Soft Tissue Replacements - Biomaterials - page 202

blood, energy converter (from electrical to mechanical energy), variable column compensator, implant-
able batteries, transcutaneous energy transmission system, and external batteries. The blood pump
configuration in these devices includes sac, diaphragm, and pusher plate devices . Materials used in
blood-contacting surfaces in these devices are synthetic polymers (polyurethanes, segmented polyure-
thanes, Biomer ® , and others). Segmented polyurethane elastomer used in prosthetic ventricles with a
thromboresistant additive modifying the polymeric surface has resulted in improved blood compatibil-
ity and reduced thromboembolic risk in animal trials (Farrar et al., 1988). Design considerations include
reduction of regions of stagnation of blood within the blood chamber and minimizing the mechanical
stresses induced on the formed elements in blood. Apart from the characteristics of these materials to
withstand repetitive high mechanical stresses and to minimize failure due to fatigue, surface interaction
with blood is also another crucial factor. An electrically powered TAH intended for long-term implanta-
tion is shown in Figure 9.8b. The details of the design considerations for the circulatory assist devices are
included in Rosenberg (1995) and details of the evaluation of the electrically powered heart are included
in Rosenberg et al. (1995).
Due to significant problems with thromboembolic complications and subsequent neurological prob-
lems with long-term implantation of TAH in humans, attention has been focused on minimizing fac-
tors responsible for thrombus deposition. In order to eliminate crevices formed with the quick connect
system, valves sutured in place at the inflow and outflow orifices were offered as an alternative in the
Philadelphia Heart (Wurzel et al., 1988). An alternative quick connect system using precision-machined
components has been demonstrated to reduce valve- and connector-associated thrombus formation
substantially (Holfert et al., 1987). Several in vitro studies have been reported in the literature in order
to assess the effect of fluid dynamic stresses on thrombus deposition (Phillips et al., 1979; Tarbell et al.,
1986; Baldwin et al., 1990; Jarvis et al., 1991). These have included flow visualization and laser Doppler
anemometry velocity and turbulence measurements within the ventricular chamber as well as in the
vicinity of the inflow and outflow orifices. The results of such studies indicate that the flow within the
chamber generally has a smooth washout of blood in each pulsatile flow cycle with relatively large turbu-
lent stresses and regions of stasis found near the valves. The thrombus deposition found with implanted
TAH in the vicinity of the inflow valves also indicates that the major problem with the working of these
devices is still with the flow dynamics across the mechanical valves. Computational flow dynamic anal-
ysis within the ventricular chamber may also be exploited to improve the design of the valve chambers
and the mechanical valves in order to reduce the turbulent stresses near the vicinity of the inflow and
outflow orifices (Kim et al., 1992). Structural failure of the mechanical valves, initially reported with
the TAH, may have been the result of increased load on the valves during closure due to the relatively
large d p /d t ( p is the pressure and t is the time) at which the TAH was operated. Attempts at reducing the
d p /d t during closure of the inflow valves have also been reported with modified designs of the artificial
heart driver (Wurzel et al., 1988). Due to the relatively large d p /d t at which TAHs are operated, there
is increased possibility of cavitation bubble initiation, and subsequent collapse of the bubbles may also
be another important reason for thrombus deposition near the mechanical valve at the inflow orifice.
Introducing synthetic valves to replace the mechanical valves (Chandran et al., 1991b) may prove to be
advantageous with respect to cavitation initiation and may minimize thrombus formation.
9.1.4 Vascular Prostheses
In advanced stages of vascular diseases such as obstructive atherosclerosis and aneurysmal dilatation,
when other treatment modalities fail, replacement of diseased segments with vascular prostheses is a
common practice. Vascular prostheses can be classified as given in Table 9.4.
9.1.4.1 Surgically Implanted Biological Grafts
Arterial homografts, even though initially used in large scale, resulted in aneurysm formation espe-
cially in the proximal suture line (Strandness and Sumner, 1975). Still, a viable alternative is to use the