Biomedical Engineering Reference
In-Depth Information
appear to be in a good agreement with HA/PLLA biocomposite unit
cell model predictions [378].
On their own, PGA and PLA are known to degrade to acidic products
(glycolic and lactic acids, respectively) that both catalyze polymer
degradation and cause inflammatory reactions of the surrounding
tissues [379]. Thus, in biocomposites of poly(α-hydroxyesters) with
calcium orthophosphates, the presence of slightly basic compounds
(HA, TTCP) to some extent neutralizes the acid molecules, provides
with a weak pH-buffering effect at the polymer surface and, therefore,
more or less compensates these drawbacks [168, 380-382]. However,
additives of even more basic chemicals (e.g., CaO, CaCO
) might be
necessary [173, 381, 383, 384]. Extensive cell culture experiments
on pH-stabilized composites of PGA and carbonateapatite were
reported, which afterwards were supported by extensive
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pH-
studies [385]. A consequent development of this approach has led to
designing of functionally graded composite skull implants consisting
of polylactides, carbonateapatite, and CaCO
in vitro
[386, 387]. Besides the
pH-buffering effect, inclusion of calcium orthophosphates was found
to modify both surface and bulk properties of the biodegradable
poly(α-hydroxyesters) by increasing the hydrophilicity and
water absorption of the polymer matrix, thus altering the scaffold
degradation kinetics. For example, polymer biocomposites filled
with HA particles were found to hydrolyze homogeneously due to
water penetrating into interfacial regions [388].
Biocomposites of poly(α-hydroxyesters) with calcium
orthophosphates are prepared mainly by incorporating the inorganic
phase into a polymeric solution, followed by drying under vacuum.
The resulting solid biocomposites might be shaped using different
processing techniques. One can also prepare these biocomposites by
mixing HA particles with L-lactide prior the polymerization [380] or
by a combination of slip-casting technique and hot pressing [389].
Addition of a surfactant (surface active agent) might be useful to
keep the suspension homogeneity [390]. Furthermore, HA/PLA
[314, 315] and HA/PLGA [316] microspheres might be prepared
by a microemulsion technique. More complex carbonated-FA/PLA
[391] and PLGA/carbon nanotubes/HA [392] porous biocomposite
scaffolds are also known. An interesting list of references, assigned
to the different ways of preparing HA/poly(α-hydroxyesters)
biodegradable composites, might be found in publications by
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