Small-Angle X-ray Scattering and Rheological Characterization of Alginate Gels. 1. Ca−Alginate Gels

Abstract

Ca-alginate gels were studied by small-angle X-ray scattering and rheology to determine relations between chemical composition and concentrations of the alginate and the elasticity and structure of the gels. The gels were prepared by in situ release of Ca 2+ from either Ca-EGTA or CaCO3 with total Ca 2+ concentration in the range 5-30 mM. Alginates with low (39%), intermediate (50%), and high (68%) fractions of R-L-GulA originating from the brown algae Ascophyllum nodosum, Laminaria hyperborea leaf, and Laminaria hyperborea stipe, respectively, were employed. Two to three different degrees of polymerization for each chemical composition were used in the experiments. The excess small-angle X-ray scattering for the alginates in solution yielded linear cross-sectional Guinier plots, and the cross-sectional radius of gyration, R g,c, was determined to be 3.1-4.6 Å. The SAXS profiles of the alginate gels depended on the alginate concentration, Ca 2+ concentration, and the alginate composition. The SAXS data suggested that dimerization of chain segments was the principal association mode at low fractional Ca 2+ saturation of guluronic acid of the alginate. Increasing the fractional Ca 2+ saturation of guluronic acid, either by the concentrations or selection of alginate source, yielded coexisting lateral association modes, as manifested in a curvature in the cross-sectional plots. The coexisting junction zone multiplicities occur because of a delicate balance between the block length distribution of the R-L-GulA residues, polymer concentration, and Ca 2+ . These results are quantitative extensions of the "egg-box" model used to describe ionotropic gelation of alginate and hence enhance the understanding of the structure-function relationship of alginate gels.

Drying and Rehydration of Calcium Alginate Gels

“…Lateral association of dimers has been shown to occur in alginate gels at high calcium concentration (i.e. high calcium to guluronic acid ratio) 16,17 . Calcium concentration strongly increases during dehydration of alginate gel beads, especially in the later stage of the drying process.…”

Controlling Degradation of Hydrogels via the Size of Crosslinked Junctions

“…This is likely due to the stiffer and poorly packed polymer chains that are present in binary MVG gels. [12,13] Incubating the binary MVG gels for two weeks led to limited changes in the freezing point of water (Fig. 2a), while binary MVM gels exhibited a significant increase in the freezing point, which represented an increase in the average pore diameter from 6 to 9 nm (Fig.…”

Swelling of a hemi-ellipsoidal ionic hydrogel for determination of material properties of deposited thin polymer films: an inverse finite element approach

“…Thus, experimental determination of the reduction of swelling due to the polymer coating and coating thickness allow determination of the deposited film's mechanical properties. This strategy is realized using a hemi-ellipsoidal geometry that is both coinciding with the experimental proof of concept realization 11 , and that ensures nearly uniform deformation of the polymer coating. This approach is realized by interferometric swelling measurements of hemi-ellipsoidal hydrogels as a function of ionic strengths before and after the deposition of the polymer layer coating, see Figure 1.…”