Microstructure and viscoelasticity of confined serniflexible polymer networks

The rapidly decreasing dimensions of many technological devices have spurred interest in confinement effects. Long before, living organisms invented ingenious ways to cope with the requirement of space-saving designs down to the cellular level. Typical length scales in cells range from nanometres to micrometres so that the polymeric constituents of the cytoskeleton are often geometrically confined. Hence, the mechanical response of polymers to external confinement has potential implications both for technology and for our understanding of biological systems alike. Here we report a study of in vitro polymerized filamentous actin confined to emulsion droplets. We correlate observations of the microstructure, local rheological properties and single-filament fluctuations. Enforcing progressively narrower confinement is found to induce a reduction of polymer fluctuations, network stiffening, structural heterogeneities and eventually cortex formation. We argue that the structural and mechanical effects can be consistently explained by a gradual suppression of single-polymer eigenmodes. © 2006 Nature Publishing Group.