## 3 Results
### Fabrication Results of Pt-Black MEA  
Figure 12 shows the fabrication results of the nanostructured MEAs designed for both in vivo and in vitro experimental applications. Figure 11(a) shows the fabrication results of the nanostructured microelectrodes on a polyimide layer, suitable for in vivo experiments. The final device was assembled through a wire-bonding process with 16 microelectrodes in a 4 \(\times\) 4 grid. Figure 11(b) demonstrates the device designed for in vitro experiments, fabricated on a PCB. This setup included an MEA with an inner well for cell manipulation and outer electrodes that connect to the MEA-60 recording system (Multi Channel Systems GmbH, Kusterdingen, Germany). The glass MEA comprised 61 microelectrodes, including  
Figure 10: Photograph of the fabricated stimulator chip die. (**a**) Close-up view of the stimulator chip comprising an H-bridge, operational amplifier, and capacitor. (**b**) Simplified schematic diagram of the switched capacitor structure within the stimulator IC. The red line represents the Pinit signal, and the blue line indicates the voltage charged to the capacitor.  
1 reference electrode, 58 recording electrodes, and 2 stimulation electrodes. The stimulator charge was injected through the stimulation electrodes, while the remaining 58 electrodes were used to record the neural response signals. The glass MEAs were attached to the PCB via a wire-bonding process. The PCB measured 49 mm \(\times\) 49 mm, with typically 60 microelectrodes arranged in an 8 \(\times\) 8 grid and an interelectrode distance of 200 \(\upmu\)m. The SEM images of the fabricated MEAs are shown in Figure 12c. The nanostructures of the electroplated Pt-black were well attached to the Au layer. Each microelectrode, which comprised a circular electrode with a diameter of 50 \(\upmu\)m, was fabricated by depositing an Au layer followed by a Pt-black layer. From the surface SEM image, the Pt structures exhibited a highly porous morphology, resulting in a significantly large surface area.  
Figure 13a shows the experimental setup to measure the electrode-electrolyte interface impedance and perform cyclic voltammetry (CV) to analyze the nanostructured Pt-black microelectrodes using a three-element circuit model. An impedance analyzer, CS350 (Wuhan CorrTest Instruments Co. Ltd., Wuhan, China), was used for these measurements. The MEAs were dipped in a phosphate-buffered saline (PBS) solution, and impedance measurements were taken at frequencies ranging from 1 Hz to 1 MHz.  
Figure 11: Evaluation of stimulator IC corresponding to the SiNW layers and light intensity. Voltage across the capacitor with an initializing pulse duration of (**a**) 1 \(\upmu\)s and (**b**) 10 \(\upmu\)s; variation in biphasic current pulse with (**c**) a different number of stacked SiNW layers under a light intensity of 100 lux and (**d**) varying load resistor of 4 k\(\Omega\) and 16 k\(\Omega\) under a bias voltage of 3 V; relationship between stimulus current and light intensity up to 610 lux (**e**) in the linear scale and (**f**) in the semi-log scale.  
Figure 13b summarizes the average impedance measurements at different frequencies for five distinct electrodes with a diameter of 50 \(\upmu\)m. The measured impedance at a frequency of 1 kHz was approximately 779 \(\Omega\). As the frequency increased, the measurements remained below 1 k\(\Omega\) above 1 kHz. This ensures that large currents can be driven to retinal cells despite the small surface area, which is essential for effective cell current driving. The CV measurements were conducted with a voltage sweep from \(-\)0.6 V to 0.6 V at a step rate of 50 mV/s for 5 cycles. The charge storage capacities (CSCs) of the nanostructured Pt-black MEAs were calculated from the CV curves shown in Figure 13c by integrating the current over the full potential cycle within the sweep range. The measured results indicate CSCs of 0.778 mC/cm\({}^{2}\).