**8. Electroplating**

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

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

22 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

parylene-based arrays in an animal model.

As a possible mechanism for extending the longevity of chronically pulsed electrodes, we have investigated electroplated films of high surface-area platinum. Specially designed thin-film platinum electrode arrays, consisting of sixteen 75 µm and 150 µm diameter electrodes of 3000 µm center-to-center spacing, were fabricated according to the single-layer process. Initial experiments were performed on these arrays to determine material morphologies after plating at different potentials in an aqueous ammonium hexachloroplatinate solution according to Whalen *et al.* [49]. Subsequently, arrays were immersed in the solution in a specialized jig and six were plated at a plating potential of -0.6 V (vs. an Ag/AgCl reference electrode) for 1.5 hours. The others remained unplated. Electrochemical tests were performed to evaluate the efficacy of this plating step in extending electrode longevity under chronic pulsing.

The electrode morphologies of a typical array of 16 electrodes plated at different poten‐ tials are shown in Figure 21. Note that the 4 corner electrodes (1, 4, 13, and 16) were not plated. Magnified views of some of the possible morphologies attainable via this mecha‐ nism are shown in Figure 22. These micrographs show morphologies that likely corre‐ spond to a drastically increased surface area. In order to confirm this cyclic voltammograms (CVs) in O2-free H2SO4 were taken of the electrodes before and after platinization. Accord‐ ing to [50], "real" electrode surface area can be adequately estimated by integration of the area over the hydrogen adsorption peak or likewise the area under the hydrogen desorp‐ tion peak. The two CVs in Figure 23 show that there was a more than 40-fold increase in surface area after platinization when compared with the pre-plated surface area (note the change in scale of the ordinate from nA to µA). Under pulsing, the voltage responses of both plated and unplated electrodes remained stable for approximately 29 days, at which point the unplated electrodes showed signs of failure. Voltage responses for one such electrode on day 26, 29, and 31 are overlaid in Figure 24 (left), which documents the progression of failure. The plated electrodes, on the other hand, remained intact for much longer, most surviving more than 50 days, or 430 million pulses, at which point the testing goal was met and the test was stopped. Overlaid voltage responses for one such elec‐ trode, showing the voltage responses at day 26, day 31, and day 50, are shown in Figure 25 (right). The electrochemical impedances at 1 kHz of a typical plated and unplated electrode are shown in Figure 25. A dramatic jump in impedance was observed for the unplated electrode at the time of failure, while the plated electrode demonstrated only minor variability in its lower impedance throughout the 430-million-pulse trial (most variability happened on the days that CVs were taken, as expected). These preliminary data corroborate the evidence that plating of the electrodes is beneficial to longevity, and suggest that high surface-area platinization of electrodes can have a dramatic effect on extending electrode life while lowering electrochemical impedance to charge delivery. Future work will include replication of these tests and chronic pulsing at high temperatures for longer times to further accelerate and assess the possible modes of failure.

**Figure 21.** SEM of each of the electrodes on a typical 16-electrode array after platinization.

**Figure 22.** Magnified views of possible surface morphologies after platinization.

**Figure 23.** CVs showing more than 40-fold increase in electrode surface area from before plating (left) and after plat‐ ing (right). The surface area is estimated by integrating the area under the peaks circled in red. Note change in scale of the ordinate. Scan rate: 100 mV/s. Electrolyte: O2-free H2SO4 (N2-bubbled).

**Figure 21.** SEM of each of the electrodes on a typical 16-electrode array after platinization.

24 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

**Figure 22.** Magnified views of possible surface morphologies after platinization.

**Figure 24.** Voltage responses to a current pulse for (left) an unplated electrode, documenting the process of electrode failure, and (right) a plated electrode, showing steady responses throughout the 50 day test. Note response ampli‐ tudes for plated electrodes are far lower than those for the unplated electrode, as expected [27].

**Figure 25.** Magnitude of the electrochemical impedances at 1 kHz of an unplated and plated electrode over time. The unplated electrode showed a dramatic increase in impedance around day 30, at which time the test was stopped, whereas the plated electrode showed steady impedance through day 50. The arrow denotes a temporary dip in impe‐ dance due to CV scanning.
