Ultra-stretchable hydrogels with reactive liquid metals as asymmetric force-sensors

Abstract

Stretchable electronic materials have attracted great attention due to their promising applications in flexible sensors, electronic skins and devices. This work reports the convenient use of liquid metals (LMs) as liquid fillers in hydrophilic polymer networks to realize ultra-stretchable hydrogels as asymmetric force-sensors. The existence of liquid metals endows the hydrogel with features that make them suitable for use in both convenient synthetic methods and unique sensing applications. Liquid metals unexpectedly behave as ‘reactive’ fillers that promote persulfate radical initiators for autonomous gelation. LM-hydrogels are fabricated via extremely rapid in situ gelation (20 seconds) due to the catalytic effects and special surface interaction of monomers and liquid metals. Additionally, the liquid nature of the LM fillers significantly toughens the hydrogel matrix, affording excellent extensibility (tensile strain: ∼1500%). Furthermore, the LM-hydrogels show a twenty times increase in compressive sensitivity (0.25 kPa⁻¹) compared with pristine hydrogels and exhibit remarkable electrical stability (1000 cycles) and rapid resistive response (180 ms). Different from traditional micropatterned LM sensors, the LM-hydrogels have a distinguished ability to discern a series of subtle motions (such as handwritings and personal signatures) on surfaces attributed to the asymmetric deformation due to the fluidic nature of the liquid metals.