## 2 Materials and Methods
### Fabrication of Nanostructured Platinum-Black MEA  
In retinal stimulation, a crucial objective is to decrease interface impedance while maintaining a small base area. In this paper, electrodes plated with platinum black (Pt-black) are fabricated to achieve low contact impedance with the retinal cells and to allow high current injection [30]. The Pt-black layer features a wide surface area due to its nano-porous structure, which effectively lowers the interface impedance between the microelectrodes and the retinal cells and increases the charge injection capacity. The fabrication process is illustrated in Figure 6. In this paper, the glass MEAs with an electrode size of 50 \(\upmu\)m are connected to a printed circuit board (PCB) using a wire bonding process for subsequent in vitro experiments.  
The fabrication process of the implantable nanostructured MEA begins with a Pyrex 7740 borosilicate glass wafer, used as a substrate. The initial step involves sputtering 300 A of Ti for adhesion and 500 A of gold (Au) for the metal interconnection layer.  
Figure 5: Experimental setup for measuring the optical and electrical characteristics of the SiNW photodetector. (**a**) Overview of setup. SiNWs under illumination from a halogen lamp and a monochromator. (**b**) Monochromator setup for assessing the wavelength response. (**c**) Integration of the SiNW photodetector with the stimulator IC on an FPGA board.  
Figure 6: Fabrication process of nanostructured MEA using Pt-black: (**a**) Ti/Au deposition, (**b**) 1 st photolithography, (**c**) Ti/Au wet etching, (**d**) PR strip, (**e**) SiO\({}_{2}\) TEOS CVD, (**f**) 2 nd photolithography, (**g**) SiO\({}_{2}\) etch, (**h**) PR strip, (**i**) 3 rd photolithography, (**j**) Pt-black electroplating and PR strip, and (**k**) backside grinding. (**l**) The fabricated glass MEAs are attached to the printed circuit board (PCB) using a wire bonding process for in vitro experiments. (**m**) The diced glass MEA is wire-bonded to a polyimide layer for in vivo experiments.  
These layers are then patterned using photolithography and chemical etching processes. The isolation layer is created through sequential chemical vapor deposition (CVD) of tetraethylorthosilicate (TEOS), photolithography, oxide etching, and ashing. Subsequently, a second layer of Ti/Au is sputtered to serve as a seed layer for the electroplating of nanostructured Pt-black on the stimulation electrodes and interconnection pads. Oxygen plasma treatment is used to enhance the adhesion before the electroplating of a 3 \(\upmu\)m thick Au layer and the Pt-black layer. Afterward, the Au seed layer is removed, and the glass wafer is thinned to 100 \(\upmu\)m by backside grinding.  
The type of substrate used for mounting MEAs determines whether the setup is suitable for in vitro or in vivo experiments. In the fabrication process for in vitro experiments, as shown in Figure 6l, the glass MEAs are attached to a printed circuit board (PCB) through a wire bonding process. For in vivo applications, detailed in Figure 6m, the MEA body and cover layers are fabricated using polyimide, a polymer known for its flexibility and resistance to heat and chemicals. The polyimide serves as a robust body for the implantable electrodes. In this process, laser cutting technology is used to create the polyimide body and cover, after which the diced glass MEA is wire-bonded to the polyimide layer, preparing it for implantation.