Biomedical Engineering Reference
In-Depth Information
5.5.3 Mechanical properties
Mechanical properties are important for several reasons: first, the tissue must be
strong enough to withstand the forces exerted on the valve during use. As the
peak loads on the tissue are estimated at 1MPa at the commissures, the ultimate
tensile strength (UTS) of the tissue must exceed 1MPa (Zioupos et al., 1994).
Further, continuous use can cause the tissue to break down with time, leading to
lower mechanical parameters (Sacks, 2001; Sun et al., 2004). Since no one
knows how continuous wear correlates with changes in mechanical strength in
vivo, it is not possible to define precisely what the UTS of a material should be
at the point of manufacture in order to provide a specified number of years of
durability. Suffice to say, however, that a healthy `safety factor' is appropriate,
and choosing materials with UTS values close to 1MPa could be a durability
concern. This is where porcine aortic valves fall short, their UTS values range
from 0.6 to 1.2MPa in the circumferential direction (along the axis of the major
collagen bundles), whereas bovine pericardial tissue has UTS values ranging
from 5 to 26MPa or more (Sacks et al., 2006). The increased strength of
pericardial tissue is due to the increased amount of collagen in the tissue, as well
as its lower water content. A porcine leaflet contains 94% moisture, leaving only
6% of the mass of the tissue composed of structural proteins and other com-
ponents. Bovine pericardium, however, contains only 78% moisture and
contains twice as much collagen as a porcine leaflet (Cunanan et al., 2001).
Second, natural pericardium has a wide range of intrinsic mechanical proper-
ties, and if thickness does not correlate with mechanical properties, how does
one ensure proper valve behavior? Figure 5.16 contains elongation data for nine
different pieces of bovine pericardium, each considered suitable for valve
5.16 Stress±strain curves for nine random samples of bovine pericardial tissue.
All tissues were considered suitable for use in valve production and were of
similar thickness. Note significant differences in elasticity between tissues.
(Figure reprinted with permission from Arbor Surgical Technologies, Inc.
(now owned by Medtronic.)
