Biomedical Engineering Reference
In-Depth Information
Quantitative microstructural characterization has
mainly included the apparent density of the
composite, the PAEK crystallinity (above), and the
size and volume fraction of the bioactive reinforce-
ments. If the PAEK polymer can be pyrolized at
a temperature where the bioactive reinforcements are
unaffected, the size, morphology, and volume frac-
tion of reinforcements in the composite can be
measured after shape forming
[12,26,56]
. Dispersion
of the reinforcements in the PAEK matrix and failure
surfaces has been typically assessed qualitatively
from optical or SEM micrographs. The exposure of
bioactive reinforcements on PAEK surfaces has been
qualitatively observed from SEM micrographs and
von Kossa staining
[28]
. Note that the broad fluor-
escence emission spectrum of PEEK, ranging from
400 to 600 nm
[57]
, interferes with common fluor-
ophores (e.g., alizarin, calcein) for labeling calcium.
The crystallographic and morphological orientation
of single-crystal HAwhisker reinforcements in PEEK
was characterized using quantitative texture analysis
with X-ray diffraction (XRD)
[25]
. Composites
exhibited a mechanically advantageous preferred
orientation of HA whiskers along the length of
compression-molded tensile bars, which was similar
to that exhibited by apatite crystals in human cortical
bone tissue along the principal stress direction. The
pore volume, architecture, and interconnectivity of
HA whisker-reinforced PEKK scaffolds were quan-
titatively characterized using micro-CT
[28]
.
There is a need for greater attention to quantitative
microstructural characterization in order to establish
structure
e
property relationships and rationally design
bioactive PAEK composites. A “make it and break it”
approach (processing
e
properties) that does not pay
careful attention to the composite microstructure
[10]
will be detrimental to continued progress.
Figure 11.5
Photograph showing various examples of
dense and/or macroporous bioactive PAEK composites
of varying size and shape produced by compression
molding, compared with a commercial cervical spinal
fusion cage (upper left). All specimens comprised
PEEK (Invibio LT1) reinforced with 20 vol% calcium-
deficient HA whiskers and were molded either fully
dense or with 75 vol% porosity using a sodium chloride
porogen (
Fig. 11.4
). Note that the dense beam at the
bottom has dimensions of 43 10 2.5 mm.
was unaffected (20
e
22%), but crystallinity in the
bulk “core” region of composites increased from
24% to 31% with 0 to 40 vol% HA reinforcement
[12]
. Differences in the core/skin are not unex-
pected due to differences in cooling rate and were
subsequently minimized by the aforementioned
annealing treatments. There has been little investi-
gation of the effects of the cooling rate or annealing
treatment on the crystallinity of calcium phosphate-
reinforced PEEK composites, as well as the pres-
ence or effects of an interphase layer adjacent to
reinforcement particles. This is surprising consid-
ering their known importance in carbon fiber-
reinforced PEEK
[55]
.
Bioactive PAEK composites have been machined
after shape-forming processes using standard tooling.
Porous scaffolds were also readily machined prior to
leaching the porogen
[30]
, as shown by grooves milled
onto the top surface of an implant shown in
Fig. 11.5
.
11.3 StructureeProperty
Relationships
11.3.1 Biological Properties
PAEK polymers are well known to be biocom-
patible and bioinert
[2,3,58
e
64]
. PAEK and carbon
fiber-reinforced PAEK were encapsulated by a layer
of fibrous tissue in vivo after intramuscular implan-
tation in rabbits
[58,60]
, subcutaneous implantation
in sheep
[63]
, fixation of a canine femoral osteotomy
[60]
, and injection of particles into the spinal canal of
