Towards a wearable format for transducing responsive swelling in hydrogels using impedance

Abstract

Responsive hydrogels offer significant promise for wearable biochemical sensing owing to their tuneable chemistries and biocompatible, tissue-like properties. In this study, we demonstrate a wearable impedance configuration as a novel transduction approach for monitoring swelling dynamics in planar poly(acrylic acid-co-N, N’-methylene-bis-acrylamide) (PAAc-co-MBA) hydrogel films. Using surface-mounted gold contact pins,we extract a localised gel resistance parameter (Rgel) that was correlated directly with gravimetric swelling measurements, and was shown to provide a quantitative measure of swelling-induced changes. Impedance-based measurements exhibited superior sensitivity compared to the gravimetric measurements, attributed to localised interrogation of hydrogel regions near the electrode interface where equilibration occurs rapidly. The apparent pKa of the hydrogel, determined from impedance-monitored pH titrations, was measured as 4.1, within 0.4 pH units of the theoretical PAAc value. Systematic optimisation of starting oxidant concentrations revealed that formulations with lower oxidant content achieved optimal mechanical properties suitable for applying to tissue, demonstrating excellent skin adhesion (>350 flexion cycles) whilst maintaining requisite flexibility for conformal wearable applications. To validate tissue interfacing capabilities, hydrogel films were applied to exposed kiwi fruit tissue (pH~4.0), where the impedance response reflected hydrogen ion diffusion from the tissue into the hydrogel (neutral pH). Concurrent pH mapping and dimensional analysis confirmed ion transport across the tissue-hydrogel interface, demonstrating the material’s capacity for real-time biochemical monitoring of soft interfaces. These findings establish simplified impedance-transduced PAAc-co-MBA hydrogels as promising platforms for next-generation epidermal sensors, offering the potential of quantitative, non-invasive monitoring of tissue pH with direct applicability to wearable health monitoring technologies.