Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 336

more iron than cant.

According to Zuniga and co-workers (Zuniga and Aguilera, 2008 ), tailoring the porous
structure of an aerated gel would also offer opportunities for protection or release of a
physiologically bioactive component (or a nutrient).
The personal products and cosmetic industries have also made use of what they term

fibrillar gels made at pH < 3, so gel microstructure is also signi

' , but in practice most of these seem to fall outside our remit: they are often entirely

surfactant-based or they are simply viscous liquids, and commonly both. A typical
example is a branded commercial

gels '

, which consists of worm-like micelles
which naturally entangle. Nonetheless the work by Miyazawa and co-workers
(Miyazawa et al., 2000 ) is of relevance. They prepared novel soft microcapsules for
cosmetics, consisting of an oil

' shower gel '

oil emulsion in which the water phase was then
gelled with a commercial agar. These microcapsules were typically around 300

- water -

- 400 μ m

in diameter, and the concentration of agar in the water phase was typically 2 4% w/w.

Various agar preparations, differing in composition and molecular mass, were measured
in compression, and it was established that the best stabilized microcapsules were those
where the original agar gels had the highest modulus.

-

11.3 Biomedical applications

It was recently estimated (Peppas et al., 2006 ) that there were over 8000 medical devices
and 2500 diagnostic products employing biomaterials in various medical applications.
Hydrophilic polymers in particular have demonstrated great potential for biological
and medical applications, and the ability to engineer these with speci

c properties
is the source of new developments in tissue engineering, drug delivery and
bio-nanotechnology. The design and synthesis of hydrophilic polymers are of

interest in biomedical and nanotechnology applications. Many such

' smart '

are
particularly appealing for biological applications because of their high water content
and biocompatibility. In this volume we use the term

' hydrogels '

sparingly since, as
mentioned in Chapter 1 , it is sometimes used simply for viscous, entangled solutions.

' hydrogel '

11.3.1 Applications of chitosan

Chitosan has been studied very widely over the last decade, and here we give only a brief
summary since detailed mechanical measurements seem scarce. There are a number of
excellent reviews, including those by Berger, Dash and their respective co-workers
(Berger et al., 2004a , 2004b ; Dash et al., 2011 ). The use of ' entangled chitosan hydro-

gels is limited by their lack of mechanical strength and their tendency to dissolve but, as

noted, these will not be discussed here.

'

hydrogels are formed by irreversible
covalent links, for instance using glutaraldehyde. Unfortunately, for biomedical
applications the addition of covalent cross-linkers may decrease biocompatibility
(Berger et al., 2004b ).
Physical chitosan hydrogels can be formulated with various reversible links: ionic
interactions in ionically cross-linked hydrogels and in polyelectrolyte complexes (PEC),

' Chemical '