The material exposed to body tissues must be completely free of fibrogenic character. The most common term to describe a substance that is ignored by the body was "biocompatible." We have found that there is great confusion about this, because device and transplantation people use the term "biocompatible" to mean different things. For a vascular device, "biocompatible" means that the material induces engraftment; the vascular graft is covered with a layer of collagen fibers permitting overgrowth of endothelial cells. However, such overgrowth will starve a cellular implant! For a cellular transplant, "biocompatible" means lacking collagen fibers or any material that will promote cellular adhesion.

The difference is shown in the cartoon. The body's reaction to material appropriate for a vascular implant (and inappropriate for a bio-artificial pancreas) is shown in the left image; in consists of inflammation, granulation and neovascularization followed by collagen deposition and fibrosis. The reaction to material appropriate for a bio-artificial pancreas is shown in the right image. In both cases the initial interaction is between the material and immune system cells and fibroblasts (blue). For the vast majority of materials, the reaction shown on the left pertains and cells react to foreign bodies by laying down collagen fibers (green). A bio-artificial pancreas made of the material on the left would starve the cells inside because the collagen prevents nutrients from reaching the cells inside.

The ideal bio-artificial pancreas provokes only the reaction shown on the right. The number of materials that respond as shown in is this way is very small. Islet sheets are made from highly purified alginic acids (alginate), which we have previously shown can be made biocompatible as defined above.