Butterfly wings inspire eye implants for glaucoma patients

Inspired by tiny structures on transparent butterfly wings, scientists have developed a light-manipulating surface for more effective and longer-lasting eye implants for glaucoma patients.

Sections of the wings of a longtail glasswing butterfly are almost perfectly transparent, according to the research published in the journal Nature Nanotechnology.

Researchers at California Institute of Technology (Caltech)in the US found that the see-through sections of the wings are coated in tiny pillars, each about 100 nanometres in diameter and spaced about 150 nanometres apart.

The size of these pillars - 50 to 100 times smaller than the width of a human hair - gives them unusual optical properties.

The pillars redirect the light that strikes the wings so that the rays pass through regardless of the original angle at which they hit the wings. As a result, there is almost no reflection of the light from the wing's surface.

In effect, the pillars make the wings clearer than if they were made of just plain glass, researchers said.

That redirection property, known as angle-independent antireflection, attracted the attention of Hyuck Choo, an assistant professor at Caltech.

For the last few years Choo has been developing an eye implant that would improve the monitoring of intra-eye pressure in glaucoma patients.

Though the exact mechanism by which the disease damages eyesight is still under study, the leading theory suggests that sudden spikes in the pressure inside the eye damages the optic nerve.

Medication can reduce the increased eye pressure and prevent damage, but ideally it must be taken at the first signs of a spike in eye pressure.

Choo has developed an eye implant shaped like a tiny drum, the width of a few strands of hair.

When inserted into an eye, its surface flexes with increasing eye pressure, narrowing the depth of the cavity inside the drum.

That depth can be measured by a hand-held reader, giving a direct measurement of how much pressure the implant is under.

One weakness of the implant, however, has been that in order to get an accurate measurement, the optical reader has to be held almost perfectly perpendicular - at an angle of 90 degrees - with respect to the surface of the implant. At other angles, the reader gives an incorrect measurement.

That is where glasswing butterflies come into the picture, researchers said.

Choo reasoned that the angle-independent optical property of the butterflies' nanopillars could be used to ensure that light would always pass perpendicularly through the implant, making the implant angle-insensitive and providing an accurate reading regardless of how the reader is held.

The researchers figured out a way to stud the eye implant with pillars about the same size and shape of those on the butterfly's wings but made from silicon nitride, an inert compound often used in medical implants.

Experimenting with various configurations of the size and placement of the pillars, the researchers were ultimately able to reduce the error in the eye implants' readings threefold.

"The nanostructures unlock the potential of this implant, making it practical for glaucoma patients to test their own eye pressure every day," Choo said.

The new surface also lends the implants a long-lasting, nontoxic anti-biofouling property.