Beyond the Airbrush: Applications of Digital Image Correlation in Vascular Biomechanics

Abstract

Before digital image correlation (DIC) could find widespread application for strain measurement in biological soft tissues, there were a number of technical challenges that had to be addressed. The mechanical behavior of soft tissues depends significantly on hydration state; therefore, both application of a high-contrast speckle pattern and imaging must be achieved while maintaining the specimen in a fully hydrated state. Furthermore, soft tissues such as arteries typically undergo finite deformations under a physiologically relevant loading range. While hydration can be achieved by submerging the sample in a medium having appropriate osmolarity, pH, and ionic strength, imaging submerged objects introduces its own set of challenges due to refractive index changes. The issue of sample hydration also requires consideration of alternative approaches to speckle pattern creation, beyond the classic “airbrush method,” since ideally the specimen must not be allowed to dry out during pattern application. For samples that are submerged, the speckle pattern must be firmly bonded to the specimen and water-resistant, in addition to deforming with the specimen, often outside the small-strain regime. In some specific instances, nature provides a hand through the presence of intrinsic fine-scale structure in the specimen that, with innovative staining or imaging techniques, can serve as a satisfactory “speckle pattern.” Added to these issues is the difficulty of imaging small, often irregularly shaped specimens that can degrade rapidly over time. In collaboration with Dr. Sutton’s group, my colleagues and I have developed a number of approaches to address the issues of hydrating and speckling soft tissues for measurement of local strains in blood vessels at multiple length scales, with a particular focus on the mouse aorta and carotid artery.