Wearable devices like medical sensors could also generate energy from thermoelectric cells, which rely on ambient heat. Thermoelectric cells can take the body heat and apply it to electrons, after which it runs through the process of generating energy. The cells can generate 30 to 40 microwatts of power, roughly the same as solar cells, and when placed in a series, can generate more energy, Ramadass said.
"You can think of a wearable kind of system where you are applying a thermoelectric device and the temperature difference between the body and ambient [heat] is going to provide the energy to power a system," Ramadass said.
A lesser practical source for wearables would be piezoelectric, which relies on motion and vibration to generate electricity. Walking or running could power wearables, but its not practical to place cells in shoes, Ramadass said. This technique is more practical in industries like oil and gas, where continuous motion in a pipeline could power up sensors.
Energy could also be harvested through ambient mechanical vibrations such as electromagnetic waves. These are not good for wearables, but more for stable installations in industries or homes where few tens of microwatts of power can be generated on a consistent basis.
Wireless charging is also available via inductive coupling, but it's not "truly wireless," Ramadass said. A medical patch with a transmitter coil on a patch or t-shirt could power up and collect data from a medical implant inside a body. That is similar to RF wireless signals, in which a reader can power up and receive data from an RFID tag.
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