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Soft hydrated materials, such as vascular tissues and other biomaterials, provide a number of challenges in the field of nanoindentation. However, the ability of nanoindentation to probe local, nanoscale mechanical properties of heterogeneous materials makes it desirable to adapt this technique for application to biologic tissues. To develop the field of nanoindentation for the analysis of soft hydrated materials, the goals of this study were fourfold: develop a sample hydration system, select an appropriate tip for soft material indentation, identify a substrate to be used for blunt tip alignment, and determine an appropriate control material for the development of future indentation protocols. A hydration system was developed that maintained sample hydration for over 8 h without completely submerging the sample. Further, a 100-microm radius of curvature conospherical tip was shown to be a suitable tip for indenting a variety of soft hydrated materials and back-illuminated agarose gel was found to be an effective material for use in tip alignment. Finally, agarose gel demonstrated similar qualitative and quantitative nanomechanical behavior to vascular tissue, suggesting that it will be an appropriate control material for the development of future indentation protocols for soft biologic tissues.

Clinical events such as heart attack and stroke can be caused by the rupture of atherosclerotic plaques in artery walls. Computational modeling is often used to better understand atherosclerotic disease progression to identify "vulnerable" plaques (i.e., those likely to rupture) and to tailor treatments according to tissue composition. However, because of the heterogeneity of plaque tissue, there are limited data available on the material properties of individual plaque constituents. The goal of this study was to use nanoindentation to measure the mechanical properties of blood clots, fibrous tissue, partially calcified fibrous tissue, and bulk calcifications from human atherosclerotic plaque tissue. Fourier transform infrared (FTIR) spectroscopy was used to quantify the amount of mineral and lipid in each tissue region tested. The results demonstrate that the stiffness of plaque tissue increases with increasing mineral content. In addition, by providing the first experimental data on atherosclerotic calcifications, these data show that some of the estimated modulus values commonly used in computational models greatly underestimate the stiffness of the fully calcified tissue.