Scientists at Michigan State University have developed a flexible, film-like material that generates electrical energy when touched, meaning devices like smartphones and tablets could one day be powered simply by people using them. The researchers say the thin, flexible device could also be used in our clothing or shoes, helping us harvest energy from our body movements potentially all day long. The film, the researchers have created is known as a nanogenerator, in which energy is produced by a small-scale physical change, such as the tap or swipe of a finger. In this case, the device works on the principle of piezoelectricity, where an electric charge accumulates in response to applied mechanical stress. What makes this possible is the interaction between the substances that make up the film. The core structure is a silicon wafer, which is then layered with thin sheets of other materials, including silver, polyimide, and polypropylene ferroelectret, which serves as the active material in the device. Polypropylene ferroelectret is a thin polymer foam that contains charged particles. When pressure is applied to the device, the foam layer compresses, creating a change in what is called dipole moments – an interaction between positive and negatively charged molecules in the ferroelectret. This in turn generates an electric charge. In testing, a hand-sized sheet of the material was able to generate about 50 volts.