Measurement of mechanical properties of vascular smooth muscle and blood vessel wall as a model of functional materials

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To estimate mechanical environment of smooth muscle cells (SMCs) in the vessel wall, we measured mechanical properties of SMCs and their microscopic residual stress and strains in the wall : We measured quasi-static tensile properties of SMCs freshly isolated from the media with our original tensile tester to find that their mechanical properties have a close correlation with intracellular actin filaments (AFs). We also measured their stress relaxation properties to find that stress relaxation process could be separated into two phases : fast passive relaxation phase with a time constant in the order of a minute and slow active phase with a time constant in the order of an hour. Then, we established a method to elucidate microscopic mechanical environment for the aortic walls composed of concentric layers of SMCs and elastic laminas (ELs). To estimate stresses in the ELs and SMCs in the in vivo state, ELs were isolated enzymatically from 10-μm-thick slices of aortic walls to obtain their tensile properties. Upon isolation, corrugation of the ELs almost disappeared, indicating that the corrugation was buckling of the ELs due to their compressive residual strain. Residual stress and strain in the SMCs were estimated in their tensile test by referring to their nuclear length. Tensile stress required to restore the nuclear length in tissue and its concomitant strain was obtained as microscopic residual stress and strain and was found to be 10 kPa and 23%, respectively. In vivo stresses in the ELs and SMCs were calculated from data obtained in the present study and strain distribution obtained for homogeneous aortic wall. They were 80-40% of the mean hoop stress of the wall. Such inconsistency might be caused because 1) mechanical properties of SMCs were measured under fully relaxed state, and 2) collagen fibers were not taken into consideration.