Conductive, flexible and stretchable hydrogel sensors have attracted a lot of interest in human-machine interfaces, medical monitoring, and soft robotics. However, there are still have great challenges in the preparation of conductive hydrogels with self-adhesive, self-power and antibacterial properties. In order to meet the requirements of high sensitivity, convenient carrying and anti-allergic, so as to overcome the problems of traditional sensors, such as non-stickiness, absolute dependence on external power supply and skin irritation, the prepared poly(vinylalcohol)/poly(acrylamide-co-[2-(methacryloyloxy) ethyl] dimethyl-(3-sulfo-propyl) ammonium hydroxide) (PVA/P(AM-co-SBMA), PPS) hydrogel was obtained by UV crosslinking and freeze-thaw cycles. The PPS hydrogel has good mechanical properties (the elongation was 700% and the breaking strength was 370 kPa), high adhesion to various substrates, antibacterial properties (~99%) and wide sensory sensitivity. And the self-power sensor can be achieved by using hydrogel as the conductive medium to assemble primary battery with zinc foil and copper foil, the open circuit voltage of this self-powered system can reach 0.86 V. Furthermore, the assembled strain sensor can also convert chemical energy into electrical energy, and transform the resistance changes caused by stretching or compression into voltage signals for output. It can be used as effective wearable power supply for human motion detection. This kind of self-adhesive and antibacterial self-powered strain sensor was expected to play a great role in flexible bioelectronics.

Adhesive, stretchable and antibacterial hydrogel with external/self-power for flexible sensitive sensor used as human motion detection

Animal skin is a huge source of inspiration when it comes to multifunctional sensing materials. Bioinspired sensors integrated with the intriguing performance of skin-like steady wide-range strain detection, real-time dynamic visual cues, and self-healing ability hold great promise for next-generation electronic skin materials. Here, inspired by the skins of a chameleon, cellulose nanocrystals (CNCs) liquid crystal skeleton is embedded into polymerizable deep eutectic solvent (PDES) via in situ polymerization to develop a skin-like elastomer. Benefiting from the elastic ionic conductive PDES matrix and dynamic interfacial hydrogen bonding, this strategy has broken through the limitations that CNCs-based cholesteric structure is fragile and its helical pitch is non-adjustable, endowing the resulting elastomer with strain-induced wide-range (0-500%) dynamic structural colors and excellent self-healing ability (78.9-90.7%). Furthermore, the resulting materials exhibit high stretch-ability (1163.7%), strain-sensing and self-adhesive abilities, which make them well-suitable for developing widely applicable and highly reliable flexible sensors. The proposed approach of constructing biomimetic skin-like materials with wide-range dynamic schemochrome is expected to extend new possibilities in diverse applications including anti-counterfeit labels, soft foldable displays, and wearable optical devices.

Polymerizable Deep Eutectic Solvent-Based Skin-Like Elastomers with Dynamic Schemochrome and Self-Healing Ability.

Integration of hydrogen bonding interaction and Schiff-base chemistry toward self-healing, anti-freezing, and conductive elastomer

Deep eutectic solvents (DESs) have received widespread attention for their advantages of good electrical conductivity, stability, environmental friendliness, and easy preparation with a wide range of applications. However, it was not until recent years that DESs were used as gel electrolytes as environmentally friendly alternatives to ionic liquids (ILs). In this mini review, we first introduce the DESs related eutectogels (or ion gels), then introduce the types and applications of the gels, the opportunities and challenges of them are also summarized.

Deep eutectic solvents eutectogels: progress and challenges

Ion conductors (ICs) have gained extensive research interest in various advanced application scenarios including sensors, batteries, and supercapacitors. However, stretchable, tough, and long‐term stable ICs are still hard to achieve yet highly demanded. In this study, the authors propose a one‐pot green and sustainable fabrication of cellulose based ICs through polymerizable deep eutectic solvents treated cellulose followed by an in situ photo‐polymerization. The obtained ICs exhibit extremely high stretchability (3210 ± 302%), high toughness (13.17 ± 2.32 MJ m−3), high transparency, and self‐healing ability. Notably, the introduction of cellulose fibers greatly enhances the mechanical properties of ICs while eliminating the environmental concerns of traditional nanocellulose fabrication process. More importantly, the ICs possess good long‐term performance stability after 1 month storage. Due to these outstanding properties, the feasibility of applying ICs in human motion sensing and physiological signal detecting is demonstrated. This simple and green method will contribute to the development of tough, self‐healing, transparent, and long‐term stable ICs.

Tough and Ultrastretchable Liquid‐Free Ion Conductor Strengthened by Deep Eutectic Solvent Hydrolyzed Cellulose Microfibers

Conductive elastomers (CEs) with strong mechanical properties have been fabricated and used in flexible electronics. However, the development of CEs with both super-high mechanical strength and extreme stretchability remains challenging. This paper reports on the development of a series of mechanically strong and tough CEs based on photopolymerizable deep eutectic solvents (PDESs) with dense hydrogen bonding interactions. One of these CEs exhibits the highest reported mechanical performance for elastomers, with tensile strength, strain at break and toughness up to 31.21 MPa, 3645% and 615 MJ m−3, respectively. Notably, this CE tolerates puncture and can lift a 10.5 kg weight (9500 times its own weight) without any failure. In addition, the series of CEs also shows excellent transparency (>94% in the visible range), favorable conductivity (0.007–0.04 S m−1), tunable stretchability (strain from 22% to 6164%) and good self-healing capability (electrical healing efficiency of 99% within 0.26 s). In view of the comprehensive properties of the CEs, we demonstrate the practical suitability of CEs by including them in a strain sensor to detect human motion. Based on their extremely facile preparation process and practical functionalities, we believe that CEs will not only contribute new methods for the development of mechanically strong conductors, but also open up novel avenues for flexible electronics.

Jimin Yang

Papers

Polymerizable deep eutectic solvent-based mechanically strong and ultra-stretchable conductive elastomers for detecting human motions