Microarchitectural characterization of the aortic heart valve

Anisotropic strain transfer through the aortic valve and its relevance to the cellular mechanical environment

Aortic valve interstitial cells are responsible for maintaining the valve in response to their local mechanical environment. However, the complex organization of the extracellular matrix means cell strains cannot be directly derived from gross strains, and knowledge of tissue structure-function correlations is fundamental towards understanding mechanotransduction. This study investigates strain transfer through the valve, hypothesizing that organization of the valve matrix leads to non-homogenous local strains. Radial and circumferential samples were cut from aortic valve leaflets and subjected to quasi-static mechanical characterization. Further samples were imaged using confocal microscopy, to determine local strains in the matrix. Mechanical data demonstrated that the valve was significantly stronger and stiffer when loaded circumferentially, comparable with previous studies. Micromechanical studies demonstrated that strain transfer through the matrix is anisotropic and indirect, with local strains consistently smaller than applied strains in both orientations. Under radial loading, strains were transferred linearly to cells. However, under circumferential loading, strains were only one-third of applied values, with a less direct relationship between applied and local strains. This may result from matrix reorganization, and be important for preventing cellular damage during normal valve function. These findings should be taken into account when investigating interstitial cell behaviours, such as cell metabolism and mechanotransduction.

A combined experimental and modelling approach to aortic valve viscoelasticity in tensile deformation

The quasi-static mechanical behaviour of the aortic valve (AV) is highly non-linear and anisotropic in nature and reflects the complex collagen fibre kinematics in response to applied loading. However, little is known about the viscoelastic behaviour of the AV. The aim of this study was to investigate porcine AV tissue under uniaxial tensile deformation, in order to establish the directional dependence of its viscoelastic behaviour. Rate dependency associated with different mechanical properties was investigated, and a new viscoelastic model incorporating rate effects developed, based on the Kelvin-Voigt model. Even at low applied loads, experimental results showed rate dependency in the stress-strain response, and also hysteresis and dissipation effects. Furthermore, corresponding values of each parameter depended on the loading direction. The model successfully predicted the experimental data and indicated a 'shear-thinning' behaviour. By extrapolating the experimental data to that at physiological strain rates, the model predicts viscous damping coefficients of 8.3 MPa s and 3.9 MPa s, in circumferential and radial directions, respectively. This implies that the native AV offers minimal resistance to internal shear forces induced by blood flow, a potentially critical design feature for substitute implants. These data suggest that the mechanical behaviour of the AV cannot be thoroughly characterised by elastic deformation and fibre recruitment assumptions alone.