Different methods to alter surface morphology of high aspect ratio structures

Abstract

In various applications such as neural prostheses or solar cells, there is a need to alter the surface morphology of high aspect ratio structures so that the real surface area is greater than geometrical area. The change in surface morphology enhances the devices functionality. One of the applications of altering the surface morphology is of neural implants such as the Utah electrode array (UEA) that communicate with single neurons by charge injection induced stimulation or by recording electrical neural signals. For high selectivity between single cells of the nervous system, the electrode surface area is required to be as small as possible, while the impedance is required to be as low as possible for good signal to noise ratios (SNR) during neural recording. For stimulation, high charge injection and charge transfer capacities of the electrodes are required, which increase with the electrode surface. Traditionally, researchers have worked with either increasing the roughness of the existing metallization (Platinum grey, black) or other materials such as Iridium Oxide and PEDOT. All of these previously investigated methods lead to more complicated metal deposition processes that are difficult to control and often have a critical impact on the mechanical properties of the metal films. Therefore, a modification of the surface underneath the electrode’s coating will increase its surface area while maintaining the standard and well controlled metal deposition process. In this work, the surfaces of the Silicon micro-needles were engineered by creating a defined microstructure on the electrodes surface using several methods such as Laser ablation, focused ion beam, sputter etching, reactive ion etching (RIE) and deep reactive ion etching (DRIE). The surface modification processes were optimized for the high aspect ratio Silicon structures of the UEA. The increase in real surface area while maintaining the geometrical surface area was verified using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The best results were obtained by DRIE induced surface morphology. Decreases in impedance values of electrodes up to 76 % indicate the successful surface engineering of the high aspect ratio Silicon structures.

Characterizing Longitudinal Changes in the Impedance Spectra of In-Vivo Peripheral Nerve Electrodes

“…For the IrOx arrays, these spectra were similar to one that have been chronically implanted for at least 30 days [ 11 ], or to the uncoated sputtered iridium oxide films (SIROF) sites [ 12 ], and are thus likely aging sites, with the iridium oxide flaking off, damage to the tips, and/or a complex interaction with the encapsulation sheath. For the platinum arrays, the mixed category was the most prevalent group for all of the timepoints, and the spectra were similar to those previously characterized in vitro [ 14 , 15 ].…”

“…For the platinum arrays, the phase spectrum for the predominant mixed group initially presented with a U-shaped minimum in middle frequencies, similar to that of Pt UEAs previously shown in vitro [ 14 , 15 ]. As time progresses, this minimum flattens out and a high-frequency peak develops.…”

“…Thus, these changes are likely due the foreign body response, although the aging of the electrode may also contribute. The magnitude spectra are similar [ 14 , 15 ] or much lower than a previous study [ 19 ], with the latter difference potentially arising from the large error bars presented with the mean. Over time, we found that the impedance decreased at low frequencies (<100 Hz) and increased at very high frequencies (1 MHz).…”

Attenuated Glial Reactivity on Topographically Functionalized Poly(3,4‐Ethylenedioxythiophene):P‐Toluene Sulfonate (PEDOT:PTS) Neuroelectrodes Fabricated by Microimprint Lithography

“…A range of techniques have been explored to create defined microstructures on electrode surfaces, including laser ablation, focused ion beam, sputter etching, reactive ion etching, deep reactive ion etching, hot embossing, and electron beam lithography . Complementary to these techniques, imprint lithography is an especially attractive approach due to its simplicity, nondestructive character, and feasibility of patterning large areas with features down to 10 nm using elevated temperature and/or pressure processes to transfer a pattern into typically thermoplastic materials .…”

“…Topographical surface modification of the neural electrodes has shown promise in improving the electrochemical performance through high aspect ratio structures, increasing the active surface area of the electrodes . The electrochemical performance and the evaluation of the resulting effective‐active surface area of the functionalized microelectrodes were subsequently assessed.…”