Butterfly wing inspires nanotech glaucoma sensor

Nanostructures that give a species of butterfly almost perfectly transparent wings  have inspired the proof-of-concept manufacture of an implant that promises to ease the lives of glaucoma sufferers.

Light passes through the wings of the glasswing butterfly (Greta oro) without causing reflection, because they are covered in pillars that average only 100 nanometres high – about 50 to 100 times smaller than the width of a human hair – and spaced about 150 nanometres apart.

The pillars direct the light that hits them straight through the butterfly’s wings – regardless of the direction from which the rays travel. This means there is no reflection, and thus the wings appear as if made of glass.

The nanostructures were first described in 2015 by a team led by Radwanul Hasan Siddique, then of the Karlsruhe Institute of Technology in Germany. The butterfly wings’ “omnidirectional anti-reflection behaviour”, he and his team wrote, is unlike any other structure found in nature. The researchers credited the absence of reflection to the fact that the surface nano-pillars differed slightly in height and spacing, both values being distributed randomly.

Siddique is now resident at Caltech in the US, where he has teamed up with electrical engineer Hyuck Choo to develop a revolutionary new form of glaucoma monitor that can sit inside a patient’s eye.{%recommended 1535%}

Glaucoma is a disease that causes blindness because increasing pressure within the eye inflicts irreversible damage on the optic nerve. It is the world’s second leading cause of sight loss, with the global number of cases expected to top 53 million by 2020.

Medication exists that helps control the sudden rises in pressure that cause the nerve damage, but works best when taken at the very start of a spike. This is only detectable by measuring changes in the intra-ocular fluid – something generally done by doctors, and typically only once or twice a year.

Last year Choo and colleagues announced the invention of novel monitor to allow patients to measure their eye pressure at will. The system comprises a sensor just a few micrometres in diameter, and a hand-held reader. The sensor is inserted into the eye, where it remains. By using the reader, the user is able to obtain instant pressure data, allowing him or her to judge when medication is necessary.

The system, however, has one severe problem. In order to obtain an accurate read-out the angle between the sensor and the reader had to be almost perfectly perpendicular – 90 degrees, with just a 5% margin for error.

This was, needless to say, difficult to achieve, and the risk of a false reading was therefore high.

Now, the issue seems to be solved. Choo teamed up with Siddique, along with another researcher, Vinayak Narasimhan, and got to work figuring out how the nano-pillar arrangement used by the glasswing butterfly could be adapted for the glaucoma monitor.

The logic was simple: just as the distribution of size and location on the butterfly wing meant light always passed through without reflection, a similar arrangement on the surface of the implant should mean that it became much less sensitive to the relative angle between it and the reader.

The nanostructures are made from silicon nitride, often used in medical devices, and the result, after a bit of trial and error, is a threefold reduced error level in read-outs.

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

The design and the materials used also deliver a second, no less important advantage. Many medical implants eventually lose function and have to be replaced for the simple reason that the body attempts to assimilate them, causing them to be encased by cells. 

The standard way to avoid this is to coat implants with chemicals that prevent cells from attaching. At best, however, this is a temporary solution, because the coatings always eventually wear off.

The monitor made by Choo and his colleagues offers a different, more permanent solution. The monitor is extremely hydrophilic, meaning it attracts water to itself. This results in a dense, slippery, inert and safe layer that prevents cells attaching to it.

The research is published in the journal Nature Nanotechnology.