Citation:
Abstract:
The “materials properties” of a biological material include its composition and microscopic structure and the relationship between its structure and its mechanical properties. For living cells, the motor-driven internal motion also significantly impacts the properties, even independently of any remodeling of the cell structure that can occur. These materials properties dictate the passive mechanical response of the material to an applied force. The mechanical properties of cells and tissues are essential for their function and health and affect how cells actively respond to mechanical force in important biological processes, ranging from motility to differentiation and morphogenesis. The mechanical properties of bulk tissues can be determined by traditional rheological techniques that measure the force required to stretch, compress, or shear macroscopic tissue. However, individual cells are too small to be measured by such methods and have highly heterogeneous structures; thus techniques are required that can probe soft materials at the micrometer scale. A variety of microrheological techniques, developed to determine the materials properties of cells, reveal that living cells have materials properties that are quite unusual compared with common inert materials. Cells are active, nonequilibrium materials with a highly nonlinear elasticity. This article presents a subset of microrheological techniques that involve optical imaging of micrometer-sized probes on or within individual cells, describes how to analyze probe motions, and discusses limitations of the techniques.