Strain stiffening and negative normal stress in alginate hydrogels

Abstract

Alginate hydrogels are polysaccharide biopolymer networks widely useful in biomedical and food applications. Here, we report nonlinear mechanical responses of ionically crosslinked alginate hydrogels captured using large amplitude oscillatory shear experiments. Gelation was performed in situ in a rheometer and the rheological investigations on these samples captured the strain-stiffening behavior for these gels as a function of oscillatory strain. In addition, negative normal stress was observed, which has not been reported earlier for any polysaccharide networks. The magnitude of negative normal stress increases with the applied strain amplitude and can exceed that of the shear stress at large-strain. Fitting a constitutive relationship to the stress-strain curves reveals that the mode of deformation involves stretching of the alginate chains and bending of both the chains and the junction zones. The contribution of bending increases near saturation of G blocks as Ca 21 concentration was increased. The results presented here provide an improved understanding of the deformation behavior of alginate hydrogels and such understanding can be extended to other crosslinked polysaccharide networks. V C 2016

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“…In general, CNDs-alginate hydrogels display strain stiffening behavior, similar to that observed for ionically crosslinked gels. 30 The strain stiffening behavior was more pronounced for the gels with the lowest modulus. For example, G 0 in m-CNDsalginate gel increased more than two times before the material failed in shear-mode.…”

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“…Since the CNDs can have multiple amine groups, those can theoretically associate with multiple chains. However, the size of CNDs ($10 nm) and the persistence length of alginate chains ($15 nm) 30 will dictate the number of chains that can link with a single CND. Because of large persistence length, it is unlikely that the alginate chains wrap around the CNDs.…”