# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## 3 Results  
The Boston Retinal Implant team has created three generations of retinal prostheses with microfabricated components since its inception. Our team was the first to employ flexible polyimide-based epi-retinal electrode arrays for early acute clinical testing in blind patients [1,2]. The psychophysical outcomes from that acute study in blind volunteers, which were obtained using multi-electrode arrays constructed without the inorganic encapsulating layers that were later found to be essential to long-term biostability, nevertheless encouraged our team to create our first 16-channel wirelessly powered, chronically implantable stimulators that had many of the attributes of our current devices (see Figure 1 and [25] for example). As mentioned, the primary components in retinal prostheses that require microfabrication technology are the high-density multi-electrode arrays, and/or the structures that form the neural interfaces. These devices ideally have a high charge capacity, owing to the current densities required to achieve useful phosphenes--especially for small electrodes. This led our team to employ Iridium Oxide electrode coatings early in our device development because of the margin of stimulation safety they provide. The deposition process for IrOx and early electrode characterization has been presented elsewhere [31], but Figure 8 shows cyclic voltammograms and voltage waveforms from identical area planar- and penetrating-post electrodes, such as on the \"hybrid\" multi-electrode arrays shown in Figure 7.  
Figure 7: (**a**) A cross section diagram of a flexible, microfabricated electrode array with a penetrating post formed from photo-imageable SU-8 epoxy (in dark blue) whose sidewall is insulated with Parylene-C (light blue). The fabrication process is similar to the planar electrode steps of Table 3, except that the SU-8 films are patterned prior to the SIROF deposition and liftoff, and Parylene-C coating; (**b**) A SEM micrograph of a completed, hybrid penetrating- and planar-electrode array, containing posts that are either 50 or 100 \(\upmu\)m-tall (two separate 50 \(\upmu\)m layers of SU-8 were applied). The expansion of the charge capacity and enclosed area in the voltammograms are typical of SIROF electrodes with repeated cycling in saline solution, and this outcome was similar for both  
Figure 8: (**a,b**) Cyclic voltammograms versus time for a 40 \(\upmu\)m diameter microfabricated SIROF-coated penetrating post electrode. Note the evolution of the charge capacity, proportional to the enclosed area; the corresponding voltage waveforms for 1 msec bi-phasic current pulses are shown at right. (**c,d**) CV plots and voltage waveforms over time for a corresponding 40 \(\upmu\)m diameter planar electrode, respectively; (**e**) charge capacity versus time for a 40 \(\upmu\)m diameter planar electrode (#3/19; blue trace) and for three, 40 \(\upmu\)m diameter penetrating post electrodes (#’s 10/17. 11/14. and 14/6; red, green, and purple traces) showing gradual improvement over 1000+ hrs. of pulsing, likely due to hydration of the SIROF films. planar- and penetrating-electrodes. The changes to the electrode voltage waveforms upon repeated bi-phasic current pulsing can also be seen. The differences are attributed to hydration of the SIROF films and expansion of the effective surface area of the sites from the films' as-deposited state. This feature is best taken into account when \"burning in\" a new electrode array on the bench top prior to implantation, for example. Platinum electrodes deposited by conventional physical vapor deposition processes, which we fabricated in the early years of our team's device evolution, did not require such treatment. SIROF charge capacity was also found to continue to increase even after more than 1000 hrs of continuous pulsing, indicating that the beneficial effect of exercising the electrodes in saline environments continues beyond an initial break-in period.