Biomedical Engineering Reference
In-Depth Information
Polymer fibers
properties of UHMWPE fibers are such that few
resins bond well to the fiber surfaces, and so the
structural properties expected from the fiber are
often not fully realized in a composite. The low
melting point of the fibers (about 147
C) impedes
high-temperature fabrication. Bulk UHMWPE has
extensive applications in medicine for the fabrication
of bearings for joint prostheses, displaying excellent
biocompatibility but with lifetime restricted by its
wear resistance. PE fibers are used to reinforce acrylic
resins for application in dentistry (
Ladizesky
et al.
,
1994; Karaman
et al.
, 2002; Brown, 2000
), or to
make intervertebral disk prostheses (Kotani
et al.,
2002). They have been also used for the fabrication
of ligament augmentation devices (
Guidoin
et al.
,
2000
).
Dacron is the name commonly used to indicate PET
fibers. These fibers have several biomedical uses,
most in cardiovascular surgery for arterial grafts. PET
fibers, however, have been proposed in orthopedics
for the fabrication of artificial tendons or ligaments
(Kolarik
et al.,
1981) and ligament augmentation
devices, as fibers or fabrics alone, or imbedded in
different matrices in composites. Other proposed
applications include soft-tissue prostheses,
intervertebral disks (
Ambrosio
et al.
, 1996
), and
plastic surgery applications.
PLA and PGA and their copolymers are the principal
biodegradable polymers used for the fabrication of
biodegradable fibers. These fibers have been used for
a number of years in absorbable sutures. Properties of
these fibers depend upon several factors, such as
crystallinity degree, molecular weight, and purity
(
Migliaresi and Fambri, 1997
). Fibers and tissues
have been proposed for ligament reconstruction
(
Durselen
et al.
, 2001
) or as scaffolds for tissue
engineering applications (
Lu and Mikos, 1996
).
They also have been employed in composites, in
combination with parent biodegradable matrices.
Examples are the intramedullary biodegradable pins
and plates (
Vert
et al.
, 1986, Middleton and Tipton,
2000
) and biodegradable scaffolds for bone
regeneration (
Vacanti
et al.
, 1991
, Kellomaki
et al.
, 2000).
Whereas carbon fibers have been used for their superior
mechanical properties, polymer fibers are not compara-
bly strong or stiff as reinforcements for other polymers,
with the possible exceptions of aramid fibers or ultrahigh-
molecular-weight polyethylene (UHMWPE) fibers. For
biomedical applications, biocompatibility, of course, and
high strength and fatigue resistance are compulsory,
while stiffness is a design parameter to be adapted to the
specific conditions. This is why for some applications
PET fibers have been used. In addition, thanks to their
absorbability, not to their mechanical superiority, certain
absorbable fibers have been employed.
Aramid
is the generic name for aromatic polyamide
fibers. The most well known aramids are Kevlar
and Nomex (DuPont trademarks), and Twaron
(made by Teijin/Twaron of Japan). Kevlar is
produced by spinning a sulfuric acid/poly(
p
-phenylene
terephthalamide) solution through an air layer into
a coagulating water bath. Aramid fibers are light
(density
ΒΌ
1.44 g/cm
3
), stiff (the modulus can go up
to 190 GPa), and strong (tensile strength about
3.6 GPa); moreover, they resist impact and abrasion
damage. A negative point that can be relevant for
biomedical applications is that aramid fibers absorb
moisture, and also worth noting is their poor com-
pressive strength, about 1/8 of the tensile strength.
Aramid fiber composites are used commercially
where high tensile strength and stiffness, damage
resistance, and resistance to fatigue and stress
rupture are important. In medicine, these
composites have not seen extensive use, due perhaps
to some concerns about their biocompatibility or
long-term fate. Main applications have been in
dentistry (
Pourdeyhimi
et al.
, 1986; Vallittu, 1996
)
and ligament prostheses (
Wening
et al.
, 1994
).
Commercially available high-strength, high-modulus
PE fibers include Spectra from Honeywell Perfor-
mance Fibers (Colonial Heights, VA), Dyneema
from DSM (Heerlen, The Netherlands), and Toyobo
fibers from Toyobo (Shiga, Japan). UHMWPE fibers
are produced by a gel-spinning technique starting
from an approximately 2-8 wt.% solution of the ul-
trahigh-molecular-weight polymer (M
w
>
10
6
)in
a common solvent, such as decalin. Spinning at 130-
140
C and hot drawing at very high draw ratios
produces fibers with the highest specific strength of
all commercial fibers available to date. UHMWPE
fibers possess high modulus and strength, besides
displaying light weight (density about 0.97 g/cm
3
)
and high energy dissipation ability, compared to
other fibers. In addition PE fibers resist abrasion and
do not absorb water. However, the chemical
Ceramics
A number of different ceramic materials have been used
to reinforce biomedical composites. Since most bio-
compatible ceramics, when loaded in tension or shear, are
relatively weak and brittle materials compared to metals,
the preferred form for this reinforcement has usually
been particulate. These reinforcements have included
various calcium phosphates, aluminum- and zinc-based
