Researchers have been working to come out with biodegradable electronics that dissolve harmlessly after a certain period. The main application would be in medical implants. Currently, implant needs to be surgically removed after it is done or when it needs a new battery. Rather than surgery, a biodegradable battery couples with biodegradable implant device will simply melt away once they are done.
Of course, we are not there yet. But the pieces of the puzzle are slowly falling into place. Recently, researchers revealed a working battery that is made entirely of bio-compatible materials that dissolve in water after about 3 weeks.
The battery is quite small. A 1 square cm cell is able to generate 2.4mAh of current. The anodes are made of magnesium foil and the cathodes of iron, molybdenum or tungsten. The electrolyte (like the acid-water inside our car battery) between the two electrodes is a phosphate-buffered saline solution. All of this is packaged in a biodegradable polymer called polyanhydride.
Once dissolved, the battery releases less than 9 milligrams of magnesium which researchers believe it is unlikely to cause problems in the body.
Right now the lab versions of these batteries can maintain voltage for about a day, and the researchers are working on improving power-density to make the batteries even smaller.