Biomedical Engineering Reference
In-Depth Information
Fig. 13.7
A photograph of a bifurcating Dacron vascular graft. The structure is corrugated to allow
for expansion when placed in vivo. The black line is for X-ray identification
Fig.
13.7
. To improve its resistance to infections, silver-coated polyester grafts have
also been employed.
Polytetrafluoroethylene [-(CF
2
-CF
2
)-]
n
, PTFE or Teflon, becomes of a more
microporous nature (standard pore size is 30
m) by extrusion and sintering to form
the so-called expanded PTFE (ePTFE). ePTFE is also nonbiodegradable, with an
electronegative luminar surface that renders it antithrombotic. The electronegativity,
and thus the improvement of its antithrombotic properties, especially an antiplatelet
effect, is enhanced further by carbon-coating, or heparin attachment (see Chap.
12
).
Heparin can either impregnate the ePTFE grafts, or be bound covalently, to provide
a controlled release, or to be attached to fibrin glue on the grafts. Teflon is also
highly crystalline (
90%) but less stiff than Dacron, having a tensile strength of
14 MPa and a tensile modulus of 0.5 GPa. It is widely used as lower limb bypass
grafts.
Polyurethanes [-NH-O-C=O-R-]
n
contain urethane groups -NH-CO-O- and
depending on the composition of their hard and soft segments exhibit tensile
strengths from 20 to 90 MPa (much more compliant than either Dacron or Teflon),
while their tensile modulus ranges from 5 to 1,150 MPa. Both polyester urethanes
and polyether urethanes have been shown to degrade in vivo, this fact being a
major disadvantage to this otherwise promising biomaterial. A newer version of PU
was based on carbonate linkages, having, that is, no ester linkages that have been
prone to oxidation. A poly(carbonate-urea)urethane (Cardiotech
R
>
) was found to be
resistant to hydrolytic and oxidative stresses [
434
].
Apart from the above-mentioned three categories of stable, nonbiodegradable (at
least with the intention of being so) polymeric materials, another class of materi-
als, which are intended to degrade in the human body has been used for vascular
grafts. The rate of degradation of such biodegradable polymers depends on several
