A new contact lens enables humans to see near-infrared light without night vision goggles or other bulky equipment. The lens, which incorporates metallic nanoparticles that “upconvert” normally-invisible wavelengths into visible ones, could have applications for rescue workers and others who would benefit from enhanced vision in conditions with poor visibility.
The infrared (IR) part of the electromagnetic spectrum encompasses light with wavelengths between 700 nm and 1 mm. Human eyes cannot normally detect these wavelengths because opsins, the light-sensitive protein molecules that allow us to see, do not have the required thermodynamic properties. This means we see only a small fraction of the electromagnetic spectrum, typically between 400‒700 nm.
While devices such as night vision goggles and infrared-visible converters can extend this range, they require external power sources. They also cannot distinguish between different wavelengths of IR light.
Photoreceptor-binding nanoparticles
In a previous work, researchers led by neuroscientist Tian Xue of the University of Science and Technology of China (USTC) injected photoreceptor-binding nanoparticles into the retinas of mice. While this technique was effective, it is too invasive and risky for human volunteers. In the new study, therefore, Xue and colleagues integrated the nanoparticles into biocompatible polymeric materials similar to those used in standard soft contact lenses.
The nanoparticles in the lenses are made from Au/NaGdF4: Yb3+, Er3+ and have a diameter of approximately 45 nm each. They work by capturing photons with lower energies (longer wavelengths) and re-emitting them as photons with higher energies (shorter wavelengths). This process is known as upconversion and the emitted light is said to be anti-Stokes shifted.
When the researchers tested the new upconverting contact lenses (UCLs) on mice, the rodents’ behaviour suggested they could sense IR wavelengths. For example, when given a choice between a dark box and an IR-illuminated one, the lens-wearing mice scurried into the dark box. In contrast, a control group of mice not wearing lenses showed no preference for one box over the other. The pupils of the lens-wearing mice also constricted when exposed to IR light, and brain imaging revealed that processing centres in their visual cortex were activated.
Flickering seen even with eyes closed
The team then moved on to human volunteers. “In humans, the near-infrared UCLs enabled participants to accurately detect flashing Morse code-like signals and perceive the incoming direction of near-infrared (NIR) light,” Xue says, referring to light at wavelengths between 800‒1600 nm. Counterintuitively, the flashing images appeared even clearer when the volunteers closed their eyes – probably because IR light is better than visible light at penetrating biological tissue such as eyelids. Importantly, Xue notes that wearing the lenses did not affect participants’ normal vision.
The team also developed a wearable system with built-in flat UCLs. This system allowed volunteers to distinguish between patterns such as horizontal and vertical lines; S and O shapes; and triangles and squares.
But Xue and colleagues did not stop there. By replacing the upconverting nanoparticles with trichromatic orthogonal ones, they succeeded in converting NIR light into three different spectral bands. For example, they converted infrared wavelengths of 808, 980 nm and 1532 nm into 540, 450, and 650 nm respectively – wavelengths that humans perceive as green, blue and red.
“As well as allowing wearers to garner more detail within the infrared spectrum, this technology could also help colour-blind individuals see wavelengths they would otherwise be unable to detect by appropriately adjusting the absorption spectrum,” Xue tells Physics World.
According to the USTC researchers, who report their work in Cell, the devices could have several other applications. Apart from providing humans with night vision and offering an adaptation for colour blindness, the lenses could also give wearers better vision in foggy or dusty conditions.
At present, the devices only work with relatively bright IR emissions (the study used LEDs). However, the researchers hope to increase the photosensitivity of the nanoparticles so that lower levels of light can trigger the upconversion process.
