Smart watches suffer from the small volume available for energy storage. Xingbin Yan, Chinese Academy of Sciences, Lanzhou, China, and colleagues therefore designed a watchband that can serve as energy storage device.

They fabricated a flexible asymmetric supercapacitor using reduced graphene oxide (rGO) coated on TiNi alloy (TNA) flake as negative electrode, MnO₂ deposited on an ultra-thin Ni foil  as positive electrode, and a gel electrolyte as separator. TNA is an shape memory alloy, which remembers a presupposed shape at high temperatures, is free to bend, and can hold the corresponding bending shape below its phase transition temperature (PTT). It restores to the presupposed shape when temperature is higher than the PTT.

As the PTT of TNA used is 15 °C, the shape recovery process could be initiated by the human skin temperature. Hence, the watchband wraps around the human wrist as it touches the warm human skin, enabling to adopt the fixation function.

The watchband was successfully connected to a real electronic watch and shows good electrochemical performance reliability during bending, which was verified by statical and dynamical bending tests.

• Watchband-Like Supercapacitors with Body Temperature Inducible Shape Memory Ability,
  Lingyang Liu, Baoshou Shen, Dan Jiang, Ruisheng Guo, Lingbin Kong, Xingbin Yan,
  Adv. Energy Mater. 2016.
  DOI: 10.1002/aenm.201600763