Before long, sensors may be implanted in our bodies to do things like measure blood-glucose levels in diabetics or retinal pressure in glaucoma patients. But to be practical, they’ll have to both be very small–as tiny as a grain of sand–and use long-lasting batteries of similarly small size, a combination not commercially available today.
Now researchers at the University of Michigan have made a processor that takes up just one millimeter square and whose power consumption is so low that emerging thin-film batteries of the same size could power it for 10 years or more, says David Blaauw, professor of electrical engineering and computer science at Michigan and one of the lead researchers on the project.
But when this processor, dubbed the Phoenix, is coupled with a battery, the whole package would only be a cubic millimeter in volume. At this scale, Blaauw says, it could be feasible to build the chip into a thick contact lens and use it to monitor pressure in the eye, which would be useful for glaucoma detection. It could also be implanted under the skin to sense glucose levels in subcutaneous fluid. More broadly, this low-power approach to processor design could be used in environmental sensors that monitor pollution, or structural health sensors, for instance.
The processer uses only about 30 picowatts (a picowatt is one-millionth of one-millionth of a watt) of power when idle. When active, the processor consumes only 2.8 picojoules of energy per computing cycle. That amount is about a tenth of the energy used by the most energy-efficient chips on the market, says Jan Rabaey, a professor of electrical engineering and computer science at the University of California, Berkeley, who was not involved in the research.
The Michigan team’s main idea was to design a chip that runs at an extremely low voltage. While microprocessors for personal computers may require two volts of electricity per operation, the Phoenix only needs 500 millivolts, or 75 percent less.
At this voltage, parts of the chip don’t operate well, explains Blaauw, so his team redesigned the chip’s memory, which is smaller than most processor memory, and its internal clock so that it could operate with minimal electrical input. The chip’s clock–the timepiece that synchronizes number-crunching operations–has been reduced to an extremely slow rate of 100 kilohertz, as opposed to the gigahertz rates of personal computers. This approach makes sense for sensors, says Blaauw. “If we wanted to monitor pressure in the eye … we only need to take readings every few minutes,” he says.
When designing an embedded system choosing which tools to use often comes down to building a custom solution or buying off-the-shelf tools.