Interface Engineered ZnO Nanowire/Liquid-Metal PVA Nanocomposites with Dual-Pathway Conduction for Tensile Strain and Humidity Sensing

Abstract

Wearable strain sensors based on conductive polymer composites often cannot distinguish mechanical deformation from environmental stimuli, which limits their use in complex on-body scenarios. We address this problem through interface engineering: high-aspect-ratio ZnO nanowires (NWs), grown for 12–36 h (length 1–4 μm; diameter 100–250 nm; aspect ratio 7–28), are integrated into a poly(vinyl alcohol)/eutectic gallium–indium (PVA/EGaIn) matrix to create a nanocomposite with two coupled conduction pathways, each governed by a nanoscale interface, that respond preferentially to different stimuli. Pathway I, strain-induced coalescence of discrete EGaIn microdroplets (mean diameter 34 μm at 1500 rpm shear mixing), produces a normalized current change ΔI/I0 of 1.2 under 60% tensile strain, approximately twice the cycle-averaged response of the pristine PVA-LM control measured in parallel, while compressive loading leaves the two devices indistinguishable. Pathway II, humidity-gated ionic transport consistent with water-mediated Grotthuss proton hopping across the PVA-ZnO NW interface, yields an on/off ratio of 3.12 at 90% relative humidity (RH) and increases with NW aspect ratio at every RH tested, reflecting the larger hydroxyl-rich surface area available for interfacial water adsorption. Because the mechanical transients (seconds) are far faster than ambient humidity drift (tens of seconds to minutes), the single current output can be decomposed into a humidity-encoding baseline and a strain-encoding transient; stretch-release cycling at 45–90% RH confirms that the baseline-normalized tensile response is preserved for the 12 and 24 h devices across this range. These properties are demonstrated in a wearable human-machine interface that resolves biceps contraction against inactive-muscle baselines during exercise and converts the raw signals into quantitative workout-intensity feedback. Reversible Ga2O3 reoxidation and PVA hydrogen-bond reformation restore the insulating resting state after each stretch–release event, and alkaline treatment disassembles the composite with direct recovery of the liquid-metal phase. These results establish the nanowire aspect ratio as a practical geometric design parameter for tuning mechano-ionic transduction in end-of-life recoverable soft-matter sensors.