19F nuclear magnetic resonance imaging (MRI) can be used as a non-invasive tool to simultaneously determine the location, the integrity and the oxygen supply of Ba2+-alginate implants. This requires that the beads (implants) are pre-loaded with the perfluorocarbon compound F-44E. Implantation of solid 19F-labelled beads into the peritoneum, below the kidney capsule or into the muscle of Wistar WU rats demonstrated that these beads could be detected by 19F-MRI for up to 18 months after implantation. This indicated that F-44E is not considerably released from the beads during implantation. The signal to noise ratio of liquid-core beads was higher by a factor of 4 than the signal to noise ratio of solid beads, but liquid-core beads were more fragile and also too large for implantation under the kidney capsule and into the intramuscular tissue. Quantitative 2-dimensional 19F-T1 maps (resolution 0.5 x 0.5 mm) could be deduced from 19F-MRI measurements. These T1-maps correlated to the local pO2-values. The partial oxygen pressure estimated in F-44E-loaded Ba2+-alginate beads showed that the oxygen supply inside the beads was very poor when they were implanted below the kidney capsule or into the peritoneal cavity. These low pO2-values obtained for the renal subcapsular site and the peritoneum may explain the failure of previous immunoisolated islet transplantation studies using these locations.
Several types of microcarriers suitable for large-scale cultivation of mammalian cells are commercially available. However, many of these carriers have disadvantages, e.g., the need for enzymatic digestion for cell harvesting, size limitations and insufficient biocompatibility. These limitations have been overcome by the development of collagen-coated Ba(2+)-alginate microcarriers. Ba(2+)-alginate microspheres, made with the air-jet droplet generator technique, were collagen-coated by incubation in a 0.5% collagen solution, with subsequent gelling of the collagen layer around the alginate microspheres. Human chang liver (CCL-13) and mouse fibroblast (L929) cell lines were cultivated in stationary, unstirred cultures as model systems. After a lag phase of nearly 24 h, the cells grew rapidly on these microcarriers and reached confluence after 3 days. The microcarrier cultures were stable for an additional 4-9 days and longer. Cells were harvested either by trypsinization or by dissolution of the alginate matrix using 5 mM EDTA. The main advantages of this new microcarrier system are that the preparation procedure is easy and can be accomplished on demand with standard laboratory equipment.
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