  A network of semiflexible biopolymers, known as the cytoskeleton, and
molecular motors play fundamental mechanical roles in cellular activities. The
cytoskeletal response to forces generated by molecular motors is profoundly
linked to physiological processes. However, owing to the highly nonlinear
mechanical properties, the cytoskeletal response on the microscopic level is
largely elusive. The aim of this study is to investigate the microscopic
mechanical response of semiflexible biopolymer networks by conducting
microrheology (MR) experiments. Micrometer-sized colloidal particles, embedded
in semiflexible biopolymer networks, were forced beyond the linear regime at a
variety of conditions by using feedback-controlled optical trapping. This
high-bandwidth MR technology revealed an affine elastic response, which showed
stiffening upon local forcing. After scaling the stiffening behaviors, with
parameters describing semi-flexible networks, a collapse onto a single master
curve was observed. The physics underlying the general microscopic response is
presented to justify the collapse, and its potentials/implications to elucidate
cell mechanics is discussed.
