Moth-eye-inspired materials could reduce X-ray dosages

The work, led by City University of New York’s Prof Yasha Yi in collaboration with professors Bo Liu and Hong Chen of Tongji University, Shanghai, was published on 3 July in Optics Letters.

Moths and butterflies have large compound eyes made up of thousands of ommatidia structures consisting of primitive cornea and lenses connected to photoreceptor cells.

Moth eyes, unlike those of butterflies, are anti-reflective, bouncing back very little of the light that strikes them. The adaptation helps the insects be stealthier and less visible to predators during their nocturnal flights. Because of this feature, engineers have looked to the moth eye to help them design more efficient coatings for solar panels and anti-reflective surfaces for military devices.

Yi and his colleagues used the moth eye as a model for a new class of materials that improves the light-capturing efficiency of X-ray machines and similar medical imaging devices.

In particular, the researchers focused on scintillation materials, namely compounds that, when struck by incoming particles (such as X-ray photons), absorb the energy of the particles and then re-emit that absorbed energy in the form of light.

In radiographic imaging devices, such scintillators are used to convert the X-rays exiting the body into the visible light signals picked up by a detector to form an image.

One way to improve the output is to increase the input, which means a higher X-ray dosage that could be detrimental to a patient’s health as a result of increased levels of radiation.

An alternative is to improve the efficiency with which the scintillator converts X-rays to light, which is achieved with the new material.

It consists of a 500nm thin film made of cerium-doped lutetium oxyorthosilicate. These crystals were encrusted with pyramid-shaped bumps or protuberances made of silicon nitride. Each protuberance, or corneal nipple, is modelled after the structures in a moth’s eye and is designed to extract more light from the film.

Between 100,000 and 200,000 of the protuberances fit within a 100 x 100μm square. The researchers then made the sidewalls of the device rougher, improving its ability to scatter light and enhance the efficiency of the scintillator.

In lab experiments, Yi and colleagues found that adding the thin film to the scintillator of an X-ray mammographic unit increased the intensity of the emitted light by as much as 175 per cent compared with that produced using a traditional scintillator.

The current work, Yi said, represents a proof-of-concept evaluation of the use of the moth-eye-based nanostructures in medical imaging materials. He estimated, however, that it will take at least three to five years to evaluate and perfect the film and to test it in imaging devices.