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.