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Arterial stiffening occurs as part of the natural ageing process,and is thought to be related to the changes in the extracellular matrix,namely accumulation of collagen and degradation of elastin.However,little is known about how regional variations in arterial structure and mechanical properties contribute to arterial stiffening.This study compared localised differences in the nano-structure and mechanical properties in the internal mammary arteries(IMA)from two groups of patients; those with high and low pulse wave velocity(PWV).PWV is a widely used in vivo technique for assessing arterial stiffening,where the speed of propagation of pulsatile flow of blood along a length of an artery is measured.It is considered a gold standard clinical technique but does not provide information on localised changes within arteries.In this study 6 internal mammary arteries(IMAs)were collected from coronary bypass operations and the patients were grouped according to their carotid-femoral PWV; high(14.6 ± 1.4 m/s)and low(8.7 ± 0.5 m/s).The nano-topography and elastic modulus were determined by atomic force microscopy(AFM)operated under the Peakforce QNM mode using 5 μm cryosections.The localised mechanical properties were mapped across the different layers across the artery samples.Overall,IMAs in the high PWV group were significantly stiffer than in the low PWV group(High; 2234.7 ± 72.3 MPa,Low; 2015.3 ± 58.4 MPa),(p < 0.0001).Although no significant difference was found in the intimal or medial layers,the adventitia was stiffer in the high PWV group(High; 2597.6 ± 135.7 MPa,Low; 2215.6 ± 110.2 MPa),(p < 0.001).Furthermore,the collagen fibrils in the adventitia of the high PWV group were found to have a smaller diameter(High; 118.44 ± 1.1 nm,Low; 123.81 ± 1.3 nm),(p < 0.01)).Overall,the nanomechanical data was found to associate well with PWV data.The high PWV group exhibited higher nanomechanical stiffness alongside morphological alterations within the adventitial layer.