Fabrication of a 3D high-resolution implant for neural stimulation - challenges and solutions

Abstract

Background - Tissue-integrated micro-electronic devices for neural stimulation hold a great potential in restoring the functionality of degenerated organs, specifically, retinal prostheses, which are aimed at vision restoration. The fabrication process of 3D polymer-metal devices with high resolution and a high aspect-ratio (AR) is very complex and faces many challenges that impair its functionality. Approach - Here we describe the optimization of the fabrication process of a bio-functionalized 3D high-resolution 1mm circular subretinal implant composed of SU-8 polymer integrated with dense gold microelectrodes (23µm pitch) passivated with 3D micro-well-like structures (20µm diameter, 3µm resolution). To this end, a nickel (Ni) evaporated silicon (Si) wafer was sequentially spin-coated with SU-8 and photolithographed layer-by-layer, with a sharp electrode formation achieved through a two-step bi-layer lift-off process using LOR/AZ, followed by Cr/Au thin-layer sputter deposition to increase the adhesion. Next, the device was released by overnight Ni wet-etching using nitric acid, after which it was bio-functionalized with N2 plasma treatment and the addition of the bio-adhesion molecule arginine-glycine-aspartic acid (RGD). Main results - In-vitro and in-vivo investigations, including SEM and FIB cross section examinations, revealed a good structural design, as well as a good integration of the device in the rat sub-retinal space and cell migration into the wells. The reported process and optimization steps described here in detail can aid in the design and fabrication of similar neural implants. Conclusions - The reported process and optimization steps described here in detail can aid in the design and fabrication of retinal prosthetic devices or similar neural implants.

Three-dimensional electro-neural interfaces electroplated on subretinal prostheses

“…Such process was demonstrated for 100 µm pixels, but it is difficult to scale it down to much smaller pixel pitch, such as 20 µm. Another approach could be based on 3D polymer structures, which can be formed by 2photon laser lithography and subsequently metalized for conduction [30][31][32]. However, polymer features (pillars or walls) of high aspect ratio tend to deform over time, leading to cracking of the metal coating.…”