Cross-linking properties of alginate gels determined by using advanced NMR imaging and Cu2+ as contrast agent

The entrapment of enzymes, drugs, cells or tissue fragments in alginates cross-linked with Ca2+ or Ba2+ has great potential in basic research, biotechnology and medicine. The swelling properties and, in turn, the mechanical stability are key factors in designing an optimally cross-linked hydrogel matrix. These parameters depend critically on the cross-linking process and seemingly minor modifications in manufacture have a large impact. Thus, sensitive and non-invasive tools are required to determine the spatial homogeneity and efficacy of the cross-linking process. Here, we show for alginate microcapsules (between 400 µm and 600 µm in diameter) that advanced 1H NMR imaging, along with paramagnetic Cu2+ as contrast agent, can be used to validate the cross-linking process. Two- and three-dimensional images and maps of the spin-lattice relaxation time T1 of Ba2+ cross-linked microcapsules exposed to external Cu2+ yielded qualitative as well as quantitative information about the accumulation of Cu2+ within and removal from microcapsules upon washing with Cu2+ free saline solution. The use of Cu2+ (having a slightly higher affinity constant to alginate than Ba2+) for gelling gave a complementary insight into the spatial homogeneity of the cross-linking process together with information about the mechanical stability of the microcapsules. The potential of this technique was demonstrated for alginates extracted from two different algal sources and cross-linked either externally by the conventional air-jet dropping method or internally by the "crystal gun" method.