Chitosan Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 40

crucial to the success of the reaction because the reducing agent must reduce imines selec-
tively. NaBH 3 CN is widely used in reductive alkylation systems because it is more reactive
and selective than standard reducing agents. However, it is highly toxic and generates
toxic by-products such as HCN or NaCN. Therefore, the use of this reducing agent is not
acceptable in green synthesis [1,2].
The existence of hydrophobic interactions between alkyl chains improves the physico-
chemical properties of solutions of modified chitosans. Rinaudo et al. [3] have studied the
bulk and interfacial properties of a series of alkylated chitosans having different alkyl
chain lengths (C3, C6, C8, C10, and C12) and two degrees of substitution (DSs), 2% and 5%.
The optimum alkyl chain length was C12 and the degree of grafting was 5% to obtain
physical gelation based on the formation of hydrophobic domains. The cross-linking was
essentially controlled by the salt concentration. Hydrophobic interactions produced highly
non-Newtonian behavior with large thinning behavior; this behavior was suppressed in
the presence of cyclodextrins (CDs) able to cap the hydrophobic alkyl chains. The interfa-
cial properties of chitosan derivatives were tested for air/aqueous solution interfaces.
Specifically, the role of their structure in the kinetics of film formation was examined,
showing that an excess of external salt favors stabilization of the interfacial film. Derivatives
with a higher DS and longer alkyl chains were more efficient and gave a higher elastic
modulus compared to the model surfactant as a result of the chain properties.
Apart from rheological studies and the application of chitosan alkyl derivatives as rheo-
logical modifiers, especially in aqueous-based formulations in various industrial domains
such as paints, oil recovery, cosmetics, and food, alkyl chitosan derivatives have been
used in drug delivery systems, tissue engineering, and several technological applications.
Klotzbach et al. [4,5] modified chitosan with butanal, hexanal, octanal, or decanal alde-
hydes to prepare a biocompatible and biodegradable hydrophobic chitosan membrane
that can replace Nafion® ® for electrode coatings in both sensor and fuel cell applications.
Several enzymes such as glucose oxidase, alcohol dehydrogenase, formate dehydroge-
nase, lactic dehydrogenase, glucose dehydrogenase, and formaldehyde dehydrogenase
were successfully immobilized and voltammetric studies were carried out. This is the
first evidence which shows that hydrophobically modified chitosan can be used at the
anode of a biofuel cell.
N -Succinyl- N -octyl chitosan (SOCS), which can form micelles in aqueous media, has
been prepared by modifying the amino group with hydrophobic long-chain alkyl func-
tionality and a hydrophilic succinyl moiety [6]. Doxorubicin (DOX), a model antitumor
drug, was successfully loaded into SOCS micelles and a sustained release pattern was
observed. The in vitro antitumor activity studies indicated that DOX-loaded SOCS micelles
were more cytotoxic than free DOX.
2.3 Quaternized Chitosan
Chitosan has been extensively evaluated for its mucoadhesive and absorption enhance-
ment properties. The positive charge on the chitosan molecule gained by the acidic envi-
ronment in which it is soluble seems to be important for absorption enhancement. However,
chitosan is not soluble in medium except below pH 5.6. This limits its use as a permeation
enhancer in body compartments where the pH is high. In this regard there is a need for
chitosan derivatives with increased solubility, especially at neutral and basic pH values.