# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Mechanical Properties and Design Considerations for Retinal Prostheses  
Each fabrication and surgical challenge that we encountered along the way influenced not only the CAD design of the components and their multilayered construction, but also the order of fabrication and the choices of materials for processing. As well, we obtained practical feedback on both the layout and the stiffness and handling of the components by performing many pre-clinical animal surgical procedures in miniature pig models, and these too informed our design choices; see e.g., [16]. Further feedback came from in vitro testing, particularly accelerated lifetime testing in biological saline,  
Figure 1: Three generations of the Boston Retinal Prosthesis with micro-fabricated components. (**a,b**) First generation 16-channel stimulator with early micro-fabricated electrode array in mini-pig model; (**c**) second-generation hermetically packaged 16-channel device with bidirectional telemetry; (**d,e**) third-generation hermetically-packaged 256+ channel unit in minipig, and bonded array-feedthrough interconnections prior to final silicone overmolding—the electrode array is 5 mm at widest with segmented leads, and the package is 11 mm in diameter; (**f**) a simplified cross-sectional cartoon of the implant showing the package mounting on the eyeball, the leads along the outside of the eye traversing the eye wall into the subretinal space (inset); (**g**) a 5 mm wide, 15 mm long and 75 μm thick polyimide surgical guide, and the guide in use during array insertion. which influenced our material selection. The design elements and challenges that we addressed also interacted with one another; the mechanical design problems facing the creation of micro-fabricated implantable components for a sub-retinal visual prosthesis are, in fact, multi-faceted. For example, our multi-electrode arrays for retinal stimulation must cross the sclera and the choroid for their leads to convey signals from a peri-ocular, hermetically packaged implanted stimulator unit to the distal electrodes in the sub-retinal space. The peri-ocular portion of our implantable visual prosthesis is subject to different mechanical stresses during and after the surgical procedure than the intra-ocular portion. For instance, the extra-ocular portion may rub against and possibly erode overlying tissue such as the conjunctiva and [PERSON]'s capsule, which we observed early in the development of our second-generation device [25]. This outcome resulted in modifications to the most anterior part of the peri-ocular implant to reduce its profile as much as possible. For example, the 19 mm diameter secondary radio frequency coil, which in our case is implanted and sutured around the cornea, was at one time encapsulated in a ~2 mm thick layer of polydimethylsiloxane (PDMS). This gave rise to undue tension on the overlying conjunctiva at the end of the procedure. This recurrent outcome led to an effort to micro-fabricate the coils using a SU-8 epoxy mold and gold electroplating techniques. However, ultimately we found that the best solution to mechanical stresses on the RF coil and the surrounding tissue was to wind the coils from 75 \(\upmu\)m diameter gold wire on a spherical form of the same diameter as the target eye. By careful attention to the layering of the turns, we were able to reduce the height of the coil assembly to <0.5 mm, which provided long-term biocompatibility. We later elected in our third-generation device to microfabricate a coil support structure from electroplated Au and polyimide that provided attachment points for suturing the coil to the sclera. Even this simple structure, shown assembled in Figure 1 and discussed further below, had mechanical design considerations that were resolved through trial-and-error in multiple implantations in mini-pig animal models. For example, eye size impacts design and materials requirements, not only in the animal models but also among the (frequently myopic) human patient candidates whom we hope to treat [26].