Rod-like percolation of conducting polymers via vapor-phase polymerization into cellulose nanofibril hydrogels

Conducting polymers are essential for soft bioelectronics, but they are challenging to process into homogeneous, low-solidity hydrogels due to their poor solubility and tendency to agglomerate. Here, we utilize cellulose nanofibril (CNF) hydrogels as a percolating template for the vapor-phase assisted polymerization of conducting polymers. Pyrrole efficiently polymerizes within the hydrated CNF network, forming a conformal polypyrrole (PPy) coating that yields conductivities approaching 100 S/m and charge storage of 16 mAh/g (dry) or capacitance of 52 F/g solids at 93 wt% water content. The CNF framework induces rod-like percolation of the PPy phase, producing unusually low percolation thresholds and non-universal transport exponents. Long-aspect-ratio fibrils further enhance conductivity by increasing the number of effective junctions, and PPy stiffens the hydrogels (0.2-1.5 MPa) by locking these junctions. Glycerol could be used as the liquid phase to prevent evaporation and these gels remained conductive and dimensionally stable in air. Comparison with liquid-phase polymerization highlights that vapor delivery minimizes skin formation and enables more uniform bulk coverage. Finally, we demonstrate 2D/3D patterning and conductive filament fabrication, underscoring vapor-phase polymerization as a versatile route for soft conducting materials, electrodes, and patterned hydrogel devices.