Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 329

11.1.3 Gel micro- and nanoparticles

As mentioned in Chapter 4 , the rate of swelling (and de-swelling) of a spherical particle
varies approximately inversely with the square of its radius, and this general behaviour
holds even for arbitrary shapes. The release of an entrapped material is controlled by
similar factors

the surface area to volume ratio being much larger, the smaller the
particle. For that reason there has always been an advantage in producing enhanced
swelling rate micro- and nanogel particles. It was realized in the 1950s that intramolec-
ularly (chemically) cross-linked macromolecules (ICMs), particularly based on metha-
crylates, could have useful properties in the paint industry. According to Graham and
Cameron ( 1998 ), the term

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goes back to Baker in 1949, who from his work on
emulsion polymerizations recognized that his systems were indeed intramolecularly
cross-linked. In

' microgel '

these microgels behaved more like Einstein spheres than
dissolved linear polymer coils, and exhibited much lower than expected solution vis-
cosities. Subsequently ICM microgels became widely used in paints. By controlling
molecular mass by change of initiator concentration, for example, species could be
produced of radius <10 nm, referred to as nanogels.
Arguably the

' solution '

cial fruit products
produced in the 1960s by adding Na + alginate plus fruit puree drop-wise into a Ca 2+ bath
( Chapter 5 ). This technique is still used, although interest now is more in micro- and
nanogels, particularly for controlled release of active ingredients in the biomedical area.
The Trondheimgroup (Thu et al., 2000 ) has extended its work on alginates to investigate
the homogeneity of such gel particles, and small spherical alginate beads (1.0

first commercial physical gel ' spheres ' were those arti

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0.7 mm
diameter) were obtained under various conditions. Micro-images of these obtained by
magnetic resonance imaging (MRI) ( Figure 11.3 ) illustrate the polymer concentration
gradient inside the beads, and reveal differences which depend on the exact gelling
procedure. For example, low calcium concentration during gelation gave rise to more
inhomogeneous beads. The images show that there is no smooth polymer gradient
throughout the gel but that micro-heterogeneities exist, although, because of resolution
limitations, their sizes are dif cult to obtain. That said, for controlled-release properties,

such heterogeneities are valuable, and the ability to design them becomes very useful.
Traditional ionic or thermally induced gelation of polysaccharide drops is a reasonably

ef gel microspheres for application as drug

delivery systems, enzyme carriers for detergents or supports for biocatalysts, but
the characteristics can be improved. Several routes have been tried, with different
emphases for speci

cient way to prepare simple ' monodisperse '

designs, in
which the microsphere has an outer layer with various properties, either for mechan-
ical/chemical stability or to alter release characteristics, and some of these are described
below. The texture of the polysaccharide gel plays a key role in the kinetics of drug
release or in support for catalysts in

c applications. These include so-called ' core-shell '

fine chemistry reactions or live tissue, and relevant
properties can be optimized using core-shell structures.
Another approach that has been used particularly for certain physical gel systems

such as the carrageenans and agarose ( Chapters 5 and 7)

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is simply to agitate a
fast-gelling system, producing arbitrarily shaped gel pieces. Contrary to some

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