A new way to harvest electricity from body heat could inspire new wearable devices that never need to be plugged in. The millivolts of electricity this thermoelectric technology produces mandates slim power usage from any electronics plugged in to its feed. However, the developers say there already are fitness trackers and medical monitors today that could work within their device’s power envelope.
The new, wearable thermoelectric generator is also sourced from non-toxic and non-allergenic substances, making it a viable candidate for wearable technology.
More precisely, it’s a vapor-deposited strip of cotton fabric—coated with a material called, brace yourself, “persistently p-doped poly(3,4-ethylenedioxythiophene)” a.k.a. PEDOT-Cl. One end of the fabric touches a person’s skin and is thus at a person’s body temperature. The other end, ideally, is exposed to the open air. The greater the difference in temperature between the two ends, the greater the electrical output.
Andrew says other labs have previously experimented with very efficient energy harvesting devices—that unfortunately were also made of toxic and expensive rare earth materials like bismuth telluride. Alternately, she says, more biocompatible thermoelectric generators made from polymers have also been considered. But these are often such poor energy harvesters that they can only muster small fractions of a single millivolt.
However, Andrew and her graduate student and co-author, Linden Allison, may just have threaded the proverbial thermoelectric needle. The innovation here was to vapor deposit their polymer only onto the surface of the cotton fibers—and not soak the entire cloth in the polymer.
“The process is very much akin to how semiconductors are made,” Andrew says. “But our lab developed a chemistry to translate that process to make fully organic materials.”
By keeping the semiconducting material on the surface, they could allow for charge to flow through the material while still thermally insulating one end of the generator from the other.
“The coating is the important part” of the new technology, Andrew says.
This stems from the competing demands of a good thermoelectric conductor. The ideal material must somehow keep one side hot and the other side cold—in other words, the material must be thermally insulating. However, it must at the same time conduct electrons. Electrical current needs to flow, or it’s not a very good generator.
With this vapor deposition trick, she says, “The polymer can be really, really electrically conductive." And PEDOT-Cl fills that bill. However, because the polymer is only coated on the outer surface of the cotton fibers, the bulk of the material (i.e. the cotton) is still able to perform its thermally insulating role.
Andrew and Allison, whose research has been published in the journal Advanced Materials Technologies, say they’ve applied for patents for their technology. And they’re already in talks with electronics companies who are considering this technology for possible use in future wearable products. The researchers said they could not name any companies they’re in touch with. However, Andrew says, “you’ve heard of some of them.”
One realm of early applications could be in the medical device field, says Andrew. The prototype they’ve done most of their testing on is a wristband that Allison made with a slit to allow one end of the PEDOT-Cl-infused fabric to touch the person’s skin, with the other end open to the air.
Allison says most applications for the technology will likely come from room temperature or colder weather environments. (In fact, she points out, sweat helps to increase the conductivity, so it’s possible the device could be used in athletic or workout-related wearables.)
However, she says, there’s no reason why the end of the device at human body temperature couldn’t instead be the cool end of the thermal gradient. In other words, in a hot desert environment where the air temperature is consistently above 37 degrees C (98.7 degrees F), the device could potentially work as well.
Whatever the case, Andrew says, it’s unlikely the generator would be plugged directly into a device. It’s too variable a current source, she says. Rather, the technology seems better suited to being a steady trickle charger for a battery that, in turn, powers one’s wearable electronics.
For instance, Andrew says, some wearable heart rate and glucose level monitors rely on a 0.5-volt battery. The 10 millivolts that even their wristband lab demo generated would be sufficient to serve as a trickle charge source for the battery—possibly freeing the device owner from worrying about plugging their device in.