Anisotropic, ultra-sensitive, self-adhesive, biocompatible, and conductive hydrogels prepared for wearable sensors

Conductive hydrogels (CHs) stand as ideal candidates for flexible electronic devices. However, existing CHs cannot integrate the high sensitivity under low deformation, adhesion capability and biocompatibility into one system, which greatly retard their applications. Inspired by muscle structures, anisotropic conductive hydrogels were fabricated to address this challenge. The hydrogels were obtained through a stretch-induced orientation strategy, that is, freezing - thawing the aqueous mixture of poly(vinyl alcohol) (PVA), carbon nanotubes (CNTs), tannic acid (TA) and deionized water two cycles to obtain the precursor hydrogel; disrupting the H bonds between PVA chains by thermal stretching at 60 degrees C to induce anisotropic microstructures; and finally reforming them by freezing the pre-stretched gel at -20 degrees C for 4 h to fix the regular structures. This strategy resulted in anisotropic conductivities and mechanics. For instance, the mechanical anisotropy ratio (defined as the ratio between parallel and orthogonal directions) was 2.83. In addition, the electrical anisotropy ratio was 2.19. Meanwhile, CNTs and regular conductive channels imparted high sensitivity (GF = 8.5 within 50% strain), fast response (0.2 s), and low detect limit (0.2% strain). Additionally, the existence of catechol groups of TA enabled self-adhesion ability (maximum adhesion strength = 72.63 kPa). Due to non-toxicity of raw materials and physical process, the hydrogel was also biocompatible (cell viability = 100%). All of these merits hold great potential in human - machine interactions, medical monitoring, and smart electronic skins.