A polypyrrole-based ultrastable monolithic supercapacitor for medical implants

With the development of implantable electronic medical devices, there is a demand for biocompatible power sources with long-term stability and safety. Conductive polymer-based supercapacitors (SCs) have emerged as a promising choice. However, the traditional laminated structure compounded with the repeated volumetric change compromise their long-term durability, which has long been an outstanding issue blocking the development of fully conductive polymer-based SCs. Carbon nanotubes and graphene are frequently used to increase the electrode surface area and stability, which, however, may represent potential cytotoxicity if the electrodes disintegrate in vivo . To address these critical issues, a unique strategy was developed to fabricate a durable and biocompatible SC solely based on bacterial cellulose (BC) hydrogel, polypyrrole (PPy) and saline. By controlling BC culture time and PPy polymerization inside the BC hydrogel, a monolithic BC-PPy SC was formed, abandoning the classical laminated structure. Benefiting from the monolithic structure and the thin coating of PPy on individual cellulose nanofibers, the SC showed an outstanding cyclic durability, with 90.0% capacitance retention and 97.6% Coulombic efficiency after 50,000 charge-discharge cycles. The SC also showed a rapid charge-discharge behavior (≈ 13–35 s) under the current density range of 0.08–0.2 mA/cm 2 with the areal power density of 32–80 μW/cm 2 , and the specific capacitance and the energy density of ≈1.4 mF/cm 2 and ≈0.12 μWh/cm 2 , respectively. Being able to use tissue fluid as electrolytes, the BC-PPy SC does not need packaging to prevent the loss or toxicity of the electrolyte. The cytotoxicity test confirmed the biocompatibility of the BC-PPy SC. Therefore, the BC-PPy SC has an excellent potential to be used as a long-term implantable power source. • A record charge – discharge stability for conductive polymer-based supercapacitors: 100 % after 20 k and 90% after 50 k cycles. • Only containing established biocompatible materials: Bacterial cellulose, polypyrrole, saline. • A novel approach to build supercapacitors. • Inherent structural stability, attributing to the monolithic structure.