4.1.2.2. Hollow beads. With the sam

4.1.2.2. Hollow beads. With the same solutions as above, hollow beads may be obtained by just switching the positions (Spieker- mann et al., 1987): A crosslinking solution is supplemented with a viscosity-enhancing polymer such as sodium carboxymethylcel- lulose that does not participate in the reaction and then dripped into an alginate solution. Again, at the surface ionotropic gelation occurs, but this time, calcium ions will diffuse from the core to the surface crosslinking ‘‘from inside to outside’’ (Fig. 3C). The thickness of the calcium alginate layer around the core may be controlled by the concentrations of the reactants and the crosslink- ing time. Cells may be placed either into the core and/or into the alginate layer. The core can also become solid if a warm gelatin or agar solution is used instead of carboxymethylcellulose. Regard- ing biological control, this method was applied to the encapsula- tion of an entomopathogenic nematode into calcium alginate hollow beads (Patel and Vorlop, 1994).
A methodological alternative for the production of hollow beads for the encapsulation of bacteria, fungi, nematodes and viruses is the use of coextrusion where a core solution is extruded into a sur- rounding shell solution (Digat, 1993)