Biodegradable electronics that dissolve harmlessly after a certain number of days, weeks, or months could be quite useful. The main application would be with biomedical implants that need to go in the body to monitor things for a while, but that don't need to stay permanently. Rather than remove them with more surgery, they could simple slowly be reabsorbed by the body. They could also be used to monitor the environment - like ocean conditions after an oil spill - without leaving e-waste behind.
We're not there yet, though. But the pieces of the puzzle are slowly falling into place. A big one just did when researchers revealed a working battery that is made entirely of biocompatible materials that dissolve in water after about 3 weeks.
The battery is quite small: A one-square-centimetre cell with a 50-micrometre-thick anode and an 8-micrometre-thick cathode produced 2.4 milliamps of current. The anodes are made of magnesium foil and the cathodes of iron, molybdenum or tungsten. The electrolyte 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 — roughly twice as much as a magnesium coronary artery stent that has been successfully tested in clinical trials, and a concentration that is unlikely to cause problems in the body, says [John Rogers at the University of Illinois at Urbana-Champaign].
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.
If these can be made cheaply and reliably, they could be used to track oil spills and the water concentration of various chemicals. Just drop thousands of them in the oil and track them wirelessly. After their work is done, they would all just dissolve harmlessly.