Interfacial impedance engineering in ionic hydrogels for integrated enzymatic and electrophysiological sensing

Interfacial charge-transfer resistance remains a critical bottleneck in wearable enzymatic biosensors, particularly in soft, deformable systems, where impedance instability limits signal amplification. Here, we introduce an impedance-engineered dual-network ionic hydrogel designed to deliberately modulate ionic conductivity and charge-transfer resistance (R_ct) at the bioelectronic interface. Incorporation of tetrabutylammonium bromide (TBAB) into a chitosan/PEGDE/AAm-co-AA matrix increased ionic conductivity to ∼8.5 × 10^-3 S cm^-1 and reduced R_ct to ∼190 Ω, demonstrating a quantitative structure-impedance-sensitivity relationship. The optimized hydrogel achieved a glucose sensitivity of 31 ± 2 μA mM^-1 cm^-2 over the 0.1-10 mM range, a detection limit of 18 μM, and a rapid response time (t₉₀ = 3.2 s), representing a >3-fold enhancement over the undoped control. Beyond biochemical sensing, the ionically continuous interface enabled strain-dependent electrical modulation (ΔR/R₀ ≈ 0.65 at 60% strain) and high-fidelity ECG acquisition with a signal-to-noise ratio of ∼47 dB. A dual glucose-lactate configuration demonstrated selective and additive responses within the same electrolyte layer. The lactate-sensing channel exhibited a linear response over the range 0.1-6 mM, a detection limit of approximately 12 μM, and rapid response kinetics, confirming efficient multimodal sensing within an integrated soft biointerface. Supporting machine-learning analysis identified R_ct and ionic conductivity as dominant determinants of sensitivity, validating impedance modulation as a scalable materials-level design principle for multifunctional wearable biointerfaces.