## 4 Electroceticals--Intelligent Environments
### Artificial Retina
Reprinted with permission from [PERSON] et al., Proc. R. Soc. B (2011) 278, 1489–1497. Copyright 2011 The Royal Society. First, a human trial, a novel suprachoroidal retinal prosthesis, was conducted by the team of Bionic Vision Australia (Figure 10) [117]. During the experimental study, the device revealed a key advantage, which is reducing the risk of retinal damage. The benefit attributable to the suprachoroidal site is that the electrodes do not contact the neural retina, which is clearly shown in Figure 10a. In addition, the surgical technique does not require integrating the device within the vitreous cavity, negating the need for a corneal incision, intraocular lens extraction, vitrectomy, and retinal surgery [117]. This implant does not disturb natural vision because it does not interact with the eye optic neurons. Unlike epiretinal devices, which block natural vision, developed implantable device output instigated vision could coexist with a natural vision. The materials used in this device are platinum, titanium, and titanium-copper-nickel-alloy (TiCuNi) (hermetic encapsulation material for the first developed pacemakers) and biocompatible silicon-elastomer. The device consists of a medical-grade silicon substrate, over which the platinum electrode array of 19 mm \(\times\) 8 mm and two large electrodes were developed. The fabricated intraocular electrode array is illustrated in Figure 10b. In this study, a percutaneous connector was used as a direct electrical connection, allowing flexible neurostimulation and electrode monitoring without the need for any implanted devices [117]. Though the number of developments with different electrode configurations revealing reliable retinal prostheses, more recently, the advanced materials-inspired electrode systems are attracting for implantable devices, neural interfaces, and other biomedical fields due to their enhanced electrochemical performance, electrical activity, and biocompatibility [118]. For example, a hydrogel-electrode system was developed on the platinum substrate with the modified poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS) via electrochemical gelation process, employing copper as a sacrificial layer [118]. The morphology helps the hydrogel-based electrode to achieve a significantly low impedance down to 10.09 \(\Omega\) cm\({}^{2}\) at 1 kHz. This conductive polymer hydrogel electrode material provides an extensive large surface area with good adhesion. Such material presents a great potential for preparing flexible, functional electrodes imploying promising applications in neural implants [118]. Combining photo-capacitor-inspired energy storage devices with hydrogel-based electrodes would open up novel solutions to restore vision with high resolution while maintaining biocompatibility in patients [115; 116]. Furthermore, integrating a photovoltaic array with hydrogel electrode-inspired stimulating sources or micro photodiode array adopted hydrogel electrode systems would be useful for efficient retinal prosthesis. How  
Figure 10: **(a)** The concept of intraocular electrode array-based implantable device, **(b)** Fabricated intraocular electrode array. Reprinted with permission from [PERSON] et al., PLoS ONE 9(12), e115239 (2014). Copyright 2014 PLoS ONE. ever, developed devices with clinical authentications must be implemented in real-time applications as implantable devices. Although the above introduced retinal prosthesis systems and sub-systems, such as the bionic eye, retinal implants, and their electrode systems, etc., are authenticating with clinical experiment results, they do not restore vision/sight to the extent [119]. The limitation is largely due to the current implants that have a limited number of electrodes in single or double digits. Millions of electrodes would be needed to provide high-resolution vision equivalent to that of natural vision with the help of electronic retinal prosthesis systems and subsystems. This might be a complex challenge now.