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

• Young’s modulus E of Ca alginate-oligoguluronate gels depend on c of oligoguluronate.
• Effects of oligoguluronate on E of Ca-alginate gels depend on alginate chain length.
• Oligoguluronates in Ca-alginate gels perturb the network junction functionality.

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 20 mM total Ca2+ 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.