Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 644

Adsorption capacity and strength of binding are the two major factors that affect the
selection of a suitable support material. Adsorption capacity varies between 2 mg/g
(porous silica) and 250 mg/g (wood chips). Porous glass carriers provide adsorption
capacities (l0 8 e 10 9 cells/g) that are less than or comparable to those of gel-entrapped

cell concentrations (l0 9

e 10 11 cells/mL). The binding forces between the cell and support
surfaces may vary, depending on the surface properties of the support material and the
type of cells. Electrostatic forces are dominant when positively charged support surfaces
(ion-exchange resins and gelatin) are used. Cells also adhere on negatively charged
surfaces by covalent binding or H bonding. The adsorption of cells on neutral polymer
support surfaces may be mediated by chemical bonding, such as covalent bonding, H
bonds or van der Waals forces. Some specific chelating agents may be used to develop
stronger cell e surface interactions. Among the support materials used for cell adsorption
are porous glass, porous silica, alumina, ceramics, gelatin, chitosan, activated carbon,
wood chips, polypropylene ion-exchange resins (DEAE-Sephadex, CMC-), and Sepharose.
Adsorption is a simple, inexpensive method of cell immobilization. However, limited cell
loadings and rather weak binding forces reduce the attractiveness of this method. Hydro-
dynamic shear around adsorbed cells should be very mild to avoid the removal of cells
from support surfaces. Covalent binding is the most widely used method for enzyme
immobilization. However, it is not as widely used for cell immobilization. Functional
groups on cell and support material surfaces are not usually suitable for covalent binding.
Binding surfaces need to be specially treated with coupling agents (e.g. glutaraldehyde or
carbodiimide) or reactive groups for covalent binding. These reactive groups may be toxic
to cells. A number of inorganic carriers (metal oxides such as titanium and zirconium
oxide) have been developed that provide satisfactory functional groups for covalent
binding. Covalent binding forces are stronger than adsorption forces, resulting in more
stable binding. However, with growing cells, large numbers of cell progeny must be
lost. Support materials with desired functional groups are rather limited. Among the
support materials used for covalent binding are CMC plus carbodiimide; carriers with
aldehyde, amine, epoxy, or halocarbonyl groups; Zr(IV) oxide; Ti(IV) oxide; and cellulose
plus cyanuric chloride. Support materials with e OH groups are treated with CNBr, with
e NH 2 are treated with glutaraldehyde, and with COOH groups are treated with carbodii-
mide for covalent binding with protein groups on cell surfaces.
The direct cross-linking of cells by glutaraldehyde to form an insoluble aggregate is more
like cell entrapment than binding. However, some cells may be cross-linked after adsorption
onto support surfaces. Cross-linking by glutaraldehyde may adversely affect the cell's meta-
bolic activity and may also cause severe diffusion limitations. Physical cross-linking may
also be provided by using polyelectrolytes, polymers such as chitosan and salts [CaC1 2 ,
Al(OH) 3 ,FeC1 3 ]. Direct cross-linking is not widely used because of the aforementioned
disadvantages.
Some examples of cell immobilization by entrapment and by surface attachment (binding)
are summarized in Tables 12.3 and 12.4 , respectively. A good support material should be
rigid and chemically inert, should bind cells firmly, and should have high loading capacity.
In the case of gel entrapment, gels should be porous enough and particle size should be small
enough to avoid intraparticle diffusion limitations. The effect of particle size and perme-
ability inside the particle is discussed in Chapter 17.