Figure 2.

(A) A schematic illustration of the reversible swelling of a 3-dimensional cross-linked hydrogel network. Upon a change in temperature or pH, the water molecules embedded in the swollen hydrogel on the left are expelled out of the gel and the networks collapse. (B) A schematic illustration of the non-covalent physical cross-linking between poly(ethylene glycol) and poly(methacrylate) in a solution: as the acidity of the solution increases, hydrogen bonding occurs between the oxyethylene groups of one polymer and the protonated methacrylic acids of the other polymer, leading to gel formation.