Battery-less smartwatch runs on body heat

Battery-less smartwatch runs on body heat

Technology News |
By Rich Pell

Monitoring your activity (calories burned, steps taken) and sleep quality, the PowerWatch as the company calls it is built with power-efficient components such as a 1.2″ diameter LCD memory display and Ambiq Micro’s ultra-low power Apollo MCU, so it can draw all of its operating power from a small thermoelectric unit that generates electricity out of body heat. Still the device can connect to a smartphone via a built-in Bluetooth 4.0 LE module to wirelessly sync with the MATRIX iOS and Android apps for more in depth data interpretation (peaks, trends).

Calorie counting is performed through a sensor-fusion based algorithm, reading data not only from the voltage information provided by the Seebeck effect (when the wearer’s body heats up), but also from two temperature sensors (near the skin and away from it on the outer side of the watch) and from an accelerometer.

But at this early stage of the company’s existence, the PowerWatch is really only a public demonstrator, admitted CEO and Co-Founder Akram Boukai when interviewed by eeNews Europe. The bigger story is really about the very low cost and rugged thermoelectric unit that the former academic invented when doing research on clean energy sources in the Chemistry Department of the University of California, Berkeley.

Back in 2008, Boukai and his research colleagues published a paper in Nature, “Silicon nanowires as efficient thermoelectric materials”, which was our first hint for this interview.

“Actually we don’t use silicon nanowires, this was a lab discovery and a curiosity. We had found that while bulk silicon is a good heat conductor, by processing it into nanowires, its thermal conductivity decreased by a hundred times and became similar to that of glass while retaining a very good electrical conductivity” revealed Boukai.

“Then I became professor at the University of Michigan where I was trying to figure out a way to manufacture these silicon nanowires in a scalable way to build efficient thermoelectric units”.

But as you would imagine, stacking silicon nanowires vertically to create PN junction thermoelectric generators was proving difficult.

“We had developed a process to create the silicon nanowires, but arranging them into a device was difficult, so we decided to take a different approach, in fact we inverted the process. We took a silicon wafer between 500µm and 1mm thick, and drilled lots of nanoscale holes in it to create a silicon mesh”, explained Boukai, “this yields a much more mechanically robust device”.

After this breakthrough, Boukai co-founded Silicium Energy late 2011, receiving seed funding from various investors. The company was renamed Matrix Industries about a month ago, before launching the Indiegogo PowerWatch campaign.

“Nobody was able to pronounce Silicium Energy correctly, and we wanted to make it easier on consumers”, the CEO recalls.

Although he wouldn’t reveal too much about the patented process nor about the actual figure of merit achieved, Boukai told us this was a CMOS-compatible process involving some form of electrochemical etching, maybe a way to hint at further device integration. After all, one could integrate power management and conversion electronics into the very same micro-drilled die that serves for the thermoelectric unit, on a small reserved non-drilled area.

By creating a very fine mesh (in fact removing most of the silicon) out of a PN doped wafer, a thermoelectric junction is simply created by metallizing the end faces of the wafer to exploit the Seebeck effect, no stacking needed here, a simple and robust planar architecture that can be sized up on demand.

Coming back to the PowerWatch launch Boukai said: “it is just a way for us to come out of stealth mode, to show that thermoelectrics are now a viable source of energy for wearables. There has been a lot of failures in the past, the pain point being that for most wearables you have to replace the batteries or you have to recharge them very often”.

An early PowerWatch prototype with the thermoelectric
unit built-in and all the electronics out on an external
board: since then fully integrated into the watch.

“We wanted to see how the market would respond by showing our technology in the most demanding form factor. But in the long term, we’ll take this technology into other devices, sensors, beacons with energy harvesting” Boukai said, adding that he would certainly be open to licensing it.

The response on Indiegogo has been quite impressive so far, raising over USD 370,000 in less than ten days, with still a month to go.

But would that watch be feasible with off-the-shelf thermoelectric generators from competition? We asked.

“Certainly”, answered Boukai, “but off-the-shelf Bismuth-Telluride type generators are much more expensive, so you may be able to integrate them in one-off consumer products, but they would be too expensive to be adopted for large-scale industrial IoT deployment. Our technology is about twenty times less expensive, and being silicon it can operate at much higher temperatures too”.

Powering the PowerWatch is a 1cm2 holey slab of silicon, 1mm thick. “Of course, there has to be metal on each side, the mechanical design of the device around the thermoelectric unit is an important aspect to ensure a good temperature differential, from the wearer’s skin contact to the outside”, clarified the CEO, adding that at room temperature, the thermoelectric unit could generate between 40 and 50µW/cm2/ºC.

As soon as the Indiegogo campaign is over, the startup plans to discuss other applications with potential customers in the medical and IoT markets. Also on Matrix Industries’ roadmap are differently sized thermoelectric generators, most probably connecting the same little silicon dies in series to scale up on power.

Boukai is also tinkering on reference kits to make thermoelectric units more accessible to makers and others to use.

Visit Matrix Industries at

Check out the PowerWatch on Indiegogo  

Related articles:
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Smart shoes could power wearables from walking
Energy harvesting chips are next billion-dollar market, says report
Flexible power generators promise self-powered wearables
Thoughts on energy harvesting for wearable equipment

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