Gels are a special type of material that behave like a solid yet by mass and volume are mostly liquid. The solid forms a network that traps the molecules of liquid and prevents the bulk from assuming the shape of its container as a liquid would. The special properties of gels make them attractive for a number of high-tech applications. Water-based hydrogels, for example, are being widely investigated for tissue engineering applications.

Fig. 1: Cross-linking (left) and dethreading (right) in a polyrotaxane mechanically bonded gel© 2011 Wiley-VCH

The formation of gels can be achieved in a variety of ways. One of the most common is to use a cross-linking polymer, which develops strong inter-polymer covalent bonds. Alternatively, gels can be formed using small molecules that bind together through multiple weak intramolecular interactions such as hydrogen bonds. Yet another approach is to form cross-links with mechanical bonds, where molecules are threaded together and dumbbell-shaped cross-links form between chains containing many of these threaded macrocycle rings (see image).

A common goal of gel research is to produce a gel that can be switched between the gel and solution states in response to a simple stimulus. Hydrogen-bonded gels are often only stable under a limited range of conditions and are therefore widely used in such sol–gel applications. Breaking the cross-links in a mechanically bonded gel, however, is much more difficult. Toshikazu Takata and co-workers from the Tokyo Institute of Technology in Japan have now designed mechanically cross-linked gels in which the cross-links can be easily dethreaded1.

In the polyrotoxane system reported by Takata and his colleagues, the dumbbell-shaped unit contains an ammonium salt that is hydrogen-bonded with a polyether macrocycle. “We designed new dumbbell units where the size of one end-group closely matches the size of the macrocycle,” explained Takata. “In this way, if we weaken the interaction between the dumbbell unit and the macrocycle, we can break the cross-links because the end group can now slip through the ring.” Several methods for weakening the interaction and breaking the gel down into solution are possible: the ammonium salt could be deprotonated, a polar solvent could be added to weaken the hydrogen bonds, or a different ammonium salt that competes for binding with the macrocycle could be added.