Hydrogel electronics: New horizons of flexible, wearable, and implantable devices
Jun Fu
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 hydrogels as stretchable strain sensors. 2D materials serve as toughening conductive fillers to entitle not only high stretchability, but also conductivity and strain sensitivity. It prospects the challenge of comfortableness, reliability, power supply, and long-term stability of hydrogel-based strain sensors for future investigation. Micro-/nanostructures have vital influences on the sensory performances of conductive hydrogel electronics. The Review by Xie and coworkers overviews the latest progress in patterning technologies to engineer functional gels at micro/nanoscales for soft devices. It introduces patterning strategies, including additive and laser manufacturing, patterned molding, and directed dewetting for fabrication of gel arrays with a resolution of hundreds of micrometers. It also presents printing methods, including inkjet printing, direct writing, and extrusion-based 3D printing, for engineering functional gel arrays with sub-micron to hundreds of micron resolutions. Based on these methods, micropatterned conductive hydrogel arrays with high-density hydrogel pixels have been developed for precise sensing of signal distribution on flat or curved surfaces. They explain how protrusions, including pyramids, wrinkles, and graded intrafillable architectures, provide additional compressibility for hydrogel electrodes to achieve ultrahigh sensitivity. Exploring the relationship between micro-/nanostructures and sensory performance remains a fundamental challenge. High throughput and uniformity are challenges for the scale-up of the patterning strategy. Moreover, massive production of hydrogel arrays with varying dimensions, compositions, and functions is required to achieve high throughput collection and data analysis. To fabricate hydrogel sensors with nano-structures, Zhang et al. synthesize biphasic hydrogels for flexible sensors. Microphase separation of gels in oil/water mixtures generates structures at nano- to micro-meter scale, which imparts excellent stretchability and resistance against cyclic loadings. The microphase-separated structures define conductive channels to provide high resistive sensitivity. The sensory performances are drastically changed around body temperature. Flexible sensors based on the biphasic gels are promising for monitoring human body motions. In order to tackle the problems of low sensitivity, poor environmental resistance, and low-air permeability of wearable strain sensors, Liu et al. develop a novel cryo-spun encapsulation method to fabricate a coaxial fiber comprised of a PVA/PANI core and a thermoplastic elastomer sheath. It leads to improved stretchability, high conductivity, and high-yet-linear sensitivity. The fibrous sensor shows a stable response within broad temperature range and can identify fast and slow bending of dummy joints. A woven fabric of the core-shell fibers show linear sensing of stretching and produce stable sensory signals over a broad temperature range for 6 months. During long-term service, damages are inevitable for flexible electronics. Self-healing is a crucial property that helps maintain the integrity and performance, thus extending the life span of hydrogel electronic devices. The Review article by Zeng et al. focuses on the synthesis and mechanisms of self-healing conductive hydrogels for flexible electronics. It summarizes representative strategies for synthesizing self-healing hydrogels using dynamic chemical and non-covalent bonds to build reversibly crosslinked networks. Dynamic chemical bonds, including Schiff base reaction bonding, borate ester bonds, and Diels-Alder reaction bonding, are reversible under specific conditions. Thus, hydrogels with dynamic bond crosslinks can be injectable and subsequently recovered upon exposure to triggering reagents. Non-covalent bonds, including hydrogen bonding, ionic bonding, metal ion coordination, and host-guest recognition, are usually combined to synthesize self-healing hydrogels with multiple interactions that work synergetically to augment mechanical properties and gain self-healing. Self-healing conductive hydrogels are fabricated into flexible sensors with different configurations for in vitro and in vivo monitoring of human motions. The Research Article by Chen et al. used potassium ion crosslinked κ-carrageenan microgels to composite with hydrophobically crosslinked polyacrylamide to produce physically crosslinked double network hydrogels. The reversible non-covalent interactions generate outstanding mechanical strength, toughness, fatigue resistance, and self-healing. The hydrogels are sensitive to stretching and show rapid healing. To improve the stability of mechanical and electrical properties of hydrogel sensors, Sun et al. developed a novel strategy to introduce natural zwitterionic osmolyte and ammonium chloride as anti-freezing agent into physically crosslinked polyvinylpyrrolidone (PVP)/PAAm hydrogel. Both zwitterionic moieties and salt provide high ion conductivity and excellent tolerance to low temperatures. Sensors based on this hydrogel are flexible at very low temperatures and maintain excellent sensitivity to stress and strain. The broad and reliable working range may enable its use to monitor robotic motions in harsh and dangerous environments. This collection of Review articles and novel research papers on hydrogels for flexible sensors provides comprehensive accounting of latest progress in different aspects of this rapidly developing field and novel studies on high-performance hydrogel sensors. The critical and insightful comments and perspectives are inspiring for new ideas toward solutions to fundamental problems and novel functional devices for practical applications. We are indebted to all authors, reviewers, and editorial staff for their significant contributions to this special issue.