Biomedical Engineering Reference
In-Depth Information
pressures inside the artery itself (ΔD/DP) X 100). The two main structural proteins, collagen
and elastin, confer several properties, enabling arteries to distend more at low pressure and
become less elastic at high ones. Both properties are extremely important, since inadequate
resistance may lead to rupture, and absence of elasticity may disturb flow, leading to
thrombosis. In detail, the velocity of propagation (v) of pressure waves depends on the
elasticity of vessel walls (E), their thickness (s) and diameter (D), and on the density (P) of
blood, as described by the Moens-Korteweg equation, v= Es/ P D. When a section of artery
and a replacement conduit have different radial elastic properties, two consequences may
ensue. Discontinuity in the speed of propagation of pressure waves leads to turbulence in
the area between the natural and artificial vessels which, in turn, leads to local overpressure,
the cause of new aneurisms. Apart from the different degree of distension of arterial sections
(original artery/prosthesis), this may put greater stress on sutures.
Different compliance has been associated with intimal hyperplasia in an arterial
replacement and the artery itself (Stewart SF et al 1992).
Synthetic tissues are far less elastic than arteries, but absorbable materials which could be
replaced by the normal vasal extracellular matrix are believed to avoid this problem.
Resorption does very often trigger a response similar to that of a foreign body, which leads
to the formation of scar tissue which may deprive the original construct of its elasticity (Zilla
PD et al 2007).
From the viewpoint of experimental models, elastic components are more or less the same in
various species, in spite of changing sizes. Wolinsky and Glagov (1967) have shown that,
whereas the total circumferential tension in the vascular wall of the aorta increases 26 times
from mouse to pig, defined by Laplace's law as T=PR, tension per lamellar unit (elastic
lamina) is similar, being about 1/ 3Nm-1 in various animal aortas, but unfortunately the
above study does not compare peripheral vessels. Arterial elasticity causes a reduction in
the pressure gradient generated during cardiac systole/distole (Wolinsky et al 1967;
Shadwick RE 1999). In peripheral vessels, flow is continuous, thanks to the reservoir role
played by the large vessels during cardiac systole. In humans, the ratio between flow
gradient and mean flow falls from a value of 6 near the aortic arch to 2 distal to the femoral
artery (Wolinsky et al 1967; Shadwick RE 1999). There is therefore a difference between the
elasticity of vessels in relation to their size and position within the circulatory system which
may influence the design of ideal replacements for them.
According to the above, and in view of the complexity of the cardiocirculatory system and
interactions with blood flow, some components are essential for neovessels if they are to
guarantee sufficient resistance and compliance and, at the same time, avoid thrombosis and
intimal hyperplasia (Mitchell SL et al 2003). At the present time, always in terms of totally
absorbable products, there are many approaches involving heterologous tissues, synthetic
polymers, biopolymers and totally engineered products (Isenberg BC et al 2006). Every
experiment leads to different conclusions, and associations between materials to exploit
desired properties while avoiding problems have also been proposed. For example, PGA is
a generally stiff product and is frequently associated with other substances to achieve the
necessary elasticity (Shinoka T et al 2008).
4. Materials
Many research groups all over the world have approached the problem of developing the
ideal prosthesis in a variety of ways. As in all tissue engineering fields, research ranges round
