Potential health monitors like this one made of interlocking nanofibres are great—it’s flexible and conforms to your body. But what you don’t usually see is how these monitors might be powered. Commercially available power sources are generally bulky and take away the flexibility of the original device. So it’s no wonder that researchers are spending significant time and effort on developing stretchable batteries to power these conformable electronics.
One approach is being developed by researchers at the City College of New York. Abhinav M. Gaikwad and colleagues made a stretchable battery with electrochemically active materials embedded in a conformable conductive fabric. Their work is published online in the July 3, 2012 issue of Advanced Materials.
A conventional battery has two non-conformable conductors (current collector), anode and cathode electrodes with limited flexibility, a separator to prevent electronic contact in between electrodes, electrolyte to provide ionic connection. According to the authors, the key to their stretchable battery is a stretchable current collector capable of maintaining constant electrical conductivity as it’s stretched. But just like conventional alkaline batteries, the new stretchable battery is made of zinc (Zn) and manganese dioxide (MnO2).
A commercial available stretchable silver fabric was embedded with MnO2 particles (cathode piece) and Zn particles (anode piece). The fabric acted as a current collector and support for the electrochemically active MnO2 and Zn particles. The two pieces of embedded fabric (cathode/anode) were placed side by side inside an elastomeric pouch, along with a polyacylic acid based polymer gel electrolyte
The two images on the left give a good look at the fabric: the original silver coated fabric and the different materials in the fabric (embedded MnO2 particles in purple and the silver coating in pink). I think it looks pretty cool because you can see the individual fibres in the fabric but also where the embedded MnO2 particles are. (More about this elemental mapping technique another time…) Click here to see the images of the fabric where the Zn particles are identified.
The battery demonstrated a potential of 1.5 V and a capacity of 3.875 mAh/cm2. The authors identify the battery capacity to be determined by the amount of MnO2 embedded. They showed a red LED could be continuously powered—even when stretched by 150% and twisted by 90 degrees—by connecting two cells in series (see the last photo in the image above).
They also reported that the embedded particles didn’t flake off as the fabric was stretched up to 100%. I think this is an important reason why the battery was able to maintain electrical conductivity (i.e. constant capacity) as it was stretched.
Flexible displays and sensors require significantly more power to operate than an LED bulb, so it might be awhile before these devices are solely powered by stretchable batteries. But this new design brings us closer to being able to power any stretchable electronics with a power source that’s equally as compliant.
Abhinav M. Gaikwad, Alla M. Zamarayeva, Jamesley Rousseau, Howie Chu, Irving Derin, & Daniel A Steingart (2012). Highly Stretchable Alkaline Batteries Based on an Embedded Conductive Fabric Advanced Materials DOI: 10.1002/adma.201201329
Featured image of the stretchable battery developed by Gaikwad et al. (2012) (Source)