Most batteries that power wearable and implantable medical devices are rigid and don’t move with the body. Researchers are developing flexible batteries and energy-storing supercapacitors to overcome that limitation. However, current ones use electrolytes containing strong acids and bases or toxic and flammable organic solutions. If these materials leaked in or on the body, they could cause harm. Researchers in China have now created bendable sodium-ion batteries (SIBs) with biologically friendly aqueous electrolytes. They built batteries with a Na0.44MnO2 cathode and a carbon-coated NaTi2(PO4)3 anode in contact with a sodium-ion solution. The team tested a variety of biologically friendly solutions as battery electrolytes.