Flexible Leaflet Polymeric Heart Valves - Cardiovascular and Cardiac Therapeutic Devices - page 98

Room temperature vulcanizing silicone (RTV-615; General Electric) was used
by Chetta and Lloyd to produce two valve designs with 256 lm leaflets using a
cavity moulding method and achieved 65 and 280 mc respectively at 21 Hz [ 38 ].
Pulse duplication studies highlighted the need for test sections to be standardized
and as close to physiological shape as possible, as the geometry of the test section
significantly affects the valve performance.
2.2 PTFE and ePTFE Valves
Polytetrafluoroethylene, commonly known by the tradename Teflon , is a highly
crystalline fluorinated polymer that is well known for its inertness, low surface
tension (high water contact angle), and low coefficient of friction.
Nina Braunwald and Andrew Morrow reported on clinical trials using flexible
tricuspid Teflon fabric prostheses, based on the Muller-Littlefield valve [ 39 ] for
aortic valve replacement in 23 patients between 1961 and 1963 [ 40 ]. The pros-
theses were either of plain Teflon fabric valves, Teflon fabric valves with a Teflon
dispersion, or single Bahnson leaflets of jersey-knit Teflon. In most cases valves
removed at reoperation or autopsy were stiffened, with regurgitation through holes
in the leaflets and tears of varying sizes from small cracks to complete shredding
and disruption of the fabric integrity. Nodular calcific deposits were evident but
extensive calcification was not observed.
Gore-Tex trileaflet valves (23-25 mm; WL Gore & Associates) were rein-
vestigated in an animal study in 1990 (weanling sheep tricuspid up to 34 weeks)
[ 41 ]. Leaflets were all unretracted, but half the valves had one or more stiffened
leaflet, and macroscopic calcification was evident in the commissural areas. Ando
and Takahashi reported the use of hand-made valves comprising Gore-Tex leaflets
suspended in Dacron conduits for the treatment of 139 paediatric patients under-
going pulmonary reconstruction due to Ross procedures and complex congenital
heart disease [ 42 ]. The valves showed functional in vivo durability for up to
10 years and are considered to be a good alternative to homo and xenografts in this
position.
The experiences with textile and expanded PTFE thus indicate that PTFE
valves calcify and stiffen, and it seems, apart from the hand-made valves men-
tioned above, that the use of this material for heart valves is limited to employment
as chordae tendineae for mitral valve repair [ 43 , 44 ].
2.3 Polyurethane Valves
Polyurethanes are a broad class of materials with a very wide range of properties
depending on the chemistries involved in their manufacture. They can be either
thermosets or thermoplastics; in the latter case the polymers may form virtual