Biomedical Engineering Reference
In-Depth Information
we may point out as a general characteristic of the MSFB, its capability to
maintain the stabilization of the bed (suppressing bubbling and slugging) for
a much larger range of velocities than that of conventional fluidized beds (i.e.,
maintaining the fixed bed behavior much longer) while keeping a low pressure
drop at all times (a characteristic which is typical of fluidized beds). MSFB also
enables to keep the mixing of the solids while processing solutions containing
suspended solids. Also, very important in biomedical and biotechnological
applications is the ability of MSFB to process biological cultures in a gentle
and harmless way.
12.3.2 MSBs and MFBs as Porous Media
Particles used in MSBs and MFBs may be porous or nonporous. Beds of non-
porous magnetic particles seem to be more resistant to diffusional limitations,
attrition, and fouling than beds of porous particles (Halling and Dunnill 1980).
Several nonporous particles have been used, as, for example, iron, cobalt, and
their oxides; and there are four main methods for their preparation: direct use
of a coupling agent, adsorption, encapsulation, and formation of a layer of a
thin polymer film. However, porous particles are the most interesting media for
biotechnological and biomedical applications, and in many cases such applica-
tions would be impossible to perform with nonporous magnetic particles. The
main feature of the magnetic porous particles, when used as media for MSFB,
is their adsorption capabilities, which, when properly controlled, have a direct
action and influence in the separation/purification of important biological sub-
stances. The protein adsorption is one of the examples of their spreading use.
MSFBs are replacing existing packed-bed operating systems, as they have the
advantage of a low and constant pressure drop, which is independent of the
flow rate. Besides the ability to process feed streams with suspended solids
(as previously discussed), magnetic stabilization of fluidized beds also reduces
axial dispersion and enhances separation (Lucchesi et al. 1979; Rosensweig
1979; Rosensweig et al. 1981a,b; Siegell et al. 1984; Burns and Graves 1985;
Siegell 1987, 1989; Geuzens and Thoens 1988a,b; Sajc et al. 1994; Rosensweig
and Ciprios 1991; Wallace et al. 1991).
The porous particles are usually polymeric (generally having a magnetite
core and a polymeric shell) and contain biologically active centers that help
in the adsorption/separation of biological materials. Details of these beads
will be discussed later in Section 12.5. Magnetic porous particles have a high-
magnetic susceptibility, a large reactive polymer surface area, and pores on
which functional molecules, such as enzymes, antibodies, antigens, cells, drugs,
and biotins, among many others, can be immobilized (Ugelstad et al. 1992;
Ding et al. 1998; Qiu et al. 2006). These magnetic particles have many advan-
tages as supports for cell immobilization due to their controlled surface area
and porous structure, good thermal resistance, reasonable chemical inertness,
and the possibility of being reused (Qiu et al. 2006). These polymer beads
can be formed either by suspension polymerization and specific treatment
with acid and base or by seed swelling polymerization in the presence of the
magnetite core (Bahar and Celebar 2000).
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