Biomedical Engineering Reference
In-Depth Information
and polyurethane, and its superior inertness due to its oxidation resistant Carbon-
Carbon backbone [
78
,
79
]. In addition to the percutaneous valve described in
Sect. 4
, the group also worked on surgically implantable valves. Early SIBS
impregnated Dacron mesh valves (240 lm thickness) showed no significant dif-
ference to mechanical and bioprosthetic controls regarding thrombogenicity using
a platelet activation state (PAS) assay in a VAD device [
80
]. Valves, with and
without precoating with dimyristoyl phosphatidylcholine (DMPC), implanted for
20w in the sheep aortic position, failed due to stent deformation and cracks in the
leaflets due to creep, with accompanying exposure of the PET fabric, with severe
blood and tissue interactions, including extrinsic calcification [
81
]. A new SIBS
valve design subsequently evaluated in vitro for thrombogenicity and platelet
activation rates (PAR) was multiples lower than control bioprosthetic valves using
the PAS assay and flow cytometry [
82
]. Innovia has also developed a thermoset
crosslinked version of the material, dubbed xSIBS, to overcome creep problems
associated with the non-crosslinked material, and to potentially make leaflets
without the need for Dacron reinforcement [
83
]. The devices were recently opti-
mised by the group at Stony Brook University (in collaboration with groups in
Aachen and Arizona) for reduced stress, improved hemodynamics and reduced
thrombogenicity using device thrombogenicity evaluation (DTE) [
84
,
85
].
The group of Frank Baaijens in Eindhoven has been working on another olefinic
material, namely crosslinked ethylene-propylene-dimer-monomer rubbers
(EPDM; K520 from DSM), for the construction of heart valves. The valve com-
prises multiple layers of EPDM reinforced with wound fibres in a design that
allows for the moulding of the leaflets (200 lm) as well as sinuses and part of the
aortic root [
86
,
87
].
Prototype heart valves have also been made from polyvinyl alcohol (PVA, a
water soluble polymer) rendered insoluble by a one-piece cavity moulding/freeze
thawing procedure (to form a so-called cryogel) using well-defined geometries
discussed in
Sect. 5
[
88
]. Devices were also fabricated from a combination of PVA
with bacterial cellulose reinforcement to mimic the anisotropic mechanics of
natural valve leaflets [
89
-
91
].
3 VAD/TAH Valves
Although some of the latest generation non-pulsatile centrifugal and axial flow
VADs are also designed for (permanent) destination therapies (DT) [
92
], pulsatile
VADs used in bridge to recovery (BTR) and/or bridge to transplant (BTT) treat-
ments [
93
,
94
] require valves that would function for a relatively short time.
Reduced time of blood contact, reduced durability requirements and employment
of systemic anticoagulation make the VAD market a logical entry point for
polymeric valves, although the successful application of the device as a whole
depends not only on the valve, but also on the large potentially thrombogenic
blood-contact area of the pumping chamber.
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