Currently, the only way for patients with glaucoma to keep tabs on the disease is to go to the doctor’s office. There, a clinician administers one of several tests to measure glaucoma’s main risk factor, intraocular pressure (IOP), and prescribes medication accordingly. But such visits normally occur two or three times a year, and there’s no take-home monitoring device for patients who may experience pressure spikes between visits.
Now scientists at the University of California, Davis, have designed a contact-lens prototype with a built-in pressure sensor, using a novel process that etches tiny electrical circuits within a soft polymer material. The lens’s designer, Tingrui Pan, assistant professor of biomedical engineering, says that the design may eventually be fashioned into disposable contact lenses, enabling patients to continuously monitor glaucoma at home.
In glaucoma, drainage of the fluid that normally delivers nutrients to and removes metabolic waste from the eye is blocked. Elevated pressure in the eye ultimately presses on the retina, compromising neural activity and damaging the optic nerve, resulting in loss of vision. Doctors manage glaucoma by measuring patients’ IOP and prescribing drugs to lower it.
“It’s very different from situations like cardiac disease or diabetes, where patients can wear devices that measure heart rate or blood pressure 24 hours a day for a week or more to get a better idea of what’s going on,” says James Brandt, a professor of ophthalmology at UC Davis and Pan’s collaborator. “We don’t have that for glaucoma, and that’s one of the biggest clinical frustrations we have.”
Pan’s team recently made a contact-lens prototype from PDMS, an organic polymer commonly used to make contact lenses and breast implants. “This material has been widely used in biology because it’s easy to work with and can bend and flex like skin,” says Pan. “But the problem is, it’s not conductive, and if you want to make it sensing, it has to be conductive.”
Because it’s difficult to adhere metal wires directly to the polymer’s surface, Pan looked for ways to embed metal within the polymer. He first made the polymer sensitive to ultraviolet (UV) light by mixing it with a chemical agent. When exposed to UV light, the polymer solidifies, forming a soft, rubberlike material. Without UV light, the polymer remains in its liquid form.
The team then created a negative cutout in the pattern of a small circuit and shined UV light through the cutout, onto a layer of polymer mixture. Areas exposed to light gelled, while those under the cutout did not. Researchers were able to easily wash away the liquid polymer, leaving an imprint of a small, nanoscale circuit within the solidified polymer.
Pan filled the pattern with a solution of powdered silver, a nontoxic metal conductor. After polymerization, the silver formed a continuous circuit within the soft polymer. In initial laboratory tests, the team found that voltage within the tiny circuit changes slightly as the polymer is bent. Pan says that measuring this change could provide a good monitor for IOP: as pressure within the eye increases, the shape of the contact lens would distort, causing a change in voltage within the wires. The researchers published their results in the recent issue of the journal Advanced Functional Materials.
“This device is really a breakthrough in real-time IOP monitoring,” says David Calkins, associate professor of ophthalmology at Vanderbilt University Medical Center, who was not involved in the research. “We don’t have a means right now to measure pressure in real time outside of the clinic. Because of that, we are missing the fluctuations in IOP that could be pertinent to the pathogenesis of glaucoma.”
However, several hurdles remain before the prototype can be fashioned into a practical contact lens. In the current version, the silver circuit is opaque and would obviously obstruct vision. Pan says that such a visible circuit could still be used for short-term, sit-down tests in the clinic. However, he is also looking for materials that may be made into transparent circuits, for longer-term use.
Powering the lens also presents a problem. Ideally, Brandt says, a “smart” contact lens would consist of an electrical pressure sensor as well as an RFID tag to wirelessly transmit information to a computer, along with a small battery to power the device. “Getting energy to the device, and pulling information off of it, is not a trivial task,” he says.