Butterfly's wing inspires nano-material for intraocular glaucoma sensor

How can a butterfly help measure a glaucoma patient’s intraocular pressure? Researchers were inspired by the optical properties of the transparent wings of the longtail glasswing butterfly, and it was just the inspiration needed to improve their ocular implant.

Here’s how it happened.

A research team led by Hyuck Choo, PhD, assistant professor, Electrical Engineering, California Institute of Technology, Pasadena, CA, has been developing a sensor that can be implanted in the eye to measure intraocular pressure (IOP) in real time.

“Right now, eye pressure is typically measured just a couple times a year in a doctor’s office,” Dr. Choo said. “Glaucoma patients need a way to measure their eye pressure easily and regularly.”

An implantable sensor, which measures IOP continuously and directly, can do just that. By comparison, current IOP measurement techniques, such as tonometry, measure IOP indirectly and so are generally more prone to error due to factors such as corneal thickness, curvature, and biomechanics.

To obtain an IOP measurement from Dr. Choo’s sensor, a light must be directed at it at a nearly 90-degree angle. If the light is just a few degrees off, the readout will provide an incorrect pressure measurement.

Here’s where the butterfly comes in. Caltech postdoctoral researcher Radwanul Hasan Siddique, PhD, had studied the longtail glasswing butterfly to understand how its transparent wings do not reflect light in the way that glass reflects light. He used high-resolution scanning electron microscopy on the transparent area of the butterfly’s wings to reveal tiny, dome-shaped nanopillars, each about 100 nm in diameter and spaced about 150 nm apart.

The nanopillars’ construction gives them unusual light-scattering optical properties. The pillars redirect the light that strikes the wings so that the rays pass through regardless of the original angle at which the light hits the wings (referred to as angle-independent scattering). As a result, there is almost no reflection of light from the wing's surface.

Learning of this, Dr. Choo reasoned that the angle-independent optical property of the butterflies' nanopillars could be used to pass light perpendicularly through the IOP sensor, which would make the implant angle-insensitive and provide an accurate reading regardless of the angle of light directed at it.

The investigators copied the butterfly wing’s nanopillars onto a silicon nitride substrate and incorporated it into the IOP sensor. After experimenting with the size and placement of the nanopillars, the researchers were able to reduce the error in the implants’ IOP readings by threefold.

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

In addition, the lab-made nanopillars are extremely hydrophilic, which creates an aqueous barrier to ward off the adhesion of proteins, bacteria, and eukaryotic cells. The investigators performed tests with the IOP sensor implanted in the eyes of rabbits and found that, as they wrote in a recently published article in Nature Nanotechnology, “the nanostructures effectively suppressed biofouling and inflammation 12-fold, resulting in a highly practical implant for long-term IOP monitoring.”

“Further development of our bio-inspired work, including continuous IOP monitoring using mobile devices with integration of features such as memory-based tracking, will improve glaucoma treatment outcomes and lower the risk of visual impairment and blindness,” Dr. Choo and coauthors concluded. “With these promising results, we envisage numerous medical technologies and devices will benefit greatly from the multifunctionality of biophotonic nanostructures.”