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Weiterentwicklung von kardiovaskulären Implantaten und finite Element Modellierung der Degradation von Mg-Legierungen

Improvement of cardiovascular Implants and a FE-Framework for Degradation of Mg-Alloys

Led by:  P. Wriggers, J. Lamon, S. Besdo
Team:  M. Weidling
Year:  2011
Funding:  DFG within IRTG 1627
Is Finished:  yes

Background:

Lesioned myocardial tissue due to ischemic incidents is a wide-spread disease in industrial nations. Surgical substitution of damaged cardiac tissue with biological grafts is a promising regenerative approach. An anticipated physiological in vivo remodelling of the often heterotopically applied grafts would allow for full recovery the heart`s performance. However, the strength of most biological grafts is initially not sufficient for left ventricular application. An implant can help that mechanically supports these grafts and gradually loses its function as the graft develops its strength. Our approach is to develop magnesium alloy scaffolds. Magnesium and some of its alloys degrade in a physiological environment. Magnesium is needed for the metabolism; therefore the whereabouts of the elements in the body is uncritical. Various magnesium alloys are developed by the Institut für Werkstoffkunde (Material Science), Leibniz Universität Hannover, for use as bioresorbable implants. Scaffolds are manufactured from extruded sheets by abrasive waterjet cutting. A dynamic testing rig is used for in vitro tests while in vivo studies are done in animal tests.

 

Tasks:

Previously designed myocardial scaffolds do not achieve the required durability. Fractures occur at an early stage after implantation. Therefore, it is needed to adjust the scaffold design, to better endure loads during cardiac motion and thereby prevent premature failure due to fatigue.

Progressing degradation influences the durability of the implant and therewith its functionality. Thus, degradation needs to be taken into account.

 

Approach:

The finite element method is used to simulate deformations of myocardial scaffolds according to cardiac movement. Hereby, areas on the implant can be identified that are highly stressed and strained. Knowing these, the design can be changed to reduce stresses. Further, it is assumed that preforming of the scaffold in line with the heart curvature can significantly reduce resultant stresses within the implant. Therefore flat and preformed scaffolds are compared. Additionally new shapes ideas are designed and simulated.

In a second step, the degradation behaviour of the used magnesium alloys is included in the FE simulation. To this end it is necessary to understand the corrosion mechanisms of magnesium and its alloys. Based on that, a numerical model is developed and implemented in FE code.

The simulations need to be validated with experiments.

 

Results:

A model was developed that mimics myocardial movement of the anterior basal heart region. It was shown that preformed scaffolds are significantly lower stressed in comparison to flat ones. This was confirmed by experiments. A few new shapes were designed, some of them are promising. Detailed results can be found in the below mentioned publications.

Magnesium corrosion is a very complex process, composed of several mechanisms. Reduced to the main mechanisms, a model concept was abstracted. Work in progress is to find suitable numerical models and implement them.