# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Mechanical Properties and Design Considerations for Retinal Prostheses

Table 3: Schematic microfabrication process flow summary for a flexible multielectrode array with Iridium oxide stimulating sites and two layers of metallization for signal routing. Features are not drawn to scale, for visibility. eye including its length (which correlates with the natural refractive state of the eye) and the insertion sites of the extra-ocular muscles, which differ between porcine and human subjects. Specifically, we sought a design that could accommodate variations in our animal model (needed for pre-clinical testing) that also would be suitable for human eyes that also have variations. And, we wished to build electrodes that would be usable across a range of eye sizes without having to customize arrays for individual patients. Thus, a degree of adjustability was desirable in our design. After much trial-and-error over the course of a number of early surgical trials, we developed two array designs that were workable. A first design evolved which employed a medical grade silicone tube as a physical support for a helically wound coiled-cable type lead, which was inspired in part by flexible, implantable helix type wire-based leads that are in common use in other medical devices. The use of the coiled arrays required a much longer physical length of the arrays, because much of the length was consumed in the coiling process but the coiled arrays were easily re-positioned without significant creasing or sharp bending of the coiled structures. And, the coiled assemblies could easily be passed beneath the eye muscles by simply elevating them with a muscle hook and passing the array beneath with the free hand. An example of an inactive coiled-cable array mockup fitted around a human cadaver eye can be seen in Figure 4.  
Ultimately, our \(\sim\)4 mm wide, two-dimensional implementation of a 2-layered multi-electrode array was refined through pre-clinical testing. This required dealing with a fundamental problem that micro-fabricated structures for medical applications often encounter, and that is particularly true for organs such as the eye. The micro-fabricated structures are frequently created on planar surfaces, such as silicon wafers, for ease of handling and processing. However, the host tissue, i.e., the mini-pig eye in the case of our pre-clinical tests, had a complex 3-D shape and a diameter of \(\sim\)23 \(\pm\) 2 mm. The issues that were related to laying our fabricated 2-D structures down onto 3-D surfaces can easily be visualized by wrapping a strip of paper around a ball. There is a natural tendency for the 2-D object, such as the paper in this example, to buckle or to protrude in some manner when portions of 2-D objects are affixed to the 3-D surface, and the edges of the very thin 2-D parts, such as the flexible electrode arrays here, can be sharp. This deformation with a sharp leading edge can lead to erosion of the conjunctiv when thin, flat prosthesis components are implanted beneath it, which we in fact initially did experience [25]. To facilitate a degree of adjustability to different host eye sizes, and to allow the 2-D multi-electrode arrays the ability to lay flat on the sclera surface, we eventually adopted a serpentine shape for the arrays, which can be seen in Figure 1. Even these structures, which lay relatively flat on the eye surface, required segmentation of the two-layered multi-lead structure into strips or groupings of  
Figure 4: Photographs of the implanted components of our prosthesis. (**a**) A previous-generation coiled-cable electrode array that accommodated a variable distance between the implanted neuro-stimulator package and the insertion site into the sub-retinal space. A flexible, 12 \(\upmu\)m-thick array is helically wound around a medical grade silicone tube; (**b,c**) The same generation device is shown implanted around a human cadaver eye after its enucleation.