Biocapsule delivery

Biocapsule containing insulin-secreting cells. KIRK WOELLERT/NSF

Tejal Desai (who has since moved to Boston University) and Lara Leoni have constructed a device that effectively encapsulates insulin-secreting (insulinoma) cells (IEEE Trans. Biomed. Eng. 2001, 48 (11), 1335–1341). The capsule, which is made of a microfabricated silicon membrane that has nanometer-scale pores and is bound to a polymer well, has potential as an implantable system for steady and continuous insulin production and delivery into the bloodstream.

The researchers measured the in vitro diffusion of glucose, a vital nutrient for insulin production, and insulin through the free silicon membrane. Constant and unimpeded passage of these species was observed, even in the presence of albumin protein, a good candidate for pore blockage.

Following up on these results, the scientists probed the behavior of insulinoma cells in the complete silicon–polymer biocapsules to determine an ideal cellular environment for effective therapy. Cells loaded into the capsule in their own suspension resulted in less insulin diffusion than those put into the capsule in a collagen or collagen–chitosan matrix. The latter, more viscous matrix environments allow for a more effective overlap between the cells and the membrane pores. The collagen–chitosan matrix slowed cell proliferation compared with the collagen matrix, which may be an advantage in avoiding excess insulin production and the occurrence of hypoglycemia.

A major challenge in the development of this biodelivery device is protecting the transplanted cells from attack by the body’s immune system while allowing the passage of needed nutrients. The 24.5-nm pores are believed to be small enough to block the entrance of antibodies and other immune components. According to the authors, in fact, 30-nm pores have been previously shown to be successful in retaining several important immune species. The long-term effectiveness of the capsules in vivo, however, remains to be demonstrated.