Alginate is a natural biopolymer that forms, in the presence of divalent cations, ionic-bound gels typifying a large class of biological gels stabilized by non–covalent cross-links, and displaying a consistent restructuring kinetics. Alginate has several unique properties that have enabled it to be used as a matrix for the entrapment and/or delivery of a variety of biological agents (e.g. proteins, cells and DNA). A full comprehension of its gelation mechanism is therefore desirable to control the structure and the release kinetics from its gels.
In this work, the kinetics of formation of alginate gels has been investigate by means of large deformation rheology. The stress/strain behavior can be explained by assuming that the cross-links between guluronic sequences form at the early stages of the gelation process and are weakly affected both in number and strength by gel ripening, whereas additional links due to guluronic-mannuronic sequences increases in number and strength with aging and therefore enhance the overall stiffness of the gel.
The presence of a monovolent electrolite such as sodium chloride 0.1M does not impact on the strongly stereospecific bounds provided by the guluronic sequences. The Na+ cations favorably compete with calcium in binding to the guluronic-mannuronic sites, strongly hindering their liability to form cross-links.
Stress relaxation test always shows an asymptotically decay to zero, implying a fully plastic behavior for all the tested samples. At long delay time, the normalized stress displays a simple exponential decay. At short time, however, the stress relaxation curve is significantly non-exponential, thus proving that very fast relaxation mechanisms contribute to σ(τ) too.
A slow creeping behavior of the gel has been observed after a first rapid stress relaxation, which can be attributed to the concurrence of many superpositions of local relaxation modes yielding an overall SE decay and which is consistent with the occurrence of intermittent global rearrangements.
