# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Process Flows for Microfabrication
Rather, electroplated Au metallization was instead used for its ability to strengthen the suture holes that were etched around the perimeter for affixing the coil to the sclera. Once the coil supports were freed from the host wafer, the wire-wound coils were attached to them using Kwik-cast, a medical grade silicone adhesive. Completed coil supports are shown in Figure 6.  
Most recently, our team has made advances in microfabricating and implanting subretinal multi-electrode arrays that contain both planar electrodes, and electrodes atop penetrating posts for stimulation of the inner retinal layers, each of which are SIROF-coated; these have been well tolerated in a number of animal trials [16]. The process for creating \"hybrid\" penetrating and planar electrode arrays differed from the basic planar multi-electrode array process in that prior to sputtering the SIROF electrode material (but after the exposure of the metal underlying the electrode sites), SU-8 photo-imageable epoxy was applied to the entire wafer surface and patterned; special care needed to be taken in the extended soft baking step. On the one hand, this prevented adhesion between the photomask with the desired post pattern and the coated wafer surface, and on the other hand, the soft baking process improved the adhesion of the posts themselves to the host surface after developing the pattern. A SIROF film was sputtered onto the passive penetrating posts, creating a conducting path from the base of the posts to the tips; the 50 \(\upmu\)m and 100 \(\upmu\)m-tall posts had a top-to-bottom impedance of \(<\)40 \(\Omega\), which introduced negligible additional series resistance to the sites. After the SIROF patterning, the arrays were coated in 1 \(\upmu\)m of Parylene-C by vapor deposition, and the tops of the posts were exposed by reactive ion etching in an O\({}_{2}\) plasma. Special care was needed to properly protect the posts and the surrounding electrodes during this etching step with photoresist, and applying the resist using a spraying technique with a Suss Microtec Gamma Cluster Tool with a spray coating module provided this protection. A schematic cross section diagram of a multi-electrode array with an incorporated penetrating post and a micrograph of a completed hybrid array with mixed planar- and penetrating electrode sizes is in Figure 7.  
Figure 6: Microfabricated supporting structure for supporting and suturing a 19 mm diameter wire wound secondary radio frequency coil around the cornea (see also Figure 1). (**a**) A base layer of HD Microsystems PI-2611 polyimide was applied to a host Si wafer, electroplated Au was added to mechanically reinforce the structure, and the support perimeter and suture holes were etched by O\({}_{2}\) RIE; (**b**) An Au wire coil wound on a spherical form so that it would lay flat on the sclera surface; (**c**) The assembled coil support with an incoming silicone tube to protect the coil wires from handling and to allow flexibility of coil placement relative to the electronics package.