Imagine being able to generate electricity, just by walking along.
German scientists have made two energy-collecting devices tiny enough to put in the soles of shoes.
One device collects energy when your heel hits the floor, while the second works as the foot is swinging; so every step you take generates power.
At the moment they can only generate a very small amount of power: enough for tiny gadgets. Hundreds of times more would be needed to charge a smartphone.
The people behind the tech hope to be able to use the devices to create shoes that tie their own laces, to help older people who find tying shoelaces difficult.
The invention was published in the journal Smart Materials and Structures.