Application of Chitosan-Based Biomaterials in Tissue Engineering - Chitosan Based Hydrogels: Functions and Applications - page 406

Figure 9.4
SEM micrographs of the chitosan nanofibrous scaffold (a) using HFIP [13] and (b) TFA as a spinning solvent.
(From Schiffman, J. D. and Schauer, C. L. 2007. Biomacromolecules 8: 2665-2667. With permission.)
The choice of solvent is crucial to obtain the ideal chitosan-based fiber scaffold. Min
et al. [13] created chitosan nanofibers by first electrospinning chitin nanofibers with
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as the solvent, followed by deacetylation of the
as-spun chitin fibers with NaOH solutions at various temperatures. The mean diameter is
ca. 180 nm ( cf. Figure 9.4a). As for the pure chitosan system, two direct spinning methods
from chitosan solutions were reported. One is the concentrated acetic acid solvent system;
Jang and coworkers [14] found that only chitosan with 106 kDa can be electrospun to give
bead-free thin fibers with pump-feeding of the chitosan-aqueous 90% acetic acid solu-
tion. In this system, the repulsive forces between ionic groups within the chitosan back-
bone that arise due to the application of a high electric field during electrospinning are
very strong, which restrict the formation of continuous fibers and often produce particles.
The other is the trifluoroacetic acid (TFA) solvent system. Chitosan dissolved in TFA can
be electrospun into randomly oriented, continuous, bead-free fibers ( cf. Figure 9.4b) [15].
There are two possible reasons why the electrospinning of chitosan is successful when
using TFA: (a) TFA forms salts with the amino groups of chitosan and this salt formation
destroys the rigid interaction between chitosan molecules, making them ready to be elec-
trospun; (b) the high volatility of TFA is advantageous for the rapid solidification of the
electrified jet of chitosan-TFA solution [16,17]. The addition of dichloromethane to the
chitosan-TFA solution improved the homogeneity of the electrospun chitosan fibers. The
diameter of chitosan fibers is influenced by the solution concentration and molecular
weight (MW) of chitosan. In general, as the solution concentration decreased, the average
fiber diameter linearly decreased ( cf. Figure 9.5) [18]. It was found that a 7% concentration
produced beads on the fibers, while an 8% concentration produced an almost uniform
fiber network. The average fiber diameter ranges from 74 to 108 nm and increased as the
MW increased. The low MW fibers exhibited some branching, which was possibly due to
the intrinsically low MW of bulk chitosan [19]. In addition, the average diameter decreased
with narrower diameter distribution as the applied electric field increased.
Recently, electrospun chitosan composite nanofibrous scaffolds have been fabricated
using chitosan and synthetic biodegradable protein, polysaccharide, and synthetic polymer.
These composite fiber mats have advantage over the electrospinning of pure chitosan due
to their intrinsic character [20]. Table 9.1 summarizes the experimental conditions adapted
for the fabrication and the obtained average size of electrospun chitin nanofibers.