Our eyes take in the dazzling greens of dense forests, blues of alpine lakes, and reds and purples of sunset. Yet there’s an entire world hidden from sight. Our eyes only perceive a narrow sliver of the spectrum—wavelengths between 400 and 700 nanometers make up what we call “visible” light. Infrared light, just beyond the 700-nanometer mark, is invisible to us.
Over half of the spectrum is mysterious to people, wrote Tian Xue and team at the University of Science and Technology in China. And while we can augment our vision in the dark with night-vision goggles, the equipment is bulky, relatively low resolution, and monochromatic—hardly a natural visual experience.
This week, Xue and colleagues introduced contact lenses that convert near-infrared light into visible light. They’re like normal contact lenses in size and shape and don’t require external power. Because near-infrared light can penetrate our eyelids, wearers were able to see letters in multiple previously imperceptible wavelengths of light—even with their eyes closed.
Mice and humans wearing the contacts could detect the presence and direction of flashing near-infrared light. Shown a series of letters that looked black and white in visible light, people wearing the lenses detected multiple colors associated with near-infrared wavelengths.
“Our research opens up the potential for non-invasive wearable devices to give people super-vision,” said Xue in a press release.
While super-vision is cool and all, the nanoparticles used to make the contacts have other potential uses. “Flickering infrared light could be used to transmit information in security, rescue, encryption, or anti-counterfeiting settings,” said Xue.
Seeing Is Believing
We rely on sight to make sense of the world, but humans only see a fraction of the spectrum. Shorter wavelengths are the ultraviolet light that our eyes, and sunglasses, filter out to protect against damage. Infrared wavelengths are just outside the opposite end of the visible spectrum and generally aren’t harmful. However, they’re too weak to activate the light-sensitive cells—or photoreceptors—in our eyes that translate light into electrical signals the brain comprehends.
Xue has long wanted to expand our eyes’ capabilities. In 2019, he learned about an unusual material during a chat with Gang Han, a biochemist at the University of Massachusetts Medical School. Tiny particles in the material converted low-energy infrared photons into higher-energy visible light.
Adding energy to a system is difficult. Xue looked to a group of elements called rare earth metals. You may have heard of them. They’re common in powerful magnets, laptops, smartphones, camera lenses, and batteries. These elements have a special quirk. Most elements absorb energy from photons, rapidly spit it out, and then settle back into their original energy state. Rare earth metals can absorb multiple low-energy photons, such as those in the infrared range, build up energy, and then release a high-energy photon of visible light.
The duo tapped two rare earth metals for their nanoparticles. One absorbs infrared light and transfers this energy to the other, which converts it into visible green light. They then added a “guide” protein to this nanoparticle, so it homed in on photoreceptors in the eyes. In theory, the system should convert infrared light to green colors that can be perceived by the brain.
It worked. Directly injecting the nanoparticles into the eyes of mice seemingly gave them super-vision. In multiple tests, they found a hidden platform in a swimming pool, guided only by near-infrared light signals. Those without the nanoparticles swam around aimlessly.
Super-vision sounds intriguing—the thought of a needle in the eye less so. The new study, published in Cell, made the technology wearable in the form of contact lenses—but with an upgrade.
Eyes Wide Shut
Embedding the nanoparticles into contact lenses could alter their light-sensing behavior. The team screened multiple materials already used in contact lenses and found a harmless, flexible “canvas” in which to embed the nanoparticles. Importantly, the lens material didn’t interfere with the nanoparticles’ light conversion.
The finished contact lenses were safe and comfortable. Mice wearing tiny versions for 14 days roamed around as usual and showed no signs of eye strain or damage. The lenses gave the mice super-vision. In one test, the team flashed near-infrared light in front of the animals. Electrical chatter in the visual cortex showed their brains were receiving the signals.
Most rodents prefer dark corners to brightly lit rooms. In one test, the team “lit up” a room using near-infrared light. Those with the contacts skittered toward the dark side. Those without didn’t seem to care. Incredibly, the experiment worked even when the mice’s eyes were shut.
This is likely because “near-infrared light [can] penetrate the eyelids easily,” wrote the team.
But mice can’t tell us about their experience. People can. Human participants who wore the contact lenses perceived near-infrared light, along with visible light, when staring at a constant or flickering LED light source. With their eyes shut, however, they were able to pick up near-infrared signals by almost four-fold.
Most people could see horizontal and vertical lines, shapes resembling the letters “S” and “O,” triangles, and squares, all illuminated in near-infrared light.
“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,” said Xue.
Night-vision goggles only perceive infrared signals in green. The team next tapped into their light-converting nanoparticles to capture multiple light wavelengths within the infrared zone. Like visible light, near-infrared light also has its own rainbow of colors. With a sprinkling of other elements to fine-tune their physical properties, the nanoparticles can specialize in specific wavelengths—all normally undetectable to the human eye. For example, they converted some infrared light wavelengths to blue, others to green, and yet more to red.
Adding these nanoparticles allowed the participants to see a range of near-infrared colors in bright visible colors when wearing the contact lenses.
The technology could also help colorblind people. The color-coding nanoparticles could transform visible light wavelengths that they can’t see into ones they can. “By converting red visible light into something like green visible light, this technology could make the invisible visible for colorblind people,” said Xue.
The technology isn’t super-vision on demand. It only works with infrared light projected from an LED source rather than ambient lighting. But the team plans to increase the system’s sensitivity—like building a better sensor in a camera to detect lower lighting.
“Although this work has its limitations, it opens the door to a richly informative, colorful NIR [near-infrared] world that can be directly perceived by humans,” wrote the team.