Batteries are getting smaller, lighter and more powerful all the time. This is good news for manufacturers and buyers of products ranging from electric cars to mobile phones and fitness trackers. But for some applications the conventional shape and structure of a battery, with a rigid form and metallic components, is simply too clunky to be of use. Some personal electronics, for example, such as skin patches that monitor health conditions or brain-computer implants that decode neural signals to control electronic prosthetic devices, require more intimate contact.