Arterial Walls under Supraphysiological Loading

This research project deals with the analysis and the modeling of traumatic degenerations of overstretched arterial walls that occur in therapeutical interventions. The data base for the qualitative and quantitative description of arterial tissues is obtained from biaxial extension tests performed on the tissue components of individual arterial layers loaded far beyond the physiological domain. Such tests enable the analysis of the macroscopic mechanical response of the tissues. In addition, structural analysis techniques such as Fourier transfer infrared spectroscopy and scanning electron microscopy are used to study damage on the smaller length scale. The macroscopic response of the fiber-reinforced tissues is described by a formulation based on micro-mechanical models characterizing the individual tissue components. These models take into account alterations of stochastic distributions of fiber properties as a consequence of the tissue overstretch. In order to obtain a quantitative prediction of the material response the model parameters are adjusted to the performed experiments based on leastsquare minimization. Finally, we validate the models by comparing finite element calculations with experiments performed on whole arterial wall segments.

This research project dealt with the analysis and modeling of traumatic degenerations of overstretched arterial walls that occur in therapeutical interventions. For example, clinical interventions such as balloon angioplasty are performed to treat atherosclerotic degeneration which result in luminal narrowing. This invasive procedure involves the inflation of a catheter with the aim to increase the lumen dimensions by pushing the obstructing plaque into the vessel wall. Naturally, for this procedure a much higher pressure than the physiological blood pressure is required, and this causes microscopic-level tissue damage and results in stress-softening of the collagenous tissue.A database for the qualitative and quantitative description of arterial tissues was obtained from uniaxial and biaxial extension tests performed on the tissue components of individual arterial layers loaded far beyond the physiological domain. Towards this end cyclic tension-tests were carried out on healthy as well as on collagenase and elastase treated arterial tissues to investigate the component-specific stress softening behavior. Such tests enable the analysis of the macroscopic mechanical tissue response. In addition, structural analysis techniques such as Fourier transform infrared spectroscopy, scanning electron microscopy and nano-tomography were used to study damage at the microscopic length scale, with a special focus on damage induced changes in the interfibrillar collagen distances and the angular proteoglycan distributions.The macroscopic response of the fiber-reinforced tissues was described by a formulation based on micro-mechanical models characterizing the individual tissue components. These models take into account the alterations of stochastic distributions of fiber properties as a consequence of the tissue overstretch. In order to obtain a quantitative prediction of the material response, the model parameters were adjusted so as to fit data from the performed experiments based on least-squares minimization.Finally, the models were validated by comparing finite element calculations with experiments performed on whole arterial wall segments.