The Intrinsic Fatigue Mechanism of the Porcine Aortic Valve Extracellular Matrix
Liao, J., Joyce, E. M., Merryman, D. W., Jones, H. L., Tahai, M., Horstemeyer, M., Williams, L. N., Hopkins, R. A., & Sacks, M. S. (2012). The Intrinsic Fatigue Mechanism of the Porcine Aortic Valve Extracellular Matrix. Cardiovascular Engineering and Technology. 3(1), 62-72. DOI:10.1007/s13239-011-0080-4.
In the last two decades, decellularized native aortic valve (AV) has been investigated as tissue engineered heart valve replacement due to its potential to develop into a viable valve. However, it is not known what the intrinsic durability of the AV extracellular matrix (ECM) is and how this relates to decellularized AV functional limits when tissue remodeling does not take place. In this study, decellularized AVs were subjected to cardiac exercising and the fatigued AV leaflets were characterized structurally and mechanically to assess the ECM durability in the absence of cellular maintenance. A flow loop cardiac exerciser was designed to allow for left side heart pulsatile flow conditions while maintaining sterility. Porcine AV conduits were decellularized with 0.1% sodium dodecyl sulfate (SDS). The acellular valve conduits were then sutured into a silicone root with sinus design and subjected to cardiac cycling for 2 weeks (1.2 million cycles) and 4 weeks (2.4 million cycles). PBS solution was added with 3% antibiotic–antimycotic and protease inhibitor PMSF (Phenylmethyl Sulfonyl Fluoride) and changed every two days to maintain sterilization. We found that the overall morphology was maintained and leaflets were able to coapt and support load. However, exercised AV leaflets exhibited unfolded and thinned morphology. The unfolding of locally wavy fiber structure was observed by both H&E and picrosirius staining. The straightening of fiber network was also demonstrated by small angle light scattering (SALS), in which flow loop exercised leaflets showed higher alignment due to cyclic stretching. Biaxial mechanical testing showed that (i) leaflet extensibility along both circumferential and radial directions increased after SDS treatment, however, (ii) extensibility reduced by solution storage effects (static control groups), and (iii) extensibility was further reduced by cardiac exercising (2 weeks and 4 weeks cardiac cycling). Mechanically, the structural alteration changed the reference status of tissue and resulted in a lower net tissue extensibility. We conclude that, in absence of cellular maintenance, decellularized valve leaflets experience rapid structural deterioration due to lack of exogenous stabilizing crosslinks and the structural disruption is irreversible and cumulative.