Parylene microprobes with engineered stiffness and shape for improved insertion

This paper reports on a new structure and fabrication technology for Parylene-based recording and stimulating penetrating microprobes with engineered mechanical stiffness, low cross-sectional area and sharp tips. Taking advantage of significant benefits from polymer microprobes is often prevented due to their troublesome insertion. This paper addresses this issue and describes a process that allows integrating vertical Parylene stiffeners in the shank without adding process complexity, increasing probe thickness and volume or compromising use in 2D/3D arrays. A metal mask formed around the tip allows for the local thinning down of Parylene, creating a sharp tip during an etch step. These features increase the stability of the probe while reducing both the load on the shank during implantation as well as the dimpling of the brain surface. Shanks 2 mm long, 20 μm thick and 380 μm wide were rendered stable enough for insertion into cadaver lamb brain through the pia by employing five stiffeners. Probes were fabricated in a three mask process with high yield (>;80%), and under soak-tests it was determined that they remain electrically functional over a period of three months.