Direct laser carbonization of parylene-C toward microelectrodes for in vivo action potential detection

Integrating recording electrodes into optical imaging devices presents significant fabrication and material challenges, particularly for lightweight platforms used in freely moving applications. Here, we report an optimized laser carbonization process for the direct fabrication of high-performance carbon microelectrodes compatible with on-chip imaging platforms. We systematically investigated the influence of laser power and repetition rate on the carbonization of parylene-C to maximize graphitization while avoiding ablation. Raman spectroscopy confirmed the formation of graphitic carbon, with the lowest average D-to-G band intensity ratio (I _D /I _G = 0.63) obtained at 303.86 μW. However, this laser condition resulted in excessive material loss from ablation. The optimal balance between carbon quality and material retention was achieved at 212.69 μW and 30 Hz. Using these parameters, we fabricated implantable carbon microelectrode probes of different sizes (100 × 100, 20 × 20, and 10 × 10 μm²) on flexible polyimide substrates. The impedances of these electrodes were 12.10 ± 0.22 kΩ, 596.38 ± 84.86 kΩ, and 8.969 ± 0.991 MΩ for 100 × 100, 20 × 20, and 10 × 10 μm² laser carbonized electrodes, respectively. While larger electrodes offered lower impedance, spontaneous single-unit action potentials were only detectable using the 20 × 20 μm² electrodes, which provided the necessary spatial selectivity to isolate individual neurons. These results establish a maskless fabrication process for high-quality, flexible carbon microelectrodes and demonstrate their suitability for electrophysiological recording.