Biomedical Engineering Reference
In-Depth Information
5.7.2 Cardiovascular Devices
The primary requirements for biomaterials used for blood-contacting circulatory applica-
tions such as heart valves and blood vessel replacements or stents are resistance to platelet
and thrombus deposition, biomechanical strength and durability, and biocompatibility and
nontoxicity. These key requirements were identified by studying the primary mode of
action of the tissue to be replaced. For example, arteries consist of three layers that perform
various biological functions: the intima, media, and adventitia. The intima is on the blood
vessel interior and has a nonthrombogenic surface; it prevents blood contact with the
thrombogenic media tissue. The cells of the intima produce a myriad of biomolecules,
including growth factors, vasoactive molecules, and adhesion molecules. The media or
parenchymal tissue is the middle muscular layer that provides the required strength while
remaining viscoelastic. The media layer is made up of multiple layers of aligned smooth
muscle cells. The outer adventitia layer acts as a stiff sheath that protects the smooth muscle
media layer from biomechanical overload or overdistention.
As with most tissues, autograft is the preferred biomaterial for vascular tissue replace-
ment. For example, one of the patient's own veins can be harvested and used to replace a
clogged artery. Vein grafts have a failure rate as high as 20 percent in one year. Since vein
grafts from the patient are unavailable and unsuitable in approximately 30 percent of all
patients, synthetic graft materials have been developed. The observation that the intact
lining of blood vessels (intima) does not induce coagulation has led scientists and engineers
to produce more blood-compatible biomaterials by mimicking certain properties of the
endothelium. For example, very smooth materials, surfaces with negative charges, and
hydrophilic biomaterials are now used with limited success in blood-contacting applica-
tions. Knitted Dacron
W
(polyethylene terephthalate) and Gortex
W
(polytetrafluoroethylene
[PTFE]) vascular grafts are commonly used. The use of synthetic grafts has resulted in
reasonable degrees of success (approximately 40 percent experience thrombosis at six
months when synthetic grafts are used to bypass arteries that are smaller than 6 mm in
diameter). Improved vascular products have incorporated anticoagulants such as heparin
on the blood-contacting surfaces.
As discussed in the section on wound healing, most biomaterials are recognized by the
body as foreign and lead to platelet deposition and thrombus or coagulation (blood clot
formation). This limits their use in blood-contacting applications. Therefore, the tissue engi-
neering approach of growing and implanting a living multilayered cell construct holds
great promise for cardiovascular applications. It is only recently that the cell culture condi-
tions and scaffold material selection that will promote the smooth muscle cell alignment
and tight endothelial cell packing of blood vessels have been identified. A great deal of
additional research must be completed prior to commercial availability of a functional
tissue-engineered artery. For example, the mechanical burst strength of the highly cellular
tissue-engineered arteries is not yet sufficient to withstand what the heart can generate,
although these artificial arteries are nonthrombogenic.
Diseased human heart valves can currently be replaced with mechanical prostheses (syn-
thetic biomaterials) or bioprostheses (made of biological tissue). The two mechanical heart
valves shown in Figure 5.17 have four essential components: an occluder such as a disc
or ball; a seating ring against which the occluder sits when the valve is closed; a capture
