Porous Gold Surfaces for Implantable Neural Electrodes

Abstract: Neural electrodes used for measurements of action potentials in brain cells are often made of gold. These electrodes need to be suciently small to cover only one brain cell, but this induces the problem of increased impedance since impedance scales inversely with surface area. This means that if the surface area is too small, only noise will be measured. One way to overcome this problem is to try to enlarge the real surface area by making the gold nanoporous. In this way, the geometric surface area can be kept small while the real surface area becomes several times larger. In this project, attempts to increase the surface area by anodization has been made. First, a silicon wafer with a large number of chips containing six electrodeseach was fabricated using techniques such as evaporation, UV exposure and etching. Then the chips were diced out of the wafer and each chip was glued onto a circuit board. The copper and gold were connected by a thin aluminum wire and covered by silicone for protection and isolation. Different anodization modes and times were tested on the electrodes, and the results were evaluated by both an optical microscope and a SEM. At the best combination of anodization mode and time, the impedance was measured both before and after anodization. Also, cyclic voltammetry was used in order to calculate the real surface area of the electrodes. The results of the measurements show an increase of the real surface area with up to approximately seven times. This was conrmed by the impedance measurements, which clearly showed that the impedance sharply decreased after anodization compared to the impedance measured before anodization. This indicates that surface enlargement of gold electrodes by anodization is a viable method, but it should be carried out on more electrodes in order to get reliable results.