Hydrogel electronics: New horizons of flexible, wearable, and implantable devices

Abstract

Hydrogel electronics have recently attracted extensive research interest for great potential for flexible, wearable, and implantable devices for robotics, human-machine interface, and advanced healthcare. Unlike silicon and metal-based traditional electronics, hydrogel electronics are highly stretchable, flexible, and adaptive to biotissues and organs. Conductive hydrogels coated on medical devices can bridge bioelectronics and biotissues to collect biophysical signals or work as electrodes to provide electrical stimulation. Tough, stretchable, and fatigue-resistant hydrogels are needed to endure cyclic loadings. The sensitivity, linearity, and detection of limit are key parameters for hydrogel sensors. Besides, tolerance against harsh environments usually requires resistance against low or high temperatures without losing the mechanical and sensory performances. The sensory performances depend on not only the intrinsic network structures but also the micro-/nanostructures of hydrogel devices. Delicate network designs are needed to achieve high conductivity, high sensitivity, and linear sensing, while diverse micro-/nanostructures are fabricated to amplify the sensitivity. Polymer hydrogel electronics have emerged as new horizons of flexible, wearable, and implantable devices. However, it is still at a very early stage. Key fundamental issues like the sensing mechanisms and structure-sensory performance relationship need extensive and intensive studies. On the other hand, although numerous potential applications are demonstrated, more endeavors are needed to tackle critical issues for practical applications. This Special Issue on Polymeric Hydrogels for Flexible Electronics collects a series of review articles accounting latest progress in this rapidly-developing field and research articles with novel ideas to tackle urgent problems of hydrogel electronics. The Review article by Pan and coworkers presents a comprehensive overview of the preparation and properties of conductive hydrogels and applications for flexible electronics. It focuses on conductive polymer hydrogels that use conductive polymers to transport electrons. Three methods are introduced to synthesize conductive polymer hydrogels, in situ polymerization, direct polymerization, and pure conductive polymer hydrogels. It points out that hydrophilic modification of conductive monomers or polymer chains promotes hydrophilicity and homogeneous dispersion of conductive polymers in hydrogels. The interpenetrating network structures improve the toughness, strength, and conductivity of hydrogels. It discusses self-healing and environment tolerant hydrogels that remain stable against mechanical or environmental challenges, which can extend the lifetime of flexible devices. Processible conductive hydrogels are described for the fabrication of devices with different shapes or structures. Representative applications of conductive hydrogels are reviewed, including supercapacitors and flexible stress/strain sensors. It compares the capacity and sensory performances of supercapacitors with various architectures, including fiber-shaped, sandwich-type, and micro-structured capacitors. Besides, it also describes typical flexible stress/strain sensors based on conductive hydrogels. The biomimetic hydrogel networks provide an environment for biological and/or electrochemical reactions, which is used as biosensors. Gao et al. reviewed recent progress in tough conductive hydrogel-based strain sensors. Tough hydrogels crosslinked by hydrophobic association are conveniently converted into anti-freezing and ion conductive hydrogels by adding electrolytes. They further explain how to utilize multivalent metal ion coordination to synthesize processible tough hydrogels. The hydrogels are ion conductive. On the other hand, charges on crosslinked polyelectrolyte chains can provide ion conduction channels for ionic conductive sensors. It accounts for recent progress on 2D materials composite hyd