The cellular cytoskeleton has a large number of functions, due to the varying structure of cytoskeletal proteins and filaments, and their various interactions. Two major proteins in the cytoskeleton are actin and tubulin, which comprise microtubules and microfilaments respectively. Actin and tubulin have different structures and functions, and interact with each other to create emergent properties. They have very different stiffnesses. When together in high concentrations, they have steric interactions that conform to the “reptation” or “tube” model. Each filament is spatially restricted to a tube-like area, which is formed by the constraining filaments around it. To relax and decrease the straining forces on it, it reptates, (sliding curvilinearly) out of its tubular space. There are other methods by which the filaments can partially relax, such as bending fluctuations. The interactions that occur in cells between actin and tubulin provide controlled, structured support of the cytoskeleton. Their interactions are also important in cytokinesis and cell motility. Microtubule strength is also reinforced by the presence of supporting actin; they can withstand greater forces without buckling in the presence of actin than they can on their own. Studying the interactions of these filaments has different potential applications. Potentially, one could learn to identify an ideal ratio of soft to rigid rods to be used in the synthesis of a material that effectively combines light weight with durability. Combinations of actin and tubulin also provide the possibility for increased control over large-scale mechanics. Previous research studies have shown that actin is compressible in the presence of microtubules, and that low concentrations of microtubules added to cross-linked actin cause strain-stiffening in the actin, as opposed to the normal strain-softening that usually occurs in cross-linked actin. These previous studies, however, were limited in a few ways. The parameter space of the composite matter was limited, so differences between varying concentrations and ratios of actin and tubulin were not measured to be contrasted. The studies also measured large-scale strain and micro-scale strain, but not mesoscale strain which would be more useful based on the mesh size of actin and tubulin. These studies also used microtubules that were pre-polymerized before they were added to the actin, which oftentimes encourages actin bundling, preventing the possibility of a truly isotropic composites. In contrast, the study “Co-Entangled Actin-Microtubule Composites Exhibit Tunable Stiffness and Power-Law Stress Relaxation”methodically varies the relative concentrations of tubulin and actin and characterizes mesoscale mechanics of the filaments by displacing optically trapped microspheres by 30 µm at a rate that is very high compared to the normal relaxation rate of the filaments, and measuring the restoring force that the composite applies to the sphere. These measurements disrupt the equilibrium of the composite and allow exploration for possible buckling, rupture, and rearrangement.
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