Researchers are developing batteries and energy-storing supercapacitors that flex with the body while powering wearable and implantable medical devices. But so far these power sources have used electrolytes containing strong acids, strong bases, or toxic and flammable organic solutions, which can cause harm if they leak. Yonggang Wang, Huisheng Peng, and coworkers at Fudan University have now created bendable sodium-ion batteries containing biofriendly aqueous electrolytes (Chem 2017, DOI: 10.1016/j.chempr.2017.05.004). The batteries have a Na0.44MnO2 cathode and a carbon-coated NaTi2(PO4)3 anode in contact with one of three sodium-based electrolytes: a sodium sulfate solution, intravenous saline, or a cell culture medium. In a belt-shaped design, the cathode and anode sandwich an electrolyte-soaked separator. And a fiber-shaped design has carbon nanotube electrodes embedded with cathode and anode nanomaterials and surrounded by electrolyte. The batteries have charge capacities and power outputs per unit volume comparable to those of previously reported flexible lithium-ion batteries and supercapacitors. But for now their power output per unit mass and maximum operating voltage are too low to be commercially practical. In future work, the researchers hope to improve the batteries by using higher capacity electrode materials and to test their ability to energize actual medical devices.