Photoreceptors made from titanium dioxide nanowires coated with gold nanoparticles could restore vision in blind mice. The devices, which respond to green, blue and near-ultraviolet light when interfaced with the retina of the animals, might help in the development of improved ocular prosthetics that do not require external power sources.
Diseases like retinitis pigmentosa or age-related macular degeneration can irreversibly damage retinal photoreceptors. This leads to vision impairment and eventually blindness, even though the retinal neurons involved in signal processing and the optic nerve remain functional. There is currently no medical treatment for these diseases but researchers have been working on replacing these lost photoreceptors with artificial ones, such as those made from photodiode arrays, for example.
While showing promise, these devices are far from being optimized, with the most important problem being that they require an external power supply and microelectronic processing units. Wiring such hardware into the eye is not only very challenging technically, it is also, needless to say extremely traumatic for a patient.
1D semiconductor nanowire arrays
A team of researchers led by Gengfeng Zheng and Jiayi Zhang of Fudan University in Shanghai, China, have been studying 1D semiconductor nanowire arrays made from titanium dioxide (TiO2) and gold (Au) as possible alternative implants here.
In contrast to previous photoresponsive structures, the 1D nanowire arrays are extremely well aligned in one direction, which means that they are structurally very similar to biological photoreceptors. They can thus very efficiently absorb light and separate charges (electrons and holes) to produce a photocurrent (in the same way as photoconversion devices such as solar cells and photodetectors), so eliminating the need for trans-ocular cables or power supplies. The photocurrent they produce can then be passed to neighbouring retinal neurons to stimulate them so that they fire a signal to the brain.
Restoring the visual response
Zheng and Zhang made their oriented Au-TiO2 arrays by growing them on fluorine-doped tin oxide (FTO) or flexible polymer substrates using a hydrothermal technique. They then decorated the surface of the arrays with gold nanoparticles, which allow the arrays to efficiently photoconvert light in the visible range (as measured by UV-visible absorption spectroscopy). This is because the particles amplify the light electrical field and inject “hot electrons” generated by surface plasmons (collective excitations of conduction electrons at the surface of the gold) into the TiO2 conduction band. These hot electrons then recombine with holes to produce a photocurrent.
In their experiments, the researchers interfaced the nanowire arrays with degenerated retinas in the right eyes of blind mice. The left eyes served as a control.
They found that the arrays are able to restore electrical signalling in the mice retinal ganglion cells when the rodents are exposed to green, blue and near UV light. “More excitingly, we saw that sub-retinally implanted nanowire arrays evoke activity in the primary visual cortex in vivo as well as improved pupil dilation in response to light in the animals. Light-sensitivity, and thus visual function, is restored in about 4–8 weeks in the implanted eyes compared to controls,” says Zhang.
New treatment options for people at risk
Zheng and Zhang believe that their new study could open new treatment options for people at risk of long-term visual degeneration. “Our work could help in the development of a new generation of optoelectronic tool kits for sub-retinal prosthetic devices in human patients one day,” they tell nanotechweb.org.
The researchers, reporting their work in Nature Communications doi:10.1038/s41467-018-03212-0, say that they are now busy improving the nanowire arrays’ sensitivity and their response to the colour red. “We will also be performing more experiments that measure the visual acuity in mice with degenerated retinas,” adds Zheng