Soft Electronic Skin Research Articles

Mormyroidea-inspired electronic skin for active non-contact three-dimensional tracking and sensing.

The capacity to discern and locate positions in three-dimensional space is crucial for human-machine interfaces and robotic perception. However, current soft electronics can only obtain two-dimensional spatial locations through physical contact. In this study, we report a non-contact position targeting concept enabled by transparent and thin soft electronic skin (E-skin) with three-dimensional sensing capabilities. Inspired by the active electrosensation of mormyroidea fish, this E-skin actively ascertains the 3D positions of targeted objects in a contactless manner and can wirelessly convey the corresponding positions to other devices in real-time. Consequently, this E-skin readily enables interaction with machines, i.e., manipulating virtual objects, controlling robotic arms, and drones in either virtual or actual 3D space. Additionally, it can be integrated with robots to provide them with 3D situational awareness for perceiving their surroundings, avoiding obstacles, or tracking targets.

Bioinspired Ultra‐Stretchable and Highly Sensitive Structural Color Electronic Skins

AbstractOrganisms possess remarkably adaptive ability to complex environments. For example, chameleons can alter their skin color to adapt to varying environments, which has inspired significant advances in bioinspired soft electronic skins (E‐skins), and their wide applications in wearable sensors, intelligent robots, and health monitoring. However, current bioinspired E‐skins face challenges in ultra‐stretchability, high sensitivity, and long‐term stability owing to the intrinsic limitations associated with their mismatched interface between soft matrix and hard conductive fillers, hindering their practical applications. Here, it is reported that bioinspired structural color electronic skins (SC E‐skins) that consist of liquid metal particles (LMPs), periodical ordered colloidal crystal arrays, and ultra‐stretchable hydrogel, imparting synergistic and durable electrical–optical sensing capabilities. Such SC E‐skins demonstrate outstanding performances including superior flexibility (elongation at break > 1100%), high sensitivity (gauge factor = 3.26), fast synergetic electric–optical response time (≈100 ms), outstanding durability (over 1500 cycles), and high accuracy (R2 > 99.5%). These bioinspired SC E‐skins with excellent capability of converting mechanical signals into synergetic electrical–optical outputs hold great promise for smart wearable devices, affording a new horizon in developing advanced health monitoring technologies.

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.

Soft electronic skin for self-deployable tape-spring hinges

Thin-walled structures utilizing the release of stored strain energy for self-deployment have gained popularity in deployable space structures. However, the integration of traditional rigid or bulky sensors to monitor their mechanical behavior presents challenges due to large local deformations and strains involved in folding and deployment. Here we introduce a concept of structural electronic skin (e-skin) that is soft, lightweight, and designed for sensing the folding and deployment of tape-spring hinges. The e-skin demonstrates the capability to accommodate significant hinge deformation and enables multimodal sensing, including strain measurements, motion sensing, and dynamics monitoring. The research showcases a promising approach that leverages the design and manufacturing of soft electronics to fulfill the requirements of thin, lightweight, and soft functional devices for multifunctionality in deployable space structures.

Technological trends in medical robotic sensing with soft electronic skin

Medical robotic sensing is a developing field that combines mechanical technology with medical engineering.

Recyclable, anti-freezing and anti-drying silk fibroin-based hydrogels for ultrasensitive strain sensors and all-hydrogel-state super-capacitors

Multifunctional conductive hydrogels have triggered extensive research interest owing to their potential in artificial intelligence, human motion detection, soft electronic skins, and energy storage. However, unattainable reusable properties and unsatisfactory environmental tolerance seriously hinder potential applications in these emerging fields. Herein, an anti-freezing, anti-drying, and recyclable conductive hydrogel has been successfully prepared by integrating silk fibroin (SF) into polyvinyl alcohol (PVA)/glycerin (Gly)/lithium chloride (LiCl) hydrogel. Silk fibroin provides a variety of physical interactions for the construction of polymer hydrogels due to its abundant binding sites (–COO–, –NH2, and –OH) on its peptide chain. The obtained SPGL (SF/PVA/Gly/LiCl) hydrogels demonstrate outstanding anti-freezing properties (below −70 °C) and excellent anti-drying properties (17.4% weight loss after 30 days). Meanwhile, the prominent remolding property allows the SPGL hydrogel to be recycled, and the ionic conductivity of the remolded hydrogel remains at 84.7%. The important roles of Gly and LiCl in inhibiting the freezing and dehydration of hydrogels have been further revealed by density functional theory (DFT) simulations at the molecular scale. The SPGL hydrogel-based strain sensors possess relatively high sensitivity with a gauge factor (GF) of 2.18, enabling them to monitor human motions precisely, including macro movements and micro movements. Furthermore, we designed a circuit diagram with the sensor to detect human motion in real-time. In addition, the SPGL hydrogel-based supercapacitors display favorable cycle stability (88.5% capacitance retention after 10,000 cycles) and excellent energy density of 15.4 μWh cm−2 at a power density of 50 μW cm−2. The strain sensors and supercapacitors also have anti-freezing and anti-drying properties, maintaining excellent sensing and electrochemical properties even in extreme environments. Therefore, these SPGL hydrogel-based soft electronics have promising applications in health monitoring, human motion detection, human-machine interactions, and energy storage.

Transparent and conformable organic light-emitting diodes for skin-attachable invisible displays

A skin-attachable invisible display offers a new opportunity in next-generation electronic application, such as smart contact lenses, human-machine health monitoring interfaces and soft electronic skins. In response to this trend, transparent organic light-emitting diodes (OLEDs) offer enormous advantages in future skin-attachable invisible displays, including excellent flexibility and superior efficiency. Although several approaches reported recently attempt to manufacture transparent and flexible OLEDs, it is still a tremendous challenge to simultaneously meet the desired optical and mechanical properties. Herein, we develop a feasible strategy to achieve transparent and conformable OLEDs (TC-OLEDs). Benefiting from the intrinsic high transparency and outstanding deformability of our electrodes, the whole device shows high transmittance up to 85.0% at a wavelength of 400 nm and high mechanical flexibility with skin-like conformability. More strikingly, the resulting device maintains excellent photoelectric performance with a maximum current efficiency (CE) of 56.8 cd/A and external quantum efficiency (EQE) of 17.1%. Based on the versatile TC-OLEDs, we also report an all-organic and all-conformable invisible display driven by transparent and conformable organic thin film transistors (TC-OTFTs) for the first time. This work offers a meaningful guidance for the construction of TC-OLEDs and the development of skin-attachable invisible displays.

Effect of active and passive protective soft skins on collision forces in human–robot collaboration

Soft electronic skins are one of the means to turn an industrial manipulator into a collaborative robot. For manipulators that are already fit for physical human-robot collaboration, soft skins can make them safer. In this work, we study the after impact behavior of two collaborative manipulators (UR10e and KUKA LBR iiwa) and one classical industrial manipulator (KUKA Cybertech), in presence or absence of an industrial protective skin (AIRSKIN). In addition, we isolate the effects of the passive padding and the active contribution of the sensor to robot reaction. We present a total of 2250 collision measurements and study the impact force, contact duration, clamping force, and impulse. The dataset is publicly available. We summarize our results as follows. For transient collisions, the passive skin properties lowered the impact forces by about 40 %. During quasi-static contact, the effect of skin covers -- active or passive -- cannot be isolated from the collision detection and reaction by the collaborative robots. Important effects of the stop categories triggered by the active protective skin were found. We systematically compare the different settings and the empirically established safe velocities with prescriptions by the ISO/TS 15066. In some cases, up to the quadruple of the ISO/TS 15066 prescribed velocity can comply with the impact force limits and thus be considered safe. We propose an extension of the formulas relating impact force and permissible velocity that take into account the stiffness and compressible thickness of the protective cover, leading to better predictions of the collision forces. At the same time, this work emphasizes the need for in situ measurements as all the factors we studied -- presence of active/passive skin, safety stop settings, robot collision reaction, impact direction, and, of course, velocity -- have effects on the force evolution after impact.

Learning‐Based Damage Recovery for Healable Soft Electronic Skins

Natural agents display various adaptation strategies to damages, including damage assessment, localization, healing, and recalibration. This work investigates strategies by which a soft electronic skin can similarly preserve its sensitivity after multiple damages, combining material‐level healing with software‐level adaptation. Being manufactured entirely from self‐healing Diels–Alder matrix and composite fibers, the skin is capable of physically recovering from macroscopic damages. However, the simultaneous shifts in sensor fiber signals cannot be modeled using analytical approaches because the materials viscoelasticity and healing processes introduce significant nonlinearities and time‐variance into the skin's response. It is shown that machine learning of five‐layer networks after 5000 probes leads to highly sensitive models for touch localization with 2.3 mm position and 95% depth accuracy. Through health monitoring via probing, damage and partial recovery are localized. Although healing is often successful, insufficient recontact leads to limited recovery or complete loss of a fiber. In these cases, complete resampling and retraining recovers the networks’ full performance, regaining sensitivity, and further increasing the system's robustness. Transfer learning with a single frozen layer provides the ability to rapidly adapt with fewer than 200 probes.

Large-Area 3D Printable Soft Electronic Skin for Biomedical Applications.

Soft electronic skin (soft-e-skin) capable of sensing touch and pressure similar to human skin is essential in many applications, including robotics, healthcare, and augmented reality. However, most of the research effort on soft-e-skin was confined to the lab-scale demonstration. Several hurdles remain challenging, such as highly complex and expensive fabrication processes, instability in long-term use, and difficulty producing large areas and mass production. Here, we present a robust 3D printable large-area electronic skin made of a soft and resilient polymer capable of detecting touch and load, and bending with extreme sensitivity (up to 150 kPa-1) to touch and load, 750 times higher than earlier work. The soft-e-skin shows excellent long-term stability and consistent performance up to almost a year. In addition, we describe a fabrication process capable of producing large areas and in large numbers, yet is cost-effective. The soft-e-skin consists of a uniquely designed optical waveguide and a layer of a soft membrane with an array of soft structures which work as passive sensing nodes. The use of a soft structure gives the liberty of stretching to the soft-e-skin without considering the disjoints among the sensing nodes. We have shown the functioning of the soft-e-skin under various conditions.

All-printed soft human-machine interface for robotic physicochemical sensing.

Ultrasensitive multimodal physicochemical sensing for autonomous robotic decision-making has numerous applications in agriculture, security, environmental protection, and public health. Previously reported robotic sensing technologies have primarily focused on monitoring physical parameters such as pressure and temperature. Integrating chemical sensors for autonomous dry-phase analyte detection on a robotic platform is rather extremely challenging and substantially underdeveloped. Here, we introduce an artificial intelligence-powered multimodal robotic sensing system (M-Bot) with an all-printed mass-producible soft electronic skin-based human-machine interface. A scalable inkjet printing technology with custom-developed nanomaterial inks was used to manufacture flexible physicochemical sensor arrays for electrophysiology recording, tactile perception, and robotic sensing of a wide range of hazardous materials including nitroaromatic explosives, pesticides, nerve agents, and infectious pathogens such as SARS-CoV-2. The M-Bot decodes the surface electromyography signals collected from the human body through machine learning algorithms for remote robotic control and can perform in situ threat compound detection in extreme or contaminated environments with user-interactive tactile and threat alarm feedback. The printed electronic skin-based robotic sensing technology can be further generalized and applied to other remote sensing platforms. Such diversity was validated on an intelligent multimodal robotic boat platform that can efficiently track the source of trace amounts of hazardous compounds through autonomous and intelligent decision-making algorithms. This fully printed human-machine interactive multimodal sensing technology could play a crucial role in designing future intelligent robotic systems and can be easily reconfigured toward numerous practical wearable and robotic applications.

Preparation and characterization of multi-network hydrogels based on sodium alginate/krill protein/polyacrylamide—Strength, shape memory, conductivity and biocompatibility

Sodium alginate/krill protein/polyacrylamide (SA/AKP/PAM) hydrogel with “covalent bond―ion complex―hydrogen bond” multi-network structure was prepared by covalent cross-linking and complexion ion crosslinking using SA, AKP, and acrylamide (AM) as raw materials. The effects of ion species (Fe3+, Ba2+, Sr2+, Ca2+, and Zn2+) on the structure, morphology, and properties of multi-network hydrogels were studied in detail. The results showed that the mechanical strength of ionic cross-linked hydrogels increased significantly. The compressive strength of Fe3+ cross-linked hydrogels was 5.56 MPa, 16.13 times that of non-ionic crosslinked hydrogels. The results of ionic conductivity measurements showed that hydrogels had significant ionic conductivity and were sensitive to external forces. Interestingly, the hydrogel can be used as a capacitive pen in mobile phone writing, painting and dialing numbers. Moreover, ionic cross-linked hydrogels had a unique three-dimensional porous structure with gradient distribution, excellent shape memory effect, and good biocompatibility. Fe3+, Ba2+, Sr2+, and Ca2+ cross-linked hydrogels were nontoxic and conducive to the adhesion and growth of Schwann cells. These excellent properties of ionic cross-linked SA/AKP/PAM hydrogels have broad applications prospects in flexible electronic devices, sensors, soft electronic skins, and tissue engineering.

Flexible strain sensor with ridge‐like microstructures for wearable applications

AbstractThe flexible stretchable sensors have great potential for implementation in various applications, such as intelligent soft robots, health monitoring, and motion detection. However, most of the flexible stretchable sensors with microstructures and high sensitivities are fabricated by expensive templates and complex processes. In consideration of large‐scale fabrication, a low cost and efficient way is in great demand. Herein, electroless plating on Nafion films with decent swelling ratios are proposed to fabricate stretchable sensors with wrinkle‐structured electrodes. By adding isopropanol (IPA) to the electroless plating process, the H2O‐IPA sensor with larger swelling ratio shows deeper surface wrinkles, higher surface roughness, and better sensitivity to strain. At the same time, the H2O‐IPA sensor exhibit good durability (500 cycles). By mounting the sensor on the joint of human finger, the motion of the finger bending and even the bending degree can be accurately detected, indicating the potential use in the field of wearable devices and soft electronic skins.

Suppressing Interface Strain for Eliminating Double-Slope Behaviors: Towards Ideal Conformable Polymer Field-Effect Transistors.

High-mobility polymer field-effect transistors (PFETs) are being actively explored for applications in soft electronic skin and low-cost flexible displays because of their superior solution processability, mechanical flexibility, and stretchability. However, most of high-mobility PFETs often deviate from the idealized behavior with variable mobility, large threshold voltage, and high off-state current, which masks their intrinsic properties and significantly impedes their practical applications. Here, it is first revealed that interface strain between polymer thin film and rigid substrate plays a crucial role in determining the ideality of PFETs, and demonstrate that various ideal conformable PFETs can be successfully fabricated by releasing strain. It is found that strain in film can be released by one-step peeling strategy, which can reduce π-π stacking distance and suppress generation of oxygen doped carriers, thereby obtaining linearly injected charge carriers and decreased carrier concentration in channel, eventually realizing ideal PFETs. More impressively, the fabricated ideal conformable PFET array displays outstanding conformability to curved objects, and meanwhile showing excellent organic light-emitting display driving capability. The work clarifies the effect of the interface strain on the device ideality, and strain can be effectively released by a facile peeling strategy, thus offering useful guidance for the construction of ideal conformable PFETs.

Recent Progress in Essential Functions of Soft Electronic Skin

AbstractInspired by the human skin, electronic skins (e‐skins) composed of various flexible sensors, such as strain sensor, pressure sensor, shear force sensor, temperature sensor, and humility sensor, and delicate circuits, are emerged to mimic the sensing functions of human skins. In this review, the strategies to realize the versatile functionalities of natural skin‐like e‐skins, including strain‐, pressure‐, shear force‐, temperature‐ and humility‐sensing abilities, as well as self‐healing ability and other functions are summarized. Some representative examples of high‐performance e‐skins and their applications are outlined and discussed. Finally, the outlook of the future of e‐skins is presented.

GSH-responsive self-healable conductive hydrogel of highly sensitive strain-pressure sensor for cancer cell detection

Herein, a selective strain- and pressure-sensitive hydrogel-based wireless soft-electronic skin sensor with self-healing properties was designed for detection of cancer cells. The polydopamine-loaded glutathione-responsive polymer dot (PDA@PD) selectivity towards CD44 receptor in hydrogel matrix was identified to play vital role in strain-pressure selectivity as well as self-healing behavior in the presence of cancer cells. The hydrogen bonding released PDA from disulfide cross-linked PD affected controllable conductivity of PDA@PD-PAAm hydrogel in different cellular microenvironments. Under applied strain and pressure, this sensor could distinguish electronic signals from cancer cell-treated hydrogels by producing higher electronic signals (ΔR/R0 ≈ 23%) compared to normal cell-treated hydrogel (ΔR/R0 ≈ 15%). Moreover, the presence of cancer cells could be indicated by self-healing phenomenon that did not occur in normal cell-treated hydrogel. This sensor demonstrated excellent selectivity towards various cancer cell types, with distinct strain and pressure responses, even in cancer-normal cell co-culture with various ratios. Additionally, real-time electronic signal from strain-pressure response and self-healing can be monitored easily by using wireless devices and smartphones. Therefore, wireless soft skin sensor has the potential to be used for simple and sensitive cancer detection, such as for circulating tumor cells (CTC) or tumor tissue during surgery, allowing for point-of-care diagnostics.

Ultrasensitive and robust self-healing composite films with reinforcement of multi-branched cellulose nanocrystals

Herein, self-healing multi-branched cellulose nanocrystals/polyvinyl alcohol (MCNC/PVA) composite films with excellent sensitivity to human motions was synthesized by employing MCNC as reinforcing agent, where the potassium chloride (KCl) was added to enhance the conductivity and strain sensitivity of the composite film. There was a synergistic effect between MCNC and PVA, which could promote the slip and recombination of hydrogen bonds, so that the films had robust strength (2.2 times to pure PVA) and high self-healing efficiency (69.54% after 20 min). Especially, the flexible composite film could clearly monitor both large and subtle human motions (finger bending, swallowing, breathing, and insect crawling), and showed larger subtle strain sensitivity (strains <24.5%) with a gauge factor (GF) of 2.4, which was relatively larger than the GF of ionic hydrogel and film-based skin sensors, which could solve the current problems of poor mechanical flexibility and low sensitivity of electronic skin, and had great potential application in the fields of self-healing soft electronic skin sensors for human motion detection and healthcare.

Biofuel-powered soft electronic skin with multiplexed and wireless sensing for human-machine interfaces.

Existing electronic skin (e-skin) sensing platforms are equipped to monitor physical parameters using power from batteries or near-field communication. For e-skins to be applied in the next generation of robotics and medical devices, they must operate wirelessly and be self-powered. However, despite recent efforts to harvest energy from the human body, self-powered e-skin with the ability to perform biosensing with Bluetooth communication are limited because of lack of a continuous energy source and limited power efficiency. Here, we report a flexible and fully perspiration-powered integrated electronic skin (PPES) for multiplexed metabolic sensing in situ. The battery-free e-skin contains multimodal sensors and highly efficient lactate biofuel cells that use a unique integration of zero- to three-dimensional nanomaterials to achieve high power intensity and long-term stability. The PPES delivered a record-breaking power density of 3.5 milliwatt-centimeter-2 for biofuel cells in untreated human body fluids (human sweat) and displayed a very stable performance during a 60-hour continuous operation. It selectively monitored key metabolic analytes (e.g., urea, NH4 +, glucose, and pH) and the skin temperature during prolonged physical activities and wirelessly transmitted the data to the user interface using Bluetooth. The PPES was also able to monitor muscle contraction and work as a human-machine interface for human- prosthesis walking.

Self-powered flexible and transparent smart patch for temperature sensing

Wearable electronics that can deform with human skin and monitor physical information are promising next-generation electronics. Here, we demonstrate a self-powered smart patch for temperature sensing by integrating a triboelectric nanogenerator, power management circuit and temperature sensor together. Benefitting from the functional nanomaterial and fabrication process of spray coating, the main part of the device shows extraordinary mechanical flexibility and visible transparency. The temperature sensor presents a high sensitivity of 0.54% per Kelvin scaled by relative resistance change. Owing to the high efficiency of the implemented power management circuit, a capacitor of 100 μF can be easily charged to 1 V within 105 s through the triboelectric nanogenerator contacting with cotton, which can drive the sensor continuously to work for more than 100 s. The large-scalable fabrication process and integration design give this smart patch potential applications in human-machine interfaces and soft electronic skins.

A bimodal soft electronic skin for tactile and touchless interaction in real time

The emergence of smart electronics, human friendly robotics and supplemented or virtual reality demands electronic skins with both tactile and touchless perceptions for the manipulation of real and virtual objects. Here, we realize bifunctional electronic skins equipped with a compliant magnetic microelectromechanical system able to transduce both tactile—via mechanical pressure—and touchless—via magnetic fields—stimulations simultaneously. The magnetic microelectromechanical system separates electric signals from tactile and touchless interactions into two different regions, allowing the electronic skins to unambiguously distinguish the two modes in real time. Besides, its inherent magnetic specificity overcomes the interference from non-relevant objects and enables signal-programmable interactions. Ultimately, the magnetic microelectromechanical system enables complex interplay with physical objects enhanced with virtual content data in augmented reality, robotics, and medical applications.