An anisotropic conductive hydrogel for strain sensing and breath detection

Abstract

Conductive hydrogels, with their excellent conductivity and flexibility, are highly suitable for use as strain sensors. However, they often exhibit weak mechanical properties and isotropy, which can limit their application potential. Moreover, the incorporation of self-powering capabilities within hydrogels would provide a significant advantage for sensor applications. In this study, we utilized hydrophilic delignified wood as a scaffold and impregnated it with FeCl3 solution to fabricate conductive wood. Subsequently, a precursor solution of polyvinyl alcohol (PVA), chitosan (CS), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) was prepared by heating and mixing. The conductive wood was then fully infiltrated with the precursor solution and transformed into a conductive wood-based hydrogel (CWH) using a freeze-thaw method. The aligned fibers in the conductive wood scaffold imparted anisotropic mechanical and electrical properties to the hydrogel, with a fracture stress anisotropy ratio of 47.5 (ratio of values parallel to the fiber direction to values perpendicular to the fiber direction) and an electrical conductivity anisotropy ratio of 1.55. The hydrogel remained soft after bonding with the wood scaffold, preserving its potential as a sensor. Experimental results demonstrated that the CWH sensor exhibited stable strain sensitivity (gauge factor of 0.2805 with R²=0.998 in the range of 0∼90°), enabling the monitoring of human joint bending and capturing facial smiles. The CWH also exhibited excellent resistance to expansion while maintaining high water content (79.4%), making it more suitable for use in humid environments compared to traditional hydrogels. Furthermore, we assembled the CWH into a humidity-driven generator and conducted power generation tests, revealing its promising power generation potential. The voltage significantly increased when exposed to humid air (from 6.2 mV to 66.1 mV). Lastly, we integrated the humidity-driven generator with a mask to create a breath detector, which was connected to an electrochemical workstation for testing. The test curve demonstrated the successful capture of orderly breathing activities from volunteers. The combination of these advantages positions CWH for a wide range of applications, including wearable flexible strain sensors, breath detection, bioelectrodes, and green energy generation devices.