A new class of nanoscale materials modeled after a moth's eye could improve the light-capturing efficiency of x-ray machines and similar medical imaging devices.

Like butterflies, moths have large compound eyes composed of many thousand ommatidia — structures made up of a primitive cornea and lens, connected by photoreceptor cells. However, unlike those of butterflies, moth eyes are remarkably antireflective, bouncing back very little of the light that strikes them. This adaptation makes the insect less visible to predators during their nocturnal flights. It is because of this feature that engineers have looked to the moth eye to help design more efficient coatings for solar panels and antireflective surfaces for military devices.

Now Yasha Yi, a professor at the City University of New York, and his colleagues have taken a step ahead: They have used the moth eye as a model for developing new nanoscale materials that someday could reduce the x-ray radiation dosages received by patients, while improving the resolution of the resulting images.

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Scanning electron microscope image of the eye on a leaf miner moth. (Image: Dartmouth College)

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The scientists focused their experiment on scintillation materials — compounds that, when struck by incoming particles, absorb the energy of the particle and then reemit that absorbed energy in the form of light. Such scintillators are used in radiographic imaging devices to convert the x-rays exiting the body into visible light signals picked up by a detector to form an image.

Higher x-ray dosage improves output but is not healthy for patients. As an alternative, Yi’s team discovered that improving the scintillator’s efficiency at converting x-rays to light improved the output. Their new nanomaterial does just that.

The new material consists of a 500-nm-thick thin film composed of a cerium-doped lutetium oxyorthosilicate crystal. The crystals are encrusted with tiny pyramid-shaped silicon nitride protuberances. Each protuberance, or corneal nipple, is modeled after the structure in a moth’s eye and is designed to extract more light from the film.

Within a 100 x 100 micrometer square, about the same density as the actual moth eye, the scientists can fit between 100,000 and 200,000 protuberances. They made the sidewalls of the device rougher, improving its ability to scatter light and enhancing the scintillator’s efficiency.

During lab experiments, Yi discovered 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 percent compared with that produced using a traditional scintillator.

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(a) The self-assembly of SiO₂ nanoparticles on the top of high-index light extraction layer Si₃N₄, which is deposited on Lu₂SiO₅:Ce thin film. (b) The scanning electron microscope image of the improved bio-inspired moth eye nanostructures with certain degree roughness on the sidewall, which shows interesting nano-on-nano features. (Image: Optics Letters)

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This work represents a proof-of-concept evaluation of the use of the moth-eye-based nanostructures in medical imaging materials.

"The moth eye has been considered one of the most exciting biostructures because of its unique nano-optical properties," he said. "Our work further improved upon this fascinating structure and demonstrated its use in medical imaging materials, where it promises to achieve lower patient radiation doses, higher-resolution imaging of human organs, and even smaller-scale medical imaging. And because the film is on the scintillator, the patient would not be aware of it at all."

Yi estimates that it will take at least another three to five years to evaluate and perfect the film, and to test it in imaging devices.

The work, which appeared in Optics Letters, was done in collaboration with researchers at Tongji University in Shanghai.

For more information, visit: www.cuny.edu