Biomedical Engineering Reference
In-Depth Information
polymer backbone, which can act as sites for bridging to calcium
orthophosphates, are more promising in this respect [53].
In order to improve the situation, various supplementary reagents
are applied. Namely, if the primary effect of a processing additive is
to increase the interaction between the phases, such additives can be
regarded as coupling agents [1140]. These agents establish chemical
bridges between the matrix and the fillers, promoting adhesion
among the phases. In many cases, their effect is not unique; for
example, it also might influence rheology of the composites [282]. In
the case of calcium orthophosphates, a hexamethylene diisocyanate
coupling agent was used to bind PEG/PBT (Polyactive
TM
) block
copolymers [293] and other polymers [1133] to HA filler particles.
Thermogravimetric and infrared analysis demonstrated that the
polymers were chemically bonded to the HA particles through
the isocyanate groups, making it a suitable approach to improve
the adhesion [1133]. Other researchers used glutaraldehyde as a
cross-linked reagent [470, 474, 598, 600, 601, 619, 624, 699, 736,
778, 781, 1141]. Besides, the interfacial bonding among calcium
orthophosphates and other components might be induced by
silanes [242, 268, 269, 293, 406, 640, 1142-1145], zirconates [282,
406, 408, 1146, 1147], titanates [282, 406, 1146], phosphoric acid
[643], alkaline pretreatment [877, 880], polyacids [143, 144, 293]
and some other chemicals. Furthermore, some polymers might be
grafted onto the surface of calcium orthophosphate particles [656].
Structural modifications of the polymeric matrices, for instance, with
introduction of acrylic acid [245, 268, 269, 293], have also proved
to be effective methods. For example, application of polyacids as a
bonding agent for HA/Polyactiveâ„¢ composites caused the surface
modified HA particles to maintain better contact with the polymer
at fracture and improved mechanical properties [143, 144, 293].
The use of titanate and zirconate coupling agents appeared to be
very dependent on the molding technique employed [282]. Silane-
coupled HA powders were tested before applying them as fillers in
biodegradable composites [1143-1145]. This treatment allowed
HA withstanding the attack of water without impairing overall
bioactivity. Besides, a chemically modified reinforcement phase-
matrix interface was found to improve the mechanical properties of
the biocomposites. The examples include chemically coupled HA/PE
[268, 269], chemically formed HA/Ca poly(vinylphosphonate) [349]
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