A neural cell-compatible electroactive hydrogel Interface for reversible stiffness modulation at physiological temperatures.

The mechanical properties of neural tissues are inherently dynamic and play a crucial role in regulating cellular behavior during development, disease progression, and response to injury. However, common in vitro models cannot generally replicate these spatiotemporal mechanical changes, which limits their physiological relevance, especially in studies of complex central nervous system pathologies such as brain tumors, glial scarring, and traumatic brain injuries. To address this limitation, a novel mechanobiology-on-a-chip platform was developed, incorporating a reversible-stiffness hydrogel composed of poly(N-isopropylacrylamide) (pNIPAM) and polypyrrole (PPy; pNIPAM-PPy). Building on prior work, the hydrogel was engineered to actuate at its volume-phase transition temperature (VPTT), where stiffness changes are most pronounced and reversible. The VPTT was tuned to reflect a physiologically relevant microenvironment of 37 °C. Comprehensive material characterization confirmed robust thermo-responsiveness and revealed key mechanisms underpinning pNIPAM-PPy actuation. Electrochemical impedance spectroscopy suggested that ion mobility is a major contributor to VPTT-associated actuation. At 37 °C, the hydrogel exhibited a clear reduction/oxidation-dependent stiffness shift, with a Young's modulus of 2.4 ± 1.3 kPa in the reduced state and 3.7 ± 1.4 kPa in the oxidized state. The mechanobiology-on-a-chip platform was further used to culture and electrically stimulate human neuroblastoma SH-SY5Y cells on pNIPAM-PPy substrates, where electrical perturbations simultaneously modulate the electrochemical state of PPy and the coupled mechanical state of the hydrogel. Under electrochemical oxidation, cell metabolic activity increased to 147.5 ± 5.5% compared to the unstimulated control (100.0 ± 8.7%), whereas under electrochemical reduction, it decreased to 60.7 ± 14.2%. These findings indicate that SH-SY5Y responses depend strongly on the electrochemical state and on coupled material changes. In particular, the elevated metabolic activity observed under oxidized conditions may reflect a pro-survival effect of a stiffer substrate on cultured neuroblastoma cells. Overall, this dual-function system enables on-demand, reversible tuning of electroactive hydrogel mechanical properties during cell culture, providing a versatile tool for studying neural cell behavior in dynamic microenvironments.