# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Mechanical Properties and Design Considerations for Retinal Prostheses
If the secondary radio frequency coil were to be maintained at a fixed distance from the anterior edge of the hermetic electronics package among the peri-ocular components, e.g., using a fixed-length micro-fabricated connecting structure, a smaller-diameter host eye would force the posterior edge of that electronics package far back into the orbit, thus creating a challenging trade-off for the surgeon during implantation. This design element was optimized through collaboration between our surgical and engineering teams. The micro-fabricated coil support structure shown in Figure 1 was built to accommodate varied host eye shapes by encapsulating the RF coil's leads in flexible silicone surgical tubing, and by fixing the ends of that tubing segment to the coil support with sutures and to the electronics package via silicone over-molding. These modifications allowed considerable flexibility in the relative placement of the electronics package and the RF coil because of the tubing's flexibility, while at the same time allowing the surgical field to be less crowded, since the coil assembly could be \"flipped over\" out of harm's way until it became time to affix the coil to the sclera. Turning to considerations for microfabricated intra-ocular components, there are two competing needs. On the one hand, our electrode array must be stiff enough to pass through a flap created in the sclera and an incision in the choroid, which is one of the most vascular tissues in the body. The electrodes must also be able to be advanced to the target sub-retinal location. On the other hand, once the array is advanced to its final resting spot, it is desirable for the array to be flexible, so as to conform to the contour of the sub-retinal space. This segment of the device must be stiff enough to breach an incision in the choroid, yet at the same time, if the distal end of the electrode array retained that same level of stiffness, it might cause long-term biocompatibility issues for the device, including potentially a retinal tear. Our solution to this problem was in part to keep the intra-ocular portion of the flexible electrode array to <15 \(\upmu\)m in thickness, and to perforate the electrode array by design using reactive ion etching (RIE) so as to allow fluid exchange through the openings that rest within the sub-retinal space. Further processing details are below. Additionally, to assist in the surgical procedure, we employed a custom-fabricated 75 \(\upmu\)m-thick shaped polyimide surgical guide that was laser-cut from a DuPont Kapton(r)sheet. The distal aspect of the guide was dull (i.e., had rounded edges) but semicircular in shape, which aided implantation of the flexible multi-electrode array through the flap and into the sub-retinal space (see Figure 1). The procedure proceeded by first introducing the stiffer polyimide guide through the cauterized choroidal incision, and then implanting the electrode array by sliding it over and along the guide into the eye, having first raised the retina out of harm's way by creating a bleb (by injection through a small retinotomy from inside of the eye). The guide was then removed, the electrode array was sutured in place to anchor it to the sclera outside the incision site; the bleb was slowly resorbed. In this manner, a flexible multi-electrode array with or without penetrating posts was deployed through an incision that the array would not otherwise have had enough stiffness to pass through. Further details about the implantation procedure have been recently presented [16]. A final mechanical design consideration for a retinal prosthetic's electrode array and other micro-fabricated implantable components is the selection of encapsulating materials to protect the leads and other structures from ingress of moisture.