Cross-linking properties of alginate gels determined by using advanced NMR imaging and Cu(2+) as contrast agent.
The entrapment of enzymes, drugs, cells or tissue fragments in alginates cross-linked with Ca(2+) or Ba(2+) 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 microm and 600 microm in diameter) that advanced (1)H NMR imaging, along with paramagnetic Cu(2+) 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 T(1) of Ba(2+) cross-linked microcapsules exposed to external Cu(2+) yielded qualitative as well as quantitative information about the accumulation of Cu(2+) within and removal from microcapsules upon washing with Cu(2+) free saline solution. The use of Cu(2+) (having a slightly higher affinity constant to alginate than Ba(2+)) 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.
D000464:Alginates / Q000737:chemistry* / Q000648:ultrastructure; D001464:Barium; D055592:Biophysical Phenomena; D001703:Biophysics; D002214:Capsules; D003287:Contrast Media; D003300:Copper; D003432:Cross-Linking Reagents; D005782:Gels; D020723:Glucuronic Acid / Q000737:chemistry*; D006603:Hexuronic Acids / Q000737:chemistry*; D008279:Magnetic Resonance Imaging
D000464:Alginates; D002214:Capsules; D003287:Contrast Media; D003432:Cross-Linking Reagents; D005782:Gels; D006603:Hexuronic Acids; D001464:Barium; D003300:Copper; D020723:Glucuronic Acid