Although most materials slightly expand when heated, there is a new class of rubber-like material that not only self-stretches upon cooling - it reverts back to its original shape when heated, all without physical manipulation. The material is like a shape-memory polymer because it can be switched between two different shapes. “However, unlike other shape-memory polymers, the material does not need to be programmed each cycle - it repeatedly switches shapes, with no external forces, simply upon cooling and heating,” says Mitchell Anthamatten, an associate professor of chemical engineering at the University of Rochester. Anthamatten and his team built on the success of a recently developed polymer that can also stretch when cooled. The other polymers need to have small loads - or weights - attached in order to direct the shape to be taken. That is not the case with the new polymer, because Anthamatten’s team “tricked it into thinking” a load was attached. The team envisions the material being applied to a number of areas in which reversible shape-changes are needed during operations, including biotechnology, artificial muscles, and robotics.