Biomedical Engineering Reference
In-Depth Information
ductus arteriosus, coarctation of the aorta, or in the stenting of pulmonary veins, an inter-
vention often plagued by in-stent stenosis (Mohanchandra and Carman, 2009).
In both in vivo and in vitro testing of the TiNi, thin film-covered stents were conducted
by Mohanchandra and Carman (2009). The in vitro tests demonstrate the feasibility of
successful stent deployment and the stent immobility under pulsatile flow of 1.5 L/min.
For in vivo testing of the covered stent a swine animal model was used. All arterial planta-
tions were performed in the descending aorta. Venous implants were performed in either
the superior vena cava or inferior vena cava and evaluated at different time intervals.
After animal euthanization, stent-containing vessels were harvested, as well as speci-
mens from major organs (heart, lungs, liver, stomach, kidney, pancreas, and spleen). No
significant in-stent neointimal hyperplasia was found on thin film TiNi covered stents
placed in the arterial circulation. Moderate-to-severe neointimal proliferation was seen
in all venous stent implants after 3 weeks. Notably, several of the stent covers were found
to have defects that were likely caused during deployment (Mohanchandra and Carman,
2009). In the arterial circulation, the thin film nitinol cover did prevent in-growth of
neointima and supported growth of endothelial cells over the first month after implanta-
tion. In the venous system, there was angiographic evidence of less in-stent stenosis in the
first 3 weeks after implantation (when compared with an uncovered stent). However, by
4 and 6 weeks, all venous stents (covered and uncovered) had significant in-stent stenosis
from neointimal growth (Mohanchandra and Carman, 2009). The vastly different thermo-
dynamic environments encountered in the arterial versus the venous circulation likely
contributed significantly to this differential result. The pulsatile arterial circulation, reach-
ing peak flow velocities of approximately 1 m/s, is likely to have had a significant influence
on the type and extent of neointimal proliferation when compared with the continuous
low low state present in the venous circulation. It is likely that the covered stents placed
in the venous circulation prevented direct growth of the vessel wall through the stent but
not onto and around the device (Mohanchandra and Carman, 2009). It may be possible
to obtain improved results by covering both the outside and inside of the stent with thin
film nitinol. Because thin film nitinol seems to promote a very thin layer of endothelializa-
tion, an inner covering may improve our results by shielding the lumen of the vessel from
exposure to the bulk nitinol or stainless steel present in the stent scaffolding. As thin film
nitinol is much smoother and contains many less contaminants than bulk nitinol, this
inner covering could allow for a more favorable biological response when compared with
bulk nitinol (Mohanchandra and Carman, 2009).
Heart Valve
One medical device recently proposed for thin film TiNi is a heart valve (Stepan et al.,
2005). The TiNi film in the valve is used as the heart leaflets for a surgically placed heart
valve replacing the mechanical or bioprosthetic heart valves currently on the market. First,
the TiNi film may provide increased longevity compared to current bioprosthetic valves
(i.e., average 10-year lifespan). Second, the TiNi thin film valve may not require anticoagu-
lation therapy necessary in mechanical valves (Mohanchandra and Carman, 2009). For
percutaneously placed heart valves, the TiNi leaflets could be substantially thinner than
bioprosthetic allowing the heart valve to be collapsed into a substantially smaller cath-
eter for delivery. This is extremely important in pediatric patients. The surgically placed
TiNi prosthetic heart valve (see Figure 9.72) was designed and constructed by inserting
an elliptically shaped piece of shape memory TiNi thin film into a Teflon scaffold made
of two Teflon tubes with different diameters (Mohanchandra and Carman, 2009). After
