Elastomer-based microcable electrodes for electrophysiological applications

Abstract

Compliant microelectrodes have been designed in a microcable geometry that can be used individually or in an array and either as a shank-style electrode or as a string-like electrode that can be threaded around features such as the peripheral nerve. The fabrication process, using spin-cast micromolding (SCuM), is simple and adaptable to different patterns. The microcables were fabricated using polydimethyl siloxane (PDMS) for the insulating substrate and thin-film gold for the conductive element. The thin, metal film and the low tensile modulus of the PDMS substrate created an electrode with a composite tensile modulus lower than other compliant electrodes described in the literature. The gold film increased the composite modulus approximately three-fold compared to the unaltered PDMS. The durability of the electrodes and tolerance for stretch was also tested. The microcables were found to be conductive up to 6% strain and to regain conductivity after release from multiple applications of 200% strain. The tolerance for high-strain shows that the electrodes can be deployed for use and stretched or pulled into place as needed without damaging the conductivity. The microcable electrode recording sites were electrically characterized using frequency-based impedance modeling and were tested for electrophysiological recording using a peripheral nerve preparation. A suitable insertion mechanism was designed to use the microcables as shank-style cortical electrodes. The microcables were coated on one side with fibrin, which, when dry, stiffens the microcables for insertion into cortical tissue. A 28-day implant study testing the inflammatory response to fibrin coated PDMS microcable electrodes showed a positive, but relatively low inflammatory response, as measured by glial fibrillary astrocytic protein (GFAP; indicating activated astrocytes) immediately at the tissue edge of the implant site. The response of the control, silicon shank-style electrodes, was varied, but also trended toward low levels of GFAP expression. The GFAP staining was possibly due to the clearance of the fibrin from the implant site in addition to the presence of the PDMS-based electrode. Implant studies extending beyond 28 days are necessary to determine whether and to what degree the inflammation persists at the implant site of PDMS-based electrodes.