A microindentation technique to measure rheological properties of the vascular intima

A microindentor was developed to measure the depth of indentation of the intimal surface of arterial tissue loaded by flat-ended, cylindrical probes. The depth of indentation depended on the initial stretch of the tissue which required a rigid support (plaster of Paris) beneath the adventitial surface. Probe tips used ranged from 550-mum down to 65-mum diameter while loads ranged from 800 to 15 mg. The depth of indentation was markedly time dependent; that obtained 30 s after loading (variation of 30) was reproducible and served as a useful parameter of viscoelasticity of the aortic intima and supporting tissues of dog and man. The mean variation of 30 (0.19-mm diam tip, 120-mg load), obtained from longitudinal series of indentations of nine dog aortas, ranged from 40.8 to 68.8 mum while coefficient of variation in these series ranged from 4.8 to 15.9%. Intimal pads were found to have greater resistance to identation than adjacent tissue; likewise the tissue on the dorsal, intimal surface of the aorta had lower variation of 30 values compared with the rest of the intima. Lipid-filled intimal regions were about twice as complaint as macroscopically spared areas. The technique should prove useful in understanding the microrheological response of the blood vascular interface to hydrodynamic stresses.