287
(beads) with better physicochemical properties: they are easily swollen, being insoluble in water and organic solvents, and stable in alkaline solutions.
These networks become insoluble provided that the “gel point” has been reached
during the cross-linking process, and, because of their hydrophilic character, they
will have the ability to absorb high amounts of water. They can be considered, therefore, as hydrogels. Obviously, the presence of cyclodextrin units in these hydrogels
makes them capable of forming inclusion complexes with suitable host molecules.
Nevertheless, the structure of the polymer network needs now to be taken into
account, looking at other factors that influence the diffusion of molecules. In order
for the sorbate molecules to be trapped by the polymer, they must interact with the
cyclodextrin cavity and/or with the polymer network. Therefore, the amount of
cyclodextrin in the polymer, the amount and type of cross-linker used, the swelling
capacity (a consequence of the latter), and the possible further chemical modifications thereof need to be considered. These cyclodextrin polymers, in addition to the
“rigid” cavities of the cyclodextrin moieties, have other cavities constituted by the
cross-linking bridges between the cyclodextrin units (secondary cavities). Those
additional cavities can be considered more flexible than those of cyclodextrin, at
least when the cross-linker is the self-polymerizable epichlorohydrin molecule.
Sorption in these polymers takes place then by inclusion in the cyclodextrin cavities
or by specific interactions in the secondary ones. In addition, the cross-linker tails
are also capable of interacting with the sorbate molecules. As potential drug delivery devices, cyclodextrin-epichlorohydrin hydrogels made with a controlled geometry are useful to obtain kinetic parameters by fitting experimental data to
mathematical models suitable for drug release and to obtain diffusion coefficients
according to the contact surface of the polymeric matrix (Machín et al. 2012).
Among the most frequently used non-epoxide cross-linkers are diisocyanates,
first introduced by Buckler and co-workers (1969), as mentioned above.
Dihalogenated acid dihalides or dihalogenated dicarboxylic acids of different sizes
have also been used as space arms (Buckler et al. 1969; Zemel and Koch 1990),
besides other agents such as dihalogenated alkenes or, more recently, maleic anhydride (Girek et al. 2000). Shono’s group prepared insoluble porous polymers with a
high cyclodextrin content, polymerizing α- and β-cyclodextrin with diisocyanates
as cross-linking agents in pyridine or dimethylformamide, and studied their capability to absorb aromatic derivatives (Mizobuchi et al. 1980; Tanaka et al. 1981). Since
those initial investigations, the most commonly used diisocyanates to produce
cyclodextrin polymers are hexamethylene diisocyanate and toluene diisocyanate.
After those, other groups also prepared cyclodextrin polymers with these same
cross-linkers and certain properties, and applications have been studied thereof: stationary phases in chromatography (Lee et al. 2002), artificial cholesterol receptors
(Asanuma et al. 1998), solid phase for the extraction, and subsequent analysis of
carcinogenic aromatic compounds (Bhaskar et al. 2004). Ma’s group, which also
used these two diisocyanates, postulated the presence of interconnected nanoporosity in these polymers (Li and Ma 1999; Ma and Li 1999), which increases the apparent inclusion constant with respect to other cyclodextrin polymers such as those
cross-linked with epichlorohydrin. The use of difunctional cross-linkers with longer
6 Cyclodextrin-Based Polymers for Food and Pharmaceutical Applications…
(beads) with better physicochemical properties: they are easily swollen, being insoluble in water and organic solvents, and stable in alkaline solutions.
These networks become insoluble provided that the “gel point” has been reached
during the cross-linking process, and, because of their hydrophilic character, they
will have the ability to absorb high amounts of water. They can be considered, therefore, as hydrogels. Obviously, the presence of cyclodextrin units in these hydrogels
makes them capable of forming inclusion complexes with suitable host molecules.
Nevertheless, the structure of the polymer network needs now to be taken into
account, looking at other factors that influence the diffusion of molecules. In order
for the sorbate molecules to be trapped by the polymer, they must interact with the
cyclodextrin cavity and/or with the polymer network. Therefore, the amount of
cyclodextrin in the polymer, the amount and type of cross-linker used, the swelling
capacity (a consequence of the latter), and the possible further chemical modifications thereof need to be considered. These cyclodextrin polymers, in addition to the
“rigid” cavities of the cyclodextrin moieties, have other cavities constituted by the
cross-linking bridges between the cyclodextrin units (secondary cavities). Those
additional cavities can be considered more flexible than those of cyclodextrin, at
least when the cross-linker is the self-polymerizable epichlorohydrin molecule.
Sorption in these polymers takes place then by inclusion in the cyclodextrin cavities
or by specific interactions in the secondary ones. In addition, the cross-linker tails
are also capable of interacting with the sorbate molecules. As potential drug delivery devices, cyclodextrin-epichlorohydrin hydrogels made with a controlled geometry are useful to obtain kinetic parameters by fitting experimental data to
mathematical models suitable for drug release and to obtain diffusion coefficients
according to the contact surface of the polymeric matrix (Machín et al. 2012).
Among the most frequently used non-epoxide cross-linkers are diisocyanates,
first introduced by Buckler and co-workers (1969), as mentioned above.
Dihalogenated acid dihalides or dihalogenated dicarboxylic acids of different sizes
have also been used as space arms (Buckler et al. 1969; Zemel and Koch 1990),
besides other agents such as dihalogenated alkenes or, more recently, maleic anhydride (Girek et al. 2000). Shono’s group prepared insoluble porous polymers with a
high cyclodextrin content, polymerizing α- and β-cyclodextrin with diisocyanates
as cross-linking agents in pyridine or dimethylformamide, and studied their capability to absorb aromatic derivatives (Mizobuchi et al. 1980; Tanaka et al. 1981). Since
those initial investigations, the most commonly used diisocyanates to produce
cyclodextrin polymers are hexamethylene diisocyanate and toluene diisocyanate.
After those, other groups also prepared cyclodextrin polymers with these same
cross-linkers and certain properties, and applications have been studied thereof: stationary phases in chromatography (Lee et al. 2002), artificial cholesterol receptors
(Asanuma et al. 1998), solid phase for the extraction, and subsequent analysis of
carcinogenic aromatic compounds (Bhaskar et al. 2004). Ma’s group, which also
used these two diisocyanates, postulated the presence of interconnected nanoporosity in these polymers (Li and Ma 1999; Ma and Li 1999), which increases the apparent inclusion constant with respect to other cyclodextrin polymers such as those
cross-linked with epichlorohydrin. The use of difunctional cross-linkers with longer
6 Cyclodextrin-Based Polymers for Food and Pharmaceutical Applications…