Chitosan Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 60

H 3 PO 4 /Et 3 PO 4 /P 2 O 5 /butanol phosphorylation reaction system, at 30°C, for periods up to
48 h [170]. The phosphorylation method is based on the H 3 PO 4 /Et 3 PO 4 /P 2 O 5 /hexanol reac-
tion route optimized by Granja et al. [171] for the synthesis of highly phosphorylated cel-
lulose derivatives and originally proposed by Touey and Kingsport [172] in 1956 for the
synthesis of water-soluble and nondegraded cellulose phosphates. This method has the
advantage of being carried out at room temperature, with low degradation of the polymer
thus resulting. The H 3 PO 4 /Et 3 PO 4 /P 2 O 5 /butanol method is an alternative to the H 3 PO 4 /
urea/dimethylformamide method described for the phosphorylation of chitin and chito-
san [146,173], which is the phosphorylation pathway usually followed to introduce phos-
phate functionalities into chitin fibers and chitosan films/sponges and which involves the
use of high temperatures, typically higher than 120°C [174-176].
The influence of two water-soluble anionic derivatives, namely sodium carboxymethyl-
chitin (CM-chitin) and P-chitin, on the crystallization of calcium phosphate by seeded
growth and turbidity was studied in [177]. Macroporous scaffolds were obtained through a
freeze-drying technique using both NMPC and CS [178]. Biomimetic mineralization was
carried out in different media, that is, simulated body fluid (SBF) or CaCl 2 and Na 2 HPO 4
solutions [116]. NMPC with phosphonic groups led to clear differences in the ability of scaf-
fold wall surface to support heterogeneous calcium phosphate nucleation and growth.
Chitosan scaffold incubated in SBF for 20 days did not display mineral growth, whereas
NMPC scaffold showed increasing mineral particulates, surface coverage with an essential
continuous mineral layer. The biomineralization behavior of NMPC was superior to that of
chitosan at higher soaking cycle number. Moreover, these scaffolds only deposit apatite
slightly in SBF. This result provided NMPC derivatives constituting the biocomposite scaf-
fold with improved compressive stiffness. Similarly, it has been reported that calcium phos-
phate mineral was formed on P-chitosan membranes after soaking with Ca(OH) 2 [179].
2.8 Thiolated Chitosan
Thiomers are hydrophilic macromolecules bearing free thiol groups on their backbone.
Due to the immobilization of thiol groups on already well-established polymers, their
mucoadhesive, enzyme-inhibitory, permeation-enhancing, and efflux pump-inhibiting
properties are strongly improved. Thiol-bearing ligands can be covalently immobilized on
the primary amino groups at the C-2 position of the glucosamine subunits of chitosan.
According to this, chitosan-cysteine [180], chitosan-glutathione [181], chitosan-thioethyl-
amidine [182], chitosan-thioglycolic acid [183], chitosan-4-thio-butyl-amidine [184], chito-
san- N -acetyl cysteine conjugates [185], and chitosan-6-mercaptonicotinic acid conjugate
[186] as well as other thiolated chitosans [187,188] have been synthesized ( cf. Figure 2.17).
The primary amino group at the 2-position of the glucosamine subunits of chitosan is
the main target for the immobilization of thiol groups. As outlined in Figure 2.17, sulfhy-
dryl-bearing agents can be covalently attached to this primary amino group via the forma-
tion of amide or amidine bonds. In the case of the formation of amide bonds, the carboxylic
acid group of the ligands cysteine and thioglycolic acid reacts with the primary amino
group of chitosan mediated by a water-soluble carbodiimide. The formation of disulfide
bonds by air oxidation during the synthesis is avoided by performing the process at a pH
below 5. At this pH range, the concentration of thiolate anions, representing the reactive
form for oxidation of thiol groups, is low, and the formation of disulfide bonds can be