| Description |
Implantable microelectrode arrays are biomedical devices used in-vivo serving as neural interfaces between the nervous system and external systems such as neuroprosthetics. They are designed to be chronically implanted in the central or peripheral nervous system and record or stimulate neural signals. The Utah electrode array (UEA) is a representative example of silicon-based neural interfaces. They are typically encapsulated with the USP Class VI biocompatible material, Parylene-C, on the inactive areas to insulate and encapsulate the electrodes and minimize damage to the neural tissue. In order to record or stimulate neural signals, the active electrode sites must be deinsulated. Tip deinsulation of Parylene-coated UEAs is typically performed by a reactive ion etching (RIE) process using an O2 plasma, and an aluminum foil mask. This technique has limitations due to nonuniform tip exposure lengths contributing to large impedance variations (o > 0.5 MQ), and difficulty in controlling the magnitude of tip exposure, especially for tip exposures less than 40 ^m, which are needed to increase its selectivity in recording or stimulating single or multiple neurons. Moreover, foil masks cannot be used for more complex electrode geometries, such as variable height electrodes. In this work, excimer laser ablation of Parylene from a UEA using a tip metallization of iridium oxide (IrOx) was investigated as an alternative deinsulation technique. A hybrid method of etching Parylene-C using a combination of laser ablation and the O2 RIE was investigated in the efforts to minimize electrode damage and remove carbonaceous residues. The median impedance for fine tip (< 20 ^m) electrodes was ~ 4.6 MQ immediately after laser ablation. However, significant amounts of carbon residue on the exposed surface were removed, generating improved impedances to ~ 0.7 MQ by including a maskless RIE process using 1 minute O2 RIE. We also observed that reduction of IrOx could occur at relatively mild (> 180 °C) temperatures in reducing ambients, resulting in dramatic changes to the structural and electrical properties of the tip metallization. The reduced IrOx material was found to tolerate significantly more laser irradiation than the fully oxidized material. The median impedance, cathodal charge storage capacity (CSCc), and charge injection capacity (CIC) for the reduced electrodes with 40 |im exposure were ~ 25 kQ, ~ 40 mC/cm2, and ~ 0.8 mC/cm2, respectively. These results suggest that a hybrid laser ablation using an excimer laser and RIE is promising for deinsulation of UEAs. |
