They've used a thin film made of a layer of polymer and another of aluminum. Both layers have tiny structures etched on them at the nanometer scale. When the plastic and the metal come in contact with each other, they accumulate a static electric charge. Flexing them generates a current.
The etched nanostructures increase the surface area, which gives electrons a lot more room to gather and boosts the charge accumulated. The efficiency with which the material turns the mechanical motion of flexing into electricity can go as high as 40 percent, according to the paper.
Wang has done similar work before: in 2009 he demonstrated that a hamster
could wear a jacket that generated power in a similar way.