Entanglements have been often understood as knot-like constraints which stretch a polymer chain when sheared. Despite the highly elastic response of uncrosslinked solutions of actin filaments to shear, no constraints can be found that act in a knot-like fashion to cause a tension along polymer chains. Actin elasticity is solely caused by the affine shear distortion of tube-like constraints formed around each polymer by its neighboring chains. While this finding is of significance to polymer physics, crosslinked arrays of actin filaments are more relevant for cytoskeleton research. Thus, actin networks in the presence of crosslinking proteins are a hybrid between a polymer solution and a polymer gel with unique polymer dynamics, which we will investigate by the technique described above.
Molecular motors have an even larger impact on polymer dynamics.
Polymer solutions are well studied at thermal equilibrium. In this case, the overall motion of a polymer chain is determined by diffusive transport described by reptation. Since reptation is very slow and thus the relaxation of deforming stresses is delayed, solutions of long polymer chains elastically resist all deforming motions faster than the stress relaxation time. The energy consuming molecular motor myosin II opens a new route to control the viscoelastic behavior of actin solutions. Thus, myosin practically fluidizes the polymer solution.