Biomedical Engineering Reference
In-Depth Information
with few crystalline defects and a relatively large
crystal size, although the particle size may be tailored
by grinding and/or sorting. Low-temperature synthesis
(
Compounding and injectionmolding is amenable to
low-cost, high-volume commercial manufacturing of
net shapes with a dense microstructure. Standard
PAEK beads for injection molding may be used since
bioactive reinforcements are mixed into the molten
polymer by shear flow during compounding. However,
the increased melt viscosity with the addition of
calcium phosphate reinforcements limits reliable
mixing andmolding to less than 30
e
40 vol%, and high
reinforcement fractions may cause equipment wear.
Cold pressing and pressureless sintering requires
low overhead equipment costs and is amenable to
almost any level of reinforcement during processing.
Similarly, electrophoretic deposition and pressureless
sintering was used to prepare uniform PEEK coatings
reinforced by bioglass (45S5) particles
[35]
. The
absence of applied pressure during sintering results
in residual microporosity on the size scale of the
starting powders. This microporosity may be bene-
ficial for fluid entrapment and cell attachment, but is
detrimental to mechanical properties
[20]
.
Compression molding is similar to injection
molding in relatively low-cost, high-volume
commercial manufacturing of net shapes with
a dense microstructure, except that productions rates
are slightly lower and machining may be required to
attain nongeometric shapes. Like cold pressing and
pressureless sintering, compression molding is
amenable to nearly any level of reinforcement during
processing. However, unlike cold pressing and pres-
sureless sintering, the resultant microstructure is
fully dense, resulting in improved mechanical prop-
erties. Furthermore, tailored macroporosity has been
200
C)
d
including hydrothermal synthesis and
precipitation
d
generally enables greater control over
crystal defects (disorder), doping, size, and
morphology. For example, calcium-deficient, carbon-
ated, and other defective HA crystals prepared by low-
temperature synthesis exhibit greater solubility than
stoichiometric HA prepared by high-temperature
synthesis (
Table 11.2
), which may lead to greater
bioactivity
[48]
. Powders prepared by high-tempera-
ture solid-state reactions or calcination are generally
equiaxed or spherical (
Fig. 11.3
). Single-crystal HA
whiskers or platelets, which mimic the morphology of
natural apatite crystals in mineralized tissues
(
Fig. 11.3
), have been prepared by hydrothermal
synthesis
[49
e
51]
and molten salt synthesis
[52]
.The
size of hydrothermally synthesized HA has ranged
from the extremes of nanoscale (
100 nm)
[53]
to
several millimeters
[54]
.
11.2.3 Composite Manufacturing
and Microstructure
PAEK composites have been prepared by (1)
compounding and injection molding
[11
e
15,21]
, (2)
cold pressing and pressureless sintering
[19,20,33]
,
(3) compression molding
[25
e
32]
, and (4) selective
laser sintering (SLS)
[16
e
18,21
e
24]
(
Table 11.1
).
The latter three methods can also be used to produce
macroporous PAEK scaffolds. Each method has its
own advantages and disadvantages.
Figure 11.3
Schematic diagram
(not to scale) showing common
morphologies of natural and
synthetic hydroxyapatite (HA)
crystals. The SEM micrographs
show equiaxed HA crystals
prepared by calcination, as well
as whisker and plate-like calcium-
deficient HA crystals prepared by
hydrothermal synthesis.
Equiaxed
(synthetic)
Whiskers or Needles
(enamel & synthetic)
Platelets
(bone & synthetic)
[210]
a
-axis
[100]
c
-axis [001]
c
-axis [001]
1 μm
10 μm
1 μm
