Fabrication of Out-of-Plane High Channel Density Microelectrode Neural Array with 3D Recording and Stimulation Capabilities

Abstract

The Utah Electrode Array (UEA) and its different variants have become a gold standard in penetrating high channel count neural electrode for bi-directional neuroprostheses (simultaneous recording and stimulation). However, despite its usage in numerous applications, it has one major drawback of having only one active site per shaft, which is at the tip of the shaft. In this work, we are demonstrating a next-generation device, the Utah Multisite Electrode Array (UMEA), which is capable of having multiple sites around the shaft and also retaining the site at the tip. The UMEA can have up to 9 sites per shaft (hence can accommodate 900 active sites) while retaining the form factor of the conventional UEA with 100 sites. However, in this work and to show the proof of concept, the UMEA was fabricated with one active site at the tip and two around the shaft at different heights; thus, three active sites per shaft. The UMEA device is fabricated using a 3D shadow mask patterning technology, which is suitable for a batch fabrication process for these out-of-plane structures. The UMEA was characterized by in-vitro tests to showcase the electrochemical properties of the shaft sites for bi-directional neuroprostheses in contrast to the traditional tip sites of the standard UEA. The UMEA not only improves the channel density of conventional UEAs and hence can access a larger population of neurons, but also enhances the recording and stimulation capabilities from different layers of the human cortex without further increasing the risk of neuronal damage.

Keywords: Microelectrodes; Neural Electrodes; Shadow Mask; Utah Electrode Array; Utah Multisite Electrode Array.

Fig. 1.

Scanning electron micrographs of (a) a standard Utah electrode array (UEA) and (b) a single microneedle in the UEA, where the exposed active site is coated with Platinum and the remainder of the device is coated with a biocompatible polymer Parylene-C. (c) Schematics of the Utah Multisite Electrode Array (UMEA) with three active sites per needle for a total of 75 active sites.

Fig. 2.

Fabrication steps of the UMEA device: (a) starting with a 10 × 10 device with backside bond pads and insulated glass lines among the needles, (b) deposition of Al₂O₃ and silicon nitride insulation layers, (c) aluminum foil encapsulation, (d) etching of silicon nitride layers from the tips, (e) dicing into a 5×5 device, (f) shadow mask, (g) a 5×5 device inside a shadow mask, (h) deposition of front side metal, (i) deposition of parylene-C layer and (j) exposure of active sites.

Fig. 3.

(a) SEM image of a shadow mask (left) and a patterned hollow needle in the mask (right), (b) SEM image of an UMEA device just after front side metallization through a shadow mask (left) and a zoom in image of a single shaft in the UMEA device (right).

Fig. 4.

(a) SEM image of few exposed sites through parylene-C layer on a UMEA device, (b) zoom in image of one shaft with one tip site and two sites around the shaft, (c) zoom in image of a tip site, and (d) zoom in image of a shaft site.

Fig. 5.

Voltage transient of an electrode of the UMEA device, illustrating the driving voltage V_D, the access voltage V_ACC, and the maximum cathodic potential excursion E_MC during a pulse, where E_MC is calculated by subtracting V_ACC from V_D.

Fig. 6.

Bode plots of tip sites and shaft sites on an UMEA device. The curves represent mean values of (a) impedance and (b) phase with standard deviation as error bars.

Fig. 7.

Representative cyclic voltammograms of one tip site and one shaft site on an UMEA device.

Fig. 8.

Representative voltage transient for one tip site and one shaft site of the UMEA device, in response to a biphasic current pulse. The curves are normalized by subtracting the offset idle voltage value at t = 0 s.