a piston is slidably received in a chamber . the chamber is filled with an expandable medium , such as wax , that expands and contracts significantly as it undergoes a phase change from a solid to a liquid state . a peltier effect chip selectively adds and removes heat energy from the chamber , hence the medium , to cause the medium to change phase . a temperature control maintains the temperature of the medium substantially at the phase change temperature such that the addition of heat causes an isothermal melting and the removal of heat causes an isothermal solidification . the chamber defines a plurality of thin branches such that the heat is transferred more effectively .