Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 642

12.4.1. Active Immobilization of Cells
Active immobilization is entrapment or binding of cells by physical or chemical forces. The
two major methods of active immobilization are entrapment and binding.
Physical entrapment within porous matrices is the most widely used method of cell immo-
bilization. Various matrices can be used for the immobilization of cells. Among these are
porous polymers (agar, alginate, K-carrageenan, polyacrylamide, chitosan, gelatin, and
collagen), porous metal screens, polyurethane, silica gel, polystyrene, and cellulose triacetate.
Polymer beads should be porous enough to allow the transport of substrates and products
in and out of the bead. They are usually formed in the presence of cells and can be prepared
by one of the following methods:
(1) Gelation of polymers : Gelatin and agar beads may be prepared by mixing the liquid form of
these polymers with cell suspensions and using a template to form beads. Reduction of
temperature in the templates causes solidification of the polymers with the cells
entrapped. Gel beads are usually soft and mechanically fragile. However, we can use
a hard core (glass and plastic) and a soft gelatin shell with entrapped cells to overcome
some mechanical problems associated with polymer beads. Because of diffusional
limitations, the inner core of such beads is often not active, so this approach does not
necessarily decrease the amount of product made per bead.
(2) Precipitation of polymers : Cells are dispersed in a polymer solution, and by changing the
pH or the solvent, the polymer can be precipitated. The starting solution of the polymer
has to be prepared with an organic solvent or a water-solvent mixture. Ethanol and
acetone are examples of water-miscible solvents. Polymers used for this purpose are
polystyrene, cellulose triacetate, and collagen. The direct contact of cells with solvents
may cause inactivation and even the death of cells.
(3) Ion-exchange gelation : Ion-exchange gelation takes place when a water-soluble
polyelectrolyte is mixed with a salt solution. Solidification occurs when the
polyelectrolyte reacts with the salt solution to form a solid gel. The most popular example
of this kind of gelation is the formation of Ca-alginate gel by mixing Na-alginate
solution with a CaCl 2 solution. Some other polymers obtained by ion-exchange gelation
are Al-alginate, Ca/Al carboxymethyl cellulose, Mg pectinate, K-carrageenan, and
chitosan polyphosphate. Alginate and K-carrageenan are the most widely used polymers
for cell immobilization purposes. Ionic gels can be further stabilized by covalent cross-
linking.
(4) Polycondensation : Epoxy resins are prepared by polycondensation and can be used for cell
immobilization. Polycondensation produces covalent networks with high chemical and
mechanical stability. Usually, liquid precursors are cured with a multifunctional
component. Functional groups usually are hydroxy, amino, epoxy, and isocyanate
groups. Some examples of polymer networks obtained by polycondensation are epoxy,
polyurethane, silica gel, gelatin-glutaraldehyde, albumin-glutaraldehyde, and collagen-
glutaraldehyde. Severe reaction conditions (high temperature and low or high pH values)
and toxic functional groups may adversely affect the activity of cells.
(5) Polymerization : Polymeric networks can be prepared by cross-linking copolymers of
a vinyl group containing monomers. Polyacrylamide beads are the most widely used