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of the alginates is a temperature-independent process; working temperatures
range from 0°C to 80°C. For encapsulation, cells are mixed with alginate (1-
8%) and are usually extruded through a syringe into a Ca
+
2
(or Sr
+
2
) solution to
generate beads. A 21-gauge syringe produces alginate beads of approximately
3-mm diameter [37]. Additional structural integrity can be achieved by coating
the alginate beads with poly-L-lysine [51].
The carrageenans are produced by red algae. There are three types (
ι
,
λ
,
κ
), all of which have a common backbone of alternating
β
(1,3)-D-galactose
and
(1,4)-D-galactose [11]. The difference in the three types is the degree
and location of sulfonation on each sugar moiety. In contrast to the alginates,
gelation is temperature dependent, but the strength of the matrix can be adjusted
with the addition of K
+
or Al
+
3
ions [26].
α
-Carrageenan (2-5% in the presence
of 0.1-0.3 M KCl) is the most widely used because of its firmer gelling prop-
erties [36]. For immobilization,
κ
-carrageenan is heated to 80°C to dissolve
the polysaccharide and then cooled to 45°C. The cell suspension is also heated
to 45°C and mixed with the
κ
κ
-carrageenan. Similar to the alginate, the cell-
κ
-carrageenan mixture is extruded through a syringe into cold KCl solution to
make small beads [11]. The obvious disadvantage of this method is the heating
and rapid cooling of the cells. Any cell culture would have to tolerate 45°C for
a short period of time. There are numerous examples in the literature of cell
immobilization in these natural polysaccharides [12, 47-49].
Sol-gel
Sol-gel matrices are porous wet gels obtained by the hydrolysis and condensa-
tion/polymerization of metal and semimetal alkoxides (SiO
2
materials) [4, 29].
Sol-gel characteristics include thermal stability, controllable surface area and
pore size, transparency, and nontoxicity [4]. Microbial cells as well as enzymes
have been immobilized by sol-gels [18, 19, 32, 63]. Armon et al. [4] achieved
film thicknesses of 0.1-0.2 mm. Chia et al. [14] reported that sol-gel films
of 0.1
m can be achieved. However, our experience has shown that sol-gel
thin-films (100
µ
µ
m) are extremely fragile.
Acrylate-Vinyl Acetate Copolymers
A more recent approach is the use of latex (acrylate-vinyl acetate copolymers)
for the immobilization of bacterial cells [39, 40, 41]. The advantages of these
copolymers include mechanical strength at very thin film thicknesses (10-
100
m), high cell retention, and higher mass cell loadings compared to natural
polysaccharides. Lyngberg and co-workers [39] demonstrated latex films with
microbial cell densities as high as 50 vol% and 2
µ
10
11
cells/cm
3
of coating
volume. These thin layers minimize mass transfer limitations of analytes (target
compound, nutrients, O
2
) to the bioreporter. Additionally, photons produced by
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