Butterfly wings to the aid of glaucoma patients

Print edition : May 25, 2018

A patient at a glaucoma screening camp. Photo: C.V. SUBRAHMANYAM

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.

Researchers at the California Institute of Technology (Caltech) in the United States found that the transparent sections of the wings of a longtail glasswing butterfly are coated in tiny pillars, each about 100 nanometres in diameter and spaced about 150 nanometres apart. According to the research published in the journal “Nature Nanotechnology”, 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.

That redirection property, known as angle-independent antireflection, attracted the attention of Hyuck Choo, an assistant professor at Caltech, who has been involved in developing an eye implant that can better monitor intra-eye pressure in glaucoma patients. Sudden spikes in the pressure inside the eye, it is suggested, 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’s eye implant is 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.

However, to get an accurate measurement, the optical reader has to be held almost perfectly perpendicular to the surface of the implant. 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, non-toxic, anti-biofouling property.