Biomedical Engineering Reference
In-Depth Information
In patent WO2007/051307, a variation of
compression molding and salt leaching was
described for PEEK and other members of the
polyaryletherketone family
[11]
. Here, the materials
are dry blended as particles, as commonly done, and
then loaded into a compression mold. The novelty is
that after the melt phase and cooling occur, the
compound is subjected to additional pressure during
the cooling off period. Another variation in this
method has been described by researchers at Notre
Dame University using PEEK powder and salt
leaching to fabricate a porous scaffold, but to addi-
tionally add HA whiskers in the powder blend to act
as a structural and bioactive filler. The theory here is
that any exposed HA on the struts of the pores would
permit
other porous materials used in the CMF application
area, such as porous hydroxyapatite (HA), are that
the polymer has superior strength and lighter weight.
In heat sintering, particles of polymer are held in
a mold in contact with each other and the polymer
heated to just allow thermal bonding of the adjacent
polymer particles at their contact points. However,
disadvantageously, the pore diameter is dictated by
the particle sizes of the selected polymer and this
may not always be optimal for bone ingrowth.
Machining postsintering can be made more difficult
since there are open pores on the surface. Further-
more, since the sintered particles are only bonded at
their contact points, the bond between the particles
may be relatively weak leading to potential friability
of the porous material. This will vary depending on
whether solid-state or liquid-state sintering was used.
However, modifications of this technology have
been reported in the patent literature as methods of
creating porous osseoconductive PEEK, potentially
suitable for medical use.
Patent filing US2010/0255053 describes how
a porous PEEK material can be made by compressing
100
e
600
m
m diameter PEEK particles in a hot
compression mold with a defined time, temperature,
and pressure (45 kg), which results in heat sintering
them together
[17]
. Additionally, it shows that PEEK
particles may be cold loaded in a mold and
compressed with the temperature ramped over 6 h to
the melting point of PEEK (343
C). The porous
material can also act as a vehicle for the delivery of
a bioactive compound. The types of applications that
may capitalize on such a porous PEEK material are
described by the author in an additional patent
US2008/0161927 that shows some elegant designs
for spinal cages possessing aspects of porous mate-
rials
[18]
. In recent times, further patent applications
from other parties incorporating porous PEEK spinal
device designs can be found, for example US2010/
0094426 and separately, US2010/0042218
[19,20]
.
The latter patent suggests the methods that the
porous PEEK could be manufactured by (e.g., by the
stacking and bonding together of porous sheets of
PEEK). They both show spinal device designs that
are hybrids of solid and porous PEEK, to benefit from
the strength of solid PEEK while still allowing
ingrowth in porous regions.
In another patent (WO 2009/099559), both material
and device aspects are addressed
[21]
. PEEK particles
100
e
500
m
m are mixed with a 10% wt/wt biocom-
patible powder (such as tricalciumphosphate). This is
osseointegration
and
osseoconductivity
[12,13]
.
Abu Bakar et al.
[14]
also reported using porogen
leaching to create a 60% porous scaffold manufac-
tured using an HA and PEEK powder compound.
They additionally showed in vivo data where the
surrounding tissue had migrated into the material.
Recently, Choi et al.
[15]
published an in vivo study
involving a porous PEEK-sleeved fixation device for
hips. Although little is disclosed on the actual method
or material characteristics in the article, according to
the authors the method for producing the porous
PEEK sleeve was salt leaching.
12.4.2 Heat Sintering
Various means of sintering exist such as heat,
laser, or microwave. All have the aim of fusing PEEK
particles together to make a structure. The success of
sintering can depend upon several factors including
the available surface free energy, which is lower in
polymers than in metals or ceramics. This can require
higher levels of energy that should be controlled to
prevent polymer degradation. Sintering is commonly
used in the manufacturing of ceramics and metals,
including porous metal from powders. Some patent
literature relating to thermal sintering of polymers is
provided by Porex, who manufacture porous poly-
ethylene (Medpor) for use in CMF applications
d
namely mid-face reconstruction. The materials are
prepared from sintered polyethylene and have varied
through the inclusion of metal mesh to allow shaping
of the implant or in combination with 45S5 bioglass
with the polymer in an admix prior to sintering to
improve fibrovascular ingrowth (WO/2004/009000)
[16]
. Some of the benefits of these materials over