Some like working with clay, some like carving from stone, others etch out of silicon, but the team at CENIMAT apparently really likes plain 'ol paper. The researchers there proved they could print transistors on the stuff back in 2008, and now they're making paper batteries too. But that's nothing new, others have made mache cells for years. What's exciting here is that these batts charge with water, and they don't need very much of the stuff to juice up.
