Highly Stretchable and Self‐Healable MXene/Polyvinyl Alcohol Hydrogel Electrode for Wearable Capacitive Electronic Skin

Abstract

body motion monitoring, covering subtle and large-strain ranges.Capacitive strain sensor has a simple structure with two electrodes separated by a dielectric material. Metals [19] and semiconductors [20] can be used as electrodes for constructing capacitive sensors. However, these electrode materials have poor mechanical properties, limiting the sensing range of capacitive strain sensors. [21] To solve the problem, geometrical engineering of rigid materials into buckled, [22][23][24] wrinkled, [16,25] and kirigami [26] forms is widely employed. For example, Someya's group shaped gold film into a wrinkled form for developing capacitive strain sensors, with a stretchability up to 140%. [16] In addition to the geometrical engineering, the usage of intrinsic stretchable materials as electrode for soft capacitive strain sensors is the other strategy widely adopted. These intrinsic stretchable materials include liquid metal, [27] nanomaterial/elastomer composites, [28] and conductive textiles. [29] Dickey and co-workers developed a liquid-metal-based capacitive strain sensor having a high stretchability (≈100%). [8] A capacitive strain sensor using conductive textile electrode was developed by Walsh's group, with a stretchability of 100% and a sensitivity of 1.23. [29] Cohen et al. reported a carbon nanotube (CNT)/elastomer-based capacitive sensor, owning a stretchability of 100% and a sensitivity of 0.99. [7] Although the above strategies can provide capacitive sensors with high stretchability, most of the electrode materials are not intrinsically self-healable. To fully mimic the functionality of human skin, the self-healability for sensory devices is highly demanded. [30] Hydrogels which consist of 3D networks with large quantity of water or ionic liquid have been proposed as electrode materials for soft electronics, due to their large stretchability, self-healability, and biocompatibility. [31,32] For sensing applications, hydrogel-based resistive strain sensors have been widely studied. [33][34][35][36] These devices have achieved large sensing limits, but the hydrogels have relatively long self-healing time and low self-healing efficiency. [33,36,37] Additionally, some of them needs external heating to accelerate the self-healing process. [35,38] Through the dynamic complexing interaction between metal ion and OH group, polyvinyl alcohol (PVA) based hydrogel has been investigated as active materials for constructing sensors. [39,40] CNT/PVA-based resistive strain Capacitive strain sensors could become an important component of electronic skin (E-skin) due to their low hysteresis and high linearity. However, to fully mimic the functionality of human skin, a capacitive strain sensor should be stretchable and self-healable. The development of such a sensor is limited by electrode materials which generally lack self-healability and/or stretchability. A highly stretchable and self-healing MXene (Ti 3 C 2 T x )/polyvinyl alcohol (PVA) hydrogel electrode is developed for use in capacitive strain sensors for E-skin. The...

Highly Conductive, Scalable, and Machine Washable Graphene‐Based E‐Textiles for Multifunctional Wearable Electronic Applications

“…Wearable strain sensors, which can detect the mechanical deformations have attracted significant interests for motion monitoring of human body parts. [ 57–61 ] In order to demonstrate other potential applications of graphene‐based highly conductive textiles as skin‐mounted strain sensors, graphene‐coated, compressed and encapsulated textiles were attached on different parts of the body, for example index finger, wrist, and elbow joint (Figure 5d−f). The change of the resistances with the movement of various parts of the body before (Figure 5a−c) and after wash samples (Figure 5g−i) are measured.…”

Advances in Rational Design and Materials of High‐Performance Stretchable Electromechanical Sensors

“…The structural design of dielectric layers is important to control the sensing performance of such sensors. These designs include wrinkled structures, [ 22,123,127 ] pyramid shapes, [ 45,134,135 ] air gaps, [ 122,129 ] conformal structures, [ 114,120 ] micropillars/microtower patterns, [ 128,131 ] bionic komochi konbu structures, [ 132 ] parallel lines, [ 57 ] nanofibers, [ 126 ] microporous structures, [ 133 ] and ecoflex layers. [ 136,137 ] The materials and designs of capacitive pressure sensors are chosen to achieve high stretchability, high sensitivity, wide pressure range, low detectable pressure limit, and high linearity.…”

Highly Sensitive and Flexible Pressure Sensor Prepared by Simple Printing Used for Micro Motion Detection

“…Clearly, as the pressure increased, ∆ R / R shows a downward trend, this also means the resistance is decreasing, especially when the pressure ranges from 0 to 0.1 kPa, the resistance change presents a certain linear relationship, and according to the calculation formula of sensitivity, GF is about 12.96 kPa −1 , when the pressure is about 0.1–1 kPa, the GF is about 1.33 kPa −1 , when the pressure reaches about 1 kPa, the resistance change value is not so obvious, indicating that the sensing range of the sensor is 0–1 kPa. Compared to other sensors, our sensor displays superior sensitivity, due to the good conductivity of 0.47 S m −1 . More importantly, the structure of face‐to‐face assembled method has double conductive layers which offers more recoverable and deformable space when subjected to an external force than a single conductive layer sensor, what's more, the rough surface of the conductive layers also facilitates larger contact area.…”