A transfer process to fabricate ultra-compliant neural probes in dissolvable needles

A fabrication approach for ultra-miniature ultra-compliant neural probes with parylene-C insulation that are embedded in biodissolvable insertion needles was previously established by the authors. However, that approach required application of a peeling process to release the probe-needle assembly from its handle wafer. The use of thermal annealing in vacuum to improve encapsulation properties of parylene-C results in increased adhesion to the substrate that undermines the peeling process. In this paper, we introduce a transfer process step that eliminates the peeling process and allows the potential use of a wide range of sacrificial release materials. The transfer step increases the versatility of the overall fabrication approach since it allows the integration of insertion needle and sacrificial release materials that otherwise would not have been compatible with the high-temperature annealing. Several sacrificial release materials, including photoresist, polydimethylsiloxane, mounting adhesive, and liquid wax, are investigated and characterized for suitability in the transfer process. Considering compatibility with the biodissolvable needle attachment, a liquid wax is identified to be an effective material because of its strong adhesion to relevant surfaces, its ability to be spin coated, and its dissolvability in isopropyl alcohol.