Page 86 - Advances in Textile Biotechnology

Smart materials containing enzymes or enzyme substrates 65

different release kinetics for swelling-controlled systems. For drug delivery
systems, mostly hydrogels are used. Hydrogels are glassy in their dehy-
drated state. Drug release generally involves simultaneous absorption
of water and desorption of drug via swelling controlled mechanism
(Peschier et al., 1993, Rao and Devi, 1988). The presence of solvent in a
glassy polymer causes the development of stresses that are accommodated
by an increase in the radius of gyration and end-to-end distance of polymer
molecules, which is seen macroscopically as swelling. The movement of
solvent molecules into the dry (glassy) polymer matrix takes place with a

well-defined velocity front and a simultaneous increase in the thickness of
the swollen (rubbery) region with time in the opposite direction. The exist-
ence of a slow macromolecular relaxational process in the swollen region
is believed to be responsible for the observed non-Fickian behaviour (Kim
and Lee, 1992).
However, most systems are designed to control the kinetics of release of
the active agent in a site- and time-dependent manner. In this instance, the
release of the active compounds can be triggered by the local conditions in
the target environment (Albin et al., 1987, Brownlee and Cerami, 1979,
Fischelghodsian et al., 1988, Kost et al., 1989, Hsieh et al., 1981, Jeong et al.,
1985, Kim et al., 1994, Merlin, 1991, Pinnaduwage and Huang 2002) or by
externally applied triggers (Amidon, 1997, Brannon-Peppas, 1995, Cox,
1993, Jain, 1989, Pothakamury and BarbosaCanovas, 1995, Vingerhoeds
et al., 1994) (Fig. 3.3). Much effort is focused on creating biodegradable
polymers for enzymatic drug-delivery systems that permit release of the
entrapped drug only during degradation of the polymer matrix. For this to
occur, the polymer matrix must be vulnerable to biodegradation by a com-
ponent in the surrounding media. In a model system using dextran, enzy-
matic degradation of dextran by dextranase led to the release of insulin in
a controlled manner (Moriyama and Yui, 1996). A similar system used
polycaprolactone copolymerized with PEG to form enzymatically degrada-
ble gels which can be degraded by lipase (Rice et al., 2006).

Another possibility is the preparation of films of different enzymatically
degradable polymers such as chitosan (Yomota et al., 1990). Chemicals of

interest were incorporated into chitosan films and degradation by lysozyme
and release of the loaded chemicals were investigated. The enzymatic deg-
radation rate of the fi lms was dependent on the degree of deacetylation of
chitosan used and decreased with an increase in deacetylation. The chemi-
cals were released only in the presence of lysozyme as trigger, and their
release rates were controlled by the degradation rate of the fi lms (Yomota
et al., 1990).
Other natural materials such as gelatine, collagen, alginate or fi brin were
described recently for various controlled-release applications (Patel and
Mikos, 2004, Young et al., 2005). Apart from these polymers from natural

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