**7.3. Implantation and testing**

Chronically implantable retinal electrode arrays comprising 1024 75 µm diameter electrodes arranged in a complex biomimetic pattern that closely mimics the density of RGC in the human retina [48] were designed (the electrode density varied radially in a ratio matched to that of the RGCs), as shown at the left in Figure 15. These arrays (shown at right in Figure 15) were fabricated according to the dual-layer process, with 60 of the electrodes connected via two traces each to facilitate electrical conductivity verification. The strength of metal adhesion was verified using a Scotch tape test, which demonstrated that direct platinum evaporation is feasible without the need for a titanium adhesion layer. Electrical testing demonstrated a typical line impedance of a contact-electrode-contact circuit to be approximately 5 kΩ, which included two 8 µm wide traces of 20 mm length, as well as two via step junctions connecting

**Figure 14.** Fabrication process for parylene-based dual-metal-layer flexible MEAs.

underlying traces to the overlying electrode. Two types of via and electrode configurations were tested. Some electrodes (electrode SEM given in Figure 16 (left)) had vias connecting to the underlying trace near the center of the electrode, hence enabling charge spreading from the center of the electrode. One possible drawback to this configuration is that the contact from trace to electrode over the sidewall is a potentially vulnerable point of the circuit during processing (e.g., subsequent RIE processes) and during pulsing in electrolyte because the metal may be thinner there. The other electrode configuration had vias located adjacent to the electrode (electrode SEM given in Figure 16 (right)), with the possible advantage that it would be protected during RIE and subsequent pulsing by the overlying conformal parylene layer. An SEM showing the morphology of a single central via is given in Figure 17. This clearly depicts the sidewall coverage responsible for the electrical contact between the first and the second metal layers. In both configurations, each via had an impedance of less than 12.5 Ω. The best final configuration has not yet been determined.

**Figure 15.** (left) Design of biomimetic dual-metal-layer retinal electrode array showing biomimetic arrangement of electrodes. (right) Fabricated biomimetic chronically implantable arrays with 60 of 1024 75 µm diameter electrodes connected through dual-layer process with U.S. dime for size comparison.

**Figure 16.** Two possible dual-layer electrode configurations. Electrode with central vias (left), and electrode with abutting vias (right).

**Figure 17.** Magnified view of trace to electrode via showing sidewall coverage.

underlying traces to the overlying electrode. Two types of via and electrode configurations were tested. Some electrodes (electrode SEM given in Figure 16 (left)) had vias connecting to the underlying trace near the center of the electrode, hence enabling charge spreading from the center of the electrode. One possible drawback to this configuration is that the contact from trace to electrode over the sidewall is a potentially vulnerable point of the circuit during processing (e.g., subsequent RIE processes) and during pulsing in electrolyte because the metal may be thinner there. The other electrode configuration had vias located adjacent to the electrode (electrode SEM given in Figure 16 (right)), with the possible advantage that it would be protected during RIE and subsequent pulsing by the overlying conformal parylene layer. An SEM showing the morphology of a single central via is given in Figure 17. This clearly depicts the sidewall coverage responsible for the electrical contact between the first and the second metal layers. In both configurations, each via had an impedance of less than 12.5 Ω.

**Figure 15.** (left) Design of biomimetic dual-metal-layer retinal electrode array showing biomimetic arrangement of electrodes. (right) Fabricated biomimetic chronically implantable arrays with 60 of 1024 75 µm diameter electrodes

The best final configuration has not yet been determined.

**Figure 14.** Fabrication process for parylene-based dual-metal-layer flexible MEAs.

18 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

connected through dual-layer process with U.S. dime for size comparison.

The arrays were successfully molded to the approximate curvature of the canine retina (Figure 18 (top)) using heat-annealing and a custom mold in a vacuum chamber, and sterilized using ethylene oxide gas. Two biomimetic arrays were implanted in the right eye of two canines through a 5 mm pars plana incision after vitrectomy, and were affixed to the retina (Figure 18 (bottom)) using a retinal tack modified by the addition of a PDMS washer (to account for the thin nature of the parylene arrays).

**Figure 18.** Heat-molded and annealed retinal electrode array with retained spherical curvature (top), and intraopera‐ tive photographs of tacking in each canine (bottom).

Follow-up in both animals was conducted for six months using fundus photography, fluores‐ cein angiography (FA), in which blood is fluorescently stained to assess vessel perfusion in the retina, and optical coherence tomography (OCT), an interferometric technique that enables cross-sectional imaging of the retina. Fundus photography and FAs of both animals, examples of which are shown in Figure 19, consistently demonstrated that vessel filling underneath the array was normal. Obstruction and vessel leakage would have been visualized if the array were placing excessive pressure on the retina. In addition, OCT demonstrated that the electrodes were consistently less than 50 µm away from the ganglion cell layer in both animals

18 (bottom)) using a retinal tack modified by the addition of a PDMS washer (to account for

**Figure 18.** Heat-molded and annealed retinal electrode array with retained spherical curvature (top), and intraopera‐

Follow-up in both animals was conducted for six months using fundus photography, fluores‐ cein angiography (FA), in which blood is fluorescently stained to assess vessel perfusion in the retina, and optical coherence tomography (OCT), an interferometric technique that enables cross-sectional imaging of the retina. Fundus photography and FAs of both animals, examples of which are shown in Figure 19, consistently demonstrated that vessel filling underneath the array was normal. Obstruction and vessel leakage would have been visualized if the array were placing excessive pressure on the retina. In addition, OCT demonstrated that the electrodes were consistently less than 50 µm away from the ganglion cell layer in both animals

the thin nature of the parylene arrays).

20 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

tive photographs of tacking in each canine (bottom).

**Figure 19.** Fundus photographs (left) showing parylene MEAs tacked to the right retina of both animals and FAs (right) showing normal vessel perfusion under the arrays. Arrows point to retinal tacks.

(typical OCTs of both animals are shown in Figure 20), an outcome that theoretically would afford excellent electrical coupling between the electrodes and the electrically excitable cells of the retina. It is important to note that in the OCT of the second canine, the scan was taken along a segment furthest from the tack site, where one might expect the least proximity. Even at this location, this array remained in very close apposition throughout the six-month implantation. Post-enucleation histology has since confirmed the excellent biostability seen during follow-up.

As was briefly mentioned previously, the dual-layer process proffers considerable advantag‐ es over the more traditional single-layer approach. Design of single-layer electrode arrays is usually hindered by the need to route traces amongst the electrodes. This tends to cause crowding of traces and electrodes into groups, an organization that may not be optimal for stimulating the tissue of interest. In addition, this has a propensity to constrain the geometric area of the electrodes in the MEAs to smaller sizes, and thus reduces the number of electrodes possible in a given area. The dual-layer process obviates these problems by

**Figure 20.** OCTs of both animals showing very close apposition (<50 µm) of the arrays to the RGC layer.

enabling traces to pass under overlying electrodes without making contact to them, having the effect of both relaxing the constraints on electrode size and number and enabling more complex electrode organization (such as the biomimetic one presented in this work). Although the arrays fabricated here had just 60 electrodes of connectivity with 120 traces total, this was without making full use of both layers for wire routing and connection of electrodes. In order to not make traces unnecessarily narrow and of too high impedance, we believe an extension of this process to three or more metal layers will be necessary to achieve 1024 electrodes of total connectivity. Indeed, this fabrication process is easily extendable to create such structures through addition of extra layers of parylene and metal. Given the encouraging biostability results presented here and the ability of these arrays to stimulate retinal tissue, future studies will include chronic stimulation from implanted parylene-based arrays in an animal model.
