Figure 10.

Structure of alginate-poly-l-lysine capsules. The system is composed of two layers and not of three layers as wrongly stated in many papers. The core of the capsule is composed of a matrix of alginate and calcium in which the islets are entrapped. The second layer is obtained after incubation of the calcium bead in sodium rich solutions to form a sodium-alginate complex at the surface. Sodium can be substituted by poly-l-lysine (PLL) to form a complex alginate-PLL- alginate (APA). The PLL-alginate layer can take three different structures by intramolecular hydrogen binding, (i) random coil formation between alginate and PLL, (ii) α-helicoidal structure between amide groups of PLL, and (iii) antiparallel β-sheet structure between amide groups of PLL.