Magnetic Decoration and Labeling of Prokaryotic and Eukaryotic Cells - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 193

interact with reactive groups on the cell surface. In typical examples of cells
covalent immobilization, magnetic chitosan particles activated by glutar-
aldehyde have been used for Saccharomyces cerevisiae immobilization. 15 Al-
ternatively, magnetic cellulose microparticles, after their activation with
periodic acid, were used for the yeast cell immobilization. 15 Polyacrolein
microspheres with magnetic properties carrying reactive aldehyde groups on
their surface were used for covalent binding of fresh human red blood
cells. 50
In addition to magnetic biopolymer and synthetic polymer particles, dif-
ferent types of magnetic iron oxides have been used for covalent cell
immobilization or modification. 51-53 Carboxylate- and amino-modified
magnetic nanoparticles were used for covalent modification of Flavobacter-
ium ATCC 27551. Under optimal conditions, the magnetic cells displayed
specific activity ratios of 93% and 89% compared with untreated cells, after
the covalent coupling with carboxylate and amino-modified magnetic
nanoparticles, respectively. 51 Silanized magnetite (20-40 nm, activated by
(3)-aminopropyltriethoxysilane followed by glutaraldehyde treatment) was
covalently bound to cells of the alkalotolerant producer of cyclodextrin
glucanotransferase (CGTase) Bacillus circulans ATCC 21783 in order to in-
crease the produced enzyme activity. The highest CGTase production
was achieved after 96 h of semicontinuous process using this type of im-
mobilized cells when the specific enzyme activity was 8.4-fold higher com-
pared to that of free cells. Magnetic nanoparticles linked to the cell walls by
the covalent bond between the activated magnetite and the cells were very
stable. 52

d n 8 y 4 n g | 5

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10.2.3 Entrapment of Cells into Biocompatible Polymers
Microbial cells can be entrapped in natural or biocompatible synthetic
carriers (gels). The carriers can be grouped according to the mechanism
leading to the gel formation. Gels can be formed by polymerization (e.g.,
polyacrylamide, polymethacrylate), crosslinking (e.g., proteins), poly-
condensation (polyurethane, epoxy resins), thermal gelation (e.g., gelatin,
agar, agarose), ionotropic gelation (e.g., alginate, chitosan) and precipitation
(cellulose, cellulose triacetate). The gel is formed in the presence of the cells
and appropriate magnetic materials. There are various methods available to
obtain particles (beads) containing entrapped cells and magnetic particles: 54

- Block polymerization with subsequent mechanical disintegration into
particles. This is a simple method but it results in irregular particles of
a wide size distribution.
- Molding of particles (beads) in a template form. This method results in
a uniform preparation of immobilized cells but it is less suitable for the
preparation of large quantities of immobilized cells.
- Bead formation in a two-phase system. Spherical beads can be prepared
in large quantities by suspending an aqueous mixture of cells, magnetic