There’s a whole world out there beyond the limits of human sight. Like most creatures, we have eyes are attuned to only a narrow portion of the electromagnetic spectrum—which becomes a serious handicap in dimly lit conditions.
But someday, that might no longer be the case. In a study published today in the journal Cell, a team of researchers has expanded the visible range of mice, enabling them to see infrared light with enough precision to distinguish between different shapes. The catch? The treatment requires injecting metal nanoparticles into the back of the eye.
The procedure would need to be proven safe and effective in other species before being considered for use in humans. But if that happens, nanoparticles like these could someday be used to delivery of drugs to hard-to-reach parts of the eye—or even pave the road for new techniques in vision enhancement.
“This is extremely interesting work,” says Kameran Lashkari, an ophthalmology researcher at Harvard Medical School and the Schepens Eye Research Institute of Massachusetts Eye and Ear who was not involved in the study. “There’s always been discussion about whether this type of tech would ever be possible… and there could be big applications.”
Though invisible to mammalian eyes, infrared energy is constantly being emitted by living creatures and inanimate objects alike, even in the absence of visible light. Scoping out the world in infrared could be a powerful asset for countless pursuits, including those of soldiers or police officers working a night shift. But humans typically can’t accomplish this feat without the help of external devices.
Part of the problem is, mammals are limited by the light-sensing cells, or photoreceptors, in our eyes. Human and mouse photoreceptors absorb light with wavelengths between 400 to 700 nanometers, and convert this energy into electrical signals that are then transmitted to the brain. The realm of infrared, however, covers wavelengths beyond the upper limit of this range.
To bypass this roadblock, a group of researchers led by vision biologist Tian Xue of the University of Science and Technology of China and nanomaterials scientist Gang Han at the University of Massachusetts Medical School designed nanoparticles that could latch on to photoreceptors and translate infrared wavelengths down to a more manageable size. Once injected into the eye, the nanoparticles would capture infrared light and re-emit it at a shorter wavelength (in this case, 535 nanometers, which the brain interprets as green).
From the photoreceptors’ point of view, this artificial light source is interpreted similarly to visible wavelengths hitting the retina through more natural means—making the process akin to using an outlet adaptor to accommodate a plug from a foreign country.
To test the nanoparticles’ power, the researchers injected them into the eyes of mice and plopped the rodents into a series of lighted mazes. To Xue’s amazement, treated mice quickly navigated to “safe” zones illuminated only in infrared, while their normal-sighted peers fumbled in the dark. The same held true even when the mice had to distinguish between complex patterns and shapes to puzzle out their path. Additionally, the nanoparticles didn’t seem to interfere with the rodents’ ability to see natural visible light—it simply broadened the range of wavelengths to which they were privy.
“This was always something crazy, and we’d thought, ‘Let’s just try it,’” Xue says. “But the first time we saw that the injected animals could see in infrared, I was so excited.”
The researchers’ ultimate vision goes well beyond “supersighted” mice, though. Injectable infrared vision could play a role in military operations or law enforcement—or, Lashkari points out, even help astronauts performing delicate tasks in dimly lit outer space.
“This could be a remarkable advancement,” says Eve Higginbotham, an ophthalmologist at the Perelman School of Medicine at the University of Pennsylvania who was not involved in the study. However, she adds, caution should be exercised until the results are repeated by other groups. And there’s a long road ahead before this technology can be adapted for human use.
One of the biggest considerations going forward is safety. In their current iteration, the team’s nanoparticles contain rare earth metals, which could theoretically accumulate to toxic levels. It’s still not entirely clear what happened to the nanoparticles after they were injected, or how long they stuck around: Months after the procedure, mice still responded to infrared. Xue thinks the nanoparticles were probably being slowly swept away by the body’s natural cleanup crew. But their ultimate destinations could still be organs like the liver or spleen, where they might cause damage.
That said, the mice in this study suffered no lasting, noticeable side effects. But that’s no guarantee that humans will be similarly symptom-free.
Neena Haider, an ophthalmology researcher at Harvard Medical School and the Schepens Eye Research Institute of Massachusetts Eye and Ear who was not involved in the study, called the study “innovative,” praising the researchers for their creative thinking. However, Haider has reservations about its applicability in humans. “I’m not sure if I’d want to do this for a normal, healthy retina,” she says.
After all, every medical procedure comes with risks, says Ahmara Ross, an ophthalmologist at the Hospital of the University of Pennsylvania who was not involved in the study. “A subretinal injection is no cavalier thing,” she says.
Even if the procedure doesn’t pan out for vision enhancement, though, Ross and others think they might have promise as therapeutics. With a couple adjustments, for instance, the nanoparticles could be tailored to absorb and emit different wavelength combinations, which could help people with colorblindness. Alternatively, nanoparticles like these could act as a drug delivery system for various parts of the eye. Infrared light might even trigger the controlled release of medicine over long periods of time, reducing the need for visits to the doctor.
In cases like these, patients might be willing to incur a few risks, Ross says. People with normal vision, on the other hand, might be more loath to endure an invasive procedure.
But in the meantime, Xue says, it’s exciting to get a glimpse into what could be possible.
“When you combine the fields of material science and biology, you get a lot of things that sound like science fiction,” he says. “But we might be able to extend our visual spectrum. The limit is just our imagination.”