Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 349

Unswollen
disintegrant
particles

Wet environment

Swollen disintegrant
particles

Stress concentrated
area

Dry dosage form
(tablet)

Hydrated
dosage form

Weakened
structure

Disintegrating
dosage form

Figure 11.15 Disintegration mechanism for superdisintegrant tablets. From Omidian and Park (2008) with
permission from Editions de Santé.

eliminating the need for water, and are adapted especially for children and the elderly,
who may have dif

culty swallowing the whole tablet. Initially the polymeric material
should be in the glassy state, and form the matrix containing the drug particles. When
water comes into contact with the polymer, swelling will begin as water penetrates
between these chains. The process is self-accelerating, and a sheath of the swollen
material is formed. In the swollen state, drug release is usually diffusion-controlled for
water-soluble drugs, while water-insoluble drugs are released through a different,
erosion-controlled mechanism. This so-called ' pseudo-swelling ' is the major mechanism

of controlled drug delivery in tablet matrices.
Such anisotropic swelling behaviour can be designed into tablets. These are
generally manufactured under compression, and the polymer used in the tablet formulation
is compressed uniaxially. In the presence of water it will swell to a greater extent axially
than radially (Palleschi et al., 2006 ). This is generally observed in sustained-release tablets
where hydroxypropyl methylcellulose is used as a swelling agent (Papadimitriou et al.,
1993 ). Anisotropic swelling with scleroglucan

borax (Coviello et al., 2005 ) and xanthan
gum tablets (Talukdar and Kinget, 1995 ) has also been reported. Tablet disintegration times
are typically found to be inversely proportional to the swelling ability of the superdisinte-
grant. The disintegrant particles are generally small and porous, which allows for rapid
tablet disintegration in the mouth without undesirable mouth-feel from either large particles
or the gel texture. Such tablets prepared by direct compression in the presence of gel-
forming microparticles disintegrate in less than 10 s.

-

11.3.7 Further applications in tissue engineering

Tissue engineering has been de ned in a number of ways, but in practice it covers the whole

area pertinent to designing and growing cells on substrates of appropriate materials. In the
present volume we are only interested in cases where physical gels have been used as the
material support. Some cases, including the arti

cial pancreas using alginate gels as cell
supports, have already been described ( sections 5.5.4 and 11.3.2 ). This is a vast and very
rapidly growing field, so we can only produce a very selective summary. Many papers on

physical gels now tend to stress potential applications in this

field, even though no tissue