Direct Electrochemical Signaling in Organic Electrochemical Transistors Comprising High-Conductivity Double-Network Hydrogels

In composite hydrogels, the high electrical performance of poly(3,4-ethylenedioxythiophene) complexed with poly(styrenesulfonate) (PEDOT:PSS) is integrated with complementary structural and electrochemical functions via a rationally designed poly(acrylamide) second network incorporating phenylboronic acid (PBA). Free-standing double-network hydrogels prepared by a simple one-pot radical polymerization exhibit state-of-the-art electrical conductivity (∼20 S cm^-1 in phosphate buffered saline) while retaining a degree of hydration similar to that of biological soft tissues. Low resistance contacts to Au electrodes are formed via facile thermo-mechanical annealing and demonstrate stability over a month of continuous immersion, thus enabling hydrogels to serve as channels of organic electrochemical transistors (OECTs). Despite thicknesses of ∼100 μm, gating of hydrogel OECTs is efficient with transconductances g_m ∼ 40 mS and on/off ratios of 10³ in saturation mode operation, whereas sufficiently high conductivity enables linear mode operation (g_m ∼ 1 mS at -10 mV drain bias). This drives a shift of sensing strategy toward detection of electrochemical signals originating within the bulky channel. A kinetic basis for glucose detection via diol esterification on PBA is identified as the coupling of PBA equilibrium to electrocatalyzed O₂ reduction occurring on PEDOT in cathodic potentials. Hydrogel OECTs inherently amplify this direct electrochemical signal, demonstrating the viability of a new class of soft, structural biosensors.