Biomedical Engineering Reference
In-Depth Information
present as a smaller diameter particle that prevents
flow of the PEEK particles and permits them to sinter
together when at 400
C for 5 min. The TCP can be
subsequently left to act as an osseoinductive agent or
can be acid leached to leave the pores with a micro-
topography that may aid cell migration. The patent
filing describes several spinal device designs, again
some utilizing a hybrid solid-porous approach. These
patent filings are just some of the recent submissions
that feature methods of manufacturing a porous PEEK
or of device designs featuring a porous PEEK. This
recent activity demonstrates that there is some belief in
such a material presenting beneficial properties.
12.4.4 Microwave Sintering
Ceramics have been the main focus of sintering
usingmicrowave technology but there are some studies
examining the potential using polymers. These poly-
mer studies have mainly targeted the thermoset poly-
mers rather than the thermoplastics such as PEEK,
which have a lower dielectric loss factor and absorb
little energy. Compounding PEEK with a densifying
filler can increase the potential of thematerial to absorb
microwaves. Zhang and Leparoux
[25]
used a 10%SiC
filler in the PEEK and found that using smaller filler
particles permitted more rapid and uniform heating.
12.4.5 Micromachining
With modern machining capabilities, it is now
possible to laser or microdrill small diameter holes
into a solid PEEK substrate to a depth where they can
interconnect. Although this does create a rather
uniform result, it is also currently labor intensive for
micromachining many individual channels. Although
this can be eased through using laser drilling, there are
other concerns around the highlocalized temperatures
generated that may cause degradation of the polymer.
12.4.3 Selective Laser Sintering
Another method of fusing particles together is
laser sintering, where the energy source comes from
a laser rather than a heat source. The great advantage
to this process is that because the process builds up
the shape from the base by adding sequentially layer
by layer of material across the
x
-axis then it is
possible to make complex three-dimensional shapes.
Each layer may be up to 200
m
m thick at a time.
High-performance thermoplastics such as PEEK
have proved challenging due to their high melting
temperature. EOS (Munich, Germany) has developed
an SLS system (EOSINT P 800) that can process at
temperatures up to 385
C. It has produced solid
PEEK with tensile strengths of 95 MPa and a tensile
modulus of 4.4 GPa, thus creating parts with good
mechanical properties but additionally in highly
complex geometries. For industrial uses, the grade of
PEEK has been modified (PEEK-HP3) by EOS and
Victrex
to cater for the unique demands of laser
sintering such as the high energy needed and opti-
mizing the flow of the material for distribution. The
technology is being developed to process unmodified
PEEK, which should make the technology more
accessible to the medical implant community.
In the scientific literature, Tan et al.
[22]
at
Nanyang University used a high-power laser at
a temperature just below the glass transition of PEEK
(140
C) to sinter a PEEK and hydroxyapatite
compound. Rechtenwald et al. used a higher
temperature at around the melt temperature in order
to conduct the preheating step. Films formed using
this method were porous, possessing 24% lower
density than moldings made from solid PEEK but,
due to the irregular geometry of the PEEK powder
used, the layers were not uniform
[23,24]
.
12.4.6 Textiles
Although the discussion in this chapter mainly
focuses on the creation of structurally robust porous
PEEK, it is also worth mentioning that porous
structures can be created for soft tissue or nonstruc-
tural applications through the use of PEEK textiles.
Jarman-Smith et al.
[26,27]
showed that PEEK
monofilament and multifilament fibers were cyto-
compatible and supported cell ongrowth. Edwards
et al.
[28]
reported that porous scaffolds made from
woven monofilament and multifilament PEEK
textiles supported fibroblast proliferation. Different
constructs were knitted or woven as porous sheets or
as 3D spacer fabrics. The mechanical properties of
these materials were similar to control constructs
made from PET, a material previously used for these
applications. However, unlike PET, the PEEK would
include additional benefits such as resistance to
hydrolysis or oxidation (either in vivo or during
sterilization) and its origin of supply is more
controlled. An arthroscopic rotator cuff repair device
incorporating porous meshes of PEEK-OPTIMA
have been 510 (k) approved (PROcuff
, Synovis,
USA) and demonstrate the potential of porous PEEK
