School of Physics - Theses
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ItemCarbonaceous wires for neural interfacing: electrode development, surgical implantation, and device integration( 2017)A new generation of neural interfacing (NI) devices is needed to realize the full clinical and research potential of invasive, direct communication with the brain. Specifically, the electrode component of state-of-the-art devices is not well-tolerated by brain tissue. Deleterious processes such as inflammation, scarring, and eventual neuronal death disrupt information exchange between brain and machine. Recent studies suggest that intracortical electrodes fabricated from soft, flexible, and low-density materials evoke significantly less inflammation and scarring than their inorganic, metallic counterparts. While carbonaceous composites based on graphene and carbon nanotubes meet the aforementioned materials criteria and have already demonstrated great promise for biological applications, there is a need for a toolbox to transition these materials from benchtop prototypes into scalable and reproducible medical devices. In this thesis, fabrication techniques and surgical tools were developed to meet the unique needs of applying soft, flexible, carbonaceous wires as NI electrodes. Electrode fabrication was carried out by insulating carbonaceous wires with 2 µm films of parylene-C, followed by low power (<10 mW) laser ablation to expose the electrode site. Electrochemical characterization, in conjunction with both in vitro and in vivo assessments, suggests that these electrodes offer a highly functional alternative to conventional electrode materials for both recording and stimulation, yielding safe charge injection capacities up to 46 mC cm-2. For implantation into brain tissue, soft carbonaceous electrodes were encapsulated in a sucrose glass microneedle using a drawing lithography technique. Measurement and control of the glass transition temperature of the sucrose glass, which was dictated by the moisture content, was found instrumental to the success of the process. Sucrose carriers dissolved quickly upon implantation into brain and enabled neural recording in both short and long duration implantation periods. Insertion force characterization and histological tissue response of the sucrose glass microneedles were performed and compared with conventional cannula-based insertion methods. Tissue damage following sucrose microneedle insertion was comparable to that observed for a syringe needle stab wound. Using sucrose glass microneedles, carbonaceous wire electrodes were implanted into an epileptic rat model and high fidelity recordings of seizure events were acquired for periods up to 3 weeks with electrodes exhibiting signal-to-noise ratio values ranging from 8-10 over a 22-day implantation period. Finally, as a step towards integrating carbonaceous wire electrodes with standard electronics joining processes, a metallization technique based on active brazing was developed. Ultimately, a combination of brazing techniques was used to integrate carbonaceous electrodes into biostable and durable polycrystalline diamond circuit substrates with the capability of supporting surface mount component attachment.