By Stephen Beech
New infrared contact lenses allow people to see in the dark, even with their eyes closed.
They enable "super-vision" by converting infrared light into visible light, say Chinese scientists.
But, unlike infrared night vision goggles, the state-of-the-art contact lenses don't require a power source, and they even allow the wearer to perceive multiple infrared wavelengths.
As they're transparent, users can see both infrared and visible light simultaneously, though infrared vision was enhanced when participants had their eyes closed, according to a study published in the journal Cell.
Senior author Professor Tian Xue, a neuroscientist at the University of Science and Technology of China (USTC), said: "Our research opens up the potential for non-invasive wearable devices to give people super-vision.
"There are many potential applications right away for this material. For example, flickering infrared light could be used to transmit information in security, rescue, encryption or anti-counterfeiting settings."
He explained that the lenses use nanoparticles that absorb infrared light and convert it into wavelengths that are visible to mammalian eyes.
The nanoparticles specifically enable detection of "near-infrared light" - infrared light in the 800 to 1600nm range, just beyond what humans can already see.
The Chinese team previously showed that those nanoparticles enable infrared vision in mice when injected into the retina, but they wanted to design a less invasive option.
To create the contact lenses, the researchers combined the nanoparticles with flexible, non-toxic polymers that are used in standard soft contact lenses.
After showing that the contact lenses were non-toxic, they tested their function in both humans and mice.
The team found that contact lens-wearing mice displayed behaviours suggesting that they could see infrared wavelengths. For example, when the mice were given the choice of a dark box and an infrared-illuminated box, contact-wearing mice chose the dark box, whereas contact-less mice showed no preference.
The mice also showed physiological signals of infrared vision, with the pupils of contact-wearing mice constricting in the presence of infrared light, and brain imaging revealed that infrared light caused their visual processing centres to light up.
In humans, the infrared contact lenses enabled the study participants to accurately detect flashing Morse code-like signals and to perceive the direction of incoming infrared light.
Xue said: "It's totally clear cut: without the contact lenses, the subject cannot see anything, but when they put them on, they can clearly see the flickering of the infrared light.
"We also found that when the subject closes their eyes, they're even better able to receive this flickering information, because near-infrared light penetrates the eyelid more effectively than visible light, so there is less interference from visible light."
He said that an extra "tweak" to the contact lenses allows users to differentiate between different spectra of infrared light by engineering the nanoparticles to color-code different infrared wavelengths.
For example, infrared wavelengths of 980nm were converted to blue light, wavelengths of 808nm were converted to green light, and wavelengths of 1,532nm were converted to red light.
As well as enabling the wearers to perceive more detail within the infrared spectrum, the color-coding nanoparticles could be modified to help color blind people see wavelengths that they would otherwise be unable to detect.
Xue said, "By converting red visible light into something like green visible light, this technology could make the invisible visible for color blind people."
Because the contact lenses have limited ability to capture fine details, due to their close proximity to the retina, which causes the converted light particles to scatter, the team also developed a wearable glass system using the same nanoparticle technology, which enabled participants to perceive higher-resolution infrared information.
The contact lenses are currently only able to detect infrared radiation projected from an LED light source.
But the research team is working to increase the nanoparticles' sensitivity so that they can detect lower levels of infrared light.
Xue added: "In the future, by working together with materials scientists and optical experts, we hope to make a contact lens with more precise spatial resolution and higher sensitivity."