Figure 6.

Loading and release of biomacromolecules in a cross-linked polymer hydrogel. A cross-linked gel composed of nonionic PEG and anionic PAA polymer chains is immersed in a solution of a cationic protein cytochrome C. The initial solution has a red color is due to the presence of the cytochrome C. The cytochrome C is spontaneously loaded in a gel due to a polyion complex formation between the protein and PAA chains. The gel collapses and acquires red color of cytochrome C while the external solution becomes clear. Acidification or addition of Ca²⁺ ion results in the protein release due to either deprotonation of the carboxylic groups of the PAA (acidification) or competitive binding of Ca²⁺ ions with the carboxylic groups of the PAA. In both cases the external solution acquired the red color of cytochrome C, while the gel further collapses.