Biomedical Engineering Reference
In-Depth Information
Natural-origin absorbable polymers have also been
utilized in biomedical composites. Purified bovine colla-
gen, because of its biocompatibility, resorbability, and
availability in a well-characterized implant form, has been
used as a composite matrix, mainly as a ceramic com-
posite binder (
Lemons
et al.
, 1984
). A commercially
available fibrin adhesive (
Bochlogyros
et al.
, 1985
) and
calcium sulfate (
Alexander
et al.
, 1987
) have similarly
been used for this purpose.
Reis
et al.
(1998)
proposed alternative biodegradable
systems to be used in temporary medical applications.
These systems are blends of starch with various ther-
moplastic polymers. They were proposed for a large
range of applications such as temporary hard-tissue re-
placement, bone fracture fixation, drug delivery devices,
or tissue engineering scaffolds.
biomedical particulate composites. In some applications
composites are manufactured
in situ.
This is the case of
dental restorative composites and particle-reinforced
bone cements.
Fabrication of fiber-reinforced
composites
Fiber-reinforced composites are produced commercially
by one of two classes of fabrication techniques: open or
closed molding. Most of the open-molding techniques
are not appropriate to biomedical composites because of
the character of the matrices used (mainly thermoplas-
tics) and the need to produce materials that are resistant
to water intrusion.
Consequently, the simplest techniques, the hand lay-
up and spray-up procedures, are seldom, if ever, used to
produce biomedical composites. The two open-molding
techniques that may find application in biomedical
composites are the vacuum bag-autoclave process and
the filament-winding process.
Fabrication of composites
Composite materials can be fabricated with different
technologies. Some of them are peculiar for the type of
filler (particle, short or long fiber) and matrix (thermo-
plastic or thermosetting). Some make use of solvents
whose residues could affect the material biocompatibility,
hence not being applicable for the fabrication of bio-
medical composites. The selection of the most appro-
priate manufacturing technology is also influenced by the
relatively low volumes of the production, compared to
other applications, and by the relatively low dominance of
the manufacturing cost over the overall cost of the device.
Some biomedical composites, moreover, are fabri-
cated
''in situ.''
This is the case of composite bone
cements.
The most common fabrication technologies for com-
posites are:
1.
Hand lay up
2.
Spray up
3.
Compression molding
4.
Resin transfer molding
5.
Injection molding
6.
Filament winding
7.
Pultrusion
In principle all of the listed technologies could be used
for the fabrication of biomedical composites. Only some
of them, however, have found practical use.
Vacuum bag-autoclave process
This process is used to produce high-performance lami-
nates, usually of fiber-reinforced epoxy. Composite ma-
terials produced by this method are currently used in
aircraft and aerospace applications. The first step in this
process, and indeed many other processes, is the pro-
duction of a ''prepreg.'' This basic structure is a thin sheet
of matrix imbedded with uniaxially oriented reinforcing
fibers. When the matrix is epoxy, it is prepared in the
partially cured state. Pieces of the prepreg sheet are cut
out and placed on top of each other on a shaped tool to
form a laminate. The layers, or plies, may be placed in
different directions to produce the desired strength and
stiffness.
After the laminate is constructed, the tooling and at-
tached laminate are vacuum-bagged, with a vacuum being
applied to remove entrapped air from the laminated part.
Finally, the vacuum bag enclosing the laminate and the
tooling is put into an autoclave for the final curing of the
epoxy resin. The conditions for curing vary depending
upon the material, but the carbon fiber-epoxy composite
material is usually heated at about 190
C at a pressure of
about 700 kPa. After being removed from the autoclave,
the composite part is stripped from its tooling and is
ready for further finishing operations. This procedure is
potentially useful for the production of fracture fixation
devices and total hip stems.
Fabrication of particle-reinforced
composites
Filament-winding process
Another important open-mold process to produce high-
strength hollow cylinders is the filament-winding process.
Injection molding, compression molding, and extrusion
are
the
most
common
fabrication
technologies
for
