The key to the technology is a hyper-elastic polymer material made from isoprene, one of the main ingredients in rubber, that was combined with polystyrene to form a material called SIS. Combined with zinc oxide, this allows the batteries to stretch to twice their size, in any direction, without suffering damage.
The ink used to print the batteries is made of the zinc silver oxide mixed with SIS, and adding bismuth oxide to the batteries to make them rechargeable.
"This is a significant step toward self-powered stretchable electronics," said Joseph Wang, nanoengineering professor at the Jacobs School of Engineering at UC San Diego, where he directs the school's Centre for Wearable Sensors. "We expect this technology to pave the way to enhance other forms of energy storage and printable, stretchable electronics, not just for zinc-based batteries but also for Lithium-ion batteries, as well as supercapacitors and photovoltaic cells."
Adding the bismuth oxide was demonstrated by the same team last year. "Understanding the scientific mechanism to do this will allow us to turn non-rechargeable batteries into rechargeable batteries--not just zinc batteries but also for other electro-chemistries, such as Lithium-oxygen," said Professor Y. Shirley Meng, who directs the Sustainable Power and Energy Centre at the UC San Diego Jacobs School of Engineering
The researchers used standard screen printing techniques to make the batteries--a method that dramatically drives down the costs of the technology to just 50c. The batteries can be printed directly on fabric or on materials that allow wearables to adhere to the skin and can be printed as a strip, to power a device that needs more energy.
The prototype battery the researchers developed has about 20% of the capacity of a rechargeable hearing aid battery and takes two cells to power a 3V LED. But it is one tenth the thickness and uses commercially available materials.
The next stage is to improve the battery's performance for other applications.