Fighting Glaucoma with Butterfly Wings

A multidisciplinary team has created a biomimetic eye implant that helps reduce the risk of glaucoma. The technology reproduces the nanoscopic properties of the Greta oto, a species of butterflies with transparent wings, edged with dark-colored lines, giving them the look of stained-glass windows. Their study, entitled “Multifunctional biophotonic nanostructures inspired by the longtail glasswing butterfly for medical devices”, was published on April 30, 2018, in Nature Nanotechnology. It was co-authored by Vinayak Narasimhan, Radwanul Hasan Siddique, Jeong Oen Lee, Shailabh Kumar, Blaise Ndjamen, Juan Du, Natalie Hong, David Sretavan and Hyuck Choo.

Glaucoma is the second leading cause of blindness in the world. Screening for glaucoma, before the first symptoms appear, can prevent the gradual and irreversible sight loss associated with this disease. To do this, ophthalmologists (or optometrists) monitor intraocular pressure. Pressure in the eye can damage the optic nerve making it incapable of sending visual signals to the brain. Medication can help reduce eye pressure and prevent the progressive loss of vision. However, it must be taken as soon as signs of pressure buildup are detected.

Hyuck Choo, assistant professor in the Department of Electrical Engineering and Applied Science at the California Institute of Technology (Caltech) and researcher at the Heritage Medical Research Institute, directed the design of an eye implant that facilitates intraocular pressure monitoring in patients with glaucoma.

Biophotonic Implant as Intraocular Pressure Sensor

The implant consists of a cylindrical lens that, once inserted into the eye, flexes with eye pressure. When pressure increases, the cavity of the lens decreases in depth. A sensor emits light toward the implant and measures the depth of the lens cavity. The microlens functions as a sensor.

In the beginning, the measurement accuracy of this system was limited by the angle of the sensor. To obtain reliable measurements, the sensor must be positioned perpendicular to the implant surface. If the prescribed angle is off by 5 degrees, more or less, the sensor gives false measurements. As the goal was to have the patient take these measurements, a solution had to be found to make the measurement process easier and more effective. To this end, the team searched for a material that would give the implant optical properties for accurate reading, regardless of the angle of incidence of the light beam sent by the sensor. The material also had to meet the biomedical constraints of the lens. The team discovered a scientific study that solved this problem and invited the researcher to join their group.

The Wings of the Greta oto

Radwanul Hasan Siddique, a postdoctoral student at the Karlsruhe Institute of Technology in Germany, studied the Greta oto wing in 2015. He discovered that its surface is characterized by a nanostructure that gives it interesting optical properties.

The transparent sections of the wings are covered with small pillars. Each is approximately 100 nanometers in diameter, spaced about 150 nanometers apart. The size of these pillars, 50 to 100 times thinner than a human hair, gives the butterfly wings transparency and anti-reflective properties, as the pillars redirect light that strikes the wings. The light beam completely passes through the surface, regardless of its angle of incidence. The researcher also discovered that the nanostructure eliminates bacteria deposited on the surface of the wing.

Manufacturing the Lens

The team made a material inspired by the Greta oto wing nanostructure. Its hydrophilic and non-stick properties prevent proteins, bacteria and eukaryotic cells from sticking to the lens. To replicate the wing’s nanostructure, the researchers used two immiscible polymers, polymethyl methacrylate and polystyrene, to form pillars on a silicon nitride substrate. Their tests have shown that the measurements obtained from this technology are as accurate as those of a tonometer, the equipment used by ophthalmologists to measure intraocular pressure.