Injected nanoparticles make blind mouse retinas respond to light

Researchers have developed microscopic particles that, when injected into the eye, can make a blind retina respond to light again. In experiments with mice that had advanced retinitis pigmentosa, a disease in which the retina's light-sensing cells die off, the particles allowed light to trigger activity in the visual cortex, the part of the brain that processes vision. The mice also changed their behavior in response to light after the injection. The same particles activated nerve cells in isolated retinal tissue taken from pigs.

The work was led by Associate Professor Menglin Chen at the Department of Biological and Chemical Engineering, Aarhus University, and published in Nature Biomedical Engineering. The project began seven years ago as an open-ended question: could a material be built that acts as a wireless go-between for light and living cells, the way a solar cell converts light into electricity. The retina results mark the point where that question turns into an early step toward a specific medical device, a light-sensitive implant meant to replace lost photoreceptors.

In retinitis pigmentosa, the photoreceptor cells at the back of the eye that normally catch light gradually die, but many of the other nerve cells and connections in the retina survive intact. The new particles are designed to make use of those survivors. They are hollow spheres about 300 nanometers across, roughly a hundredth the width of a human hair, made from a light-sensitive semiconductor called graphitic carbon nitride. Their hollow shape is partly inspired by chloroplasts, the structures inside plant cells that capture sunlight.

When the particles are exposed to light, they set off physical and chemical processes in their immediate surroundings that can influence how nearby cells send signals. The researchers tested this at several scales. A tightly focused laser could activate a single particle inside a single cell and trigger a signal that spread to neighboring cells; ordinary LED light aimed at particles taken up by heart muscle cells made those cells beat more in sync with each other.

In the eye, the injected particles settled on the surface of the retina, close to the retinal ganglion cells, the nerve cells that carry visual signals from the eye toward the brain. Illuminating the eye then produced measurable activity in the visual cortex of the mice. Unlike gene therapy or optogenetics, which require modifying the genes of surviving retinal cells, this approach does not alter the cells themselves; unlike electronic retinal implants, it does not require an implanted device placed surgically. Henri Leinonen, a retina specialist and co-author of the study at the University of Eastern Finland, said the appeal of the approach is that it does not depend on the specific mutation causing a patient's blindness, unlike gene therapies, which target specific genes.

The idea of using nanoparticles to restore some retinal function is not new. A separate line of research using different, polymer-based nanoparticles has previously shown that light-activated particles injected beneath the retina could restore pupil responses to light, brain signals tied to vision, and light-driven behavior in rats with advanced retinal degeneration, even after the animals' photoreceptors had completely died and the wiring of their inner retinas had become extensively rearranged, a process that tends to occur as degeneration drags on. That work also noted this rewiring as a challenge for any retinal prosthesis, not just nanoparticle-based ones, since prosthetic devices are typically used only after degeneration is already advanced.

Broader reviews of the field describe electrode-based retinal implants, sometimes called bionic eyes, as having delivered limited sharpness of vision after more than two decades of development, largely because electrical current spreads through retinal tissue and stimulates cells indiscriminately, rather than only the ones a given electrode is meant to target. Optical approaches, including light-sensitive nanoparticles, are seen as a way to avoid that spread since light can in principle be focused more precisely. Reviews of the field also note that nanoparticle-based approaches still face open questions about how reliably particles can be delivered to precise, stable locations within the eye.

The Aarhus researchers are explicit that their mouse experiments do not amount to restored normal vision. What they have shown is a measurable biological response to light in a retina where the cells that normally detect light have largely degenerated, which Leinonen described as an early step rather than a finished prosthesis. Long-term safety and function still need to be studied in much greater detail before the technology could be tested as a treatment in humans, the researchers said.

Since the project's start in 2019 with funding from the Carlsberg Foundation, the team has moved from testing nanofibers to hollow nanoparticles, and from activating single cells to producing responses in whole animals. In 2024 the researchers filed an international patent application covering the particles as an injectable retinal prosthesis and as a way to stimulate heart cells, and in 2025 Chen received a further grant from the Novo Nordisk Foundation to develop the technology specifically for the eye. The remaining work includes improving how the particles are delivered into the eye, tracking how they behave there over longer periods, and further testing their safety, before the question of whether the material could ever help restore sight in people can be seriously addressed.