A group of researchers at Binghamton University (New York) has been developing paper fuel cells for several years, where special bacteria perform the function of an electrolyte. After the original prototype of the foldable battery, a more powerful version was created - in the form of a ninja star. The team is currently looking for a practical paper sheet shape that can be folded in a variety of configurations.
In the last version of the device, instead of a deposited nickel cathode, a new original design was proposed - with a silver nitrate gasket under a thin layer of wax. Bacterial water placed inside a conductive polymer reservoir acts as an anode. The anode and cathode are placed on opposite halves of the paper sheet. When they come into contact, as a result of cellular respiration, microbes begin to produce electricity.
By folding such sheets in different configurations, you can get different power and current. So, for example, 6 leaf batteries (option 2 x 3) give out 31.51 μW / 125.53 μA, and already with the option 6 x 6 - 44.85 μW / 105.89 μA. Of course, this is not enough for a smartphone to work, but for low-power biosensors that determine blood sugar levels, it is quite enough.