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One-Cubic-Millimeter Computer

A prototype sensor consumes just a few nanowatts and could someday take pressure readings from the eye.
February 28, 2011

A new wireless computer sensor just a cubic millimeter in size could eventually be implanted in the eyes of people with glaucoma, taking pressure readings 24 hours a day and transmitting the data to doctors.

Tiny sensor: This wireless pressure sensor could eventually be implanted in the eye of glaucoma patients.

The new device packs a processor, memory, a pressure sensor, a solar cell, a thin-film lithium battery, and a transmitter into a tiny glass rectangle. University of Michigan researchers reported on the device earlier this week at the International Solid-State Circuits Conference in San Francisco.

The sensor takes pressure readings every 15 minutes and stores them in memory until they can be uploaded an external device. The solar cell ,which measures just 0.07 square millimeters, constantly recharges the battery; it takes either 1.5 hours of sunlight or 10 hours of indoor light to charge it completely.

Dennis Sylvester, an electrical engineer at the University of Michigan, says that the real constraint in shrinking the device wasn’t the size of the circuits but the tiny amount of power its battery was capable of providing. So every component was designed to use as little power as possible.

In fact, says David Blaauw, a University of Michigan  electrical engineer who coauthored the paper, reducing the power requirements meant making the circuits larger than they had to be. Smaller circuits would have leaked more current, necessitating a bigger battery and making the whole package larger. So the circuits were made using a decade-old process that yields feature sizes of 180 nanometers, rather than the modern standard of 32 nanometers.

Meanwhile, the memory cells had to be redesigned to work at only about 400 millivolts instead of the usual one volt. The researchers also used special power-gating transistors capable of almost completely cutting off current when the device is asleep.

The result is a sensor that consumes only 5.3 nanowatts on average. But the power saving comes at a cost. The processor runs at only 100 kilohertz (compared with around 1 gigahertz in a smart phone); the memory has a capacity of only 4,000 bits; and the wireless transceiver has a range of only about 10 centimeters. Still, that’s enough to take a pressure reading and store it for several days.

In a separate paper, University of Michigan electrical engineer David D. Wentzloff reported an advanced miniature radio that could extend the transmitting range of small sensors, allowing them to network together.

Radios normally use a quartz crystal or some other oscillator to serve as a reference when adjusting the radio frequency. But the oscillator adds bulk, making it hard to shrink the radio to millimeter size. Wentzloff’s radio, however, takes advantage of the fact that every antenna has a natural resonance frequency that varies according to its dimensions. Wentzloff created a circuit that measures this resonance frequency and uses it to tune the radio frequency. Eventually, he says, the tiny radios could have a range of one to 10 meters, allowing networks of sensors to communicate with one another.

Tiny sensors could be used for other biomedical monitoring, such as tracking the progress of tumors. They might also be used to monitor the environment for pollutants, or to provide surveillance for military or security applications.

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