Human skin used as conductible material to charge smartwatch

Smartwatches are increasingly able to monitor the vital signs of health, including tracking sleep, but the devices are often disconnected from a user’s body overnight in order to be charged at the bedside.

Researchers at the University of Massachusetts Amherst have used the natural electrical conductivity of human skin to try and solve this problem.

“In this device we have an electrode that couples to the human body, which you could think of as the red wire, if you’re thinking of a traditional battery with a pair of red and black wires,” said computing engineer Jeremy Gummeson.

The conventional black wire is established between two metal plates that are embedded on the wearable device and an instrumented everyday object, which becomes coupled (or virtually connected) via the surrounding environment when the frequency of the energy carrier signal is sufficiently high – in the hundreds of megahertz (MHz) range.

The researchers tested a prototype of their technology with 10 people in three scenarios during which the individuals’ arm or hand made contact with the power transmitter – either as they worked on a desktop keyboard or a laptop, or as they were holding the steering wheel of a car.

Their research showed that approximately 0.5-1 milliwatt (mW) of direct current power was transferred to the wrist-worn device using the skin as the transfer medium. This small amount of electricity conforms to established safety regulations.

“You can think of the amount of power that gets transmitted by our technology as roughly comparable to what’s transmitted through the human body when you stand on a body composition scale, hence poses minimal health risks,” Gummeson said.

There is no sensation to the person who comes into contact with the power transmitter as it is beyond the frequency range that humans can perceive.

The prototype currently doesn’t produce enough power to continuously operate a sophisticated device such as an Apple Watch but could support ultra-low-power fitness trackers like Fitbit Flex.

The researchers aim to improve the power transfer rate in subsequent studies, but also believe smart wearable devices will become more power-efficient as technologies advance.

“We imagine in the future as we further optimise the power that’s consumed by the wearable sensors, we could reduce and ultimately eliminate the charging time,” Gummeson said.

In February, another team developed a mini-wearable device that is capable of charging up using heat from the human body.