In a unique experiment, mice received infrared vision

Special nanoparticles (shown in white) cling to rods (left) and cones (right) in mouse photoreceptors.

By injecting nanoparticles into the eyes of mice, scientists allowed them to see near infrared light - electromagnetic radiation, usually not visible to rodents (or humans). A unique breakthrough, which is even more unusual when understood, is that such a technique can be used on humans.

A science team led by Tian Xue of the University of Science and Technology of China and Gang Khan of the University of Massachusetts Medical School changed the mice 's vision so that they can see near infrared light(NIR), retaining its natural ability to see normal light. This was accomplished by injecting special nanoparticles into their eyes. The effect lasted about 10 weeks and without any serious side effects.

A series of tests showed that the mice actually saw infrared light, and not some other things. Scientists say that the human eye is not too different from the eyes of mice, which leads to a fantastic perspective on the application of this technique to humans.

Humans and mice can only see a narrow section of the electromagnetic spectrum, indicated by the rainbow band. Other animals, such as birds or bees, are able to see ultraviolet, and snakes have infrared radiation.

Humans, like mice, can see only a narrow section of the electromagnetic spectrum . The spectrum of wavelengths invisible to humans is huge, we do not see anything beyond the borders of the so-called visible spectrum (wavelengths of 380 - 740 nanometers). Infrared radiation exists in the form of longer waves, from 800 nm to a millimeter.

Objects in the world, whether people or a hot plate of soup or something cold like an ice cube, emit infrared radiation. Mammals, such as humans and mice, cannot see NIR, but we have technologies, namely night or thermal vision glasses, that can convert this invisible spectrum into light that we can see. The new technique used on mice does something similar, but instead of relying on wearable technology, scientists used a biological solution.

To allow mice to see beyond the normal visible spectrum, Tian and Ganges have developed special nanoparticles that increase the frequency of radiation and are able to function in existing rodent eye structures. Drops of liquid containing tiny particles were injected directly into their eyes, in which, using special anchors, they fit snugly against the photoreceptor cells. Photoreceptor cells - rods and cones - typically absorb the wavelengths of visible light, which the brain interprets as vision. In the experiment, the injected nanoparticles converted NIR into a visible wave, which the mouse brain could perceive as visual information (in this example, they considered NIR to be green light). Nanoparticles were in the eyes for two months, allowing mice to see both NIR and visible light with minimal side effects.



Graphic image of the process of vision. When infrared light (red) enters the photoreceptor cell (light green circle), nanoparticles (pink circles) convert NIR into visible green light.

Nanoparticles on photoreceptor cells served as a transducer of infrared light. Infrared waves were captured in the retina by nanoparticles, which then emitted them as shorter waves of visible light. Thus, rods and cones absorbing shorter waves were able to receive this signal and then send the converted information to the visual zone of the cerebral cortex. In particular, the injected particles absorbed a NIR of about 980 nm in length and converted it to 535 nm of light. The mice perceived infrared light as green. The result was similar to observing NIR in night vision goggles, except that the mice could also maintain their normal perception of visible light. As already indicated, the effect was temporary, for about several weeks, in some mice the cornea became cloudy, which quickly cleared up.

To prove that the method really works, Tian and Gang conducted a series of tests and experiments.

For example, the pupils of mice decreased when exposed to NIR, while the pupils of mice without injection did not. And when exposed exclusively to NIR, measurements of the electrical activity of the brain in mice injected with nanoparticles showed that the eyes and visual cortex function as in the presence of visible light.

Behavioral tests also showed that the technique works. Mice placed in the Y-shaped labyrinth were taught to recognize the location of the hidden shelter platform that NIR points to. During the tests, the injected mice constantly found a platform, and the mice without injections swam around the maze. Another test included a box with two compartments: one completely without light, and the other illuminated by NIR. Mice, like nocturnal creatures, gravitate toward darkness. In the tests, the mice injected with the nanoparticles spent more time in the compartment without light, and the mice without injections showed no preference.

“These extensive experiments leave no doubt that mice injected with infrared sensitive nanoparticles acquire the ability to see infrared light and receive visual information,” said Vladimir Kefalov, professor of ophthalmology and visual sciences at the University of Washington in St. Louis.

In a press release, Tian indicated that the nanoparticles clung to sticks and cones, and were activated by dipped infrared light, so “we believe that this technology will work in human eyes, not only as super-vision, but also for therapeutic purposes.” In an interview with Cell, he clarified by saying:

Unlike mice, humans and other primates have a retinal structure called fovea, which provides high-definition central vision. In human fovea, the density of cones is much greater than that of rods; while in the mouse retina the number of sticks is greater. Since cones have different spectral and intensity sensitivities compared to rods, we may need to fine-tune the UCNP emission spectrum to more effectively activate cones of the desired type in humans.

As Tian said, for this technology to work for a person, it needs to be changed, but new experiments show that its change is possible. Kefalov said that the potential for using such a concept in humans is real and exciting, but he warned that we still have a long way to go.

“The authors showed that a single injection of nanoparticles does not adversely affect the mouse retina,” Kefalov said. “However, it is still unclear whether practical infrared vision will require repeated injections and, if so, whether chronic infrared vision will affect the structure and function of our eyes.”

The ability to see infrared light seems fantastic, but that would certainly be a useful sign. We could see many things beyond the boundaries of our usual visual spectrum - and we would have a built-in night vision system. As Tian explained to Cell:

Scientists are trying to develop a new technology that allows the use of abilities beyond our natural capabilities. Visible light, which can be perceived by the person’s natural vision, occupies a very small part of the electromagnetic spectrum. Electromagnetic waves longer or shorter than visible light carry much more information. Depending on the material, the object may also have different absorption and reflection in the near infrared. We cannot detect this information with the naked eye.

Another interesting feature of this potential improvement is that a person does not need to wear bulky and energy-intensive equipment, such as night vision goggles. And technology does not require any genetic manipulation. Most likely, the military will be interested in this work.

Dayong Jin of the School of Mathematical and Physical Sciences at the University of Technology Sydney called the new work “very innovative and inspiring.” Dayong said that, to his knowledge, "this work is the first example of implantable and" wearable "optical nanodevices." He said that it is important that the mice do not have inflammation or cell death, but it is possible that some cells absorbed nanoparticles, a prospect “worthy of more scrutiny.”

Likewise, Kefalov was impressed by the study, stating that “the authors did an amazingly good job by characterizing the effect of injecting infrared-sensitive nanoparticles on the visual function of mice,” adding that “this innovative work demonstrates an original and powerful method of enhancing the ability of the visual system to detect light in the boundaries of the natural visible spectrum. " He considers it “striking” that nanoparticles most likely do not interfere with the normal function of photoreceptors in visible light.

Regarding whether this technique can be used to correct visual impairment such as color blindness, this is less clear, he said.

“Since reception is based on the ability of photoreceptors to detect and amplify light signals, using it to treat photoreceptor dysfunctions will require the development of new stages, in addition to converting light beyond the boundaries of the visible spectrum,” Kefalov said.

Looking to the future, Tian and Gang would like to improve the technique using organic-based nanoparticles consisting of substances approved by the FDA, which can lead to even brighter infrared vision. They would also like to customize the technique so that it is closer to human biology. Optimistic about the direction of technology, Tian and Ganges have already filed a patent application regarding their work.

I already present television commercials: "Ask your doctor if near-infrared vision is right for you."

www.cell.com/cell/fulltext/S0092-8674(19)30101-1