Flexible Leaflet Polymeric Heart Valves - Cardiovascular and Cardiac Therapeutic Devices - page 97

2 Surgically Implantable Valves
2.1 Polysiloxane Valves
Polysiloxanes, more commonly referred to as silicones (Dow Corning: Silastic )
are polymers of alternating Silicon and Oxygen atoms in the backbone, and can
contain a variety of pendant reactive groups on the silicon -[Si(R 2 )-O]-, with
polydimethylsiloxane (PDMS: R = CH 3 ) comprising the structural chain moiety.
Crosslinking through radicals, condensation or addition reactions leads to highly
elastic materials which have been used in a wide range of implantable devices due
to their biostability and flex and fatigue resistance [ 33 ].
Benson Roe and colleagues in San Francisco developed one of the first flexible
trileaflet valves from a silicone material (Silastic 50, Ellay Rubber Company) in
the late 1950s [ 26 , 34 ]. The 380 lm thick leaflets were originally housed in a
silicone cylinder, and valves evaluated in acute dog ascending aorta [ 34 ] showed
satisfactory function, but long-term survival could not be achieved in a subsequent
subcoronary model [ 35 ]. Later versions were made from a different silicone
(General Electric SE-555) and with composite of a steel band/silicone sponge/
Dacron sewing ring that contained slightly thicker leaflets (430-500 lm) [ 30 ].
Although some valves were shown to remain intact for 786 million cycles (mc) in
accelerated tests (58 Hz; 18 years equivalent at 80 beats per minute; bpm) the
program was discontinued due to high mortality rates in clinical trials with 18
selected patients between 1960 and 1962 [ 29 , 30 ]. The authors noted that the high
mortality in this case did not reflect failure of the leaflet mechanism, as most
deaths resulted from surgical complications, although emboli from clot formation
on the valve were evident in 3 cases. It should be noted, however, that the high
cycle rate of 58 Hz is not conducive to proper valve opening and closing, and that
lower rates were subsequently used by later researchers, as seen below.
Mohri et al. revisited trileaflet silicone valves and tested various Dow Corning
(DC) silicone formulations and leaflet designs ranging from 225 to 510 lmin
thickness with a preferred thickness of 330 lm[ 36 ]. Some of the initial combi-
nations exhibited poor durability with tearing at 3.9 years (dome shaped; DC
Silastic 372), but the preferred embodiment valves were able to withstand
17.7-23.8 equivalent years in accelerated tests (average 900 mc at 17 or 33 Hz;
triangular leaflets; DC MDX4-4059 experimental polymer). This study also
showed, however, the significant effect of valve fabrication as well as the con-
sistency of raw materials on durability, as valves made from a different batch of
the same material failed after 4-10 year equivalents. Another design reported by
Gerring et al. (the Oxford valve) comprised silicone (DC Silastic 5505) coated
Terylene (polyester) fabric and had a thickness of 120 lm[ 37 ]. In vivo evaluation
in the pulmonary position of calves showed good survival of growing animals up
to 30 months, with only 2 out of 7 animals in one series dying of thromboembolic
events.