Surabhi Nimbalkar1 2 Elisa Castagnola1 2 Luca Pazzini3 Davide Polese3 Guglielmo Fortunato3 Sam Kassegne1 2 Luca Maiolo3

1, San Diego State University, San Diego, California, United States
2, Center for Sensorimotor Neural Engineering, Seattle, Washington, United States
3, Istituto per la Microelettronica e i Microsistemi, Roma, , Italy

In this work, we introduce a new generation of Glassy Carbon (GC) microelectrodes mounted on a highly mechanically compliant elastomer substrate (E = 1.2 MPa) using a recently introduced pattern transfer method. This new class of probes leverages our recently introduced pattern transfer technology for GC microelectrode array supported on polyimide (E = 2 GPa) substrate that has been demonstrated to offer a significant advance in neuroprosthetics technology. While various neural prosthetic interfaces have been engineered for sensing electrical as well as electrochemical neural signals since 1950, the issue of tissue reaction in the form of microglial scarring due to difference in material stiffness largely remains unsolved. The glial scarring may reduce the neural sensing ability of neural probe, especially for long-term probe. To address this, flexible and elastomeric polymer materials that offer a promising solution to this problem by lowering the mismatch in stiffness of dura mater and elastomeric device have recently been considered by several researchers. With Young’s Modulus in the low MPA range, such elastomers have stretchability that enables neural probes to follow the natural internal brain movement regulated by blood flow and respiration. For long-term implants, in fact, the neural probes experience rhythmic mechanical friction as the brain is a pulsatile organ. In this study, we, therefore, integrate GC microelectrodes supported by elastomer substrate to fabricate a new generation of neural sensing and stimulation probes with high mechanical compliance. This new class of probes called GC Elastomer probes are based on a recently introduced pattern transfer technique to microfabricate GC microelectrodes array (12 channels, thickness= 2 µm, diameter= 300 µm) and supported on insulating silicone elastomer layer. The interconnecting traces made of thin-film metal (Au with thickness = 100 nm) with high stretchable properties (horseshoe shape) and good electrical conductivity are deposited by metal sputtering on elastomer substrate and are mechanically connected to the GC microelectrodes designed for recording electrophysiological signals such as Electrocorticography (ECoG). After device fabrication, in vitro characterization is carried out to evaluate the quality of the stretchable electrodes. In particular, impedance magnitude and phase of the microelectrodes are measured after uniaxial deformation cycling up to 20% and ultimately up to the device failure. Furthermore the capacitive charging and injection as well as the stability under prolonged stimulation use are quantified through Cyclic Voltammetry and Power Pulse Techniques.