Top of pageAbstract Microencapsulated cells engineered to secrete therapeutic proteins have proven effective in treating several murine genetic disorders such as dwarfism, lysosomal storage disease and hemophilia, as well as multifactorial disorders such as cancer. Microcapsules are most commonly fabricated from the naturally occurring polysaccharide, alginate, and cross-linked with Ca2+ to form a hydrogel. Beads of alginate containing the cells are coated with poly-L-lysine and a further layer of alginate to make alginate-poly-L-lysine-alginate (APA) microcapsules. A concern with APA microcapsules is loss of structural integrity during long term implantation, especially in large animals. We have now improved the stability by creating covalent linkages in these hydrogel microcapsules to form an interpenetrating polymer network (IPN) with photo-polymerized sodium acrylate and N-vinyl-pyrrolidone (NVP). For application in gene therapy, these novel microcapsules must be stable, non-toxic, supportive of cell survival, biocompatible, and effective perm-selective immuno-isolating devices. We now report on some of these biological functional properties of the novel microcapsules. While the polymers creating the IPN were safety sequestered, the viability of the encapsulated cells was adversely affected during the photo-polymerization. On the day after NVP capsule fabrication, the number of viable cells was decreased by about 50% compared to APA capsules. Alamar blue test showed the average encapsulated cell number per NVP capsule kept roughly consistent from 1 month to 9 months of implantation (530|[plusmn]|67 at 3 months, compared to 1250|[plusmn]|120 for APA capsules at 3 months). The NVP capsules were well tolerated, with no apparent toxicity clinically. Hematological monitoring showed that the neutrophil fraction at 1 week was significantly higher, reflecting an acute inflammatory reaction in the animals. However, this response was resolved after 4 weeks, and remained normal until 9 months, indicating that there was no chronic inflammatory response to the implants. Liver enzyme levels were normal for both groups throughout the experiment. The mechanical stability of NVP capsules, as measured with an osmotic pressure test at 5.2 mOsm, was twofold higher than that for classical APA capsules after up to three months of implantation. After 6 and 9 months' impanation, the NVP capsules remained stable while almost all of the APA capsules were broken. In the osmotic pressure test the retrieved NVP capsules implanted into mice for 6 and 9 months remained 45% intact (at 5.2 mOsm), compared to 60% for the capsules before implantation. In conclusion, the NVP covalently-modified microcapsules have shown a high level of biocompatibility and safety, along with an increase in stability for use as immuno-isolation devices in gene therapy.