New wearable devices are coming out almost every week, but many of them have to be charged, or run on replicable batteries—a 20th century problem for 21st century technology. Now German researchers have come up with small devices that lie buried in the heel of your shoe that generate energy as you walk, and could be used to power your wearables. The miniature power plants generate energy two ways: When your foot hits the ground one device gets energy through the shock, and the second device captures the energy generated by your foot swinging. The technology could also be the foundation to develop a self-tying shoe—now that’s futuristic.