Artificial retinas – restoring sight to the blind

In 1929 German neurologist Otfrid Foerster discovered that the electrical stimulation of visual pathways can be perceived as light or phosphenes. The field has come a long way since then, with retinal prosthesis now available, which stimulates the retinal layer of the eye to restore sight to those who are visually impaired. Such devices include Argus II, which received Conformité Européenne (CE) marking in 2011 and Food and Drug Administration approval in 2013, and Alpha-IMS, a photovoltaic-based retinal prosthesis which obtained its CE marking in July 2013.

Artificial retinas can be used to provide vision to those suffering from retinal diseases, including age-related macular degeneration (AMD) and retinitis pigmentosa (RP), which causes blindness by the degeneration of photoreceptor cells in the outer layer of the retina. Artificial retinas are based on the principle that electrical stimulation of the retinal layer can create vision. Electrical stimulation is achieved by either an array of micro-electrodes or micro-photodiodes. Retinal implants are categorised into three categories, based on which region of the eye they are placed:  

• epi-retinal (on the retina);
• sub-retinal (behind the retina); and
• suprachoriodal (between the sclera and the choroid).

The artificial retina market is expected to grow to $102.9 million by 2023 at a compound annual growth rate of 2.1%. Key factors driving the growth include increasing cases of visual impairments, growing health expenditures and, in part, the success of cochlear implants, which have proven that humans can regain significant sensory functions with limited input. According to the World Health Organisation, AMD is the third biggest cause of visual impairment and is responsible for 5% of blindness. It is estimated that 196 million people globally will have AMD in 2020, increasing to 288 million in 2040. There were 1.5 million cases of RP worldwide in 2014.   

Patent landscape
Patent filing in this domain is increasing at an average growth rate of 14.1% per year, with more than one-third of all patents having been filed after 2010. The United States is leading the research in this domain, followed by China, Japan and Germany. China is a new entrant in the field, having started filing patents only in the last decade. Key players include:

• Second Sight;
• Nidek;
• Nano Retina;
• Retina Implant;
• Pixium Vision;
• Bionics Institute Australia; and
• Newsouth Innovations.

Recent trends
Initial patent filings focused mainly on creating various configurations of retinal prosthesis and the correct regions of the eye in which to place the prosthesis for the proper stimulation of neural nerves. Before 2000, patents were filed for a retinal prosthesis which allows nutrients to flow through it.

After the structure of the prosthesis was developed, researchers’ focus shifted towards improving the arrangement of electrodes to reduce or prevent damage to the retinal layer. Decreasing power consumption and heat generation were also focused on, as improvements in microelectronics increased the chances of retina damage. Further, researchers concentrated on video conversion systems to properly utilise the limited number of electrodes in the prosthetics.

Major technical challenges faced between 2001 and 2010 included:

• improving electrode structure for successful implantation in the retina;
• using neural growth on electrodes for improved functionality;
• reducing damage to the retina when implanting the device;
• eliminating external systems and creating a self-sufficient prosthesis;
• improving the manufacturing method for flexible prosthesis;
• reducing power consumption and heat generation;
• converting high-resolution images to low-resolution images as per the electrode array;
• improving configuration mounting;
• improving the artificial retina’s power consumption;
• efficient multiplexing of plurality of electrodes in a nerve stimulator;
• reducing the size of electrodes; and
• improving the packaging of retinal prosthesis.

After 2010, researchers focused on increasing the number of electrodes while minimising the size of the implant (electrode density), as well as using different kinds of material for improving the biocompatibility and flexibility of the implant. Devices used to clamp the implant to the retina layer have also been improved.

Challenges
The main obstacles facing this technology are:

• low visual resolution – simulation results indicate that 600 to 1,000 electrodes are needed to enable various tasks, including facial recognition, whereas Argus II currently uses only 60 electrodes;
• biocompatibility of the device – this must be improved to ensure longevity; and
• image processing – there must be no delay in interpreting an object in view.

Future prospects
The artificial retina field has come a long way in the past two decades, demonstrating its capabilities; with continual advancements in biotechnology, micro-electronics and biocompatible materials, this technology is sure to grow. In light of its success, the technology may be applied in various other fields, including:

• smart biodetection systems – Lawrence Livermore National Laboratory;
• electronic tissue interface devices – Lawrence Livermore National Laboratory;
• metabolic prosthesis for diabetics – Oak Ridge National Laboratory and the University of Southern California’s Doheny Eye Institute;
• microscale enablers – Sandia National Laboratories;
• vision simulator image processing software systems – the California Institute of Technology;
• biocompatible microelectronics – Lawrence Livermore National Laboratory; and
• microchip development – the University of California at Santa Cruz.

It will be worth watching the advancements in the field of artificial retinas and related technologies.