**Conflict of interest**

*Prosthesis*

each ~70 μm<sup>2</sup>

or data [37, 89, 90].

generations of devices.

ment them in the form of biohybrid implants.

**5. Future directions**

following concerns about device longevity [86].

considerable improvements in the lifespan of this system.

There are several other groups across the world using a variety of techniques to develop retinal prosthetic systems. Currently the intelligent retinal implant system (IRIS) II (*Pixium Vision S.A. France*) is the only other *epiretinal* device that has received CE approval. The external components and power induction mechanism of this system are fairly similar to the Argus II system. However, unlike the Argus II system, the 150-microelectrode array receives stimulation commands directly from an infrared array, which is integrated into a glasses-mounted visual interface, thus facilitating high data transfer and device miniaturization [45]. While initial data from the clinical trial were promising for functional and safety outcomes with the IRIS II, the study was postponed

The aforementioned Alpha IMS and its successor, the Alpha AMS (*Retina Implant AG, Germany*), have both been approved for commercial use in Europe. Utilizing photovoltaic technology, the Alpha IMS has 1500 autonomous photodiode complexes, giving a higher theoretical resolution than the Argus II. To date, this system has yielded safety and functional results similar to the Argus II, albeit with inferior longevity, possibly due to the surgical approach, involving subretinal device placement and a tunnelled link to a postauricular coil for electromagnetic power induction [35, 36, 87]. Preliminary data from trials of the Alpha AMS report

Finally, the Photovoltaic Retinal Implant (PRIMA) bionic vision system (also *Pixium Vision S.A.*) is currently undergoing a safety and performance evaluation feasibility study [88]. This subretinal system comprises a modular array set-up with 1 mm-wide hexagonal chips, each containing 142 30 μm-thick pixel cells,

infrared light. Multiple photodiodes in series are stimulated, generating sufficient current to polarize the adjacent neuronal tissue with a local concentric return to limit signal diffusion. This system is unique in that it is scalable through insertion of additional chips and does not require any direct transscleral delivery of power

Several other groups are also investigating alternative methods of neural stimulation, including optic nerve, cortical and thalamic prostheses [91]. As increasing numbers of patient volunteers undergo implantation with these systems, more data will become available, thus guiding the optimal design characteristics for future

Advancements in visually restorative medicine are not limited to prosthetics, with other regenerative technologies, including stem cells, gene therapy and optogenetics, demonstrating exciting developments in the endeavor to treat blindness [92–97]. In particular, optogenetics, an approach that is focused on targeting microbial opsin (light-dependent ion channels) to surviving retinal neurons to rescue or restore visual function, has shown exciting results in *in vivo* animal and *in vitro* human studies [98–100]. In addition to the aforementioned organic approaches, these techniques are attractive insofar as they offer a biocompatible, autonomous and scalable alternative to inorganic systems, which could be administered earlier in the disease course as a rescue therapy. It is widely anticipated that these strategies will soon build on the initial success of synthetic prosthetics, or potentially comple-

The success of the Argus II is representative of the enormous progress that has been made in the field of retinal prosthetics over the past 3 decades. However, there remain significant challenges to be overcome before the concept of 'bionic vision' is fully realized. Despite this, retinal prostheses have delivered the most compelling

, which receive visual data from a visual interface as pulsed near

**102**

The authors declare no potential conflicts of interest or financial support with respect to the authorship and/or publication of this chapter.
