Human heart valves: advanced physiology and structure-function characterization for tissue engineering applications

The four human heart valves (HVs) exhibit substantial physiology, reflected in their structural and mechanical properties. HV diseases remain among the most challenging clinical issues in cardiovascular medicine. The mechanical and structural properties of native human HVs, such as tensile and flexural stiffness, anisotropy, or even simpler factors such as leaflet thickness are largely neglected in the design of commercially available prosthetic devices. Furthermore, these properties are frequently poorly characterized, with data primarily derived from animal or cadaveric tissues. This gap in knowledge in valve physiology and prosthetic devices has increasingly made human cardiovascular tissue characterization a priority in biomechanics and tissue engineering. The purpose of this study is to fully characterize human HVs structure at both organ and tissue scales and to compare these measurements with equivalent data obtained from porcine and ovine tissue. To achieve this aim, fresh human samples were obtained from heart transplanted patients and tested within 24 hours from the tissue explant. At the organ scale, thickness topology for the mitral, aortic, tricuspid, and pulmonary (MV, AV, TV, PV) HVs was measured and compared. Quantitative histology and collagen bundle diameter were analyzed and compared at the microscopic scale. In order to measure collagen fiber bundle diameter a novel decellularization protocol, specific for HV leaflets, was developed. Scanning electron microscopy (SEM) analysis was subsequently performed on the decellularized leaflets, enabling quantification of fiber angle distribution and diameter using a modified, custom-made algorithm in conjunction with ImageJ software. Functional analyses were conducted through biaxial mechanical tests and three-point bending tests. The bending rigidity (EI) of native valve leaflets corroborated the biaxial response, showing that the MV and TV displayed greater bending rigidity (EI) than the outflow tract valves, the AV, and the PV. This study highlights structural and functional differences in fresh human HVs. Establishing a detailed database of HV structural and functional parameters is critical for advancing valve engineering and incorporating biomimicry into the design of leaflet, supra-, and sub-valvular apparatuses. This comprehensive resource will aid in advancing the biomimetic-engineered tissue paradigm by providing detailed descriptions of HV structural and biomechanical parameters that can be replicated using current or future material processing technologies.