Biomedical Engineering Reference
In-Depth Information
When the accumulation reaches a critical point, the regional hemodynamics become
highly disturbed and may lead to thromboembolic events, or organ failure and death.
Another, less common but serious nevertheless, situation arises when the wall of
the artery becomes thinner, due to trauma, or genetic conditions or other medical
reasons, the vessel bulges (aneurysm), and is prone to rupture, leading to internal
bleeding that most often leads to death.
In both situations, the diseased part of the aortic vessel needs medical interven-
tion. Depending on the severity of the disease, the site, the vessel diameter, and on
a host of other medical indices, the treatment may be one of the following:
1.
Surgical excision of the diseased part and replacing it by suturing an autologous
part of a vein or artery, also surgically excised.
2.
Replacement of the excised diseased part by suturing a synthetic vascular graft.
3.
Nonsurgical intervention, by placing a stent intraluminously to the atheroscle-
rotic site, using a catheter, and letting the self-expansion of the stent restore the
blood flow by opening the lumen.
4.
In specific cases, especially in abdominal aortic aneurysms, a stent-graft is uti-
lized. A stent-graft is a synthetic graft, as in case 2, supported at the outside with
a metal mesh.
In this section, we shall review the biomaterials aspects of the cases 2 and 3,
as well as the current research on developing tissue engineered vessel grafts. One
more note: the surgical treatment involving the first two cases, 1 and 2 is also called
bypass surgery.
13.6.1
Synthetic Vascular Grafts
Several polymeric materials have been tested for manufacturing vascular grafts. The
ones most currently in use are from polyethylene terephthalate (known as Dacron),
expanded PolyTetraFluorEthylene (ePTFE), and Polyurethanes (PU) [
433
].
Polyethylene terephthalate [O-C=O-C-C
6
H
6
-O-C=O-CH
2
-CH2-]
n
is a poly-
ester in the form of filaments, which may be either woven or knitted into vascular
grafts. The woven grafts have small pores, while the knitted ones, formed by the
velour technique (looping fibers together) have larger pores. The large pores facil-
itate and promote tissue ingrowth, and the whole structure is more compliant, but
there is a price to pay: due to the pore size leakage may be a serious problem, espe-
cially at high pressure sites, and for this reason it is necessary to
preclot
prior to use
the knitted Dacron grafts with albumin, gelatine or even blood to prevent seepage.
While the biological coating may degrade over 2-3 months Dacron itself is non-
biodegradable. Dacron is highly crystalline, has a tensile strength of 170-180 MPa
and a tensile modulus of 14 GPa. It is used mainly as aortic and large diameter
peripheral bypass grafts. A photograph of a Dacron graft with a main large arte-
rial part leading to two 'daughter' iliac arteries, smaller in diameter is shown in
