Skin-inspired sensors have great potential applications in wearable and implantable devices to monitor human motions. Robust tissue adhesion, fatigue resistance, and biocompatibility are desired for in situ signal capture and wireless transmission. Inspired by the mussel and zwitterionic adhesion mechanisms, we prepared novel stretchable, self-healing and polydopamine zwitterionic nanocomposite hydrogels that provide reversible and robust adhesion to tissues with a strength up to 19.4 kPa and a strain sensitivity of 4.3. The hydrogel sensors robustly adhere to organs like the heart, liver, and lungs to capture signals for remote monitoring and diagnostics through wireless transmission.

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Stretchable, self-healing and tissue-adhesive zwitterionic hydrogels as strain sensors for wireless monitoring of organ motions

Recently, flexible and wearable electronics have gained considerable research interest due to their potential applications in wearable devices, energy storage materials, electronic skins, sensors, etc. Compared to elastomers, hydrogels demonstrate more potential for flexible electronics because of their biomimetic structures, suitable mechanical properties and excellent biocompatibility. Among all the designs, nanocomposite hydrogel-based strain and pressure sensors which can transmit external stimulus to electrical signals have been intensely investigated due to their high mechanical strength, considerable conductivity and outstanding sensitivity. Numerous reports have been dedicated to the designs, preparations and applications of nanocomposite hydrogels. This review provides an up-to-date and comprehensive summary of research progresses of nanocomposite hydrogel-based strain and pressure sensors including designing strategies, preparing methods and applications of the five nanofiller based hydrogel sensors including carbon nanotube based, graphene oxide based, MXene based, polymer nanofiller based and other nanofiller based sensors. Representative cases are carefully selected and discussed regarding the fabrication, merits and demerits, respectively. Finally, perspectives and challenges are presented for the designs of future nanocomposite hydrogel-based strain and pressure sensors.

Nanocomposite hydrogel-based strain and pressure sensors: a review

Flexible sensors have shown great potential in remote health monitoring, body movements track, electronic skin, human-machine interfaces, and soft robotics. Hydrogels possess exceptional stretchability, flexibility and biocompatibility that render them appealing candidates for wearable flexible sensors. Among them, considerable efforts have been devoted to developing conductive hydrogels to achieve multifunctional wearable sensing through using functional groups/additives/nanofillers to modify the hydrogel network in recent years. This review summarizes recent advances of applications of hydrogels in flexible wearable sensors, such as sweat sampling and flexible electrodes, strain/pressure sensors and touch panels, focuses on the multifunctional conductive hydrogels-based flexible wearable sensors with self-healing, self-adhesion, or anti-freezing capabilities. A brief introduction to representative synthesis methods and strategies of conductive hydrogels is also presented. In the end, we also provide a personal perspective on the future development, and address the remaining challenges in the commercialization of conductive hydrogels-based multifunctional flexible wearable sensors.

Multifunctional conductive hydrogel-based flexible wearable sensors

Recent Progress in Essential Functions of Soft Electronic Skin

Ionic conductive hydrogels with long-lasting antifreezing, water retention and self-regeneration abilities

With growing interest in flexible and wearable devices, the demand for nature-inspired soft smart materials, especially intelligent hydrogels with multiple perceptions toward external strain and temperatures to mimic the human skin, is on the rise. However, simultaneous achievement of intelligent hydrogels with skin-compatible performances, including good transparency, appropriate mechanical properties, autonomous self-healing ability, multiple mechanical/thermoresponsiveness, and retaining flexibility at subzero temperatures, is still challenging and thus limits their application as skinlike devices. Here, conductive nanocomposite hydrogels (NC gels) were delicately designed and prepared via gelation of oligo(ethylene glycol) methacrylate (OEGMA)-based monomers in a glycerol-water cosolvent, where inorganic clay served as the physical cross-linker and provided conductive ions. The resultant NC gels exhibited good conductivity (∼3.32 × 10-4 S cm-1, akin to biological muscle tissue) and an autonomously self-healing capacity (healing efficiency reached 84.8%). Additionally, such NC gels displayed excellent flexibility and responded well to multiple strain/temperature external stimuli and subtle human motions in a wide temperature range (from -20 to 45 °C). These distinguished properties would endow such NC gels significant applications in fields of biosensors, human-machine interfaces, and soft robotics.

Conductive Self-Healing Nanocomposite Hydrogel Skin Sensors with Antifreezing and Thermoresponsive Properties.

Wearable electronics have stimulated interest over the past few years due to their flexible and portable properties. Among them, hydrogel-based strain sensors demonstrate tremendous advantages because they exhibit attractive characteristics such as stretchability, skin-compliance, ion transport capability, and biocompatibility. However, their low strength compared to natural tissues, unstable sensing performance upon swelling, and a relatively low response rate arising from loose cross-linking networks still pose numerous challenges for their further applications. Here, tough hybrid hydrogel fibres with heterogeneous networks for stretchable, stable and high-performance wearable strain sensors are reported. In such gel fibres, a secondary polyaniline (PANI) hydrophobic nanostructured conductive polymer was constructed in clay/P(MEO2MA-co-OEGMA-co-NIPAM) nanocomposite gel fibres to achieve tough hybrid gel fibres with high electrical conductivity (87.99 S m−1) and good swelling resistance. Besides, multimolecular interactions among the heterogeneous structured gel fibres endowed them with high fracture energy (172.43 kJ m−2) and excellent tensile strength (7.21 MPa). Furthermore, such hybrid gel fibres allowed for integration into traditional fabrics to monitor human motions and physiological signals rapidly and sensitively in multiple environments (under air/wet conditions or underwater). It is believed that such hybrid gel fibres would create new opportunities in the design and manufacture of wearable and implantable sensors for health monitoring, innovative electronics and human–machine interactions.

Hongmei Liu

Papers

Conductive Self-Healing Nanocomposite Hydrogel Skin Sensors with Antifreezing and Thermoresponsive Properties.

Heterogeneous structured tough conductive gel fibres for stable and high-performance wearable strain sensors