approach suitable for scenarios where reconfigurability is of utmost importance. Several numerical simulations are performed in order to demonstrate the capability of the proposed dc electric concentrator in localizing steady electric fields into the desired region. Moreover, by utilizing the analogy between electrically conducting materials and resistor networks, the attained DNM is realized with low-cost resistors and then exploited for fabricating a square shape dc electric concentrator on a printed circuit board (PCB). It is demonstrated that the measurement results agree well with the theoretical predictions and numerical simulations, which corroborate the effectiveness of the propounded method. The presented idea of this paper could find applications in scenarios where highly confined electric fields/currents are of critical importance such as electronic skin devices and electrical impedance tomography.

All-polymer ultrathin flexible supercapacitors for electronic skin

Ultrathin flexible supercapacitors (UF-SCs) are highly desired for powering electronic skin devices. Most of current UF-SCs require electrochemically inactive substrates and binders to load electro-active materials, which limit the volumetric capacitance and stability of these UF-SCs. Herein, we report an all-polymer UF-SC based on conductive polypyrrole (PPy) film, which is constructed by the dynamic network formed by rigid PPy (~90 wt%) and soft polyethylene glycol (PEG, only 10 wt%) through supramolecular interactions. The network structure can effectively dissipate the destructive energy during mechanical or electrochemical deformation, making the PPy film strong and robust; and also helps to maximize the utilization of PPy, enabling the PPy film better electric conductivity and electrochemical capacitance than conventional PPy materials. After systematic optimization, we found the PEG600-PPy as the optimal electrode material, which can serve as electroactive material, current collector, and mechanical support simultaneously. Therefore, without using any substrate or current collector, these high-performance PEG600-PPy films can be directly used as electrodes for making all-polymer UF-SC. With high volumetric capacitance (547 F/cm3), outstanding mechanical and electrochemical stability, our all-polymer UF-SC (~80 μm in thickness, thinner than a piece of A4 paper) is a promising power device for electronic skin devices.

High‐Performance Flexible Pressure and Temperature Sensors with Complex Leather Structure

AbstractThe ability of the skin to be pressure‐sensitive has prompted scientists to develop materials and equipment to simulate this function. Recently, flexible and stretchable artificial electronic skin has received increasing attention with its unique ability to detect subtle pressure changes. Pressure sensing is one of the key functions of electronic skin devices. Here, a stretchy and highly sensitive pressure sensor is developed that used a polydimethylsiloxane (PDMS) film with leather composite layer as flexible part. These features enable the sensor to accurately detect a variety of human activities, such as small finger movements and bending, pulse and so on. The sensor is found to have a good sensing signal for temperature. This feature provides great promise for sensors to detect temperature.

Research progress on new materials and properties of electronic skin

Human skin is an extraordinary organ; it comprises an integrated, stretchable network of sensors that transmits information to the brain about tactile and thermal stimuli, enabling us to safely and efficiently operate in our environment. Researchers have become interested in large-scale electronic device networks inspired by human skin, motivated by the prospect of developing devices such as autonomous smart robots and bionic prostheses. Developing electronic networks consist of flexible, stretchable, and robust devices that are compliant with large-scale implementation and integrated with multiple functionalities is a testament to the progress in developing human-skin like electronic bodies. In the fields of human physiological parameter detection and robot tactile perception, electronic skin has been commonly used as a kind of flexible tactile biomimetic sensor. Conventional electronic skin tactile sensors based on metal and semiconductor materials do not meet the requirements for stretchability and portability during actual use because of poor flexibility and wearability. Attributed to the rapid development of flexible materials, and manufacturing and sensing technologies, new materials such as polydimethylsiloxane (PDMS), carbon nanotubes, and graphene have been used to prepare or support electronic skin sensors in recent years, thus enabling electronic skin to be more similar to human skin in terms of stretchability, compressibility, and spatial resolution of touch, and other properties. Now, multi-functional integrated electronic skin devices have realized interaction with smart devices to obtain further collection and processing of human body information. This study analyzed and discussed new electronic skin materials and sensing technologies used in electronic skin, including capacitive effects, piezoelectric effects, piezoresistive effects, optical effects, and wireless antenna sensing. We focused on the recent research progress in electronic skin in terms of stretch/compressibility, biocompatibility, biodegradability, self-power, self-healing, temperature sensitivity, and multi-functional integration. Moreover, we anticipate the future research directions of new electronic skin properties and possible ways to achieve large-area, low-cost, multi-function integrated electronic skin sensor arrays.

Enhancement of electronic and optoelectronic performance of multilayer InSe via strain engineering

In this letter, we report a study on the strain-tuned optical bandgap, piezoresistive effect and photoresponse of multilayer β-InSe. A redshift of photoluminescence is observed in multilayer InSe under uniaxial tensile strains and the shift rate of the optical gap is 57.7 meV/%. The multilayer InSe shows pronounced piezoresistive effect with a reduced resistance and the calculated gauge factor is 47, which is larger than those of graphene and traditional metal foils. The photoresponsivity of the photodetector is enhanced to 115.9 AW−1 measured at 1.06% uniaxial tensile strain, three times higher than that of a device measured without strain. Our results demonstrate that strain-engineering can effectively modulate electronic and optoelectronic properties of multilayer InSe, opening a door for InSe applications in strain sensors, flexible photodetectors and electronic skin devices.

Core–Sheath Stretchable Conductive Fibers for Safe Underwater Wearable Electronics

AbstractStretchable conductive fibers can be directly woven into textiles and applied in wearable and implantable electronic devices. However, for a stretchable conductive fiber that is capable of safely using in full water environment with strains when contacting to skin, it is challenging. Herein, an ultrafine core–sheath stretchable conductive fiber (CSCF in short) with insulative outer sheath and conductive inner core is designed particularly for safe underwater electronic skin devices. CSCF starts from prestrained Lycra fiber followed by spray coating 1D conductive species (carbon nanotubes (CNTs)/silver nanowires (AgNWs)) and wrapping styrene‐(ethylene‐butylene)‐styrene (SEBS) thin film. CSCF exhibits stable core conductivity (e.g., R0 ≈ 2 × 104 S m−1, ΔR/R0 ≈ 0.1 at 100% strain) as well as surface insulation upon mechanical stretching in full water environment. The thickness‐tunable SEBS not only protects skin away from leak of CNTs and AgNWs, but also efficiently suppresses leakage current to an extremely safe level (&lt;1 µA at 5 V) when applying in underwater electronic skin devices. A wireless charging patch composed by CSCF induction coil is able to light light emitting diode (LEDs) no matter when CSCFs are folding and stretching in water. These advantages highlight the promising application of stretchable conductive fiber with protective polymer skin for safe underwater wearable electronics.

Versatile Electronic Skins with Biomimetic Micronanostructures Fabricated Using Natural Reed Leaves as Templates.

Versatile electronic skin devices that enable detection of multimodal signals have revealed great potential for human health monitoring. To make a versatile electronic skin, hierarchical micronanostructures are essential to obtain improved sensing performance and multisignal detection capability. However, current strategies for developing a nanostructured electronic skin usually involve complex procedures, harsh experimental conditions, and the use of expensive equipment, which limit its practical applications. In this paper, we reported the fabrication of a multifunctional wearable electronic skin with hierarchical micronanostructures by using natural reed leaves as templates. The capacitive-type electronic skin is fabricated by double-sided coating of Au electrodes on an artificial polydimethylsiloxane reed leaf that is duplicated from natural reed leaves via soft lithography. The electronic skin features a very simple device structure yet high sensing performance. It permits multimodal signal detection, including that of pressure, deformation, and proximity, and can serve as surface-enhanced Raman scattering substrates for the detection of metabolites in sweat because of the formation of plasmonic structures. The versatile electronic skin can be attached to the human skin, and it enables effective monitoring of multiphysiological signals, revealing great potential for cutting-edge applications, such as human health monitoring.

Flexible Pressure-Sensitive Contact Transistors Operating in the Subthreshold Regime.

Organic thin-film transistor (TFT)-based pressure sensors have received huge attention for wearable electronic applications such as health monitoring and smart robotics. However, there still remains a challenge to achieve low power consumption and high sensitivity at the same time for the realization of truly wearable sensor systems where minimizing power consumption is significant because of limited battery run time. Here, we introduce a flexible pressure-sensitive contact transistor (PCT), a new type of pressure-sensing device based on organic TFTs for next-generation wearable electronic skin devices. The PCT consists of deformable S/D electrodes integrated on a staggered TFT. The deformable S/D electrodes were fabricated by embedding conducting single-walled carbon nanotubes on the surface of microstructured polydimethylsiloxane. Under pressure loads, the deformation of the electrodes on an organic semiconductor layer leads modulation of drain current from variation in both the channel geometry and contact resistance. By strategic subthreshold operation to minimize power consumption and increase the dominance of contact resistance because of gated Schottky contact, the PCT achieves both ultralow power consumption (order of 101 nW) and high sensitivity (18.96 kPa-1). Finally, we demonstrate a 5 × 5 active matrix PCT array on a 3 μm-thick parylene substrate. The device with ultralow power consumption and high sensitivity on a biocompatible flexible substrate makes the PCT promising candidate for next-generation wearable electronic skin devices.