Indium Tin Oxide (ITO) is a well-known n-type semiconductor material that is often utilized in transparent microelectrodes. ITO has high conductivity, excellent transparency over the entire visible spectrum due to a large bandgap of around 4 eV, as well as confirmed biocompatibility. Because of numerous advantages of ITO, in this dissertation, ITO as a base material will be applied in both electrophysiological recording and electrochemical sensing. Optogenetics is a revolutionary neuromodulation technique that utilizes light to excite or inhibit the activity of genetically targeted neurons, expressing light-sensitive opsin proteins. To fully realize the potential of the optogenetics tools, neural interface devices with both recording and stimulating capabilities are vital for future engineering development, and improving their spatial precision is a topic of constant research. Conventional transparent recording microelectrodes made of a single material, such as ITO, ultrathin metals, graphene, and poly-(3, 4-ethylene dioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS), have limitations and hardly possess the desired combination of broadband transmittance, low electrical resistivity, mechanical flexibility, and biocompatibility. One direction of this dissertation work is to develop multilayered electrophysiological microelectrodes with high transparency, outstanding conductivity, low electrochemical impedance, high charge storage capacity, excellent mechanical properties, and ultra-flexibility. Chapter 1 briefly introduced the background, current challenges, and motivations of this dissertation. Chapter 2 concluded a review of electrical materials for neurophysiology recording implants. Chapter 3 proposed a probe with a combined ITO-PEDOT:PSS electrode configuration by spinning thin PEDOT:PSS films on ITO microelectrodes, for applications in low-impedance neural recordings. The characteristics of the ITO-PEDOT:PSS microelectrodes were analyzed as a preliminary study for the following transparent electrophysiology recording array research. Chapter 4 reported an ultra-flexible, conductive, transparent thin film using a PEDOT:PSS-ITO-Ag-ITO multilayer structure on Parylene C deposited at room temperature. The material characterization demonstrated enhanced conductivity, remarkable and wavelength-tunable transmittance, significantly reduced electrochemical impedance, increased charge storage capacity, good stability, good adhesion, and confirmed mechanical properties of the combined film. Next, Chapter 5 demonstrated two 32-channel transparent μECoG arrays using this PEDOT:PSS-ITO-Ag-ITO multilayered thin film structure on Parylene C. These two μECoG arrays proved to work effectively in vivo for the electrophysiological detection in the living brain tissue. Last but not least, Chapter 6 first discussed the ongoing work to develop a 120-channel high spatial resolution transparent micro-ECoG array. The other subsection of this chapter is to fabricate an ITO-based transparent and miniaturized electrochemical sensor for continuous and quantitative monitoring of the concentrations of copper (Cu) and manganese (Mn) ions in bodies and soil environment by utilizing Differential Pulse Stripping Voltammetry (DPSV).
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