Biomedical Engineering Reference
In-Depth Information
chitins and chitosans was investigated. It turned out that the digestibility of chitin
by the chitinase from
Bacillus
sp. PI-7S is much higher than by lysozyme. Also
β
- chitin, and chitosan deacetylated
under homogeneous conditions was hydrolyzed by lysozyme more rapidly than
that under heterogeneous conditions [82].
In contrast to chitin, chitosan is highly soluble in diluted acids. The primary
amino groups in chitosan are protonated below pH 6.0, resulting in a water-soluble
cationic polyelectrolyte. At higher pH values, the ammonium salt gets deproto-
nated resulting in a neutral amino group and the polymer gets insoluble. On the
other hand, this solubility transition is highly dependent on the degree of
N
-
acetylation and chitosan with 50%
N
-acetylation is soluble even under alkaline
conditions [78]. In addition, the anion of the acid plays an important role for the
solubility of chitosan. While many acids such as acetic, citric, formic, hydrochloric,
lactic, and diluted nitric acid can easily dissolve chitosan, the phosphates and
sulfates of chitosan are not soluble in water [76].
Several approaches were published to solubilize chitin with and without chemi-
cal modifi cation of the polymer. 2.77 M sodium hydroxide was reported as
good solvent for chitin and the addition of urea did improve the solubility [83].
A powerful organic solvent system for chitin was fi rst described by Austin
and Rutherford. They found that lithium chloride forms a complex with the aceta-
mide carbonyl group of chitin [84]. The resulting complex is soluble in polar
organic solvents such as
N
- methyl - 2 - pyrrolidinone,
N,N
- dimethylacetamide,
N,N
-
dimethylpropionamide, and 1,3 - dimethyl - 2 - imidazolidinone. Chitin solutions
with a concentration of 5-7% (w/v) could be obtained using these conditions [85].
Another suitable solvent system is CaCl
2
-dihydrate saturated methanol as reported
by Tamura [86]. The water content is essential and anhydrous CaCl
2
in methanol
does not dissolve chitin at all. Two grams of
-chitin was digested more smoothly than
α
-chitin powder can be dissolved in
100 mL of CaCl
2
· (H
2
O)
2
- saturated methanol but just 0.5 - 1 g of
α
- chitin is soluble
under those conditions. The solubility is also affected by the degree of
N
- acetylation
and the molecular weight of chitin as depicted in Figure 7.1 [87].
Another successful strategy for chitin dissolution is the synthesis of soluble
chitin esters. The introduction of bulky acyl groups into the chitin chain yields
chitin derivatives with improved solubility [88]. Acetylchitin is readily synthesized
and spun into fi bers but still polar acidic solvents such as formic acid are necessary
to dissolve the material [89]. Butyrylchitin, with a larger substituent in the chain,
can be synthesized using methanesulphonic acid as catalyst and solvent. This
derivative is easily soluble in several organic solvents, such as acetone, methanol,
ethanol, dimethylformamide, and methylene chloride [90]. A simpler method for
the synthesis of highly substituted dibutyrylchitin with butyric anhydride uses 70%
perchloric acid as a catalyst. Dibutyrylchitin fi bers with a porous core were made
by a simple method of dry spinning its 20-22% solutions in acetone. These fi bers
have tensile properties similar to or better than those of chitin. Alkaline hydrolysis
of the butyric esters restores chitin, and even fi bers with good tensile properties
can be obtained by alkaline hydrolysis of dibutyrylchitin fi bers in 5% sodium
hydroxide at 55 °C without destroying the fi ber structure [91]. The ester cleavage
β
