Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 348

initial volume. In this highly swollen state, they are sometimes dif cult to handle without

fracturing, and a second generation of SPH composites has been developed which are
characterized by fast swelling, but a medium swelling ratio and improved mechanical
properties.

11.3.5.2 Superporous gels

These third-generation SPH

' hybrid '

are designed to exhibit improved elastic properties,
valuable for gastrointestinal devices as well as in other pharmaceutical and biomedical
applications. To synthesize these, a so-called hybrid component that can be cross-linked
after the SPH is formed is also included. This is a water-soluble or water-dispersible
polymer that can form structures with either chemical or physical cross-linking.
Examples of hybrid components are polysaccharides including sodium alginate, pectin
and chitosan, and synthetic water-soluble polymers such as PVA. The overall system is
structurally similar to an interpenetrating polymer network (IPN). An example of the
process is the synthesis of acrylamide-based SPH in the presence of sodium alginate,
followed by the cross-linking of alginate chains by Ca 2+ .
One of the unique properties of SPH hybrids is that the gels are highly elastic in
the swollen state and, compared to conventional SPHs, do not easily fail when subjected
to large tensile strains. This property makes SPH hybrids a better choice in applications
where resilient gels are preferred. They can resist various types of deformation,
including tension, compression, bending and twisting. Such a hybrid hydrogel can
also swell by up to 40 to 50 times, is very elastic in its swollen state and in this state can
be extended by up to 3 times its original length, and the loading/unloading cycle can be
repeated many times. This property could be exploited in the development of fast- and high-
swelling elastic hydrogels for a variety of pharmaceutical, biomedical and industrial
applications.

' hybrids '

11.3.6 Superdisintegrant materials

Superdisintegrants are another class of superabsorbing materials with tailor-made swel-
ling properties (Omidian and Park, 2008 ). They are not intended to absorb signi

cant

amounts of water or aqueous fluids, but to swell very fast. Such superdisintegrants are

used to accelerate disintegration of solid samples (dosage forms) by being physically
dispersed within the matrix of the dosage form, and then to allow the whole system to
expand when exposed to a wet environment. The swelling pressure and the isotropic
swelling of the particles create high stresses in localized areas, so the whole structure
breaks apart, as shown in Figure 11.15 .
To ensure that the tablet or solid dosage form breaks apart at an appropriate rate, the
disintegrant should be evenly distributed within the matrix and should swell very fast to a
size typically 10

40 times its original volume, in water or other appropriate aqueous
medium. It should also be compatible with the other components (excipients).
In recent years there has been considerable growth in the number of orally soluble
and chewable tablet products. These disintegrate rapidly in contact with saliva, thus

-