| Biological materials, such as isolated protein networks, tissues and living cells are extremely heterogeneous and contain structures on length scales from nanometers to hundreds of microns. Their complex geometries and sensitivity to environmental conditions make traditional measurements of structure and mechanics difficult, challenging experimenters to develop new techniques. In this work, we present the design of a novel microscope-based static light scattering instrument, and demonstrate its usefulness by studying striated and smooth muscle and skin. We measure the two-dimensional scattering patterns, and find that tissue structure can give rise to strong anisotropies, which can be used to identify specific classes of tissues.; We also report the development of multiple particle tracking techniques, using the thermal movements of colloids that have been embedded in soft complex materials to measure local viscoelastic response. These measurements require only tens of microliters of material, making them particularly useful for studying biological samples that are difficult to obtain in large quantities, or inherently small like living cells. For materials that are homogenous on the length scale of the probe particle, these thermal movements have been previously shown to be a direct measure of the macroscopic linear frequency-dependent viscoelastic response1. In this work, we have extended these techniques to probe the viscoelastic and microstructural properties of a number of heterogeneous materials, including structured polysaccharide gels, biopolymer networks, and isolated cytoplasm. To better understand the mechanical microenvironments of these heterogeneous materials, we examine the effect of varying both tracer size and surface chemistry, and present a novel protocol to render colloids protein-resistant using only commercial reagents.; In cases where larger sample volumes are available, we also use macroscopic, mechanical rheology methods to characterize the frequency dependent moduli and non-linear viscoelastic response. Using a combination of microscopic and macroscopic techniques we report the first characterization of the mechanical properties of isolated cytoplasm, obtained from the eggs of Xenopus laevis, and discuss the role of the three cytoskeletal filaments, actin, microtubules, and the intermediate filament cytokeratin, in the viscoelastic response.; 1Mason, T. G., D. A. Weitz (1995) Physical Review Letters 74: 1250. |
