Alginates as therapeutic and drug delivery systems
Alginate is widely used as a viscosity enhancer in many different
pharmaceutical formulations. The aim of this thesis is to
quantitatively describe the functions of this polyelectrolyte in
pharmaceutical systems. To do this the techniques used were
Viscometry, Light Scattering, Continuous and Oscillatory Shear
Rheometry, Numerical Analysis and Diffusion.
Molecular characterization of the Alginate was carried out uS1ng
Viscometry and Light Scattering to determine the molecular weight,
the radius of gyration, the second virial coefficient and the Kuhn
statistical segment length. The results showed good agreement with
similar parameters obtained in previous studies.
By blending Alginate with other polyelectrolytes, Xanthan Gum and
'Carbopol', in various proportions and with various methods of low
and high shear preparation, a very wide range of dynamic rheological
properties was found. Using oscillatory testing, the parameters
often varied over several decades of magnitude. It was shown that
the determination of the viscous and elastic components is
particularly useful in describing the rheological 'profiles' of
suspending agent blends and provides a step towards the non-empirical
formulation of pharmaceutical disperse systems.
Using numerical analysis of equations describing planar diffusion,
it was shown that the analysis of drug release profiles alone does
not provide unambiguous information about the mechanism of rate
control. These principles were applied to the diffusion of
Ibuprofen in Calcium Alginate gels. For diffusion in such nonNewtonian
systems, emphasis was placed on the use of the elastic as
well as the viscous component of viscoelasticity. It was found
that the diffusion coefficients were relatively unaffected by
increases in polymer concentration up to 5 per cent, yet the
elasticities measured by oscillatory shear rheometry were increased.
This was interpreted in the light of several theories of diffusion