Researchers at North Carolina State University are using nanotechnology to create energy harvesting and storage devices for ultra-low power sensors. The federally-funded research aims to create battery-free, body-powered wearable health monitors. The Center for Advanced Self-Powered Systems of Integrated Sensors Technologies (ASSIST) received funding from the National Science Foundation in 2012 for five years of research into health management electronics that use novel power sources.

ASSIST is working with two categories of sensors: non-invasive health sensors for bio-electric, biochemical, and acoustic monitoring; and environmental sensors that measure gasses, particulate matter, and temperature. The goals are to gather more accurate data on how the environment causes changes in physiological signals and to develop a multi-modal harvesters.

Tom Snyder, the lab's industry liaison, demonstrated an asthma monitor at CES this year, which was both activated and powered by breath. “If we understand exposure to the ozone and monitor EKG and understand motion of the body…an asthma attack could be predicted as much as 24 hours before it happens,” he said, noting that this principle is currently being studied.

These wearable sensors must be small, low power, and often transmit data regularly. To that end, ASSIST and its research partners are working on multi-modal energy harvesters combining elements that, for example, tap thermoelectric and kinetic energy. At its annual review next month, ASSIST will demonstrate an EKG monitor armband that wirelessly transmits data to an aggregator over thermoelectric power.

“If we want to do longitudinal long term studies, we don’t want interruptions in data collection. So if you can find a way to self-power…you can collect much more complete and continuous data sets,” he said, adding that power is a huge issue with wearables in general. “People that buy these devices use them for some period of time and a majority of those end up in a drawer or get discarded in under six months [because of ] the hassle associated with a battery,” he said.

Thermoelectric materials are electrically conductive but thermally insulated and, in the case of the armband, convert body heat to power. If one side of the material is colder than the skin-facing side, the temperature differential can drive voltage that can be used to harvest energy. Teams also are researching new heat syncing techniques for flexible, nano-scale materials such as stretchable electrodes to create more conformity to the skin and draw more heat from the body.

ASSIST uses piezoelectric materials for kinetic harvesting, aiming to operate at very low voltages. Snyder pointed to the Seiko kinetic watch, which has a weight inside that turns a motor to create a current and power. ASSIST hopes to use this principle, attaching a weight to a piezoelectric element to harvest rotational and kinetic energy. Both techniques have practical applications and reduce energy cost, Snyder said. For example, medical professionals could use a harvester to monitor an EKG or blood sugar levels. Motion-based harvesting could be of use in asset tracking or shipping where monitoring vibration of products is helpful.

ASSIST wants to develop a complete system that requires less than a milliwatt of power – preferably “a couple hundred microwatts so we can harvest enough to run the system continuously in a very small form factor,” Snyder said. “We do anticipate there are times where we’re able to capture more energy than we need. They’re doing supercapacitor research at Penn State looking at devices that are a hybrid of traditional capacitors and lithium ion batteries,” Snyder said. “Those supercapacitors have world record performance in terms of energy density and low leakage currents,” he added Separately, ASSIST working with colleagues at the University of Michigan developed a custom radio that consumes 1,000 times less power than Bluetooth Low Energy. The researchers are currently testing hypotheses on a number of low power techniques for radios, sensors, and other hardware.

— Jessica Lipsky, Associate Editor, EE Times