Power supply continues to be the Achilles’ heel of many wearables, but help could be at hand in the form of a promising approach whereby “on-body energy” is harvested using a concept similar to that used in smart cities.
Wearables can be found in almost all areas of life these days, from our daily routines and professional lives to exercise, healthcare and medicine. So it’s hardly surprising that over 500 million of these smart mini companions are expected to have been sold by 2024. However, ever since wearables have entered the scene, they have been dogged by one real sticking point: their power supply. Developers have now given up hope of achieving batteries that are half the size but boast double the output and are hence focusing on energy-saving electronics and wireless technologies that are powered by rechargeable button cells such as Varta CoinPower.
Scientists are of course not satisfied with that. Those not involved in battery research strive to obtain the necessary energy from the environment—known as “energy harvesting”. And almost anything can be an energy source here: sun, heat, movement, radio waves, vibration, noise, and moisture. However, the downside is that you cannot expect high energy densities. And of course, even these solutions also require a form of battery, usually a supercapacitor.
The user’s own body is a potential energy reservoir for electronic devices that are located near to their body or are worn on or even in their body, as it continuously provides around 100 watts of thermal energy alone. In addition, there is also the kinetic energy that can, for example, be “harvested” using piezo elements. However, solutions to date have not been very satisfactory.
Nanoengineers at the University of California San Diego (UCSD) are therefore combining various different complementary sources of energy together in a so-called “wearable microgrid”. It is based on the idea of a smart city’s microgrid which is predominantly powered by wind and solar energy.
The on-body system developed by the scientists in San Diego is, firstly, composed of biofuel cells (BFCs), which produce bioenergy with the help of the enzymatic oxidation of the high lactate concentrations in sweat. The power density reaches several milliwatts per square centimeter (mW/cm2), however the current flow is delayed. At the same time, as and when the individual moves, triboelectric generators (TEGs) on the T-shirt provide energy which is immediately available but is also unstable. The energy storage function is performed by a supercapacitor which regulates both the high-voltage input from the TEG and the low-voltage input from the BFC and continuously supplies the device with power.
The system operates in both a synergistic and a complementary way, meaning that the shortcomings of both generators can be mutually offset. When the user moves, the triboelectric generators immediately produce energy. When the user starts to sweat, the biofuel cells spring into action. They even continue to supply power for a long time after the individual sporting the smart clothing has stopped moving.
The flexible, washable electronic components of the microgrid are printed on a T-shirt and positioned in such a way as to ensure that they recover as much energy as possible. As such, the biofuel cells are, for example, located in the chest area inside the textile. The triboelectric generators, on the other hand, are located on the outside of the T-shirt around the underarms and on the sides around the hips.
In tests consisting of 10-minute exercise sessions (cycling or running) followed by 20 minutes of rest, this system managed to continuously power an LCD wristwatch and a small electrochromic display.
Co-authors include Kyeong Nam Kim, Jian Lv, Farshad Tehrani, Muyang Lin, Zuzeng Lin, Jong-Min Moon, Jessica Ma, Jialu Yu and Sheng Xu.