## 3 Results
### In Vitro Experiment  
In vitro experiments were conducted to evaluate the photoresponsivity of vertically stacked nanowires. The purpose was to observe cellular responses to higher stimulation currents as the density of vertically stacked SiNWs increases under constant light intensity. The study focused on identifying differences rather than performing a quantitative analysis. Figure 14a depicts the experimental configuration. The setup included a multilayer SiNW photodetector, a stimulator IC, an FPGA test board, and a halogen lamp. The stimulator circuit, integrated with either a 1 \(\times\) 1 or 1 \(\times\) 5 array SiNW photodetector, was tested  
Figure 12: Fabricated results of nanostructured 3D MEAs. (**a**) In vivo MEA on a polyimide layer. (**b**) In vitro MEA on a PCB with 61 microelectrodes, including 1 reference, 58 recording, and 2 stimulation electrodes. (**c**) SEM images showing nanostructured Pt-black electroplated onto 50 \(\upmu\)m diameter Au microelectrode. repeatedly across a range of light intensities increasing from 4 lux to 310 lux. The capacitor charging time was 0.5 ms, and the load resistor value was set to 4 k\(\Omega\). For the in vitro experiments, degenerated retinal tissue was extracted from the eyes of rd1 mice. It was then cut into 5 mm \(\times\) 5 mm retinal patches for use in the experiment. From the retinal patch, three cells showing the strongest and most consistent responses to electrical stimulation were selected for data analysis. As shown in Figure 14b, this retinal patch was mounted on Pt-black electrodes and used for the experiment. Surrounding the retinal patch, a well-structured cavity was formed, which was subsequently filled with  
Figure 14: (**a**) An experimental setting for the in vitro experiment. Fabricated SiNWs are illuminated by the intensity-adjustable halogen lamp. (**b**) Arrangement of fabricated MEA and retinal tissue using a multi-channel probing system. Figure 13: (**a**) Experimental setup for measuring the interface impedance and cyclic voltammetry using a three-element circuit model. (**b**) Electrical characteristics of a Pt-black MEA with a diameter of 50 \(\mu\)m, average impedance measurement results from five distinct electrodes. (**c**) Cyclic voltammetry. cerebrospinal fluid (CSF). This setup enabled the retinal cell to maintain its functionality over an extended period. The stimulator IC injected a stimulation current into the RGCs every 1 s for 1 min at fixed light intensities, and the experiment was repeated with the 1 \(\times\) 1 and 1 \(\times\) 5 array SiNW photodetectors. The SALPA algorithm was used to reduce the overall noise level of recorded responses, suppressing various types of background noise that might interfere with spike detection. After noise reduction, the first spike was detected using the method reported in a previous study [34]. Figure 15a shows the neural response signals when the stimulation signal was applied. The effectiveness of the stimulation was assessed by observing significant changes in the number of spikes following the application of the stimulation signal. Figure 15b illustrates the post-stimulus time histogram (PSTH) of the neural signals recorded from each electrode that exhibited a strong response to stimulation in the in vitro experiment. The PSTH demonstrates that most neural spikes were evoked by current stimulation resulting from an increase in light intensity in the same SiNW photodetector, indicating high sensitivity to low light levels. Furthermore, at the same light intensity, the number of neural spikes increased with the SiNW density, revealing enhanced light sensitivity due to the higher density of SiNWs. The number of spikes after stimulation was subtracted from the number of spikes before stimulation. The average number of neural spikes per stimulation was then calculated by dividing this difference by the total number of stimulations.