Wheel-shaped robots, which are just 4 centimetres in diameter and 1 centimetre thick, were built by Shinichi Hirai and Yuuta Sugiyama at Ritsumeikan University in Kusatsu. The robots propel themselves along by continuously altering their shape.
The rim of the wheel is made of an elastic polymer, while the spokes are made of a smart material known as a shape memory alloy, which becomes shorter when heated.
Shortening the spokes towards the front of the wheel changes its shape, causing the rim’s point of contact with the ground to move backwards, behind the centre of gravity. As the wheel then tips forwards, other spokes are heated to deform the rim again and keep the robot rolling along.
Shape memory alloys store energy in the form of stresses in their crystalline structure. Passing an electric current through the spokes heats them up, releasing this energy and making them shorter. Allowing them to cool then returns the spokes to their original state.