# Micro-Fabrication of Components for a High-Density Sub-Retinal Visual Prosthesis
## Materials and Methods
### Mechanical Properties and Design Considerations for Retinal Prostheses
This consideration also intersects with other factors, such as biocompatibility in the sub-retinal space, and compatibility of the deposition process for the encapsulating material in question with the previously existing component materials that are deposited earlier in the process flow. These intersecting needs may be better understood through examination of Table 2 below. Biocompatibility data are drawn in part from our team's earlier studies in mini-pig animal models [27] and are supplemented with more recent data from pre-clinical testing of our team's retinal prosthetic device. From the development experiences of earlier generations of our team's visual prosthesis, we gradually came to appreciate the unique and intersecting challenges that were summarized in Table 2 concerning the impact of added encapsulating films on the bio-stability and biocompatibility of the devices in the eye environment. Early non-hermetically encapsulated visual prosthesis components micro-fabricated by our team had vapor-deposited protective coatings of Parylene-C applied (Figure 1a,b) including the use of organosilane adhesion promoters and careful surface preparation before coating and patterning using oxygen reactive ion etching. Despite these precautions, we found that Parylene-C coatings alone had useful lifetimes in saline environments of at best 1-2 years at 37 \({}^{\circ}\)C. This was adequate, for example, for some in vitro and early in vivo testing, but fell short of our ultimate design lifetime target of 10+ years. After evaluating a number of potential encapsulants [27], we eventually settled on \"wrapping\" the conductors in our arrays with plasma enhanced chemical vapor deposition (PECVD) of amorphous silicon carbide (a-SiC:H) at 325 \({}^{\circ}\)C, which showed excellent long-term material and electrical stability in accelerated life tests at 87 \({}^{\circ}\)C (see e.g., Figure 2). These inorganic encapsulating layers were used above and below each layer of conductors, which we kept near to the neutral axis of the arrays to minimize any effects of internal film stress. The presence of a-SiC:H layers in the multi-layered electrode array structure also contributed to their overall stiffness, which was compensated for in part by adjusting the thickness of the outer polymer layers of the micro-fabricated components for scratch protection. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Encapsulating** & **Mechanical Properties** & \begin{tabular}{c} **Biocompatibility in** \\ **Sub-Retinal Space** \\ \end{tabular} & \
\begin{tabular}{c} **Micro-Fabrication** \\ **Process Compatibility** \\ \end{tabular} &
\begin{tabular}{c} **Effectiveness in** \\ **Improving Bio-Stability** \\ \end{tabular} \\ \hline \hline Parylene-C & Pliable & Good & Good & Poor [1] \\ Al\({}_{2}\)O\({}_{3}\) & Brittle & Poor & Average & Poor [2] \\ SiN\({}_{x}\) & Strong, somewhat Brittle & Average & Good & Average [3] \\ a-SiC:H & Strong, somewhat Brittle & Good & Good & Excellent [4] \\ \hline \hline \end{tabular}  
* 1 Lasted 1–2 years @ 37 \({}^{\circ}\)C, see e.g., [25]. 2 Retina tissue responded adversely [27]. 3 Projected to last 5+ years @ 37 \({}^{\circ}\)C in accelerated saline soaking tests. 4 Projected to last 10+ years @ 37 \({}^{\circ}\)C, see e.g., [28; 29].  
\end{table} 
Table 2: Comparison of properties of potential encapsulating materials for micro-fabricated, implantable retinal prosthesis components to improve long-term biosstability. While our combined inorganic and organic encapsulation strategy was evaluated through numerous tests, a systematic study of the effects of coating thickness (and thus stiffness) on long-term device lifetime was not performed from first principles. Rather, in the case of inorganic encapsulation layers deposited e.g., by plasma enhanced CVD, their natural stiffness was so much greater than that of the flexible polyimide substrates that we had microfabricated that we limited the thickness of such layers to only that which was necessary to have reasonable confidence of pinhole-free coatings.