Full Artificial Vision for Humans Takes a Giant Leap Forward

Artificial eyes that offer two-dimensional vision capabilities are certainly remarkable but are still a poor substitute for the human eye despite many years of scientific developmental work.

However, a technological breakthrough involving the application of nanowire sensors is changing all that. These sensors have the capability to do the work normally done by the photoreceptors found in the retina of normal eyes. 

Photoreceptor cells are a form of neuroepithelial cell found in the retina capable of visual phototransduction, which is the process by which light entering the eye is changed into electrical signals that the brain can interpret transform into vision. 

There are three known types of photoreceptor cells, rods, cones and photosensitive retinal ganglion cells. The rods contribute to night vision and cones handle daylight vision.

In what is said to be the first three-dimensional artificial eye to be developed by a team of scientists at Hong Kong University of Science and Technology (HKUST), the electro-mechanical eye (ECE) as it is named, may offer better vision than a human eye with additional capabilities like the detection of infrared light during darkness. 

But what about the nanowire sensors that is making such a vision breakthrough possible? Firstly, it's worth remembering that dimensionally nanowires are approximately 10,000 times thinner than a human hair. Secondly, the nanowire sensors' key operational function is handled by that humble electronics workhorse a field effect transistor (FET).

Integrated into a nanowire sensor is a field effect that is transduced using FETs. In doing this, the FETs employ a semiconductor such as p-type silicon connected to the metal source and drain electrodes via which a current is injected and collected. 

The semiconductor's conductance between source and drain is switched on and off by a third gate electrode capacitively coupled through a thin dielectric layer38. In the case of p-Si or another p-type semiconductor, applying a positive gate voltage depletes carriers. It reduces the conductance while applying a negative gate voltage leads to an accumulation of carriers and an increase in conductance. 

The dependence of the conductance on gate voltage makes FETs natural candidates for electrically based sensing primarily because the electric field created from the binding of a charged species to the gate dielectric is analogous to applying a voltage using a gate electrode.

In an amazing piece of work the team at HKUST managed to succeed in connecting the nanowire light sensors to liquid-metal wires that served as nerves behind the artificial retina and, in addition, successfully replicated the visual signal transmission to reflect what the eye sees onto the computer screen. 

This is incredibly important because the thing that has prevented eye transplantation for decades has not been the relatively simple procedure of replacing an eye it has been the impossibility of connecting the new eye to the optical nerve through which all the electrical signals created by the phototransduction work of the retina are transmitted to the brain for conversion into images. 

In the future, it is a possibility that nanowire light sensors could be directly connected to the nerves of a visually impaired person. Unlike in a human eye where bundles of optic nerve fibres have to travel through the retina before reaching the brain, light sensors that are positioned across the whole of the artificial retina could each feed signals through its own liquid-metal wire at the back, thereby eliminating the blind spot that is a feature of natural eyes because they do not have to route through a single spot.  

Apart from that, as nanowires have an even higher density than photoreceptors in human retina, the artificial retina could receive more light signals and potentially attain a higher image resolution than human retina – providing the back contacts to individual nanowires are created in the future. With different materials used to boost the sensors’ sensitivity and spectral range, the artificial eye may also achieve other night vision functions.