Self-closing Micro-Cuff for Neuronal Recording in the Locust

Over the past few decades, neuroengineering has become one of the fast-growing fields in the biomedical industry. With the help of engineering and advances in technology, neuroscientists have focused on studying the brain's ability to record, process, store, and retrieve information. This is done by mapping neural circuits to specific behaviors by measuring the natural or stimulated electrical activity of neurons. The neural interfaces required for this approach have varying degrees of invasiveness depending on whether they penetrate the nerves (intra-neural) or are placed on the periphery of the nerves (extra-neural). Greater invasiveness leads to better selectivity of the targeted neurons; it also causes more tissue damage to the nerve, thus, reducing the lifetime of the interface. This thesis presents the development of an extra-neural cuff electrode for recording the electrical activity on the descending contralateral movement detector (DCMD) neuron of grasshoppers and other insects. The current approach for collecting extracellular recordings on the DCMD is to use two metal hooks for differential recording of DCMD spikes. The goal of this project is to extend this approach to a flexible, self-rolling, multi-electrode cuff for recording DCMD spikes in grasshoppers and other insects. It will be designed to fit between the pro- and meso-thoracic ganglia, a space of about 1 mm. To interface with existing signal processing equipment contact area will be ~ 25x50 um2. While cuffs for this thesis will have 3 contacts, the technology will support 22–40 contacts on a 1 mm wide cuff. It also features facile implantation and self-sizing; if the cuff is sized for the smallest diameter to be encountered and fitted to a larger nerve cord, it simply unrolls, while still maintaining intimate contact with the nerve cord. Moreover, the cuff is so thin that the detachment force is less than 15 mN/mm, a force that will not damage the nerve cord. The cuff is formed by thermally shrinking a bilayer film of monoaxially-oriented polycarbonate (PC) substrate and gold with controlled diameters of 70–250 µm; heating releases the built-in strain of the PC while the gold provides the stiff backing that prevents buckling. The malleability of the gold prevents it cracking during thermal processing. The diameter is controlled by the thickness of the two layers and the temperature and duration of the heat-shrinking process. To fabricate electrode lines, a gold film is deposited to the back side of the PC membrane, and a plasma-deposited, ion-sensitive resist is applied. A stencil mask is then used to define the metallization lines by 50 kV He atom beam lithography (ABL). Next, ion milling is used to transfer the resist patterns to the metal film. Conformality of the resist and large depth-of-field of the lithography are critical to high-resolution patterning on these non-flat substrates. A dielectric overcoat consisting of the exposed resist with excellent biocompatibility is applied with openings that provide contact to metal electrodes.