Artificial retinal implants have the potential to restore some vision to people stricken by degenerative eye diseases, but the technology faces a range of daunting challenges—not least the difficulty of replicating humans’ sensitive color vision.
Now researchers have created a small, flexible device that closely mimics the performance of human photoreceptors (Adv. Mater. 2019, DOI: 10.1002/adma.201900231). It uses a thin layer of a perovskite material to absorb light and generate an electrical signal, similar to the perovskite solar cells that are currently racing toward commercialization.
The device is a long way from becoming an artificial retina, and has not been interfaced with living tissue, but it offers a proof of principle that perovskites could help create better retinal implants, says Hao-Wu Lin at National Tsing Hua University, who led the work.
Retinal implants have had initial clinical successes, and can restore some vision in people. The Argus II, made by Second Sight of Sylmar, Ca., relies on a video camera mounted on a pair of glasses. The camera transmits visual signals to an implant at the back of the eye, which stimulates retina cells, enabling the wearer to see patterns of light and dark.
Some researchers hope to eliminate the bulky glasses and improve the quality of vision by installing light sensors directly into the eye. But these devices need to be flexible enough to fit the curved retina. Conventional full-color digital imaging devices are simply too rigid. “They can significantly deform the retina and damage optical nerves,” explains Nanshu Lu of the University of Texas at Austin, who works on artificial retinas. More pliable alternatives are being developed, but they are monochromatic and struggle to perform in low light.
In contrast, the human eye achieves sensitive full-color vision thanks to several types of photoreceptor cells. Three different classes of cone cells distinguish red, green and blue light, while rod cells are more sensitive to dim light and offer black-and-white vision.
To simulate the eye’s full-color sensitivity, Lin’s team created four different photodetectors from methylammonium lead iodide, a perovskite that absorbs light across the visible spectrum. Each photodetector carries a sandwich structure on its surface, containing an insulating layer a few tens of nanometers thick flanked by even thinner layers of silver, which acts as a color filter. As incoming light bounces between the metal layers, interference ensures that only a specific color band is transmitted to the perovskite beneath. The color can be fine-tuned by adjusting the separation of the metal layers, and the structure allows more light through than conventional color filters, Lin says. Mounted on a flexible backing, the four photodetectors formed a device a few millimeters wide.
The team used this device to scan a picture of a sunflower, one small section at a time, and reconstructed its electrical signals into a full-color image. Other tests confirmed that the device had a similar color response to human photoreceptors, worked well in both bright and shady conditions, and could be bent repeatedly without any loss in performance. “This all comes from the amazing properties of the perovskite,” Lin says. A layer of perovskite just 300 nm wide was efficient at converting light into electrical signals, and unlike conventional semiconductors, the material can be processed at low temperature, enabling the researchers to make the device on a flexible substrate, Lin says.
Dae-Hyeong Kim of Seoul National University, who works on flexible electronics, says that the work is a promising step. “Perovskite photodetectors have the potential to become very sensitive artificial photoreceptors,” Kim says. “A very thin layer is good enough to fabricate a photodetector with high photo-sensitivity.”
Lin’s team now plans to build an array of smaller artificial photoreceptors, but integrating such a device with a human eye is a much more distant goal. To transmit color images to the brain, an artificial retina would have to connect with individual color-specific nerve cells in the eye, Kim says, which is not yet possible.