Alginate gels cross-linked with mixtures of calcium and chitosan oligomers: Effect on swelling properties and leakage from the gel
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A new alginate gelling system was recently reported where chitosan oligomers were used to cross-link alginate (poly-M) (Khong et al., 2013). These results have been followed up by determining the gel strength and syneresis of two different commercial alginates, said leaf alginate (FM = 0.54) and stipe alginate (FM = 0.32) from Laminaria hyperborea, that were cross-linked with combinations of calcium and a chitosan oligomer mixture (Article in preparation: Alginate Gels with a Combination of Calcium and Chitosan Oligomer Mixtures as Crosslinkers ). The purpose of the present master thesis has been to determine the swelling properties of two alginate gels, the first is a leaf alginate gel cross-linked with a combination of calcium and chitosan oligomer mixture (50/50 mixture), and the second a stipe alginate gel cross-linked with a combination of calcium and chitosan oligomer mixture (75/25 mixture). These two gels where selected because they exhibited a similar gel strength as pure calcium gels without significant syneresis (Article in preparation: Alginate Gels with a Combination of Calcium and Chitosan Oligomer Mixtures as Crosslinkers ). The chitosan oligomer mixture and the two alginates were first characterized with respect to their chemical composition and for the alginate, also their diad and triad sequences and average block lengths. The swelling of the gels was determined at four different pH-values (4.5, 5.5, 7.5 and 8.5) both in buffer solutions and where 100 mM NaCl was added. In order to compare the swelling of the combined gels, the swelling behaviour of pure calcium alginate gels at the four different pH-values were first determined. Both swelling kinetics and swelling after 24 and 48 hours were investigated. At pH 4.5 and 5.5, where alginate and chitosan oligomers are both fully charged, the mixed alginate gels exhibited marginally lower swelling compared to the pure calcium gels in buffer solutions with and without salt. A significant increase in swelling was observed for the mixed alginate gels at higher pH of 7.5 and 8.5, whereas swelling behaviour of calcium alginate gels was similar to what was observed in the acidic conditions. These results suggested that chitosan oligomers contributed to cross-links, both above and below the pKa of chitosan. This was supported by a swelling control study where the calcium content of the alginate gels was reduced to a similar amount of the mixed gels (but without the chitosan oligomers). The swelling kinetic results revealed that the mixed alginate gels swelled faster at pH values above the pKa of chitosan s amino groups (pKa-value of 6.5) compared to at the lower pH values and compared to calcium alginate gels. This more non-Fickian behaviour was most likely attributed to the reduced charge density of chitosan oligomers at the higher pH-values, decreasing the ionic association with alginate which consequently increase the stiffness of the gel as the polymer chain repulsion is increased. In addition, a size exclusion chromatography (SEC) study of the chain lengths of chitosan oligomers leaking out of the gel was performed on leaf alginate combined with a chitosan oligomer mixture. It was found that higher DP chitosan oligomers (DP > 3) diffused slower out of the gel, indicating an increased interaction of longer oligomers within the leaf alginate.