Biocompatible integration of synthetic materials with living tissue remains a major challenge for bioelectronics. In this case, substrate-free conducting polymer (CP) interfaces could help bridge this gap. We report in vivo assembly of n-doped poly(benzodifurandione) (n-PBDF) using whole blood-catalyzed polymerization in awake zebrafish and mice. This approach leverages endogenous catalysts, specifically hemoproteins, to form stable, thermally and ionically sensitive CP networks, ensuring long-term compatibility throughout the lifespan. We showcase the impact of this interface through reversible, cellular, and subcellular neuromodulation using near-infrared (NIR) light, including in vivo polymerized n-PBDF. Electrophysiological studies confirmed that n-PBDF alters intrinsic sodium ion channel excitability, and NIR light stimulation amplifies this modulation through thermoionic-induced shunting, providing on-demand, millisecond-scale reversible inhibitory control of excitability, a feature recapitulated in actively behaving mice.