Hierarchical MOF–Polyaniline Hybrid Fillers Regulate Strain-Dependent Conductive Pathways in PVA Hydrogels for Wearable Sensing

Abstract

Conductive hydrogels have emerged as promising materials for wearable electronics owing to their combination of mechanical flexibility, high water content, and electrical responsiveness. However, achieving a suitable balance among conductivity, mechanical robustness, and electromechanical sensitivity remains challenging. Herein, we report a physically crosslinked poly(vinyl alcohol) (PVA) hydrogel incorporating zirconium fumarate metal–organic framework (Zr-MOF-801) particles functionalized through in situ polymerization of polyaniline (PANI), yielding a MOF-PANI hybrid filler. Incorporation of MOF-PANI increased the apparent bulk conductivity of the hydrogel while preserving its flexibility and structural integrity. Mechanical characterization revealed a marked change in the response of the materials above approximately 60% strain. Simultaneous tensile and chronoamperometric measurements showed clearly different strain-dependent electrical responses for the pristine Hydro, Hydro/MOF, and Hydro/MOF-PANI formulations. The pristine hydrogel exhibited negative ΔR/R0 values during stretching, whereas both MOF-containing systems showed positive resistance variations. PANI functionalization further increased the response magnitude and improved short-term cyclic reproducibility. The Hydro/MOF-PANI hydrogel exhibited an apparent bulk conductivity of approximately 7.0 × 10−3 S cm−1 and an approximately linear electromechanical response over the investigated 20–110% strain range, with an apparent strain sensitivity of about 2.2. The observed differences among the three formulations are consistent with filler-induced changes in the overall transport network; however, the specific microscopic contributions of ionic transport, polymer–particle interfaces, and PANI-containing conductive regions cannot be directly resolved from the present measurements. Finally, integration of Hydro/MOF-PANI into an elastic knee brace enabled proof-of-concept detection of repeated knee flexion–extension movements. These results identify MOF-PANI/PVA hydrogels as a promising platform for strain-responsive soft materials and wearable motion sensing, while further studies are required to establish long-term durability, rate dependence, hydration stability, and quantitative wearable calibration.