Inhomogeneous alginate gel spheres: An assessment of the polymer gradients by synchrotron radiation-induced X-ray emission, magnetic resonance microimaging, and mathematical modeling

It has been previously demonstrated that calcium alginate gels prepared by dialysis often exhibit a concentration inhomogeneity being the polymer concentration considerably lower in the centre of the gel than at the edges. Inhomogeneity may be a preferred structure in microcapsules due to low porosity and higher stability so that it is interesting to evaluate the polymer gradient in spherically symmetrical small alginate beads (1.0-0.7 mm diameter) obtained in different conditions. In this paper, two complementary techniques have been used to investigate this aspect. The concentration gradient of alginate has been analysed by measuring both the spatial distribution of calcium ions in sections of alginate gel spheres, by means of X-ray fluorescence spectroscopy, and the T2-relaxation behaviour on intact gel beads using MR-microimaging. The experimentally determined gradients were compared with a recently reported mathematical model for alginate gel formation which was based on a set of coupled chemical reaction-diffusion equations.