Researchers looking through the transparent wings of a longtail glasswing butterfly found inspiration to create nanostructures coatings for an implantable eye pressure sensor that could help patients with glaucoma retain their sight.
Researchers looking through the transparent wings of a longtail glasswing butterfly found inspiration to create nanostructures coatings for an implantable eye pressure sensor that could help patients with glaucoma retain their sight.
Hyuck Choo, a professor of electrical and medical engineering at the California Institute of Technology, has spent several years working on a medical implant to monitor intra-eye pressure in glaucoma patients. The eye damage caused by glaucoma is irreversible, but monitoring the patient’s eye pressure can prevent further damage. Currently, patients typically get their eye pressure measured only a few times a year in the eye doctor’s office, leaving them vulnerable to further eye damage between visits. Choo, a biomedical engineer, hopes to create a way for patients to check their own eye pressure, perhaps using their cell phones to read information transmitted by the implanted device so they can address potential issues in real time.
Choo’s implantable sensor consists of a deformable membrane suspended over a mirror-like plate sealed together to form a cavity, much like a tiny drum. Pressure in the eye deforms the membrane and changes the shape of the optical resonator. Any change in the shape of that resonator can then be measured using a light source and detector.
But Choo’s lab ran into several challenges as the implant evolved.
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The sensor and the detector, for one, had to align exactly at a 90-degree angle, give or take five degrees, making it potentially useless for people with shaky hands or arthritis. Tissue growth, another issue that affects all kinds of implants, also caused the eye pressure sensor to lose its effectiveness over time.
“The body starts to wrap around the sensors,” Choo says.
While contemplating how to address these challenges, the work of Radwanul Hasan Siddique, a graduate student at Karlsruhe Institute of Technology in Germany, caught Choo’s eye at the Materials Research Society conference in 2015. At the meeting, Siddique revealed the science behind the transparent parts of a longtail glasswing butterfly’s wings. The glass-like sections of the wing consist of tiny nanoscale pillars, about 100 nanometers wide and spaced about 150 nanometers apart, which have optical properties not seen at the micro- or macro-scale. The pillars allow light to pass through independently of the angle the light hits them, resulting in minimal light reflection off the wing’s surface and giving the wings their transparency.
Choo realized that this angle-independent antireflection property could potentially address the issue with his implantable eye-pressure sensor. He recruited Siddique and they began working with Caltech graduate student Vinayak Narasimhan to see how they might mimic, or improve upon, the butterfly’s nanoscale surface properties. They began by closely examining the optical properties of the transparent parts of the butterfly wing, then building similar nanostructures out of silicon nitride and testing their properties.
They discovered that the transparent region of the butterfly wing closest to the body allowed angle-independent transmission of the light but also reflected UV light, which made those nanostructures ideal for the sensor work.
During their research, the engineers experimented with different geometries of silicon nitride nanopillars, examining their optical properties while exposing the nanopillars to platelet cells and proteins that initiate clots when they contact the implant’s surface. They often form a crust around the implant, which is also prone to attack and infection by bacteria in the body.
The researchers knew that some nanoscale patterns make it difficult for cells to adhere to a surface. The researchers tested a variety of pillar configurations, and found that keeping the height-to-width ratio of the pillars low optimized optical coherence while minimizing optical scattering and cell adhesion.
“We do not want cells to sit on the implants, but we also don’t want them to die,” says Narasimhan, who noted that cells that die on the implant could cause inflammation. A higher height-to-width ratio began to destroy cells, so keeping it low worked for both the biophysical and optical properties. In addition, the silicon nitride material attracts water, forming a barrier on the surface of the implant that also deters cell adhesion.
The researchers reported in Nature Nanotechnology that they implanted nanostructure-coated ocular sensors in a rabbit and compared their effectiveness to flat-coated ocular sensors placed in another rabbit. The nanostructures showed a three-fold increase in angle-independent detection and a 12-fold decrease in cell adhesion to the device.
They currently have a year-long, large-scale animal trial in process. If the animal trial succeeds, the next step will be human clinical trials.
If the human trials prove successful, the surface would be easy to reproduce, the researchers said. “We can make the nanosurface on a four-inch chip, which would hold about one thousand sensors,” Narasimhan says. The coating technique takes about one minute and can be done using simple equipment, according to the researchers.
“There may be other potential applications for these nanostructures as well,” said Narasimhan. The properties of these nanostructures could be useful in other implantable devices or in medical packaging.
“We could really improve the quality of people’s lives,” Choo says.
Melissa Lutz Blouin is an independent technology writer.
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