Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 340

inhomogeneities may be preferred in microcapsules since they help create low porosities
and higher stabilities.

11.3.2.2 Beads with hierarchical macroporosity

As noted above, the formation of a shell-structured microsphere with higher density at
the surface considerably improves the retention of cells in biocatalysts. Heterogeneities
inside the gel microspheres are also expected to affect diffusion and swelling phenom-
ena. One mechanism of generation of heterogeneous structures is described by Thumbs
and Kohler ( 1996 ). When a solution of Cu 2+ salt (instead of the more usual Ca 2+ )is
placed on top of a solution of Na + alginate, the chains cross-link to form a ' capillary

gel 300 μm diameter) are examples of dissipative structures, and

a periodic pattern, due to convective motion of the aqueous solution in the neighbour-
hood of the gelation front, appears. The complete theory was proposed by Treml et al.
( 2003 ), and involves a counter-current diffusion between the polysaccharide solution
and the gelling agent.
For appropriate values of the ratio between diffusivity and gelling rate, convection
cells are formed, through which the solution of gelling cations can feed the advancing
gelation front. In alginate solutions at the interface with a Cu 2+

' . These capillaries (8 -

solution, the rising

channels of the convection cells may remain in the final gel, as a system of parallel

void channels normal to the gelation front.
Another strategy is to form cavities in the core of polysaccharide beads by dehydration
treatment. Cavities can be generated by controlled syneresis of the core, in the presence
of a shell able to prevent isotropic shrinking of the bead. Such structures can lead to
improved diffusivity inside the macroporous core, and present intermediate properties
between core-shell systems and hollow-shell microcapsules, extending the range of
materials available for biocatalysis and drug delivery.
Di Renzo et al.( 2005 ) investigated the in uence of a rigid shell on the formation of

channel structures in the polysaccharide core. Core-shell microspheres were prepared
with chitosan

silica composites. Beads of nearly 3mm diameter were obtained from
drops of chitosan solution gelling in an alkaline solution. Mixing was achieved by
rotating the

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flask around its horizontal axis, and the chitosan hydrogel could easily be
impregnated with a silica sol, allowing a shell of silica gel to be deposited on the outer
surface of the beads. The beads were then washed with water. Supercritical drying of the
gel beads was preceded by dehydration in a series of ethanol

water baths with increasing
alcohol concentration. The gel beads were dried under supercritical CO 2 conditions.
Silica-coated chitosan beads show an outer shell, 200 μm thick, which contains only
silica, and a distribution of silica inside the beads below this depth, the core of the bead
being composed of 80% silica and 20% polysaccharide. A radial pattern of channels with
an average diameter 10 μm can be observed on micrographs. The shafts do not reach the
outer surface of the sphere, but appear to run from the inner surface of the silica shell
toward the centre of the bead.
The presence of a similar structure is illustrated in Figure 11.9 , which shows, at various
levels of magni

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cation Cu - alginate gels (Di Renzo et al., 2005 ) before and after drying,