# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Mechanical Properties and Design Considerations for Retinal Prostheses
In the case of the a-SiC:H coatings used for the test devices of Figure 2, this thickness was 0.5 \(\upmu\)m, but later process optimization led to the reduction of the required SiC layer thickness to 0.1 \(\upmu\)m for top or bottom layers, and 0.25 \(\upmu\)m for any middle layers. Other materials that we investigated in Table 2 either lacked sufficient biocompatibility, biostability, or both. Another matter relating to mechanical properties of our implantable multi-electrode array components in particular was the channel count needed in order to be able to stimulate sufficient numbers of sites to have a reasonable likelihood of restoring \"useful\" vision. We estimated for our third-generation retinal prosthesis that at least 256 channels would be required, based on the findings of a number of teams in the field over the last 20+ years that have recently been reviewed [3; 4]. Since no de-multiplexer was included in our system in a distal region near the stimulation sites beneath the retina, this meant that >256 individual traces or leads would need to be incorporated along the length of the array from the stimulator package containing the circuitry, to the electrode locations within the eye. A quick calculation revealed that even for a modest center-to-center spacing of adjacent  
Figure 2: Early micro-fabricated test electrode arrays by the Boston Retinal Implant team. **(a)** The conducting traces interconnecting the contact pads on the left with the 400 \(\upmu\)m diameter Iridium Oxide electrodes on the right were encapsulated in HD Microsystems PI-2611 polyimide only, and quickly de-laminated under accelerated soaking conditions in saline solution at 87 \({}^{\circ}\)C; **(b)** multi-electrode arrays which were “wrapped” or encapsulated in 0.5 \(\upmu\)m thick PECVD a-SiC above and below the conductors (in addition to a polyimide outer coating for scratch protection) survived for over 5 months under the same conditions, which we project to be equivalent to over 10 years at 37 \({}^{\circ}\)C; similar bio-stability of SiC has been demonstrated elsewhere [28; 29]. **(c)** Cyclic voltammetry curves measured from the multi-electrode array of Figure 2b after 20 weeks of saline soaking at 87 \({}^{\circ}\)C demonstrated good, consistent charge storage capacity on all 15 sites. conducting traces in the leads of 20 um, a single micro-fabricated layer containing side-by-side leads to each electrode would need to be >5 mm wide. Not only would a multi-electrode array of this width likely not conform well to the approximately spherical contour of the eye, but it would also require a relatively large incision to access the sub-retinal space; broadly speaking, our surgical experience has shown that reliable insertion into the sub-retinal space can be achieved using electrode arrays that are <5 mm wide that are implanted through sclera flaps that are \(\sim\)6 mm in width [16]. The pitch between electrode sites at the distal end of the array, then, was determined largely due to surgical factors. Smaller- or larger-pitch arrays would have been feasible to micro-fabricate, but may not have been as feasible to implant. \"Pitch\" between electrode sites here refers to the nearest-neighbor distance in a nominal hexagonally close packed configuration, which was used to save space. In contrast, the rigid, disc-shaped hermetic package containing our stimulating electronics was limited in size as well, since a larger package would likewise have been difficult to implant behind the eye in the orbit. The pitch between signal feedthroughs in that package in turn determined the overall package's 11.5 mm diameter. In that case, it was the limitations of the fabrication process for creating high temperature co-fired ceramic parts that determined the spacing.