Surface Modification with Biocompatible Polymer Conductors for Stable and Compliant Electrode Interfaces for Communicating with Neurons

Neural interfaces for recording or stimulating cells and tissues require long-term stability, mechanical compliance, low impedance, and high charge injection capacity. However, the mechanical mismatch between stiff metal electrodes (e.g., gold, platinum, and iridium oxide) and soft biological tissue poses a significant challenge, hindering optimal performance. To address this, conductive polymers have been used as an additional interface layer, with poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) being a widely utilized material. While PEDOT:PSS offers good charge injection capacity and lowers impedance, its long-term stability when implanted in the body remains a bottleneck due to delamination, pH changes, charge injection, and immune responses.<br/>In this work, we propose a chemical approach to enhance the adhesion between gold electrodes and PEDOT:PSS by covalently attaching the polymer to the gold surface, i.e., generating polymer brushes. We designed and synthesized polymer brushes made of block copolymer scaffold using a surface-initiated living radical polymerization (grafting-from). Our polymer brushes comprise a poly(styrenesulfonate) (PSS) block and a soft poly(polyethylene glycol methyl ether methacrylate) (PPEGMEMA) segment. This scaffold serves as a platform for polymerizing PEDOT, creating an adhesive and compliant interface with the brain tissue.<br/>We successfully implemented this approach on a flexible microarrayed neural probe and evaluated its electrical charge transport and long-term stability for brain activity recording. The covalently attached polymer brushes significantly enhanced the adhesion between gold and PEDOT:PSS coating, addressing the challenges associated with delamination over time. The resulting interface exhibited improved long-term stability, and reduced impedance compared to traditional PEDOT:PSS-coated gold electrodes.<br/>Our findings offer insights into the development of more reliable and efficient neuroprosthetic devices. By enhancing the stability of PEDOT:PSS coatings, we pave the way for improved neural interfaces with long-term functionality in neuroscientific research and clinical applications.