Researchers recently unveiled the first complete millimeter-scale computing system that is about the size of the letter "N" on the back of a penny (or about the same size as the letter in this sentence).

This tiniest computer to date is a prototype of an implantable eye pressure monitor for glaucoma patients. Key to this unit linking up with other computers to form wireless sensor networks is a compact radio that needs no tuning to find the right frequency.

One day, these Lilliputian computers could track pollution, monitor structural integrity, perform surveillance, or make virtually any object smart and traceable.

Better, faster, smaller

Nearly invisible millimeter-scale systems could enable ubiquitous computing by following the trajectory of Bell's Law, a corollary to Moore's Law. (Moore's Law says that the number of transistors on an integrated circuit doubles every two years, roughly doubling processing power.)

Bell's Law says a new class of smaller, cheaper computers comes along about every decade. With each new class, the volume shrinks by two orders of magnitude and the number of systems per person increases. The law has held from 1960s' mainframes through the '80s' personal computers, the '90s' notebooks and the new millennium's smartphones. [Read also: 10 Revolutionary Computers]

"When you get smaller than hand-held devices, you turn to these monitoring devices," said David Blaauw, a professor of electrical engineering and computer science at the University of Michigan who is working on the new tiny computer.

"The next big challenge is to achieve millimeter-scale systems, which have a host of new applications for monitoring our bodies, our environment and our buildings."

Power at small scales

The new system is targeted toward medical applications, focusing on a pressure monitor designed to be implanted in the eye to conveniently and continuously track the progress of glaucoma, a potentially blinding disease. (The device is expected to be commercially available several years from now.)

In a package that's just over one cubic millimeter, the system fits an extremely low-power microprocessor, a pressure sensor, memory, a thin-film battery, a solar cell and a wireless radio with an antenna that can transmit data to an external reader device that would be held near the eye.

"This is the first true millimeter-scale complete computing system," said Dennis Sylvester, alsoa professor of electrical engineering and computer science at the University of Michigan.

The processor in the eye pressure monitor is the third generation of the researchers' Phoenix chip, which uses a unique architecture and an extreme sleep mode to achieve ultra-low power consumption.

The newest system wakes every 15 minutes to take measurements and consumes an average of 5.3 nanowatts. To keep the battery charged, it requires exposure to 10 hours of indoor light each day or 1.5 hours of sunlight. It can store up to a week's worth of information.

While this system is miniscule and complete, its radio doesn't equip it to talk to other devices like it. That's an important feature for any system targeted toward wireless sensor networks.