The structural characterisation of high water content materials such as hydrogels is challenging due to the inherent properties of water that are dependent on conditions of temperature and pressure. Cryo-scanning electron microscopy (cryo-SEM) has potential to offer the most authentic insights into the native structure of polysaccharide hydrogels, due to the ability to freeze the sample under conditions where transparent vitreous ice is formed. The advantages, challenges and constraints of cryo-SEM in characterising the porous matrix of alginate hydrogels are considered in this tutorial paper through examples from our own studies. Recommendations are proposed for the minimum methodological information that is required from researchers as part of the publication of their studies of hydrogel samples to enable critical appraisal of their cryo-SEM images.

Biopolymers are researched extensively for their applications in biomaterials science and drug delivery including structures and complexes of more than one polymer. Chemical characterization of complexes formed between chitosan (CHI) and alginate dialdehyde (ADA) biopolymers established that while electrostatic interactions dominate (as determined from X-ray photoelectron spectroscopy (XPS)) covalent cross-linking between these biopolymers also contribute to their stability (evidenced from immersion in salt solution). It was furthermore found that imine bond formation could not be directly detected by any of the techniques XPS, FTIR, (1)H NMR, or fluorescence. The layer-by-layer assemblies of the biopolymers formed on silica colloids, glass slides, and alginate hydrogel beads were evaluated using XPS, as well as zeta potential measurements for the silica colloids and changes to hydration properties for the hydrogels. It was found that the degree of oxidation of ADA affected the LbL assemblies in terms of a greater degree of CHI penetration observed when using the more conformationally flexible biopolymer ADA (higher degree of oxidation).

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