Solar-Powered Eye Sensor

A cornea-implanted computer can monitor the eyeball's pressure

3 min read

In the future, that twinkle in your loved one's eye might be an implanted solar-powered pressure monitor. At the 2011 IEEE International Solid-State Circuits Conference in February, engineers from the University of Michigan, in Ann Arbor, described their work on a cubic-millimeter-size sensor meant to monitor pressure inside the eye. The researchers have yet to test the device in human eyes or animal ones, but they hope their system will one day thwart optic nerve damage brought on by glaucoma.

To determine a glaucoma patient's treatment, doctors must monitor pressure inside the eye, says Gregory Chen, a graduate student of electrical engineering at Michigan. Today's methods gauge that pressure by pushing on the cornea, the eye's clear outer coating. The results may be inaccurate: "If you just happen to have a really thick cornea, your eye is going to be harder, no matter what the pressure is," Chen says. The engineers' prototype device would allow a microelectromechanical systems (MEMS) capacitive sensor to record pressure from inside the eye about every 15 minutes and store it to static RAM. Once a day, the system would wirelessly transmit the day's data, via two on-chip inductors, to a wand. The inductors would send the data at both 400- and 900-megahertz carrier frequencies, as a means of mitigating the signal's noise and increasing its range.

Powering this daily transmission was one of the team's biggest challenges, says Mohammad Hassan Ghaed, the Michigan grad student who designed the radio system. "The common way of transmitting data is to use an external antenna," he says, "but in this millimeter-cubed space, we don't have that luxury." The team designed the diminutive device for easy implantation, but the small size comes at a cost: The device's inductors require more power to send data than a bigger antenna would, a peak of 47 milliwatts. That was a problem, given that the peak power supplied by their device's thin-film lithium battery is only about 40 microwatts.

To make up for this shortfall, the system stores enough energy to transmit one bit in a capacitor. The device then sends that bit and refills the capacitor from the battery for another go. The bit-by-bit method provides a transmission rate of around 10 kilobits per second, but with a day's data totaling around 1.5 Kb, it's still "near instantaneous," says Dennis Sylvester, a professor of electrical engineering, who led the team with colleague David Blaauw. The battery itself lasts around 28 days before it's drained, but the device also includes a miniature solar panel. The team expects that about 10 hours of indoor lighting or 1.5 hours of sunlight daily could recharge it.

"As far as I know, nobody does solar power at this size," says Pedro Irazoqui, director of the Center for Implantable Devices at Purdue University, who is also developing an implantable glaucoma sensor. Irazoqui praises the Michigan team for its energy-harvesting system and on-chip radios. His own team's tadpole-shaped device, which is now being tested in rat eyes, uses radio waves from an external source to power a 2.7-mm antenna "tail" for transmission and to charge an onboard supercapacitor.

Irazoqui's commendation of the Michigan device comes with some caution. "There are a lot of impressive innovations here," he says, "but until they've actually implanted it in a live eye and measured the pressure, they haven't shown that the device works." Of particular concern, he says, is that surgeons would need to implant the device in the "very delicate" iris tissue.

The Michigan team notes that human testing is still several years away. "Now that we have a system that is working, we can start to expand into long-term effects," Blaauw says, noting grad student Razi Haque's work to optimize the device's packaging in collaboration with Michigan professor and IEEE Fellow Kensall Wise.

What doesn't seem to be a concern is that staring into the wearer's eyes might reveal a speck of cyborg. "We're expecting to make different colors," Chen jests. "It's going to be a fashion statement."

This article originally appeared in print as "The Sun’s in Your Eyes"

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