Assessment of the nano-mechanical properties of healthy and atherosclerotic coronary arteries by atomic force microscopy

Characterization of mouse artery tissue properties using experimental testing combined with finite element modelling

Indentation tests have been widely used to determine the material properties of arterial tissue. However, it remains a challenge to extract the relevant material parameters from the force-indentation curves that result from indentation tests. This paper presents a detailed procedure for determining the first-order Ogden parameters, μ and α, for mouse arterial tissue using a method that combines indentation tests with numerical simulations. The method builds on a previous study (Li and Masen, 2024) and has been expanded to account for the surface roughness of the indented specimen. It is assumed that hyperelastic material behaviour can be linearized for small strain increments, ɛji≤ 1%, allowing the model developed by Hayes (Hayes et al., 1972) to be applied to accommodate the contact behaviour in each increment. However, mouse arterial specimens have an irregular or rough surface which complicates the use of Hayes' model, as the thickness of the specimen is an input parameter into the model. To solve this, we introduce an 'equivalent thickness' that can be applied in Hayes' model by identifying the thickness that yields the smallest variance S2 of the shear moduli among a range of possible specimen thickness values. The shear moduli obtained for the equivalent thickness, denoted as the equivalent shear moduli Gi∗, along with the corresponding principal strains ɛj obtained from simulations, were used to calculate the principal stresses σj using Hooke's law. By combining the principal stresses σj across all increments, a nonlinear stress σj versus strain ɛj curve was generated, from which the first-order Ogden parameters μ and α were obtained. The proposed method is demonstrated by applying it to simulated force-indentation curves, successfully recovering the input parameters for both thickness and Ogden parameters. The method was subsequently applied to 26 experimentally obtained curves, yielding an average shear modulus G of 1.22 kPa for the indented mouse arterial tissue specimens, with values ranging from 0.27 to 5.02 kPa. Numerical simulations of the indentation process with the obtained values were used to verify the obtained material parameters.

Vascular smooth muscle cells can be circumferentially aligned inside a channel using tunable gelatin microribbons

The gold standard to measure arterial health is vasodilation in response to nitric oxide. Vasodilation is generally measured via pressure myography of arteries isolated from animal models. However, animal arteries can be difficult to obtain and may have limited relevance to human physiology. It is, therefore, critical to engineer human cell-based arterial models capable of contraction. Vascular smooth muscle cells (SMCs) must be circumferentially aligned around the vessel lumen to contract the vessel, which is challenging to achieve in a soft blood vessel model. In this study, we used gelatin microribbons to circumferentially align SMCs inside a hydrogel channel. To accomplish this, we created tunable gelatin microribbons of varying stiffnesses and thicknesses and assessed how SMCs aligned along them. We then wrapped soft, thick microribbons around a needle and encapsulated them in a gelatin methacryloyl hydrogel, forming a microribbon-lined channel. Finally, we seeded SMCs inside the channel and showed that they adhered best to fibronectin and circumferentially aligned in response to the microribbons. Together, these data show that tunable gelatin microribbons can be used to circumferentially align SMCs inside a channel. This technique can be used to create a human artery-on-a-chip to assess vasodilation via pressure myography, as well as to align other cell types for 3Din vitromodels.

Assessment of the nano-mechanical properties of healthy and atherosclerotic coronary arteries by atomic force microscopy

Nano-indentation techniques might be better equipped to assess the heterogeneous material properties of plaques than macroscopic methods but there are no bespoke protocols for this kind of material testing for coronary arteries. Therefore, we developed a measurement protocol to extract mechanical properties from healthy and atherosclerotic coronary artery tissue sections. Young's modulus was derived from force-indentation data. Metrics of collagen fibre density were extracted from the same tissue, and the local material properties were co-registered to the local collagen microstructure with a robust framework. The locations of the indentation were retrospectively classified by histological category (healthy, plaque, lipid-rich, fibrous cap) according to Picrosirius Red stain and adjacent Hematoxylin & Eosin and Oil-Red-O stains. Plaque tissue was softer (p < 0.001) than the healthy coronary wall. Areas rich in collagen within the plaque (fibrous cap) were significantly (p < 0.001) stiffer than areas poor in collagen/lipid-rich, but less than half as stiff as the healthy coronary media. Young's moduli correlated (Pearson's ρ = 0.53, p < 0.05) with collagen content. Atomic force microscopy (AFM) is capable of detecting tissue stiffness changes related to collagen density in healthy and diseased cardiovascular tissue. Mechanical characterization of atherosclerotic plaques with nano-indentation techniques could refine constitutive models for computational modelling.