Fatigue Modeling of Collagenous Soft Tissue

In this study, a phenomenological tissue damage model has been developed to describe the fatigue-induced stress softening and permanent set of biological tissues. Since damage evolution is an irreversible dissipative process, following thermodynamic principles, an equivalent strain proportional to the strain energy of the material is employed as the damage criterion. The maximum equivalent strain represents the value necessary to cause complete sample failure during one loading cycle, while the minimum equivalent strain is the value required to elicit the onset of fatigue damage. The damage parameter evolves from zero (below minimum equivalent strain) to one at maximum equivalent strain as a function of both the equivalent strain and number of loading cycles. The permanent set evolves as a function of the peak strain in the principal directions. The damage model is implemented into ABAQUS via a user defined material (UMAT) in conjunction with the nonlinear orthotropic Fungelastic model. For the purpose of this study, glutaraldehyde-treated bovine pericardium (GLBP), a collagenous tissue traditionally used for fabricating bio-prosthetic heart valve (BHV) leaflets, is utilized as a representative collagenous tissue due to its limited durability in BHV applications. INTRODUCTION Biologically derived, chemically-treated collagenous tissues are extensively utilized in a broad spectrum of medical applications including cardiovascular grafts and bio-prosthetic heart valves (BHV), as well as ligament, tendon, cartilage, sclera, and hernia repair and replacement. However, despite their widespread use, similar to devices made of metallic and polymeric materials, fatigue-induced degradation, wear and tearing have been identified as some of the major problems associated with collagenous tissue implant failure. Fatigue is an especially critical issue for the application of BHVs. Today, the only effective, long-term treatment for valvular heart disease (VHD) is open-chest cardiac valve repair or replacement surgery. BHVs fabricated from glutaraldehyde-treated bovine pericardium (GLBP) have been used to treat VHD for over three decades , and continue to be one of the dominant replacement valve modalities, either as a conventional prosthetic valve design or more recently for minimally invasive percutaneous delivery . BHVs display superior hemodynamics to mechanical valves, and they eliminate the need for anticoagulant therapy. Regardless of the specific design, long-term fatigue resilience remains the major limitation in the durability of the GLBP tissue leaflets (10-15 years) and the mechanisms governing this process are largely unknown. There are many challenges associated with fatigue testing of BHV materials. The experimental methods can be very time consuming, involving complex testing instruments. The current FDA requirement mandates new BHV designs to be tested up to 200 million cycles to evaluate the fatigue performance using accelerated wear testers. Although, accelerated wear testers can approximate the in vivo hemodynamics, it remains difficult to determine the specific effects of different fatigue modalities (tensile, compressive, bending, etc.) on the leaflet material properties when using this technique. To address this drawback, several groups have conducted isolated material tests in order to determine for instance, the effects of uniaxial cyclic loading on GLBP material properties 5,6 and collagen fiber orientation . Others have investigated the effects of cyclic bending fatigue on the leaflet flexural rigidity . These studies, however, are limited to less than 100 millions of cycles of fatigue.