Biomedical Engineering Reference
In-Depth Information
Ha-based composites have also been pursued with the use of synthetic
polymers. among the biodegradable synthetic polymers that have been
presented above, PLa/PGa co-polymers have been considered. in particular,
apatite-coating has been integrated on poly(
l
-lactide) (PLLa) surfaces (Jao
et al
., 2007). this study focussed on the effect of the hydrolysis of PLLa
surfaces on the formation ability of bone-like HA. PLLA films and porous
PLLa scaffolds were hydrolysed for different time periods in alkaline solution.
the rationale of using a hydrolysis process is to enrich the polymer surface
with carboxylic and hydroxyl groups as well as to increase its roughness,
features which are known to be able to catalyse Ha formation. the Ha coating
was formed by mineralising the hydrolysed PLLa in SBF for 3 weeks and
then characterising it. indeed, this hydrolysis process increased Ha formation
on PLLa when compared with the pure PLLa and this increase was more
evident with longer hydrolysis times.
in a different method, PLa/Ha composites using high-modulus polylactic
acid fibres were prepared using a cyclic precipitation technique (Kothapalli
et al
., 2008). Small calcium phosphate crystal nuclei formed after the first
soaking cycle, while large quantities of mineral particles were observed after
six cycles (35%). The amount of mineral phase deposited on the polymer
yarn increased with deposition time in the calcium and phosphorous-rich
solutions and with the number of cycles, but it decreased with stirring
rate during washing cycles. the composites were then encased in a PCL
matrix to exhibit flexural moduli within the range of that of the cortical
bone.
14.2.3 Substrate properties of functionalisation
molecules
the sections above have shown that the ability of collagen to act as a substrate
for cell adhesion and mineralisation during bone formation depends on the
presence of specific functional groups in this protein. Specific amino acid
sequences act as bioligands for cell receptors (i.e. cell membrane integrins),
while other functionalities or collagen-bound nCPs work as nucleation
points for the formation of Ha crystals. Biomaterial surface functionalisation
strategies have been designed that allow the biomimicking of the chemical,
conformational and architectural features of these macromolecules.
these functionalisation strategies have made available a new generation
of biomaterials ready for use as synthetic 3D biocompetent extracellular
microenvironments (see Section 14.5) able to mimic the regulatory
characteristics of natural extracellular matrices (ECM) and ECM-bound
growth factors (see Section 14.4). this new generation of bioinstructive
biomaterials include self-assembling macromolecules and peptide-conjugated
synthetic polymers that present bioactive ligands and that are able to respond
