Improvements in visual sensors for robotics and visual prosthetics for the blind or vision-impaired may benefit from the latest developments in nanotechnology using perovskite minerals. Zhiyong Fan, The Hong Kong University of Science and Technology, Hong Kong SAR, China, and colleagues have developed a hemisphere-shaped array of perovskite nanowires, which could be used to make a biomimetic artificial retina with a wide field of “vision” and a low level of aberration.

Perovskite Structures for Optical Applications

Silicon, with its great prowess in shuttling electrons and processing photons, has been the mainstay of electronics and photovoltaic applications for decades. However, for high-performance, high-resolution applications it needs to be highly crystalline and highly pure, which is expensive and time-consuming to achieve. Moreover, it has the rather obvious drawback of being an entirely inflexible material and so cannot realistically be fabricated into curved biomimetic structures that might emulate, for example, the retina of the eye.

Perovskites are minerals that have many of the benefits of silicon in opto-electronic applications, as well as the possibility of being fabricated into structures other than flat chips. This group of minerals is defined by the archetypal member of the class, calcium titanium oxide (calcium titanate), which was discovered in the Ural Mountains of Russia by Gustav Rose in 1839. The mineral was named after the mineralogist Lev Perovski. Today, we use the term perovskite more generically for a whole range of related materials with the same cubic crystal structure as the archetype.

Perovskites have the general formula ABX₃, where A and B represent cations that are very different in size, and X is an anion. It is possible to make perovskites almost to order in the laboratory at low cost. They can be formed into thin films and nanoscopic structures.

High-Resolution Imaging

The researchers have used a vapor-phase approach to grow their hemispherical high-density perovskite nanowire array. A room-temperature ionic-liquid electrolyte acts as the front-side common contact to the nanowires. Liquid-metal wires were used as rear contacts to the nanowire photosensors. This, the team states, mimics human nerve fibers behind the retina. The resulting device has high responsivity, a reasonable response speed, a low detection limit, and a wide field of vision.

In tests with a lens to focus an image on to the artificial retina, the researchers could show that the device can acquire an image pattern, which can then be read by a computer—analogous to the brain’s visual cortex acquiring and interpreting visual stimuli to the eye. The density of “receptors” in the team’s artificial retina is far higher than that of the photoreceptors in the human retina, and thus, the retina mimic has a higher resolution than the human eye. However, the image resolution is limited by the liquid-metal wire density in the current work. The team has developed a microneedle approach to further improve the resolution.

This work may lead to biomimetic photosensing devices that could be useful in a wide spectrum of technological applications. The resulting eye-like devices, the team muses, could lead to humanoid robots that can make better “eye contact” with a person interacting with the robot than with one that uses conventional light-sensing devices.

“In the next step, we plan to explore the application of our device for visual prosthetics,” Fan told ChemistryViews. “We need to improve the biocompatibility of our device to make sure it is safe when used in a human subject. Furthermore, we will establish collaborations with clinical researchers and identify the protocol for future clinical tests.”

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• A biomimetic eye with a hemispherical perovskite nanowire array retina,
  Leilei Gu, Swapnadeep Poddar, Yuanjing Lin, Zhenghao Long, Daquan Zhang, Qianpeng Zhang, Lei Shu, Xiao Qiu, Matthew Kam, Ali Javey, Zhiyong Fan,
  Nature 2020.
  https://doi.org/10.1038/s41586-020-2285-x