Increasing the Conductivity and Adhesion of Polypyrrole Hydrogels with Electropolymerized Polydopamine

Polypyrrole (Ppy) hydrogels are a promising new avenue for developing cheap wearable electronics and biotechnology. In particular, the use of conducting polymer hydrogels can impart elasticity and a high specific surface area, leading to great potential for sensors, cell growth scaffolds, and energy storage. However, their significantly low conductivity (compared to Ppy films and carbon or metallic microstructures), hydrophobicity, and low adhesiveness mean that they are currently unsuitable for most biological and wearable applications. Here, we show that by electropolymerizing a covalently bonded polydopamine (PDA) phase within polypyrrole hydrogels, we increased the conductivity by 2720% and adhesion by 2140% compared to pure polypyrrole hydrogels. Pyrrole monomers provided pi-bond stabilization and prevented a pi-stacked, auto-oxidized layer of PDA from forming. Instead, through potentiodynamic polarization of polypyrrole gels after dopamine incorporation, we produced covalently bonded 5,6-dihydroxyindole, producing an additional phase of conjugated polymer that interacted with the polypyrrole through noncovalent bonding. The PDA's unoxidized catechol groups also led to increased hydrophilicity and adhesiveness of the hydrogels. These results are a further step toward the realization of fully polymer wearable electronics made with a simple, scalable technique, thereby removing the need for expensive, biologically unfriendly metals or carbon structures.