Abstract
Age-related aortic stiffening is associated with cardiovascular diseases such as heart failure. The mechanical functions of the main structural components of the aorta, such as collagen and elastin, are determined in part by their organisation at the micrometer length scale. With age and disease both components undergo aberrant remodelling, hence, there is a need for accurate characterisation of the biomechanical properties at this length scale. In this study we used a frequency-modulated atomic force microscopy (FM-AFM) technique on a model of ageing in female sheep aorta (young: ~18 months, old: >8 years) to measure the micromechanical properties of the medial layer of the ascending aorta. The novelty of our FM-AFM method, operated at 30kHz, is that it is non-contact and can be performed on a conventional AFM using the ׳cantilever tune’ mode, with a spatial (areal) resolution of around 1.6μm2. We found significant changes in the elastic and viscoelastic properties within the medial lamellar unit (elastic lamellae and adjacent inter-lamellar space) with age. In particular, there was an increase in elastic modulus (Young; geometric mean (geometric SD)=42.9 (2.26)kPa, Old=113.9 (2.57)kPa, P<0.0001), G′ and G″ (storage and loss modulus respectively) (Young; G′=14.3 (2.26)kPa, Old G′=38.0 (2.57)kPa, P<0.0001; Young; G″=14.5 (2.56)kPa, Old G″=32.8 (2.52)kPa, P<0.0001). The trends observed in the elastic properties with FM-AFM matched those we have previously found using scanning acoustic microscopy (SAM). The utility of the FM-AFM method is that it does not require custom AFM hardware and can be used to simultaneously determine the elastic and viscoelastic behaviour of a biological sample.
Highlights
Ageing is associated with arterial stiffening and the development of cardiovascular disease.Stiffening in large arteries such as the aorta is typically studied at the level of the whole vessel
We show that age-related changes in the viscoelastic properties of the tissue are localised to the inter-lamellar regions of the aortic medial layer
frequencymodulated atomic force microscopy (FM-atomic force microscopy (AFM)) overcomes the various issues of contact or intermittent contact mode AFM when used for mechanical measurements, such as using the appropriate contact mechanics model for accounting for tipsample interaction (Lin et al, 2009)
Summary
Ageing is associated with arterial stiffening and the development of cardiovascular disease.Stiffening in large arteries such as the aorta is typically studied at the level of the whole vessel. There is evidence which suggests that alterations in the structure and mechanical properties of large arteries at the micron scale can have detrimental effects on the function of the aorta (Kohn et al, 2015). Due to the intricate organisation of extracellular matrix (ECM) components within the vessel wall, there is a need to develop methods that accurately measure the mechanical properties of the tissue with high spatial resolution. Due to a lack of appropriate techniques, there is still limited information on these properties at the micron length scale (Graham et al, 2011).
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