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.