Vapor-Phase Engineered Functional Polymer Interfaces for Ultrathin Transparent Bioelectronics

Functional polymer coatings play a pivotal role in advancing next-generation bioelectronic interfaces, where precise molecular-level control over polymer-metal interactions determines device performance. Here, we present a vapor-phase polymer engineering strategy using initiated chemical vapor deposition (iCVD) to fabricate ultrathin transparent electrodes for neural interfacing applications. We synthesized poly(dimethylaminomethylstyrene) (pDMAMS) as a molecular adhesion layer, exploiting its tertiary amine functional groups to form coordination bonds with gold atoms. This polymer-mediated interface engineering transforms the three-dimensional island growth of gold into continuous two-dimensional films, enabling functional electrodes at only 10 nm gold thickness. The solvent-free iCVD process preserves reactive amine groups that are often lost in conventional solution-based methods, while offering conformal, uniform coatings with sub-nanometer thickness control. The resulting pDMAMS/Au films achieve >65% optical transmittance at 550 nm, sheet resistance of 3.5 Ω sq^-1, and electrochemical impedance of 0.89 Ω·cm², yielding a figure of merit of 237.4. Fabricated on flexible parylene substrates (<5 μm total thickness), these polymer-based interfaces demonstrate excellent mechanical durability over 20,000 compression cycles and biocompatibility. In vivo validation confirmed low-noise neural recording during optical stimulation, highlighting the potential of vapor-phase functional polymers for implantable transparent bioelectronics and optogenetic neural interfaces.