text: Many mechanically interlocked molecules have been studied to construct molecular machines. Because the molecules are linked by mechanical bonds instead of conventional covalent bonds, a component can move (shuttle) or rotate around the other parent component, which results in the large amount of freedom of mechanically interlocked molecules. Polyrotaxanes, as the polymer form of corresponding rotaxanes, are also applied in molecular machines.
  For example, the shuttling behavior of the molecular shuttle can be controlled by the solvent or temperature. Due to the hydrophobic interaction between rings and strings, and the repulsive interaction between rings and linkers, conditions that are capable of influencing these interactions can be used to control the mobility of the rings in the molecular shuttle. In addition, if cationic or anionic units are employed to form the polyrotaxanes, the salts or pH in the solution also will influence the charge interactions between rings and strings, which is an alternative method to control the ring motion of the molecular shuttle.
  Poly[2]rotaxane "daisy chains" (like a string of daisies with stems linked to form a chain)is an example of a molecule that can be used to form a molecular muscle. Poly[2]rotaxane can expand or shrink in response to external stimulus, which is similar to behaviors of muscle, an ideal model to construct a "molecular muscle". The ring stations on the chain can be controlled by pH or light. Due to "daisy chain" structure, two rings on the daisy chain will shift from one station to another station, which changes the distance between two rings as well as the state of the whole daisy chain. When the rings come close, the whole size of the daisy chain will increase, which is the "expand" state. As the rings get to the further station, the molecule become the "shrink" state as the decreased size.