Competing instabilities reveal how to rationally design and control active crosslinked gels

Abstract

How active stresses generated by molecular motors set the large-scale mechanics of the cell cytoskeleton remains poorly understood. Here, we combine experiments and theory to demonstrate how the emergent properties of a biomimetic active crosslinked gel depend on the properties of its microscopic constituents. We show that an extensile nematic elastomer exhibits two distinct activity-driven instabilities, spontaneously bending in-plane or buckling out-of-plane depending on its composition. Molecular motors play a dual antagonistic role, fluidizing or stiffening the gel depending on the ATP concentration. We demonstrate how active and elastic stresses are set by each component, providing estimates for the active gel theory parameters. Finally, activity and elasticity were manipulated in situ with light-activable motor proteins, controlling the direction of the instability optically. These results highlight how cytoskeletal stresses regulate the self-organization of living matter and set the foundations for the rational design and optogenetic control of active materials.

Microscopic interactions control a structural transition in active mixtures of microtubules and molecular motors

“…1B ). After an initial instability [9,16,44], the three-dimensional active gel reaches a long-lived out-of-equilibrium steady state, forming a network of extensile microtubule bundles that continuously reconfigure, driving chaotic flows [14] ( Fig. 1D, Video S1 ).…”

“…Hence, we first tested if the transition could be due to a stiffening of the network due to an increasing fraction of molecular motors functioning as passive microtubule crosslinkers when ATP is limited. Indeed, kinesin-1 motor clusters are known to have a dual antagonistic role, fluidizing or stiffening the network depending on ATP concentration [9,44,60].…”

“…In vivo , these active processes underlie cellular force generation and mechanics [6]. In vitro , they provide a blueprint to build active machines endowed with life-like properties, such as the ability to spontaneously flow [7,8], change shape [9–11], or divide [12,13].…”

Three Dimensional Spontaneous Flow Transition in Active Polar Fluids

“…The passive Fréedericksz transition describes the change of an isotropic state to an anisotropic state under the influence of external electric or magnetic fields [10]. In active liquid crystals, the transition is driven by active molecular processes causing spontaneous material flow [11][12][13].Recent works suggest such instabilities to also exist in three-dimensional (3D) active polar fluids [14]. For example, an extensile active fluid was found to exhibit a bending instability in a simplified model [15], and flow-aligning active fluids were found to display coherent motion in 3D channels upon increased activity [16].…”

“…Recent works suggest such instabilities to also exist in three-dimensional (3D) active polar fluids [14]. For example, an extensile active fluid was found to exhibit a bending instability in a simplified model [15], and flow-aligning active fluids were found to display coherent motion in 3D channels upon increased activity [16].…”