Figure 1. Phase transition behaviour of stimuli-responsive hydrogels.

The swollen phase of the gel (left) is dominated by polymer-solvent interactions obtaining the best mixing of the polymer chains and the aqueous solution. The shrunken phase of the hydrogel (right) is determined by polymer-polymer-interactions, which remove solution out of the gel. Near the phase interface, within the range of phase transition, small alterations of a thermodynamic value result in a change of the phase of the hydrogel.