Advanced Polymer-Based Microfabricated Neural Probes Using Biologically Driven Designs.

Abstract

This research presents new designs and materials for neural recording and stimulation technology. Greater electrode density is enabling neuroscientists to study larger neuronal populations, but even higher signal stability and electrode density are needed. Addressing these issues is critically important for neuroprostheses in the treatment of spinal cord injury, ALS, or limb loss. Stimulating electrodes have already improved quality of life for those with Parkinson’s and dystonia and this technology has many new indications on the horizon. Smaller stimulating electrodes with reduced glial encapsulation would reduce the power requirements of these applications. I will discuss how our research impacts neurotechnology by enabling reduced glial encapsulation, greater design options, and improved electrical insulation.

Our first study introduced a novel neural probe with reduced chronic cellular encapsulation. We hypothesized that if a structural feature size is smaller than a reactive cell body (<7 μm), the resulting encapsulation would be mitigated by the prevention of cellular spreading. We investigated this relationship between size and tissue reactivity using a microfabricated parylene structure. Probes were implanted in the rat neocortex for four weeks followed by histological analysis. We found the non-neuronal density around the sub-cellular feature was less than half of that around the probe shank.

The objective of our second study was to identify a parylene process that would enable long-term bioelectrical insulation. We contrasted parylene-C with an alternative parylene material using electrical and mechanical tests. We present a reactive parylene (complementary layers of PPX-CHO and PPX-CH2NH2) that can be used in conjunction with parylene-C but has improved electrical insulation and wet metal adhesion.

In our third study, a new parylene-based microfabrication process is presented for neural recording, stimulation, and drug delivery applications. We introduce a large design space for electrode placement and structural flexibility with a six mask process. By using chemical mechanical planarization, electrodes may be created top-side, back-side, or on edge having three sides. Poly(3,4-ethylenedioxythiophene) (PEDOT) modified edge electrodes having an 85 µm2 footprint (smallest reported to date) resulted in an impedance of 200 kΩ at 1kHz. Edge electrodes successfully recorded single unit activity in acute animal studies.