Published : Tuesday, May 1, 2018 | 5:03 PM
Engineers at the California Institute of Technology in Pasadena, inspired by tiny nanostructures on transparent butterfly wings, are finding easier ways for glaucoma patients to measure their own eye pressure regularly.
For the last few years, Dr. Hyuck Choo, assistant professor of electrical engineering in the Division of Engineering and Applied Science at Caltech, has been developing an eye implant that would improve the monitoring of intra-eye pressure among glaucoma patients.
Glaucoma is the second leading cause of blindness worldwide. 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.
“Right now, eye pressure is typically measured just a couple times a year in a doctor’s office. Glaucoma patients need a way to measure their eye pressure easily and regularly,” says Choo, who’s also a Heritage Medical Research Institute Investigator.
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 handheld reader, giving a direct measurement of how much pressure the implant is under.
To get an accurate measurement, however, the optical reader has to be held almost perfectly perpendicular – at a 90-degree angle, plus or minus five degrees – with respect to the surface of the implant. At other angles, the reader gives an incorrect measurement.
That’s when Choo took interest in a dissertation three years ago by Caltech postdoctoral researcher Radwanul Hasan Siddique, at that time working at Karlsruhe Institute of Technology in Germany, which involved the see-through sections of the wings of the long-tail glasswing butterfly, Chorinea faunus, found mostly in South America.
Siddique found that the wings of the butterfly are coated in tiny pillars, each about 100 nanometers in diameter and spaced about 150 nanometers apart. The size of these nanopillars – 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 – an effective camouflage that keeps the butterfly hidden from predators.
Looking at Siddique’s dissertation, Choo saw that the nanopillars’ redirection property, known as angle-independent antireflection, could be used to ensure that light would always pass perpendicularly through his eye implant, making the device angle-insensitive and providing an accurate reading regardless of how the reader is held.
Choo enlisted Siddique to work in his lab, and the two, working along with Caltech graduate student Vinayak Narasimhan, figured out a way to stud the eye implant with pillars approximately 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 says.
The new surface also lends the implants a long-lasting, nontoxic anti-biofouling property.
In the body, cells tend to latch on to the surface of medical implants and, over time, gum them up. One way to avoid this phenomenon, called biofouling, is to coat medical implants with a chemical that discourages the cells from attaching. The problem is that such coatings eventually wear off.
The nanopillars created by Choo’s team, however, work in a different way. Unlike the butterfly’s nanopillars, the lab-made nanopillars are extremely hydrophilic, meaning that they attract water. Because of this, the implant, once in the eye, is soon encased in a coating of water. Cells slide off instead of gaining a foothold.
“Cells attach to an implant by binding with proteins that are adhered to the implant’s surface. The water, however, prevents those proteins from establishing a strong connection on this surface,” says Narasimhan. Early testing suggests that the nanopillar-equipped implant reduces biofouling tenfold compared to previous designs, thanks to this anti-biofouling property.
Being able to avoid biofouling is useful for any implant regardless of its location in the body. The team plans to explore what other medical implants could benefit from their new nanostructures, which can be inexpensively mass produced.
The team’s study, “Multifunctional biophotonic nanostructures inspired by longtail glasswing butterfly wings for medical devices,” was published in the journal Nature Nanotechnology on Monday. Co-authors include Caltech postdoctoral scholars Shailabh Kumar, Blaise Ndjamen, and Jeong Oen Lee; Caltech WAVE fellow Natalie Hong; and Juan Du and David Sretavan from the UC San Francisco. This research was funded by the National Eye Institute, National Institutes of Health; a Heritage Medical Research Institute Investigator award; the Caltech Rothenberg Innovation Initiative (RI2) Program; the Powell Foundation; the Caltech Beckman Institute, and the Arnold and Mabel Beckman Foundation.