Self-organized shape changes in elastic active gels

Living systems utilize fundamental physics in the form of mechanical forces and geometric cues to move and change shape. A central question motivating our research is: how does biological matter utilize mechanical forces to form ordered structures and change shape? As a prototype of active biological materials capable of self-organized shape change, we explain experimental findings on cytoskeletal gel extracts by our collaborators at the Bernheim laboratory. Despite having identical composition of the biopolymer actin, molecular motor myosin and the crosslinker fascin, these gels contract and buckle into different shapes depending on the initial gel aspect ratio: thinner gels tend to wrinkle, while thicker gels tend to form domes. By incorporating motor-generated active stresses, alignment of active fibers, and stress-dependent myosin binding kinetics into a network-fluid (poroelastic) model, we qualitatively capture the observed trends in gel contraction dynamics measured using particle image velocimetry (PIV). We then show how a geometric elastic model for thin sheets can relate the 3D buckled shapes to strain rates predicted by the poroelastic model. Our findings have implications for shape changes during tissue morphogenesis and bio-inspired soft materials design.