A team of researchers from the College of Engineering was recently awarded a patent (US 20200321948A1) for technology related to ultra-low-voltage circuits that could someday find its way into a variety of useful products, such as wearable electronics that run without batteries.
Leading the team was Matthew Johnston, associate professor of electrical and computer engineering and 2021-22 Provost Fellow. Co-inventors on the patent are former graduate student Soumya Bose, now an analog design engineer at Intel, and Tejasvi Anand, assistant professor of electrical and computer engineering.
“The context for this work is the idea of energy harvesting,” Johnston explained. “Solar is probably the example everyone is most familiar with. We use solar panels to harvest energy from the sun, right? But there are other kinds of energy all around us. Radio waves are in the air everywhere, and they carry a bit of energy. It’s not a lot, but if you can capture enough of it, you can use it to power a device. And there are many other sources of ambient energy. The one we started looking at specifically was thermal energy.”
Thanks to a phenomenon called the Seebeck effect, the movement of heat across a temperature gradient can be converted into the movement of electrons through a circuit. The first part of that conversion takes place in a thin ceramic module called a thermoelectric generator. These are inexpensive and readily available. (A corollary phenomenon called the Peltier effect, which converts an electric current into a temperature gradient, is used in thermoelectric cooling and heating devices.)
“Anywhere you have a difference in temperature — even if it’s only a few degrees — you can generate a small amount of direct current,” Johnston said. “For example, the temperature of your skin surface is usually either a bit warmer or a bit cooler than the surrounding air. So, you can use that to produce electricity.”
One area where such technology would find many applications is in biomedical devices, such as wearable sensors to monitor temperature, blood glucose levels, or other health indicators. These could be attached to a patient’s skin like a patch, powered solely by body heat, logging vital data or transmitting it to a smartphone app.
The problem, Johnston explains, is that voltages generated from minor temperature gradients are very small, ranging from the order of tens of millivolts up to a few hundred millivolts on the high end. The minimum needed to power even a simple integrated circuit is around 1 volt.
“A regular battery cell, such as a AA or AAA, produces 1.5 volts,” Johnston said. “The lithium-ion battery in your smartphone puts out 3.8 volts. So, at least an order of magnitude greater than the voltages we’re looking at. In order to get useful work out of the electricity you can generate from a thermoelectric generator attached to your skin, you first need to amplify that voltage by 10 times or more.”
Technologies for converting lower voltage to higher voltage already exist, but they require help from an external power supply or battery to reach a threshold voltage before they will begin working. Johnston compares it to starting a car’s engine.
“The car can idle on a trickle of gasoline, but you need a battery to power a starter motor that can get the engine moving first. Or, in the old days, you’d have to get out and start cranking,” he said. “So, this idea of a ‘cold start’ voltage amplifier is key to developing thermoelectric devices that don’t require a separate source of electricity to get started.”
Johnston’s patent covers what he calls “a neat trick with transistors,” to create a very low-voltage ring oscillator that allows parts of a circuit to function before that minimum voltage threshold is reached. A separate patent, for a highly efficient method of amplifying very low-voltage currents, is pending.
“This first patent is really focused on what you can do with those 50 to 60 millivolts, or less, to get anything happening,” he said. “You still need to get up to a volt eventually, but you can do something with these 50 millivolts, to get some part of a circuit moving, generating a clock, and acting. And then you can use that to grow it into bigger signals later.”
The impetus for Johnston’s research grew out of some earlier work, supported by the National Institutes of Health, to develop oral sensors, and body heat seemed like an interesting way to power them. Although Johnston has focused on thermoelectric generation, the methods elaborated in both patents are applicable to very low-voltage circuits powered from any source.
Johnston credits Bose, his former student and lead inventor on the patent, with having the key insights necessary to make their technology work.
“He looked at a bunch of different ways to get these circuits to start at very, very low voltage and build themselves up until they were operating at the normal output voltage. And this was done over several years, in several generations, and it just worked really well,” Johnston said.
