Biomedical Engineering Reference
In-Depth Information
USA). The sintered polyethylene (Medpor, Stryker)
is flexible to facilitate contouring around the complex
bones of the face. Porous polyethylene is sometimes
used for chin and mandible reconstruction as there is
some tissue ingrowth and less encapsulation than for
smooth silicone alternatives. Certain application
areas, such as orthopedics, capitalize on the structural
porous biomaterials.
Currently available porous biomaterials are not
without their limitations. Although porous metals
have many attractive benefits, such as impressive
conductivity and osteointegration, they may subside
into adjacent bone, be susceptible to corrosion, and
while cell ingrowth occurs, it may be difficult to
monitor using medical imaging technologies,
including MRI. Certain porous metals, such as the
shape memory alloy Nitinol, may include nickel in
their composition, which could stimulate an allergic
response and there is heightened concern about metal
ion release and metal toxicity, especially in ortho-
pedic circles. Porous ceramics (e.g., calcium phos-
phates and bioglasses) tend to be brittle, limiting their
use in load-bearing applications, whereas porous
biodegradables (e.g., PLLA, PLGA) have unpre-
dictable degradation profiles in vivo and limited
mechanical strength. As such, the use of porous
biodegradables is largely confined to applications
such as suture anchors in sports medicine. The
limitations of existing porous biomaterials provide
some motivation to investigate PEEK as a potential
alternative.
required to resist the mechanical forces placed upon
the material.
These are actually not readily achieved by the
transfer of existing industrial technologies used for
porosity generation, due to contaminant processes or
unsuitable resultant geometries. Production scale
methods of fabricating porous polyetheretherketones
(PEEK) are themselves limited.
Some such industrial processes to foam PEEK
have utilized chemical blowing agents such as
magnesium hydroxide, pyromellitic acid or sodium
borohydride, or are based on the degradation of
poly(phenylene sulfoxide) or agents based on the side
product of the condensation reaction between PEEK
and an amine-containing polymer. These processes
can require concentrations up to 20% of the foaming
agent and leave metal oxide or carbonized organic
residues in the closed cell porous foam.
To avoid the use of blowing agents, it is possible to
use gas-assisted injection molding such as that of
MuCell (Trexel, USA). These processes introduce
a supercritical gas such as nitrogen or carbon dioxide
during the screw-recovery phase in extrusion or
injection molding. Upon depressurization, the gas
expands and creates foam. However, the porosity
formed is closed cell and, limited to pore diameters
up to 100
m
m; a resin-rich skin may also form around
the porous core. Researchers at Imperial College
London investigated the use of using carbon nano-
fibers to additionally reinforce PEEK foams made
using a blowing agent
[8]
. Here, supercritical carbon
dioxide was used as the blowing agent. Typically, the
process of using a blowing agent forms a micro-
cellular foam with very high cell density (
>
10
9
cells/
cc) but the pores may tend toward a small closed-cell
size (
<
5
m
m). Consequently, its application for any
osseoconductivity in medical devices requiring bone
ingrowth is restricted. To overcome this, variations
on the gas-assisted injection-molding technology
have been reported in the scientific literature for other
polymers such as the biodegradable poly-lactic acid
polymers. These have combined the addition of
supercritical fluids along with salt leaching in order
to achieve the interconnected porosity
[9]
.
PoroGen (Massachusetts, USA) use Victrex
PEEK to manufacture porous membranes for sepa-
ration processes using their proprietary technology.
These membranes are available as selectively
permeable microporous, closed-cell films for gas and
vapor separation. In addition, PoroGen also manu-
facture hollow fiber membranes, which are used for
12.3 Porous Polymer Production
for Industrial Applications
The idea of porous PEEK has already been
pursued for industrial applications (e.g., membranes
and chromatography frits (Upchurch/IDEX Health
and Science, Washington, USA)). Indeed porous
PEEK has been investigated for medical applications
for several years but has been limited by either the
type of technology or its ability to fulfill the Key
End-user Requirements (KERs) needed for the
medical application or sector. The main KERs
existing for an orthopedic porous material are that
the pore interconnectivity needs to be sufficient to
allow cells and nutrients to pass into and through the
material, the pore size must be sufficiently large
(~100
e
700
m
m) to permit cells and vascularization
of the biomaterial, and some structural integrity is
