MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 74

Bonfi eld used coupling agents, such as 3-trimethoxysiyl propylmethacrylate for
HAp and acrylic acid for HDPE, to improve bonding (by both chemical adhesion
and mechanical coupling) between HAp and HDPE. The improvement in bond-
ing resulted in enhanced ductility and tensile strength. In the case of untreated
composite, only mechanical bonding exists at the fi ller - matrix interface, which
resulted from the shrinkage of HDPE around individual HAp particles during
thermal processing.
Bonfi eld et al. 79 also reported an optimum combination of mechanical and
biological performance with HAp/HDPE composite containing 40 vol% of HAp.
An in vitro cell culture study showed that 40 vol% HAp/HDPE composite
enhanced cellular activity by increasing proliferation rate and differentiation
compared to the 20 vol% HAp/HDPE composite 80 . The extensive mechanical
measurements reveal the tensile strength, E-modulus and strain-to-failure for
surface-treated composite to be 23.2 MPa, 3.9 GPa and 6.8%, while that of un-
treated composite to be 20.7 MPa, 4.3 GPa and 2.6%, respectively 81 .
In a subsequent work, in vitro study revealed that HAPEX TM attached
directly to a bone by chemical bonding (bioactive fi xation), rather than forming
fi brous encapsulation (morphological fi xation). The effect of HAp particle size on
the polymer-ceramic composite properties was also investigated by various re-
searchers 82 . It was found that HAp particles (fi ner) reinforced HDPE exhibited
higher E-modulus and hardness. It has been observed that the mechanical prop-
erties were signifi cantly increased with an increase in HAp volume fraction, while
fracture strain decreased. The tribological properties (that is, wear rate and coef-
fi cient of friction) of unfi lled HDPE and HAp/HDPE composites were investi-
gated 83 against duplex stainless steel on a tri-pin-on-disc tribometer under dry
and lubricated conditions using distilled water, aqueous solution of protein (egg
albumen) and glucose as lubricants. In general, HAp/HDPE composite exhibited
a lower COF (

0.035 - 0.045). The com-
posite with 10 vol% of HAp was found to exhibit improved friction and wear
behavior.
Recently, many researchers are attempting to improve the mechanical prop-
erties of the HDPE-based composites with minimal compromise on the biocom-
patibility component, by incorporating other ceramic phases into the polymer
matrix 84 . The partial replacement of HAp fi ller particles with PSZ particles led to
an increase in the strength and fracture toughness of HAp/HDPE composites.
The compressive stress, set up by the volume expansion associated with tetrago-
nal to monoclinic phase transformation of PSZ, inhibits or retards the crack prop-
agation within the composite. This results in an enhanced fracture toughness of
the HAp/ZrO 2 /HDPE composite 85 .
In another study, Al 2 O 3 , a bioinert material, has been investigated to replace
ZrO 2 in the HAp/ZrO 2 /HDPE composite. The excellent mechanical properties
and chemical properties of Al 2 O 3 with respect to HAp are considered as a new
trend in designing a polymer-ceramic biocomposite 86 . Other HAp - polymer com-
posites used in hard tissue replacements are HAp/PEEK 87,88,89 , HAp/EVA 90 , and
nano HAp/poly (hexamethylene adipamide) 91 composite.

∼ 0.02 - 0.04) compared to unfi lled HDPE ( ∼