Biomedical Engineering Reference
In-Depth Information
6 Manufacturing Techniques
Various techniques have been used to process the polymers into the complex 3-
dimensional leaflet shapes required. The first determinant of the technique avail-
able is the nature of the polymer used. Thermoplastic materials may be processed
using heat and solvent processing (if the polymer is sufficiently soluble), whereas
moulding techniques are required if the materials are thermosets.
Dipcasting is technologically one of the simplest techniques as it involves
dipping a mould into a solution of the polymer, removing the mould such that a
film of polymer solution adheres to the mould, and then evaporating the solvent to
yield the desired structure. In practice, repeatability in obtaining the desired
thicknesses and quality is not trivial. Solvent volatility, polymer concentration/
viscosity, dipping number (single vs. multiple), technique and speed, drying
position (upright, upside down, rotating), atmospheric conditions (pressure, tem-
perature, gasses), and mould shape and surface properties all affect the distribution
of material (leaflet thickness) and quality (surface morphology, uniformity, bubble
formation/imperfections, durability and function (Fig.
11
).
6.1 Polyurethane Valves
The J-3 polyurethane valve is interesting as it is manufactured in a half-open
configuration. In order to achieve this, the top of the metal mould was flared so that
the arms of the stent would be expanded between the minimal surface area leaflet
surfaces. Dipcoating of leaflets and stent integration occurred in one step, using
cleanroom conditions within a dry glove box, and the valve is tumbled in a
controlled motion during drying to ensure even leaflet thicknesses [
53
,
54
]. This
method also avoids shape-memory effects, which can occur when leaflets are made
in a precurved closed position.
ADIAM later improved the manufacturing technique by employing a combi-
nation of dipcoating and dropping techniques (Fig.
2
c) of polymers of different
hardnesses (soft outer layers and medium hardness core), whereby the polymer is
applied in a controlled dropwise fashion in order to achieve accurate control over
leaflet thickness (which is varied between 80 and 200 lm depending on position),
the use of flexile stent posts to ensure proper closing, and the use of a precision
laser to separate the leaflets [
55
,
56
,
58
]. The company also produced a scalloped
bileaflet valve with large anterior and small posterior leaflets (varies between 100
and 300 lm thickness) specifically for the mitral position, and using sewing rings
made from fleece-like sprayed PCU [
55
,
57
,
58
]. The Fraunhofer Institute in
collaboration with Helmholtz Institute for Biomedical Engineering of RWTH
Aachen University has recently also announced the very accurate production of
small venous valves using the precise dropping techniques [
140
].
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