Biomedical Engineering Reference
In-Depth Information
Table 11.3
Dense HA-Reinforced PAEK Composites have Exhibited an Elastic Modulus (E) and Ultimate
Tensile Strength (UTS) Similar to that of Human Cortical Bone Tissue, Porous HA Whisker-Reinforced PEKK
Scaffolds have Exhibited an Apparent Compressive Elastic Modulus ( E) and Yield Strength (YS) Similar to that
of Human Vertebral Trabecular Bone.
Uniaxial Tension
Porosity (%)
Apatite Content (vol%)
E (GPa)
UTS (MPa)
Dense HA powder and whisker-
reinforced PAEK
[12e14,25]
~0
0e40
3e19
25e118
Human cortical bone
(longitudinal)
[91,92]
~5e10
~40
16e23
80e150
Uniaxial Compression
E (MPa)
YS (MPa)
Porous HA whisker-reinforced
PAEK
[29e31]
75e90
0e40
1e190
0.002e2.7
Human vertebral trabecular
bone
[93,94]
~80e95
~40
20e500
0.5e4
(
Table 11.3
), whereas all other polymers with
bioactive reinforcements were only able to mimic the
transverse elastic modulus of human cortical bone
(
Fig. 11.6
a). The elastic modulus was increased with
increased reinforcement, as expected. HA-reinforced
PEEK was able to achieve the transverse ultimate
tensile strength of human cortical bone at a similar
volume fraction of HA
[12
e
14,25]
, similar to other
polymers, and reached the low end of the longitu-
dinal ultimate tensile strength of human cortical bone
at lower levels of HA reinforcement (
Fig. 11.6
b). The
ultimate tensile strength was decreased with
increased reinforcement for all HA-reinforced poly-
mers. HA reinforcements act as flaws in the polymer
matrices due to limited interfacial bonding. There-
fore, a design tradeoff exists between increased
bioactivity, but decreased strength, with increased
levels of calcium phosphate reinforcement. The
tradeoff can be lessened by improving load transfer
from the matrix to reinforcement.
The use of single-crystal HA whiskers was
previously shown to result in significantly improved
tensile and fatigue properties when directly
compared with conventional, equiaxed HA powder
reinforcements in HDPE composites
[76,95]
.
Compression-molded HAwhisker-reinforced PEEK
(Victrex 150XF)
[25]
exhibited a greater elastic
modulus and ultimate tensile strength compared
with injection-molded HA powder-reinforced PEEK
(Victrex 450G)
[12
e
14]
. The difference in PEEK
molecular weight was opposite to the difference in
mechanical properties; therefore, this difference
was most likely due to the HA reinforcement
morphology, but may have also been influenced by
differences in PEEK crystallinity. HA whisker-
reinforced PEEK composites were orthotropic
[25,26]
due to a preferred orientation of the HA
whiskers in the direction of flow during molding in
a channel die (
Fig. 11.4
). The degree of preferred
orientation and anisotropy were tailored to be
similar to human cortical bone
[25]
and were
strongly correlated
[26]
.
Micromechanical models have been used to study
the effects of the PEEK/HA interface on the
composite mechanical properties
[74,75]
, and the
effects of the reinforcement morphology and orien-
tation on anisotropic elastic constants
[26]
. Once
validated against experimental data, micro-
mechanical models can be useful for designing new
bioactive PAEK composites for improved perfor-
mance and elucidating the mechanisms underlying
structure
e
property relationships.
In tension
e
tension fatigue, injection-molded HA-
powder-reinforced PEEK exhibited a fatigue strength
at 1 million cycles of approximately 60, 40, 35, and
30 MPa for 0, 10, 20, and 30 vol% HA, respectively
[14,15]
. These loads were typically at least 50%
of the ultimate tensile strength. Composites failed
by debonding of the HA/PEEK interface, followed
by initiation and growth of microcracks
that
