Biomedical Engineering Reference
In-Depth Information
nonclassical topics in the sciences, in medicine, in engineering materialized in those
days (1960s) and emerged as the discipline
biomedical engineering
.
The point in fact, in our case, is that it was not sufficient to reproduce artifi-
cially only the general shape and the overall operational mechanism of the valve
using a thin, flexible, strong polymeric material. Careful histological examination
of the natural aortic valve (Sect.
12.2.1
) revealed a quite complicated structure of
several
biopolymers
, at the same time being remodeled to a certain degree. Detailed
mechanical testing of the leaflet tissue [
388
-
390
] furthermore produced evidence
that the leaflets are mechanically
anisotropic
, inhomogeneous and that the opening
and closing behavior of the valve depends on several factors, such as the dynamic
pressure difference, the flow field in the vicinity, the mechanics of the aortic ring
and the heart muscle. Consequently, if a homogeneous,
isotropic
flexible structure
is constructed and put in operation, where the dynamics of the system required an
anisotropic
material, dysfunction leading to failure is not surprising. A corollary
to such a conclusion, indeed an important one, is that eventually a candidate bio-
material to be used in a particular artificial organ should be evaluated not only
as a 'material', i.e., physicochemical characterization, but in the environment of
its designed use (in contact with blood, or bone etc.), the fluid or solid mechan-
ics in the vicinity etc. We shall return to give a brief account of matters related
to
soft tissue biomechanics
and
blood fluid mechanics
as they relate to the aortic
valve.
Mechanical Valves
It was stated before that there may be other ways to design than simply copying
Nature. Granted that (natural) optimization through evolution must have developed
a reliable operating mechanism, invention is a major human (not limited only to
humans!) exercise that has changed the course of humanity and the whole earth for
that matter in unpredictable ways. The wheel is a relevant point. With reference to
the development of substitute aortic valves, the problem was to develop a passive
valve mechanism, i.e., opening and closing due to pressure difference driving forces.
This artificial mechanism had to be contained within certain limits: to have a circular
cross section 'seat' with diameter ranging from about 20-30 cm where, somehow,
an occluding device could 'seat' and seal the passage or be lifted (and secured at a
safe distance) to let the blood through. The first totally artificial (or mechanical as
opposed to bioprosthetic) valve, which was successfully employed clinically and not
having the trileaflet configuration was the
ball valve
(a well-known typical model
is the
Starr-Edwards
). It consists of a spherical ball and an annular orifice, covered
by a suturing ring. The ball is retained within a 3- or 4-legged cage arising from
the ring (see Fig.
12.7
a). The ball is made of
Silastic
(eventually problematic due to
swelling, lipid adsorption, cracking etc.) or
stainless steel
, the annular ring and the
cage were made of stainless steel and was bare or cloth-covered. There have been
several versions of the original design.
