Tough, Resilient, Adhesive, and Anti-Freezing Hydrogels Cross-Linked with a Macromolecular Cross-Linker for Wearable Strain Sensors

Abstract

Ideal conductive hydrogels for flexible, wearable strain sensors should be tough, highly resilient, adhesive, and anti-freezing. However, such hydrogels are difficult to design. Herein, a multifunctional macromolecular crosslinker (MC) based on poly(hydroxyethyl-L-glutamine) was designed and used to synthesize the hydrogels. Cross-linking with the MC leads to a reduced inhomogeneity of the gel network. Therefore, the mechanical properties of the gels are significantly improved compared with the ordinary hydrogels cross-linked with the conventional cross-linker N,N-methylenebisacrylamide (BIS). The MCcross-linked gels also exhibit high resilience. At the same time, replacing BIS with MC significantly improves the adhesive properties of the gel, which is attributed to the introduction of a large amount of adhesive groups with the MC. The gels can stick to various substrates including skin. The good tissue adhesiveness of the gel allows it to stick to skin by itself without using any straps or adhesive tapes when used as a flexible wearable strain sensor. Both large and subtle human movements were successfully monitored using the sensor. The signals are highly stable and reliable, thanks to the high resilience of the gel. The introduction of the polar groups also improved dramatically the anti-freezing properties of the gels. Even at −20 °C, the gels still remained highly flexible and stretchable, therefore allowing the gel-based sensor to work at sub-zero temperatures. The excellent toughness, resilience, tissueadhesiveness, and anti-freezing properties of the gel make it a good choice for a flexible wearable sensor.

Assembled caseins as crosslinkers for tough, adhesive and self-healing hydrogels towards flexible sensors

“…A large crosslinker in hydrogel preparation helps in the formation of a three-dimensional network with a homogeneous structure, resulting in an excellent mechanical behavior of the hydrogel. 15,34 The mechanical performance of CAP hydrogels was quantified using a tensile machine. Casein@Ag + @BACA with varying casein contents of CAP hydrogels was systematically used to test for the mechanical properties.…”

Highly transparent, mechanical, and self-adhesive zwitterionic conductive hydrogels with polyurethane as a cross-linker for wireless strain sensors

“…A detailed comparison of PASU-Zn with reported hydrogels is shown in Table S2 (ESI †). 17,27,29,[47][48][49][50][51][52][53][54][55][56][57][58]…”

“…In addition, the introduction of the multi-functional crosslinking agent MC endowed hydrogels with excellent tissue adhesion and frost resistance. 29 In this way, it is necessary to design a WPU-based macromolecular crosslinking agent to prepare hydrogels with excellent mechanical strength and ionic conductivity instead of traditional crosslinking agents.…”

Multi‐Network Poly(β‐cyclodextrin)/PVA/Gelatin/Carbon Nanotubes Composite Hydrogels Constructed by Multiple Dynamic Crosslinking as Flexible Electronic Devices

“…Adhesion is also an important parameter for hydrogels used to prepare strain sensors. [70][71][72] For example, Zhang et al [73] used poly(hydroxyethyl-L-glutamine) as cross-Linker to prepare an adhesive hydrogel that could adhere to all kinds of substrates (such as skin), and this property is beneficial to the preparation of self-adhesive hydrogel-based strain sensors. Chen's group [74] reported a self-adhesive ionic hydrogel for multimode tactical sensing.…”