Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 346

conditions (pH, ionic strength). The release of a drug from a hydrogel that swells
involves the dissociation of the complex drug-matrix through exchange with counterions
penetrating into the particle from the dissolution medium. Such release is indeed
enhanced when the salt concentration is lowered. In addition to interactions between
the constituent polymers, the incorporation of a charged species (e.g. protein) into the
particle core introduces additional electrostatic interactions, particularly where the par-
ticle loading is high. A short incubation of the nanoparticles at elevated pH, for instance,
forms cross-links between the carboxyl groups of polysaccharides of the polymeric
mixture and the amino groups of the protein (Schiff-base linkages) that can also occur
in vivo (Prokop et al., 2002 ). Such linkages are relatively labile. Charged entrapped
proteins contribute to the structure of particles and enhance high encapsulation effi- -

ciency. When tested with animal tissue, this type of particle formulation shows reduced
cytotoxic effects.

11.3.4 Application of multi-membrane systems

The interesting multi-membrane systems of Ladet et al. 2008 ) described in
Section 11.1.4 have a number of potential applications, but their work was very much
geared towards studies of cell growth and attenuation for tissue engineering. For exam-
ple, to validate the usefulness of such systems as biomaterials, the authors carried out a
chondrocyte culture within the multi-membrane onion-like hydrogels. Cell aggregates
were observed in several inter-membrane spaces, showing that cells can be introduced
and cultured within these new systems for tissue growth.

11.3.5 Superporous hydrogels

A superporous hydrogel (SPH) differs from a superabsorbent polymer (SAP) or a normal
gel in that SPHs swell fast, within minutes, to the equilibrium (swollen) state, regardless of
their size. Their fast-swelling property is based on water absorption by capillary
forces through an open porous structure. This unique property of SPHs

- size-independent

fast-swelling kinetics follows from their interconnected, open cellular structure; using

specially designed syntheses, pore sizes of up to 100 μm can be formed ( Figure 11.13 ).

-

11.3.5.1 Harmonized foaming and gelation

The reactions involved in the preparation of SPHs are combinations of cross-linking
polymerization (gelation) and foaming, just as in the traditional RIM process (Stanford,
1998 ) for production of urethane foams, but on a microscale. In the synthesis of SAP or
SPH, the following general procedure is applied ( Figure 11.14 ). (1) the monomer, such as
acrylic acid salt or hydrophilic acrylamide, is

first diluted with water to reach the desired
concentration. (2) The monomer is partially neutralized, and (3) this is followed by
addition of an appropriate cross-linker. (4) Foaming aids and stabilizers (PEO n - PPO m -

PEO n triblock copolymers) are then added, since to produce homogeneous SPHs foam
stability is essential. To promote polymerization, (5) oxidant and (6) reductant are added
to the monomer solution. Finally, to produce large numbers of pores, (7) NaHCO 3 is