Synthesis and characterization of Titanium Nitride Nanowires for neural-electrode coatings for improving electrode/neuron interface in the brain
In neurophysiological measurements, a neural-electrode interface material plays a critical role in delivering adequate charge to elicit action potentials without damaging the tissue of interest. However, the need to minimise electrode dimensions to reduce invasiveness and increase selectivity, demands the use of materials that are able to handle larger current and charge densities than traditional noble electrodes such as platinum. Since charge density is directly affected by the surface area of the electrode, nanoscale materials have shown a great deal of potential for not just improving the electrochemical properties, but the biocompatibility at reduced electrode dimensions. Titanium Nitride thin film (TiN) has been implemented previously in neural-electrode application due to its apposite properties. The work described here is aimed towards the synthesis of a novel TiN Nanowire interface (TiN-NW) as a potential neural-electrode material. The synthesis of the nanowires involved a three-step approach: (1) sputter of TiN thin film onto a substrate to act as a seed layer for the growth of NWs, (2) the growth of titanium dioxide nanowires (TiO2-NWs) of high aspect ratio and crystallinity followed by (3) a novel plasma nitridation step using Plasma Enhanced Chemical Vapour Deposition (PECVD) which offered a lower synthesis temperature than the conventional processing temperature reported in the literature. An optimised TiN thin film, grown through Radio Frequency (RF) non-reactive magnetron sputtering, was used as a seeding layer for the growth of NWs. The properties of the seed layer and the grown NWs were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-Ray diffraction (XRD), Raman spectroscopy, X-Ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) while the suitability of the grown TiN-NWs as an electrode material was tested by studying their electrochemical performance through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). In addition, the biocompatibility of both structures were tested by culturing glioblastoma cells (GBM) in vitro, and cell viability and behaviour were studied and compared to that on TiN thin film. xv XPS and TEM results showed that TiO2-NWs were converted to TiN-NWs at a temperature of 600 °C. Electrochemical results showed 5-fold of capacitance enhancement of the synthesised TiN-NWs as compared to that of the optimised TiN film. Additionally, TiN-NWs have shown greater cyclic stability, with capacitance retention of almost 99%, and lowered susceptibility to oxidation compared to TiN thin films. The impedance of TiN-NWs electrode at low frequencies, corresponding to ion diffusion, was noticeably lower than that of the film electrode. The in vitro test showed that cells were viable and attached to both structures and the cells on NWs formed dense 3D structures and had a greater spatial distribution than those cultured on the thin film layer. These findings not only highlighted the potential use of TiN-NWs as a neural-electrode interface material but also suggested a way of reducing the nitridation temperature to obtain TiN through PECVD process, which can improve existing electrodes or be integrated into next-generation neural-electrode structures.
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