Biomedical Engineering Reference
In-Depth Information
3 Developing of New Scaffold Shapes
3.1 Improving Scaffold Shapes and Introducing New Designs
3.1.1 Model Extension
Previously presented simulations correlate to the situation outlined by Schilling et al.
where the scaffold is sewn onto the epicardium [
24
]. However, it is intended that the
scaffold protect a biological graft against bulging. Therefore, the resulting load on the
scaffold due to swelling of the graft must be taken into account. Since no material
data of an appropriate tissue was available, another approach was chosen. It was
assumed that a bulging graft induced a pressure onto the scaffold. Pressure peaks of
up to 240mmHg are possible in the left ventricle. For the simulation, 140mmHg was
chosen—representing prehypertension—and applied onto the back of the scaffold in
the systolic step [
28
]. With this approach, calculations were performed for scaffolds
of 0.5mm and 1mm thickness.
3.1.2 Design Requirements
Manufacturing techniques, operating site, and intended purpose of an implant define
its design requirements. The implant is for use on the myocardium, while its size
depends on the size of the damaged area. Diameters of 30-60mm seem to be typical;
we chose an outer diameter of 40mm. An opening, preferably in the middle of the
scaffold, is needed to allow the passage of vessels for blood supply. The intention of
the scaffold is to support a biological graft and prevent it frombulging [
22
]. Therefore,
a high contact area between graft and implant is needed, the heart movement should
not be restricted, and the scaffold should withstand the cardiac motion. Sheets down
to a thickness of 0.5mm can be manufactured from LA63. The scaffold should have
strut widths of at least 1mm. Spaces between the struts may be a minimum of 0.6mm
due to the cutting diameter of the abrasive waterjet cutting method used to form the
scaffolds [
29
].
3.1.3 Shape Improvements
Under pure bending, a reduction of structure thickness will lead to lower maximal
stress. This effect was already investigated (see Fig.
6
). Further, it is possible to locally
decrease the thickness where high bending stresses are detected and thereby introduce
a kind of 'hinge'. However, this approach increases the manufacturing expense and
is not advocated. A uniform structure thickness was chosen, with stresses reduced
by changing the structure shape.
Mattheck suggests the use of 'tensile triangles' [
30
]. Using this method, a corner
is shaped as shown in the construction in Fig.
7
. Starting with the left triangle each
