A method for efficient, rapid, and minimally invasive implantation of individual non-functional motes with penetrating subcellular-diameter carbon fiber electrodes into rat cortex.

Distributed arrays of wireless neural interfacing chips with 1-2 channels each, known as "neural dust," could enhance brain machine interfaces (BMIs) by removing wired connections through the scalp and increasing biocompatibility with their submillimeter size. Although several neural dust designs have emerged, currently reported procedures for implanting them in batches place the chips directly inside the brain, which can damage or displace large numbers of neurons. Therefore, a procedure for safely implanting neural dust in batches such that only ultrasmall microwire elements enter the brain is needed. Here, we demonstrate the feasibility of implanting batches of wireless motes that rest on the cortical surface and reach 1 mm brain depths via penetrating carbon fiber electrodes (6.8-8.4 μm diameter) without employing disruptive insertion shuttles. To simulate their implantation, we assembled over 230 mechanically-equivalent carbon fiber motes and affixed them to insertion tools with polyethylene glycol (PEG), a quickly dissolvable and biocompatible material. Then, we implanted batches into rat cortex in vivo and evaluated insertion success and their arrangement on the brain surface. When positioning motes for insertion, we discovered that they readily aggregated in molten PEG such that average array pitches were 5% longer than an individual mote's dimensions (240 × 240 μm). Overall, 187/214 (87%) motes tightly-packed in 4 × 4 (N = 4) and 5 × 5 (N = 6) square grid configurations successfully inserted into rat cortex. After implantation, measurements of how much motes tilted (22 ± 9°, X̄ ± S) and had been displaced from their original positions were smaller than those measured in the literature for devices implanted inside the brain. Collectively, these data establish the mechanical viability of assembling and safely implanting motes with ultrasmall electrodes and epicortically-situated chips, motivating the use of arrays with similar geometries in future BMIs.