Additionally, solution osmolarity can not stray far from physiological (300 mOsm), and mechanical stresses must be limited so as not to rupture cell membranes during microcapsule production. Finally, limitations speci?c to cell types must be taken into consideration for instance, calcium ion concentrations usually ranging from 0.1 to 2 wt% when alginates are employed for cell encapsulation; however, stem cells are known to be sensitive to transmembrane calcium gradients and so the application presents obstacles that otherwise, in the case of most terminally di?erentiated cells types, would be negligible (Ciccolini et al. 2003).

An important consideration for cellular encapsulation is the compromise between size and sustainable mass transport. Cells towards the inside of a capsule must also be able to exchange molecules over a large range of sizes, from soluble gases to large signaling proteins, depending on the speci?c application (Orive et al. 2003). If the capsule is too large to sustain su?cient passive (di?usionbased) mass transport given the permeability of the encapsulating medium, cells towards the inside of the cell may not develop as intended or may perish leading to the necrotic microcapsule cores.

Classically, an aqueous sodium alginate solution is dispensed into a reservoir containing a divalent cationrich solution (e.g. calcium chloride), which physically crosslinks the alginate (Fig. 1.6) to yield a solid, hydrogel bead. This bead is then incubated in a second reservoir containing a complementary polycationic solution (e.g. poly-l-lysine, chitosan) which forms a capsule wall through polyionic complexation. The remaining calcium is leached and made unavailable for competition with the polycation through chelation with citrate or EDTA, yielding a polymeric capsule with a hydrogel wall and ?uid interior. An alternative method involves extrusion of similar polymeric solutions through a needle, or a small pore, to generate a ?uid stream. The ?uid column, in air, breaks into small droplets containing the polymer-in-solution and cells due to the Rayleigh instability of the ?uid stream.