Flexible Leaflet Polymeric Heart Valves - Cardiovascular and Cardiac Therapeutic Devices - page 118

Striking examples of treatments or formulations that result in reduced calcifi-
cation are (i) the extraction of low molecular mass components from polyurethanes
on calcification of leaflets in vitro [ 60 , 61 ], (ii) the potential of nanocomposite
formulations of PCA to decrease calcification [ 75 ], and (iii) the lack of degrada-
tion, calcification and thrombosis in leaflets made from Elast-Eon [ 63 , 65 ].
Bisphosphonates are well known for their effect on mineralization [ 160 ], and
derivatization of the hard segment of medical grade polyurethanes have been
shown to decrease calcification in the rat subcutaneous model [ 161 ], as well as in
the circulating sheep pulmonary valve cusp model without adverse effects on the
polymers [ 157 ]. A summary of this group's work on anticalcification and anti-
thrombotic treatments of polymeric materials can be found in [ 162 ].
Cholesterol modification of polyurethanes seems a very promising approach, as
endothelial cell attachment and retention could be shown not only in vitro [ 163 ,
164 ], but also in vivo, employing pulmonary leaflet replacement with autologous
endothelium-seeded polyurethane cusps [ 164 ].
Treatment of SIBS with dimyristoyl phosphatidylcholine (DMPC) did show
promise in terms of decreased platelet adhesion, but calcification and degradation
meant that the coatings did not have much effect when tested in a full-scale sheep
valve model [ 81 ].
8 Summary and Conclusions
This review introduces the different types of replacement heart valves currently
available, and identifies a need for alternate valves that would provide good long-
term function and durability without the need for anticoagulation. Additionally, the
unsolved need for inexpensive valves and replacement procedures for sufferers of
rheumatic heart valve disease is highlighted. One alternative, namely the use of
stable polymeric materials to fabricate the leaflets of heart valves, is reviewed and
discussed in terms of application as surgically implantable valves, valves used in
ventricular assist devices, and those that can be inserted via minimally invasive
procedures.
In the case of surgical valves, polyurethanes were the most favoured polymer,
with silicones and polytetrafluoroethylene being the next most prevalent materials,
and polyolefins and cryogel polyvinyl alcohol materials making up the bulk of the
remainder of research. Silicone valves produced in the 1960s could withstand
accelerated testing of up to 700 mc, and 900 mc were achieved in the 1970s. In
addition to animal studies, some of these valves were also implanted clinically, but
failed due to surgical complications and thrombosis. Early PTFE fabric valves also
failed when implanted into patients, as the materials stiffened, cracked, tore and
even disintegrated. Handmade valves from expanded PTFE, on the other hand,
were and are used with great success in a centre in Japan for pulmonary recon-
struction after Ross procedures and complex congenital heart disease.