The long negatively charged alginate molecules will bind to positively charged divalent ions like Ca²⁺ and instantly form a gel network.

Alginate capsules are formed by dripping an alginate solution of 2% alginate into a solution containing Ca²⁺ and/or other ions like Ba²⁺ or Sr²⁺. In order to encapsulate cells, a cell suspension is mixed with the alginate solution before the mixture is dripped into the calcium gelling solution (Figure 1).

Microcapsules with a very narrow size distribution can be made by using an electrostatic capsule generator developed here at NTNU (Figure 2a and b). The size of the capsules can be controlled by adjusting the different parameters of the capsule generator, with diameters ranging from 200-1000 µm.

Due to the highly porous structure of the alginate gel nutrients and oxygen will be able to diffuse freely to the encapsulated cells thereby ensuring their normal function. At the same time, cell products like insulin can diffuse out of the microbeads reaching the patients blood where it is needed. The microcapsules are, however, not permeable to cells of the immune system, thereby preventing an immune attack against the encapsulated foreign cells (Figure 3).