Figure 1. Global shape changes of active actin gels.

(a) At the beginning of the experiment myosin II filaments are activated by scanning patterns with a confocal blue (488 nm) argon laser beam for n=20 consecutive cycles (640 ms for one cycle), thereby inactivating the blebbistatin. The activation of the myosin is stable for at least 90 min and no further activation is necessary during the further contraction. This observed permanent activation is attributed to the light-induced covalent modification of the myosin II by the blebbistatin. (b) Start (t_init=0 min) and end (t=45 min) configuration of the fluorescently (phalloidin-Alexa 647N) labelled actin network, imaged with a 633 nm HeNe laser line. During the stimulation process (<1 min) activated molecular motors already accumulate fluorescent actin from all directions leading to an increased fluorescence directly after stimulation is finished. Circular patterns contract shape preserving whereas various initial geometries with corners entail non-affine deformations during contraction. Scale bar is identical for all figures. (c) Corresponding results of our static spring network model (Supplementary Note 1 and Supplementary Mathematica notebook). The initial state is set by the activated region, the final state is obtained by efficient inversion of a sparse matrix with a suitable value for the ratio of spring coefficients K=0.13 on a 30 × 30 square grid for all panels.