Electronic Skin Sensors Research Articles

Intrinsically stretchable phototransistors with polymer-QD-polymer multi-layered hybrid films for visible-NIR perspective electronic skin sensors

Stretchable phototransistors represent a crucial component in the development of skin-inspired optoelectronics. Although there have been significant advancements in the development of skin-like photosensitive semiconductors, technology for stretchable phototransistors that can detect a broad spectrum of wavelengths from near-infrared (NIR) to visible light is still in its early stages. This study introduces an intrinsically stretchable phototransistor employing hybrid semiconductor films consisting of polymer semiconductors (PSC) and quantum dots (QDs). The hybrid multi-layered semiconductor film (PSC/QDs/PSC) possesses wide-ranging color sensitivity from NIR to visible light, along with high photosensitivity and photoresponsivity. Furthermore, our phototransistor exhibits remarkable strain-insensitive, photo-response characteristics, maintaining performance even under 50% strain and enduring 10,000 strain cycles. Notably, the phototransistor array, featuring a distinctive dodecagon shape of source/drain electrodes, effectively responds to NIR and visible light stimuli, even when subjected to substantial areal strains of up to 38%.

Physical Sensor for Electronic Skin Based on Nanomaterials: A Review

Electronic skin based on nanomaterials has great potential for material detection and health monitoring. This paper first outlines the current development of physical sensors for electronic skin based on nanomaterials. Secondly, according to the different kinds of propagation signals, it reviews the research on physical sensors based on different nanomaterials sensing. Strain signals, temperature signals and electrophysiological signals are the main types of nanomaterials transduction. It discusses the different mechanisms, research processes and advantages of the different ways of constituting physical sensors after sensing signals. In particular, it reviews the comparison of nanomaterials constituting sensors with conventional approaches and innovations in the performance of advanced electronic skin sensors. According to the technologies used in them, it reviews the particularities of each study in detail. It analytically summarizes their application in electronic skin as a novel bionic monitoring element. Finally, it also summarizes and proposes future directions for the development of electronic skins composed of physical sensors based on nanomaterials.

A temperature and pressure dual-responsive, stretchable, healable, adhesive, and biocompatible carboxymethyl cellulose-based conductive hydrogels for flexible wearable strain sensor

The study aimed to develop a novel temperature and pressure dual-responsive conductive hydrogel with self-healing, self-adhesive, biocompatible, and stretchable properties, for the development of multifunctional anti-counterfeiting and wearable flexible electronic materials. A conductive hydrogel based on carboxymethyl cellulose (CMC) was synthesized by simple “one pot” free radical polymerization of CMC, acrylamide (AAm) and acrylic acid (AAc). The hydrogel displayed temperature responsiveness and possessed an upper critical solution temperature (UCST) value. In addition, hydrogels also had surprising pressure responsiveness. The synthesized hydrogels were characterized by FTIR, TGA, DSC, and XRD analysis. Importantly, the obtained hydrogels exhibited exceptional mechanical properties (stress: 730 kPa, strain: 880%), fatigue resistance, stretchability, self-healing capability, self-adhesive properties, and conductivity. In addition, valuable insights were obtained into the synthesis and application of flexible anti-counterfeiting and camouflage materials by the temperature and pressure dual-responsive hydrogels. Moreover, the prepared hydrogel, with an electrically sensitive perception of external strain (GF = 2.61, response time: 80 ms), can be utilized for monitoring human movement, emotional changes, physiological signals, language, and more, rendering it suitable for novel flexible anti-counterfeiting materials and versatile wearable iontronics. Overall, this study provided novel insights into the simple and efficient synthesis and sustainable manufacturing of environmentally friendly multifunctional anti-counterfeiting materials and flexible electronic skin sensors.

Enhanced Mechanical Strength of Metal Ion-Doped MXene-Based Double-Network Hydrogels for Highly Sensitive and Durable Flexible Sensors.

Development of conductive hydrogels with high sensitivity and excellent mechanical properties remains a challenge for constructing flexible sensor devices. Herein, a universal strategy is presented for enhancing the mechanical strength of Mxene-based double-network hydrogels through metal ion coordination effects. Polyacrylamide (PAM)/sodium alginate (SA)/Mxene double-network (PSM-DN) hydrogels were prepared by metal ion impregnation of PAM/SA/Mxene (PSM) hydrogels. High electrical conductivity is achieved due to MXene nanosheets, while the strong coordination bond between metal ions and SA constructs a second network that increases the mechanical strength of the hydrogel by an order of magnitude. Mechanical tests demonstrated that the elastic modulus of hydrogels matches that of human tissues. Hence, they can be used as a highly sensitive electronic skin sensor to recognize the movement of different joints in humans and also as a pressure sensing interface to recognize characters for anticounterfeiting and information transfer. This work can promote the practical application of conductive hydrogels in high-tech fields, such as flexible electronic skin and interface interaction.

Flexible pressure sensor based on multi-layer with gradient structure P(VDF-HFP)/MXene/BaTiO3 composite film for human motion monitoring

In recent years, flexible pressure sensors have been widely used in fields such as human health monitoring, flexible energy storage, wearable electronic devices, and flexible sensing due to their advantages of simple and efficient preparation process, highly sensitive, fast response, and strong piezoelectric performance. In this paper, we studied a kind of polyvinylidene fluoride hexafluoropropylene (P(VDF-HFP))/MXene/BaTiO3 nanofiber prepared by electrospinning technology, and designed a multi-layer structure system with gradient to prepare a flexible pressure sensor. The piezoelectric properties, mechanical properties and dielectric properties are obviously improved. At the same time, the addition of nano fillers and structural design have increased the sensitivity of multi-layer composite fiber membrane-based sensors by 16.5 times compared to pure P(VDF-HFP) in the low-pressure range (<1 kPa), and by 1.5 times compared to PFP-0.5M5BT nanofiber-based sensors, reaching 0.23 kPa−1. When pressed by the palm of the hand, the output voltage of the three-layer gradient structure thin film sensor can be up to about 100 V. Compared with the output voltage of 5 V when the P(VDF-HFP) thin film is pressed by the palm, the output voltage of the flexible pressure sensor of the three-layer gradient structure thin film after adding the filler is 20 times higher. The flexible pressure sensor can analyze and recognize the form of human motion based on the peak, frequency, and shape characteristics of the voltage signals generated by different forms of human motion. This makes it possible for flexible pressure sensors to be used in pressure detection, flexible sensing, electronic skin sensors and other fields.

Micelle–Micelle Cross-Linked Highly Stretchable Conductive Hydrogels for Potential Applications of Strain and Electronic Skin Sensors

Micelle–Micelle Cross-Linked Highly Stretchable Conductive Hydrogels for Potential Applications of Strain and Electronic Skin Sensors

High strength polyurethane ionogel as a wearable strain sensor for human activity monitoring

Due to their superior elasticity and electroconductibility as soft wearable sensors, ionogels have received a lot of investigation. Electronic skin sensors based on traditional ionogel frequently have poor mechanical qualities, poor durability, and poor temperature resistance. Here, a new waterborne polyurethane based macromolecular cross-linker (MPU) is developed and used to create an ionogel containing 1-ethyl-3-methylimidazoliumdicyanamide (EMIM:DCA). Importantly, due of the internal hydrogen bonding, ionogels exhibit extraordinary mechanical properties with a strain value of 1.27 MPa, a deformation at breaking of 267%, and excellent puncture resistance. At normal temperature, EMIM:DCA gives the ionogels high electroconductibility. When combined as wireless strain sensors, the ionogel sensors exhibit exceptional sensing capabilities to track real-time mechanical conduction signals of human movement. It indicates that the ionogels as developed hold a lot of promise for wearable devices.

Flexible and Transparent Electronic Skin Sensor with Sensing Capabilities for Pressure, Temperature, and Humidity

Inspired by the interlocked biological geometry of human skin, herein, we design a flexible and transparent sensor with interlocked square column arrays with composites of Ag nanoparticles (AgNPs), citric acid (CA), and poly(vinyl alcohol) (PVA), which exhibit multisensory capabilities for pressure, temperature, and humidity. As a flexible pressure sensor, the interlocked AgNPs/CA/PVA sensor possesses a high sensitivity (-1.82 kPa-1), low detection limit (10 Pa), fast response (75 ms), and outstanding stability due to the high sensitivity of the contact resistance of the interlocked square column arrays to pressure. Because of the rigid dependence of the resistance of the AgNPs/CA/PVA composite on temperature, the interlocked AgNPs/CA/PVA sensor can also act as a temperature sensor, which exhibits high resolution (0.1 °C) and reliability in detecting ambient temperature. In addition, it is found that the amount of water molecules adsorbed by PVA and CA changes with the ambient humidity. Therefore, the interlocked AgNPs/CA/PVA sensor is also able to detect humidity in real time. This work proposes a simple but useful route to fabricate a flexible and transparent electrical skin sensor, which has great potential in the perception of pressure, temperature, and humidity.

Zinc-ion engineered Plant-based multifunctional hydrogels for flexible wearable strain Sensors, Bio-electrodes and Zinc-ion hybrid capacitors

Multifunctional hydrogels, particularly with superior mechanical properties and using green/sustainable approaches, have attracted increasing attention because of their many applications and consistency with the green chemistry principle; unfortunately, their effective fabrication process is challenging. In this work, inspired by unique functions in plant-derived components, a multifunctional polyacrylic acid (PAA) hydrogel was innovatively developed using ZnCl2 and biomass-derived materials. Ingeniously, ZnCl2 has multiple functions, in combination with cellulose, lignin, and citric acid (CA), delivering intriguing properties to the hydrogel system. As a solvent system, the ZnCl2 solution has extraordinary H-bonding-donating ability, dissolving cellulose macromolecular chains; subsequently, solubilized cellulose as green reinforced-fillers are added to hydrogel, improving the strength properties, with compression strength of ∼ 4.1 MPa, tensile strength of ∼ 795.4 kPa, and elongation at break of ∼ 620 %. The lignin-ZnCl2 catalysis system imparts a fast PAA gelation process, and the resultant hydrogel has strong/long-lasting/repeatable adhesive strength of 25.5 kPa (on Zn-carbon cloth) in 28 days. Additionally, ZnCl2 can form reversible ionic/electrostatic interactions with both lignin and CA, further improving the adhesive and mechanical properties. As anti-freezing agents, water-retaining agents and conducting medium, ZnCl2 also endows the hydrogel with desirable environmental compatibility and good conductivity (14.2 mS·cm−1). Furthermore, the presence of plant-based components endows the hydrogel with ultraviolet (UV) blocking, biocompatibility and antibacterial attribute. Significantly, the as-prepared hydrogels are suited for such diverse applications as flexible wearable strain sensors, bio-electrodes and zinc-ion hybrid capacitors (ZHCs), even can efficiently work at −65 °C. This study provides a new strategy of engineering multifunctional hydrogels using plant-based functional components, and these hydrogels show promising applications in many emerging areas including electronic skin sensors, health monitoring and energy storage devices.

Recent advances for liquid metals: Synthesis, modification and bio-applications

• Fabrication and functionalization strategies for liquid metals have been reviewed. • Bio-applications of liquid metals-based biomaterials have been mainly summarized. • Challenges for liquid metals-based biomedical development have been discussed. Considering the application requirements for modern biomedicine, research into novel biomaterials with unusual functions is highly desired. As an alternative, liquid metals (LMs), a nontraditional family of metal materials, have piqued the interest of biomedical researchers and made significant advances in biomedical areas, owing to their shape transformability, self-healing capability, excellent electrical, and thermal conductivities. In particular, many functionalized strategies for the preparation and modification of LMs or LMs-based composites to achieve extended biomedical applications have been investigated in recent years. These findings provided inspiring while constructive reference for the fabrication and engineering of novel LMs-based composites. Herein, in this topic review, we elaborate on the recent advances of LMs-based functional materials, with particular focuses on the synthesis, modification, and bio-applications, especially in antitumor therapy, antibacterial, contrast agent for imaging, bone repair, electronic skin sensor, and nerve connection agent. Further on, the current challenges and future prospects of LMs-based composites are carefully discussed.

Piezoionic transfer effect in topological borophene-bismuthene derivative micro-leaves for robust supercapacitive electronic skins

To gain high performance in electrochemical device, exquisite accelerating ion-electron transfer is an effective way. Here, a topological idea is realized to design a micro-leave electrode by patterning borophene-bismuthene derivative. The inner bismuthene offers high conductive skeleton, intermediate BiOCl and Bi2O3 derivative as ornament contribute to supercapacitive property, and the outer borophene layer not only boosts supercapacitive property, but also reserves the topological shape. Assisted by a hydrogel electrolyte on a flexible indium-doped tin oxide substrate, a robust pressure sensor is fabricated, in which a piezoionic transfer effect is demonstrated to induce fast ions extraction and migration kinetics among the topological micro-leaves. Then the specific capacitance of electrode increases by 2.483 times comparing with the micro-leaves without borophene. The resultant robust electronic skin sensor presents a comparable sensitivity of 1.60 kPa−1 in the tiny pressure range of ≈ 50–150 Pa, and sensitivity of 1.01 kPa−1 in 150 Pa-8.5 kPa, low detection limit as 0.59 Pa, fast response/recovery time (80/80 ms), wide detection range (∼220 kPa) and reliable capacitance retention of 97.92% even after 10000 cycles. This has been successfully used to monitor the physiological signals and illustrates potential application prospects in human-computer interaction.

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.

Flexible Electronic Skin for Monitoring of Grasping State During Robotic Manipulation.

Electronic skin for robotic tactile sensing has been studied extensively over the past years, yet practical applications of electronic skin for the grasping state monitoring during robotic manipulation are still limited. In this study, we present the fabrication and implementation of electronic skin sensor arrays for the detection of unstable grasping. The piezoresistive sensor arrays have the advantages of facile fabrication, fast response, and high reliability. With the tactile data from the sensor array, we propose two quantitative indicators, correlation coefficient and wavelet coefficient, to identify grasping with variable forces and slippage. Those two indicators reflect both time and frequency domain characteristics in the contact forces from the sensor array and can be obtained without large amount of calculation. We demonstrate the utility of this method under various conditions, the results indicate grasping with variable forces, and slippage can be distinguished by this method. The flexible sensor arrays are adopted for tactile sensing on a bionic hand, and the effectiveness of this method in detecting various grasping states has been verified. The electronic skin sensor array and the grasping state monitoring method are promising for applications in robotic dexterous manipulation.

Femtosecond Laser-Induced Supermetalphobicity for Design and Fabrication of Flexible Tactile Electronic Skin Sensor.

Pursuing flexible tactile electronic skin sensors with superior comprehensive performances is highly desired in practical applications. However, current flexible tactile electronic skin sensors suffer insufficient flexibility and sensitivity, as well as high-cost and low-efficiency in fabrication, and are susceptible to contamination in sensing performances. Here, a highly sensitive all-flexible tactile sensor (AFTS) is presented with capacitive sensing that combines a double-side micropyramids dielectric layer and a liquid metal (LM) electrode. The design and fabrication of LM-based AFTS are based on supermetalphobicity induced by femtosecond laser. The supermetalphobic micropyramids lead to a high sensitivity up to 2.78 kPa-1, an ultralow limit of detection of ∼3 Pa, a fast response time of 80 ms, and an excellent durability of cyclic load over 10 000 times. The used femtosecond laser enables programmable, high-efficiency, low-cost, and large-scale fabrication of supermetalphobic double-side micropyramids, which is difficult to implement using conventional techniques. Furthermore, the outer substrates are treated by a femtosecond laser, endowing the AFTS with excellent antifouling performance and stable sensing signals in the highly humid environment. Successful monitoring of human physiological and motion signals demonstrates the potential of our developed AFTS for wearable biomonitoring applications.

High strength, self‐healing, and anti‐freezing polyurethane ionogel based on multiple hydrogen bonding for wearable strain sensor

AbstractHydrogel‐based electronic skin sensors often suffer from low mechanical properties, poor durability, and low temperature resistance. Herein, an ionogel was fabricated by casting the water‐borne polyurethane (WPU) and 1‐ethyl‐3‐methylimidazoliumdicyanamide (EMIM:DCA). The imidazolidinyl urea with multiple hydrogen bonds as chain extenders was introduced into WPU systems. Owing to the multiple hydrogen bonds, the ionogels exhibited good mechanical properties with a tensile stress of 1.45–12.09 MPa, a strain of 730%–860%, and good resilience. Moreover, the mechanical properties, conductivity, and sensing ability of the ionogels could be conveniently healed with the assistance of heat. EMIM:DCA endowed the ionogels with excellent ionic conductivity at room temperature (1.8–246 μS cm−1). In addition, due to the low vapor pressure of EMIM:DCA, the ionogel exhibited excellent anti‐freezing property at −20°C. The ionogel can be assembled into a sensor to monitor the movements of human body with good responsiveness and stability.

Functionally integrated bioglass microspheres-composited double-network hydrogel with good tissue adhesion and electrical conductivity for efficient wound treatment and health detection

Hydrogel dressings have been used as an important way to maintain the moist environment of the wound and accelerate healing process. The development of double-network hydrogels with both high tissue adhesion and high biosafety for wound closure and wound healing remains an ongoing challenge. Therefore, a unique double-network xanthan gum (XG)/dopamine-modified oxidized sodium alginate (OSA-DA) composite hydrogel was prepared by a facile and rapid method, in which XG network was formed by hydrogen bonding, and Ca ions released from bioglass (BG) chelated with the carboxyl group of OSA to form the second network. The prepared injectable hydrogels had good self-healing properties, biosafety and tissue adhesion. The Si ions released from BG and fucoidan loaded in the hydrogels acted together to promote the reconstruction of blood ves·sels, collagen matrix and hair follicles. The ions in hydrogels gave the hydrogel good electrical conductivity. This hydrogel also enabled functional integration and could be used as a flexible electronic skin sensor that could monitor larger movements of the elbow or fingers and track small movements of facial expressions, vocalizations and pulses in real time.

Stretchable and transparent multimodal electronic-skin sensors in detecting strain, temperature, and humidity

Smart electronic skins (e-skins) have garnered extensive attentions owing to their potentials in health monitoring, human-machine interactions, and soft robotics. Herein, stretchable and transparent multimodal e-skin sensors composed of poly(vinyl alcohol) (PVA), citric acid (CA) and Ag nanoparticles (AgNPs) are developed by a simple in-situ reduction and solvent casting technology, which possess multiple sensing capabilities in strain, temperature and humidity. Although utilizing the nontransparent AgNPs as sensing components, the as-prepared PVA/CA/AgNPs sensor exhibits not only ultrahigh stretchability (up to 596% strain) but also outstanding transparency (the transmittance > 89%) attributed to the homogeneous dispersion of CA and AgNPs in PVA matrix. As flexible strain sensor, this sensor exhibits wide strain sensing range (1–200%), rapid response (90 ms), and excellent linearity of outputted signals (correlation coefficient: 0.998). Moreover, high-precision thermo-sensation (0.1 ℃) and outstanding temperature sensitivity (−0.076/℃ in the range of 30–40 ℃) make the sensor can detect subtle temperature variation. Furthermore, the good hygroscopicity of PVA and CA endows the fabricated sensor with outstanding humidity sensing performance, which can give human skin moisture information in real-time. More importantly, the designed sensor possesses excellent recyclability and can be reused for many times via the simple dissolution-reshaping treatment, which could reduce the electronical waste.

Visible light-responsive mechanically and electronically controllable conductive carbon dot-hydrogel-based pressure-strain sensor for wireless monitoring of antifouling performance

This work describes the visible-light-induced wireless pressure and strain sensor based on carbon dot (CD) loaded conductive hydrogel for the mechanical force-regulated antifouling behavior. The conductivity, as well as pressure and strain sensitivities of the hydrogel, were modulated by modifying the hydrophobic–hydrophilic transformation of the CD components that controlled the antifouling effect through mild mechanical forces, allowing the cells to be squeezed out from the hydrogel. The photocatalytic activity of the CD containing carbonized polydopamine (cPDA) releases the cPDA which enhances the network formation, resulting in a four-fold higher conductivity, 158% enhanced mechanical strength, and 231% improved stretchability, which enhances the pressure and strain sensing response. Furthermore, antifouling phenomenon involved in the physical deformation associated with the changes in the electrochemical behavior of the hydrogel, which was monitored by wireless sensing, demonstrated the effect of mechanical force on the antifouling behavior for the successful removal of cells from the hydrogel. Additionally, the efficacy of the antifouling effect depending on pressure or strain was evaluated by real-time monitoring of the wireless response. This antifouling property of the hydrogel together with visible-light-controlled hydrophobic–hydrophilic transformation can allow the development of hydrogel-based electronics for electronic skin sensors, prosthetics, and biosensing applications.

Skin-Inspired Hair–Epidermis–Dermis Hierarchical Structures for Electronic Skin Sensors with High Sensitivity over a Wide Linear Range

The quest for both high sensitivity and a wide linear range in electronic skin design is perpetual; unfortunately, these two key parameters are generally mutually exclusive. Although limited success in attaining both high sensitivity and a wide linear range has been achieved via material-specific or complicated structure design, addressing the conflict between these parameters remains a critical challenge. Here, inspired by the human somatosensory system, we propose hair-epidermis-dermis hierarchical structures based on a reduced graphene oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) aerogel to reconcile this contradiction between high sensitivity and a wide linear range. This hierarchical structure enables an electronic skin (e-skin) sensor linear sensing range up to 30 kPa without sacrificing the high sensitivity (137.7 kPa-1), revealing an effective strategy to overcome the above-mentioned conflict. In addition, the e-skin sensor also exhibits a low detection limit (1.1 Pa), fast responsiveness (∼80 ms), and excellent stability and reproducibility (over 10 000 cycles); as a result, the e-skin platform is capable of detecting small air flow and monitoring human pulse and even sound-induced vibrations. This structure may boost the ongoing research on the structural design and performance regulation of emerging flexible electronics.

A Load-Dependent Model of Triboelectric Nanogenerators for Surface Roughness Sensing

The triboelectric nanogenerator (TENG) has the advantages of low cost, high efficiency, flexibility, lightness, and portability. Therefore, it has caused more and more attention as an electronic skin sensor. Generally, the research on intelligent components mainly focused on flexible pressure or temperature sensors. However, the surface roughness recognition of object material still remains a challenge. Meanwhile, most of the current surface roughness recognition methods need secondary processing of the sensor signal, and rarely use the signal generated by the sensor to directly identify the surface roughness. In this paper, the load-dependent electric field model is transplanted to the conductor-dielectric contact-separation TENG (CS-TENG), and the surface roughness of conductor can be differentiated theoretically based on the output of CS-TENG. To verify its effectiveness, the TENG devices were fabricated and the home-processed steels of different roughness act as the conductor layers. By contacting the conductor layer with the dielectric layer, the electric output signal based on contact induced electrification can be generated, which can be used to quantitatively estimate the surface roughness of steels. The test results demonstrate that the output of CS-TENG decreases with the increase of surface roughness, which coincides with the simulation prediction. This method is low cost and easy to implement, which provides a new design idea to extend the functions of TENGs as tactile electronic skins.