Design and development of a fluid actuated neural probe for chronic sensing

Abstract

Neural implants have found a niche in circumventing brain dysfunction for brain-machine interfaces and in neuroscience studies for interrogation of neural tissue. The performance of implantable neural probes for chronic applications has been hampered, owing to an inflammatory immune response in the vicinity of the implanted devices, concomitant with degradation of the electrode tissue interface. Current state-of-the-art electrode technologies often fail within a duration of a few weeks or months, limiting their lifespan. The primary failure mechanism has been attributed to astroglial scar tissueformation, encapsulating the device over a course of 6 weeks. This thesis presents a novel invasive CNS sensor that utilizes fluid pressure to actuate subcellular, flexible neural probes that emerge from a stiff central shank in-vivo, to bypass this layer of scar tissue, to interact with healthy neurons outside this “kill zone”. The proposed sensor was composed of biocompatible and bio resistant materials Silicon, Thermal silicon dioxide, Gold and SUEX - to minimize the foreign body response and mechanical properties of the probes were scaled to improve compliance with brain tissue. A novel, scalable fabrication process was developed involving dual side processing of SOI wafers, lowering the unit cost, for batch manufacturing of these devices. The proposed 16-channel prototype consisted of floating ~8µm wide and thick compliant probes of 300µm length, housed in a rigid 300µm thick shank of 1mm width and 7mm length. The device consisted of a floating design using capacitive coupling as the electrical interconnect methodology. The devices were tested in-vitro for basic functionality and characterized using impedance spectroscopy. The test device showed an average impedance of 6.62 kΩ ∠-41° ± 1.04 kΩ ∠-2° at 1kHz with a capacitive behavior and probes could be translated bidirectionally using fluid drag forces.