Brain tissue oxygen pressure monitoring using carbon fiber microelectrodes protected from biofouling by polyphenylenediamine-polyurethane coating

Brain tissue oxygen pressure (PbtO₂) monitoring is critical to evaluating the status of brain metabolism in healthy or pathological states. Electrochemistry provides high spatial and temporal resolution to achieve PbtO₂ monitoring, especially platinum (so-called Clark electrodes) and carbon fiber microelectrodes (CFMEs). However, a persistent problem with in vivo PbtO₂ recordings is the significant loss of sensitivity due to sensor fouling in the living brain tissue. To minimize this issue, we tested a polyphenylenediamine-polyurethane (PPD-PU) coating to reduce electrode fouling in vivo and improve the stability of PbtO₂ recordings. PPD and PU coatings were deposited on CFMEs as an overall 1 μm thick layer and displayed less fouling in vivo than bare CFMEs, or CFMEs coated with PPD. This coating resulted in no significant change in O₂ sensitivity before and after in vivo implantation. Oxygen reduction started around -300 mV vs Ag/AgCl and at -500 mV, conventional interfering molecules like ascorbic acid, uric acid, dopamine, serotonin and their metabolites were not detected. Next, we evaluated the mechanism of oxygen reduction. Critically, it was observed that oxygen reduction did not produce hydrogen peroxide in concentrations susceptible to impact brain physiology, suggesting a 4-electron reduction reaction. These CFMEs coated with PPD-PU were implanted in vivo in anesthetized rats to monitor PbtO₂ changes during spreading depolarization. These results indicate that stable PbtO₂ monitoring can be achieved using PPD-PU coating on CFMEs and that the oxygen reduction mechanism occurring at these electrodes is suited for in vivo measurement.