Introduction - Physical Gels from Biological and Synthetic Polymers - page 16

Chapter 10 Mixed gels

Mixed gels consisting of two different polymers are classi ed into two groups: gels

' '

formed by simple phase separation and those formed by speci c (in the literature,

commonly although not always accurately referred to as ' synergistic ' ) molecular inter-

actions, where the enthalpy of the speci c interactions is suf cient to overcome the

normal tendency for phase separation.
Phase separation tends to be produced by the so-called

between the
two polymers, which then increases the effective concentration of each component
within the mixture. On the other hand, gels formed by more positive interactions include
coupled networks, which are formed by speci

' exclusion effect '

c binding between two different polymers,
and interpenetrating networks, sometimes now recognized as being formed by micro-
phase separation. The mixtures of importance mainly involve certain polysaccharides or
mixtures of polysaccharides and proteins. The microscopic structure, kinetic aspects and
morphology of the phases and the overall rheological properties are illustrated for a large
number of systems including gelatin with dextran, maltodextrin and agarose; protein -

protein mixtures; and -carrageenan, gellan and xanthan mixtures, some of the latter

showing synergistic effects.

κ

Chapter 11 Innovative systems and applications

Our final chapter is intended to serve not only as a guide to novel systems but also to more

recent and/or future potential applications. The novel systems include the cleverly synthe-
sized

' '

, polyelectrolyte complexes, gel micro- and nano-particles, recently
developedmulti-membrane hydrogels and hydrogels as ultra-sensitive cantilever materials.
As far as applications are concerned, the majority of developments, and arguably the
most exciting ones, are in the pharmaceutical and biomedical areas, including the very
fast growing area of scaffolds for tissue engineering. Here physical gels have been used
as scaffold materials that act as supports and to allow in vitro cell growth to proceed
unhindered throughout the gel sample (van Vlierberghe et al., 2011 ). Pharmaceutical
applications in drug release and in enabling ef

sliding gels

cient dispersion of dosage forms (for
example, tablets) have also shown great progress. For example, fast-swelling materials
are being used to encourage fast drug distribution (as so-called ).

That said, physical gels still have novel applications in the food and cosmetic areas.
For example, in the former, there is a great interest in developing low-fat systems. Here a
combination of fat replacement by biopolymer structurants and gel-stabilized aerated
systems has been shown to be useful.

' disintegrants '

References

Almdal, M., Dyre, J., Hvidt, S., Kramer, O., 1993. Polym. Gels Network 1 ,5 - 17.
Burchard, W., Ross-Murphy, S. B. (eds), 1990. Physical Networks: Polymers and Gels. Elsevier
Applied Science, Barking, UK.
Clark, A. H., Ross-Murphy, S. B., 1987. Adv. Polym. Sci. 83 ,57 - 192.