Local structure of Ca(2+) induced hydrogels of alginate-oligoguluronate blends determined by small-angle-X-ray scattering.

Short oligoguluronates, oligoG's, are reported to affect the ionotropic gelation of alginates both with respect to altered gelation kinetics and elastic properties of the resulting gels. The local structure of Ca(2+) induced changes in oligoguluronates and blends of oligoguluronates and alginates was determined by small angle X-ray scattering (SAXS). Calcium was introduced in the aqueous polysaccharide solutions by in situ release of Ca(2+) from Ca-EGTA. The scattering profiles of the Ca(2+)-induced structures in the alginate-oligoG blends were accounted for by a two-component broken rod-like model, also with an additional term representing structural inhomogeneity by a Debye-Bueche term. Adding oligoG to the alginate yields an increase in the largest cross-sectional radius in the region of fractional Ca(2+) saturation of α-l-GulA units from 0.5 to 1. The time-lapse characterization during the Ca-induced changes in the alginate-oligoG blends shows that oligoG delays the emergence of the more extensive laterally aggregated junction zones.

Effects of added oligoguluronate on mechanical properties of Ca - alginate - oligoguluronate hydrogels depend on chain length of the alginate.

The effect of adding shorter alginate fragments highly enriched in α-l-guluronic acid (oligoG) on the Young's modulus of the Ca-induced alginate hydrogels were determined using nanoindentation. Ca-alginate gels using two low and one high molecular weight alginate, with increasing amount of added oligoG, were prepared at constant 20mM total Ca(2+) by in situ release of the cation. Differences in the effect on the mechanical properties of increasing amount of oligoG to the various alginate samples were attributed to their different capability to support network connectivity by junction zone formation. Upon decreasing the fractional Ca-saturation of all the α-l-guluronic acid residues (G) present, Fsat, by increasing the oligoG concentration, the lower molecular weight alginates displayed the largest reduction in Young's modulus. This was suggested to be due to the few sequences of α-l-guluronic acid residues making up potential zones engaging in network connectivity of this alginate. Similar trends were observed for a low molecular weight alginate with larger fraction of G. The higher molecular weight sample displayed less reduction of Young's modulus associated with increasing concentration of oligoG. The consequences of reduction in effective, mean junction zone functionality and associated increase in sol fraction with added oligoG on the elastic properties thus depend on the chain length of the alginates. These finding suggest that effects of added oligoG on Ca-induced alginate gelation should connect the effect on junction zone formation to those mediating network connectivity.

Delaying cluster growth of ionotropic induced alginate gelation by oligoguluronate.

Alginates form gels in the presence of various divalent ions, such as Ca(2+) that mediate lateral association of chain segments. Various procedures exist that introduce Ca(2+) to yield alginate hydrogels with overall homogeneous or controlled gradients in the concentration profiles. In the present study, the effect of adding oligomers of α-l-guluronic acid (oligoGs) to gelling solutions of alginate was investigated by determination of the cluster growth stimulated by in situ release of Ca(2+). Three different alginate samples varying in fraction of α-l-guluronic acid and molecular weights were employed. The cluster growth was determined for both pure alginates and alginates with two different concentrations of the oligoGs employing dynamic light scattering. The results show that addition of oligoG slows down the cluster growth, the more efficient for the alginates with higher fraction of α-l-guluronic acid, and the higher molecular weight. The efficiency in delaying and slowing the cluster growth induced by added oligoG were discussed in view of the molecular parameters of the alginates. These results show that oligoG can be added to alginate solutions to control the cluster growth and eventually also transition to the gel state. Quantitative relation between the concentration of added oligoG, type and molecular weight of the alginate, and concentration, can be employed as guidelines in tuning alginate cluster growth with specific properties.

Improved immune responses in mice using the novel chitosan adjuvant ViscoGel, with a Haemophilus influenzae type b glycoconjugate vaccine.

An immune response to an antigen is more efficiently induced in combination with an adjuvant. Chitosan has due to documented immunostimulatory characteristics been proposed as an adjuvant candidate. However, a disadvantage with chitosan is its poor solubility at physiological pH. We have circumvented this obstacle by using a soluble type of chitosan (Viscosan), with a degree of deacetylation (DD) of 50% and a random distribution of acetyl groups. A hydrogel, ViscoGel, was made from Viscosan which was further mechanically processed into gel particles of predefined size. The first cells to infiltrate ViscoGel in mice, were identified mainly as neutrophils, detected already after 4 h. ViscoGel's impact on the immune response in mice together with a commercial vaccine against Haemophilus influenzae type b (Act-HIB) was then studied. Mixing Act-HIB with ViscoGel, induced significantly enhanced IgG1 and IgG2a titers in serum (p<0.05). We could reduce the antigen dose ten-fold in combination with ViscoGel and still obtain antibody titers similar to 2 µg Act-HIB administered alone. In addition, the Act-HIB specific cellular response was stronger in mice vaccinated together with ViscoGel (p<0.05). The cytokine response after vaccination with Act-Hib together with ViscoGel was of a mixed type. We found elevated levels of the Th1 associated cytokine INF-γ, the Th2-cytokine IL-4, the proinflammatory IL-6 and IL-17A, and the regulatory cytokine IL-10. Similar effects were seen when the adjuvant was administered either subcutaneously or intramuscularly. Taken together, using vaccination against H. influenzae type b as a model, we here show proof of concept for the novel vaccine adjuvant candidate, ViscoGel.

Small-angle X-ray scattering and rheological characterization of alginate gels. 3. Alginic acid gels.

Alginic acid gels were studied by small-angle X-ray scattering and rheology to elucidate the influence of alginate chemical composition and molecular weight on the gel elasticity and molecular structure. The alginic acid gels were prepared by homogeneous pH reduction throughout the sample. Three alginates with different chemical composition and sequence, and two to three different molecular weights of each sample were examined. Three alginate samples with fractions of guluronic acid residues of 0.39 (LoG), 0.50 (InG), and 0.68 (HiG), covering the range of commercially available alginates, were employed. The excess scattering intensity I of the alginic acid gels was about 1 order of magnitude larger and exhibited a stronger curvature toward low q compared to ionically cross-linked alginate. The I(q) were decomposed into two components by assuming that the alginic acid gel is composed of aggregated multiple junctions and single chains. Time-resolved experiments showed a large increase in the average size of aggregates and their weight fraction within the first 2 h after onset of gelling, which also coincides with the most pronounced rheological changes. At equilibrium, little or no effect of molecular weight was observed, whereas at comparable molecular weights, an increased scattering intensity with increasing content of guluronic acid residues was recorded, probably because of a larger apparent molecular mass of domains. The results suggest a quasi-ordered junction zone is formed in the initial stage, followed by subsequent assembling of such zones, forming domains in the order of 50 A. The average length of the initial junction zones, being governed by the relative fraction of stabilizing G-blocks and destabilizing alternating (MG) blocks, determines the density of the final random aggregates. Hence, high-G alginates give alginic acid gels of a higher aggregate density compared to domains composed of loosely packed shorter junction zones in InG or LoG system.