Fabrication of multi-layer, high-density micro-electrode arrays for neural stimulation and bio-signal recording

The electrode-tissue interface is of principal importance in neuroprosthesis. Indeed the successes of the cochlear implant and other therapeutic devices are directly attributable to the design and fabrication techniques of their interfaces with neural tissue, that is, the electrode or electrode array. Traditional fabrication techniques are often labor-intensive and do not lend themselves to automation thereby increasing the cost of the electrode, and owing to fabrication variability, potentially compromising the reliability of the devices incorporating them. Exacerbating the difficulties in electrode fabrication further is the fact that only a handful of materials have been demonstrated to be biologically inert. These same materials are often among the most difficult to utilize in the fabrication of neural electrodes. In the present paper, a new methodology for automated fabrication of high-density electrode arrays is presented. Using exclusively biologically-inert raw materials, laser machining techniques combined with multiple layer structuring is shown to achieve feature sizes of the order of 25 mum. As an illustrative example, a 98 electrode array for interfacing with surviving retinal tissue through a visual prosthesis for the blind is presented. Overall dimensions of the array are of the order of 8.7 times 9.4 mm, consistent with approximately 25 degrees of visual field.

I. Introduction

The single most important factor in the success or failure of biomedical neurostimulators used in the restoration of movement or sensoneural function is that of the electrodetissue interface. The principal aim of such devices is to initiate physiological events that lead to the propagation of a signal from the nerve cell (or group of cells) to adjacent neurons or to another distant part of the body. Implantable bionics such as the cochlear implant for the hearing impaired, and, more recently, the work of the authors and their colleagues on a vision prosthesis for the profoundly blind, require an interface between excitable biological tissue (nerve cells in particular) and the electronics that deliver electrical stimuli. Recently, the authors began the fabrication of micro-machined devices using laser ablation of the noble metal platinum and the silicone elastomer, polydimethylsiloxane (PDMS) as these are established biomaterials [1]. Fig. 1. Steps involved in making multiple-layer electrode structures