Fig. 1: A light-responsive hydrogel is formed by multiple crosslinks between two polymers — an azobenzene (orange) and a cyclodextrin (blue).© 2010 Wiley-VCH*

A gel forms when molecules dissolved in a solvent become crosslinked in a way that traps all of the solvent molecules within a robust molecular framework. While the majority of the mixture remains liquid, the system overall behaves like a solid and does not flow. When the solvent in question is water, the material is called a hydrogel — a type of material that is in demand for drug delivery and tissue-engineering applications. Hydrogels that can change their properties in response to an external stimulus such as light are of particular interest as they could be used as carriers that can be made to release their cargo on demand.

Akira Harada and co-workers from Osaka University and the Institute of Systems, Information Technologies and Nanotechnologies in Japan have now described the preparation of a light-responsive hydrogel that can be induced to gel or liquefy depending on the wavelength of light to which it is exposed.1 Their hydrogel is produced by host–guest crosslinks formed between two polymers — a rigid sugar-derived polymer with multiple macrocyclic cyclodextrin units attached to the backbone, and a second polymer containing multiple azobenzene units.

Azobenzenes contain a nitrogen–nitrogen double bond and the shape of the molecule means that, under normal conditions, two forms (isomers) of the molecule exist. It is possible to switch the shape of the molecule between the two isomers by irradiating it with ultraviolet or visible light. The researchers exploited the fact that only one isomer, trans-azobenzene, is a good guest molecule for the cyclodextrin host. “We wanted to use this switching effect to prepare responsive hydrogels,” explains Harada.

When irradiated with ultraviolet let, the trans-azobenzene unit can be converted into the other isomer, cis-azobenzene, which does not interact as well with the cyclodextrin host, causing the crosslinks to be broken and the gel to fall apart. This isomerization can be reversed, by the action of either heat or visible light, to restore the gel properties of the system.

“The key to the design was the use of two polymers,” says Harada. “When we used a cyclodextrin containing a polymer and an azobenzene containing a dimer to form crosslinks, we observed a change in viscosity, but not switchable gel formation.”