Electrical impedance, electrochemistry, mechanical stiffness, and hardness tunability in glassy carbon MEMS μECoG electrodes

We report on electrical, electrochemical, mechanical stiffness, and hardness tunability of patternable glassy carbon (GC) microelectrodes for applications in bio-electrical signal recording and stimulation. Results from in vivo testing of a novel μECoG GC microelectrode array with beta activity recording of stimulation signals and their power spectra is reported, demonstrating a quantifiable superior performance of these electrodes as compared to metal electrodes. The microelectrodes – made from lithographically patterned and subsequently pyrolyzed polymer precursor – have excellent electrochemical stability in ionic solutions and respond well to chemical surface property modifications. In addition, lithographically patternable GC offers a unique tailorability functionality that enables fabrication of electrodes with a range of mechanical, electrical, and electrochemical properties that closely match the behavior of soft tissues. The pyrolysis conditions that drive this flexibility (i.e., maximum temperature, duration of pyrolysis, and ramp rate) could be varied to enable useful properties such as: (i) tailorable mechanical stiffness and hardness offering a much better stiffness-matching with soft tissues, (ii) tailorable electrical impedance for better impedance-matching with tissues, and (iii) tailorable electrochemical property useful for optimized stimulation and recording. We demonstrate that pyrolysis treatment at temperature of 1000 °C offers the most favorable electrochemical characteristics (least charge transfer resistance, most charge injection capacity, and decreased polarization resistance) and least impedance, whereas 800 °C results in maximum modulus of 55 GPa and hardness of 7 GPa. We, therefore, submit that these significant and original results reported here build a strong case for making GC microelectrodes, probes of choice for bio-signal sensing and stimulation applications, particularly for μECoG systems.