Carbon-Based Composite Rod as a Potential Electrode for Stimulation of Neural Tissue: Electrochemical, Biological, and Stability Evaluation

This study demonstrated the high potential of novel carbon composite electrodes obtained by direct electrical heating of carbon fibers coated with pyrolytic carbon (PyC) and further modified with hydroxyl-functionalized carbon nanotubes (CNT-OH), for neural tissue stimulation, particularly in applications such as deep brain stimulation (DBS) for neurodegenerative disorders. Microstructural and structural characterization using SEM, TEM, and XRD confirmed a homogeneous, defect-free CNT coating and revealed the presence of turbostratic carbon structures. Electrochemical evaluations showed that CNT-OH modification significantly enhanced electrode performance. Notably, voltage transient analysis indicated a 2.5-fold increase in charge injection capacity (91.1 μC·cm⁻²·ph⁻¹) compared to unmodified electrodes, aligning with the performance of platinum electrodes. This improvement was attributed to the increased electrochemically active surface area and double-layer capacitance introduced by CNT-OH. The modified electrodes also demonstrated stable current responses and reduced capacitive artifacts during pulse stimulation, supporting their clinical applicability. The investigation of the biosafety of the electrodes for neural stimulation and recording was carried out using a primary mesencephalic cell cultures measuring cytotoxicity (biochemical assay for LDH release) and assessing the presence and morphology of the cells immunostained with dopaminergic neuron (TH+) and general neuronal marker (NeuN+) after the contact with materials. The performed tests did not show a negative impact of materials on the biological response of cells and showed some pro-survival effects especially for C-C composites modified with CNT-OH (CF/PyC/CNT-OH) in the measured distance 1000-2000 m. These effects were probably linked to the hydrophilic, oxygen-rich surface created by CNT functionalization. Durability tests, including simulated aging and accelerated degradation studies, confirmed the structural and functional stability of the electrodes. Minor surface modifications detected by XPS and Raman spectroscopy did not compromise electrochemical performance of CNF/PyC/CNT-OH electrode, as evidenced by stable charge storage capacity and impedance profiles over time. Overall, the CF/PyC/CNT-OH electrodes exhibited excellent electrochemical performance, high biocompatibility, and high degradation resistance. These findings suggest that the tested composites are promising candidates for neural stimulation and recording applications. Future studies should include in vitro and in vivo models with electrical stimulation to further validate their efficacy and long-term safety.