Power up with body heat

A thermoelectric generator that converts body heat into electricity could make replacing or recharging batteries in wearable electronics a task of the past.

As the electronics market continues to expand there is a growing need for new ways to charge devices like smart watches and wearable medical sensors. However, conventional organic-based thermoelectric (TE) generators do not produce a high enough power output for use in wearable devices. And previously reported inorganic-based systems tend to use bulky, rigid and heavy ceramic substrates that increase thermal energy loss and limit their power output and energy conversion efficiency. The TE generator developed by Byung Jin Cho and his team at the Korea Advanced Institute of Science and Technology uses a glass fabric that is thinner, lighter and more flexible than other devices reported to date.

To make the device, thick films of n-type (Bi₂Fe­₃) and p-type (Sb₂Te₃) materials are screen-printed onto a glass fabric to give dots 1.5 mm in diameter and ~500 µm thick. In the process, liquid polydimethylsiloxane  (PDMS) infiltrates the glass fibres supporting the films to form a self-sustaining structure. By not needing an external substrate the problem of serious thermal energy loss affecting conventional TE generators significantly reduces.

Cho says his team expects ‘wearable, flexible thermoelectric generators to be the next big thing in the electronics market.’ As a wearable device theirs has the advantage of being lightweight (~0.13g/cm²) and very flexible. Tests showed an allowable bending radius of 20mm and no change in performance on repeated bending allowing the generator to work on a curved thermal energy source, such as a human arm. On skin a 10×10cm generator needs a skin/air temperature difference of 10K to charge a device powered using a semiconductor chip.

Albert Tarancón, head of the Nanoionics and Fuel Cells Group at the Catalonia Institute for Energy Research in Spain, is positive about the new techniques developed in this work: ‘This simple and scalable fabrication process should open new avenues for the final commercialisation of wearable electronics with multiple functionalities.’ Cho also sees potential for application of the screen-printing technique to larger-scale energy harvesting systems, such as flexible solar cells and piezoelectric technology.