Akira Harada and colleagues at Osaka University have constructed a rotaxane with a two-station system; consisting of an -cyclodextrin (CD) and an axle molecule (that has two decamethylene units) linked by a 2-methylpyridinium group. 

Much attention has been focused on the development of such molecular machines tomimic what we observe in living systems, such as muscle expansion and contraction. Molecular machines are interlocking molecular components designed to perform mechanical-like movements in response to specific stimuli. 

Harada incorporated the methylpyridinium group into the axle to enable the rate and direction of the CD ring to be regulated. Harada and his team chose CDs as the ring component in the rotaxanes because they are rigid, well-defined nonsymmetric ring structure, with a hydrophobic cavity. In this case the CD ring shows face selectivity and in addition to this the translational rate from the first station to the second station (using the wider rim of the CD) is much faster than the narrower part.

Harada believes that it will be possible to develop future materials that can be moved by external stimuli, such as light, temperature and chemical energy. Although the continuing challenge for this area of research will be to control the speed and direction of translational and rotational movement.

Emma Shiells