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In the light of results of clinical trials with immunoisolated human parathyroid tissue Ba2+‐alginate capsules were developed that meet the requirements for long‐term immunoisolated transplantation of (allogeneic and xenogeneic) cells and tissue fragments. Biocompatibility of the capsules was achieved by subjecting high‐M alginate extracted from freshly collected brown algae to a simple purification protocol that removes quantitatively mitogenic and cytotoxic impurities without degradation of the alginate polymers. The final ultra‐high‐viscosity, clinical‐grade (UHV/CG) product did not evoke any (significant) foreign body reaction in BB rats or in baboons. Similarly, the very sensitive pERK assay did not reveal any mitogenic impurities. Encapsulated cells also exhibited excellent secretory properties under in vitro conditions. Despite biocompatible material, pericapsular fibrosis is also induced by imperfect capsule surfaces that can favor cell attachment and migration under the release of material traces. This material can interact with free end monomers of the alginate polymers under formation of mitogenic advanced glycation products. Smooth surfaces, and thus topographical biocompatibility of the capsules (visualized by atomic force microscopy), can be generated by appropriate crosslinking of the UHV/CG‐alginate with Ba2+ and simultaneous suppression of capsule swelling by incorporation of proteins and/or perfluorocarbons (i.e., medically approved compounds with high oxygen capacity). Perfluorocarbon‐loaded alginate capsules allow long‐term non‐invasive monitoring of the location and the oxygen supply of the transplants by using 19F‐MRI. Transplantation studies in rats demonstrated that these capsules were functional over a period of more than two years.

A simple procedure is described for the extraction and purification of alginate from the inner stipes of the kelp Laminaria pallida. Alginate yield was about 10-15% of the dry mass, with a 70:30 mannuronic/guluronic acid ratio. Analysis of the purified alginate revealed a low polyphenol content while proteins were below detection level. The purified alginate was highly viscous, with 10-15 mPa s and 281 mPa s for a 0.1% and 0.5% solution, respectively, indicating a very high molecular mass (larger than 250 kDa). Bead formation occurred in the presence of divalent cations, but also in the presence of artificial serum (FCSIII) without added divalent cations. The biocompatibility of the alginate was tested with the in vitro mice lymphocyte test as well as by implantation of Ba2+ cross-linked beads beneath the kidney capsule of BB/OK rats. There was no evidence for significant mitogenic activity or fibrotic reaction. Biocompatibility of the alginate was also demonstrated by the encapsulation of human chondrocytes into Ca2+ cross-linked alginate beads. Immobilized chondrocytes grew and remained functional (i.e. they produced collagen).

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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.

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