CHITIN IN THE DEVELOPMENT OF HARD TISSUE SUBSTITUTE MATERIALS
ANDREW WAN CHWEE AUN
ANDREW WAN CHWEE AUN
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Abstract
Chitin was investigated as a potential bone substitute material. Preliminary consideration
for this application involved two major aspects: fabrication methods and interaction with
calcium phosphate.
Gel-processing routes allow the production of various chitin forms including foams,
porous forms and dry plates. The porosity of the freeze-dried chitin gel could be
controlled by varying the freezing temperature or chitin gel density, a lower freezing
temperature or higher gel density giving smaller pore dimensions. The candidate found
that the novel chitin foam obtained via critical-point drying of a chitin gel possessed
better physical properties compared to the porous forms obtained by freeze-drying. The
candidate further investigated post shape modification to satisfy the different
requirements for the surface and bulk properties of chitin based biomedical materials. A
carboxymethylation reaction was applied to a variety of chitin forms, resulting in novel
products possessing different material properties, a bilayer hydrogel, a reversible, water-
swellable gel and a porous, acid-modified chitin template. In addition, post-shape
modification can be used to tailor the enzyme susceptibility of chitin hydrogels.
Water soluble carboxymethyl-chitin (CM-chitin) and phosphoryl-chitin (P-chitin) were
inhibitory towards calcium phosphate formation, as demonstrated by turbidimetry and
HA seeded growth experiments. The adsorption affinity of P-chitin onto HA was
comparable to that of phosphatidylserine and phosphoserine, leading to a potent
inhibitory activity on HA seeded growth. When CM-chitin and P-chitin were
insolubilized on porous freeze-dried chitin and incubated in a metastable calcium
phosphate solution at pH 7.4, the overall effect on calcium phosphate growth was again
inhibitory. The candidate postulates that the inhibitory effect is due to chelation of
calcium in a manner which sterically inhibits calcium phosphate formation. The
unmodified, freeze-dried chitin surface was highly calcifiable and deposition took the
form of a continuous, apatite coating.
The candidate obtained a chitin-HA composite by a solution processing method, where
HA was simultaneously dispersed with chitin, the chitin-HA blend cast into gels and the
gels solvent dried to give composite plates. Inclusion of HA provided no reinforcement,
but led instead to a decrease in the maximum tensile stress and tensile modulus of the
composites, although the composite retained a high elongation to fracture at high volume
fraction of the filler.
In conclusion, various forms of chitin with potential use in orthopaedics have been
investigated in terms of processing, morphology and mechanical properties. Modification
of chitin with acidic residues enhances its degradability and water swellability but is
inhibitory in terms of enhancing calcification. High levels of calcification can be achieved
by employing freeze-dried chitin as the substrate for mineral deposition.
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Date
1998
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