Non-affine deformations and stress relaxation in contractile active gels

The study of biological active matter, such as actin-myosin networks, offers exciting insights into the principles that govern living systems. These networks serve as essential components in cellular mechanics, exhibiting complex behaviors driven by energy-consuming processes. This microscopic-scale internal driving generates active stresses that maintain non-equilibrium steady states. Equally important in maintaining steady states is the relaxation of active stresses. Although active stress generation has been relatively well studied, active stress relaxation remains poorly understood. Here we investigate contractile active gels of reconstituted actin and myosin proteins. We adhere gels to two opposing surfaces and use fluorescence microscopy and particle imaging velocimetry (PIV) to determine the evolving displacement and strain fields. We find that the gel boundary adopts hourglass contours, indicating the presence of active stresses within the gel. Furthermore, we report the presence of non-affine deformations, which may serve as an indicator of active stress relaxation mechanisms. These findings enhance our understanding of active gel behavior and cytoskeletal dynamics while paving the way for adaptive materials in fields like tissue engineering, soft robotics, and bioinspired design.