Flexible Leaflet Polymeric Heart Valves - Cardiovascular and Cardiac Therapeutic Devices - page 114

Fig. 11 Layer thickness as a
function of number of coats
for Avcothane -51
polyurethane; 12.5 % solids
by mass (adapted from [ 52 ])
Dip-coating onto injection moulded PU or machined PEEK frames fitted to
metal forming mandrels (35-45 % Estane or Lycra in DMAc) was used by David
Wheatley et al. to produce their ellipto-hyperbolic valves, with the solvent evap-
orated under slight positive pressure at 70-80 C, vented into a water trap to
ensure solvent evacuation [ 60 , 61 , 64 ]. Variations of this method when using
Elast-Eon involved dipping and drying under inert atmosphere, and using single
(1x 27 % PSUU in DMAc) and multiple (3x 15 % PSU in DMAc) coatings on pre-
warmed steel former containing the PEEK frame [ 62 , 63 ].
The alpharabola and spherical designs of the Leeds group were made by a
combination of techniques. It comprised the solvent bonding (8 % PU in DMF) of
leaflets cut from solvent cast films of PU to PU-dip-coated (250 lm layer) PEEK
frames, and subsequent thermal forming of the leaflet geometries in custom-made
metal moulds [ 72 ]. These valves were later compared to dipcoated valves,
whereby stented mandrels were successively dipped in 25, 25 and 12 % (twice
consecutively in 25 % then in 12 %) PU solutions in DMF followed by drying
respectively in the upright, inverted (upside down) and upright positions (12 mm/
min dipping; 75 C drying for 1 h) [ 73 ]. Removal from the moulds was facilitated
by extended immersion in water (12 h), and leaflets were cut using a sharp blade.
Alex Seifalian's group originally explored electro-hydrodynamic atomization
(electrospraying) to make valves from their POSS-PCU material, but is now using
a more established dip coating process [ 141 ]. The process comprises the repeated
controlled dipping of a metal mandrel containing a contoured stent wire (0.5 mm
titanium alloy) into the polyurethane solution (18 % w/v in DMAc) followed by
drying (60 C for 1 h) and trimming of the free edges [ 77 ].
6.2 Silicone Valves
Early silicone valves were cast in one piece (with a tubular root structure sur-
rounding the leaflets) by compression moulding, heating the mould to 180 C under
high pressure and curing after removal from the die at 200 C for 4 h [ 26 , 30 ]. A
similar process was followed 15 years later, using highly polished three-piece metal
moulds into which the silicone was injected, from which excess air was removed