Encoding Mechano-Memories in Actin Networks

Understanding the response of complex materials to external force is central to fields ranging from materials science to biology. Here, we describe a novel type of mechanical adaptation in cross-linked networks of F-actin, a ubuiquitous protein found in eukaryotic cells. We show that shear stress changes its nonlinear mechanical response even long after that stress is removed. The duration, magnitude and direction of forcing history all impact changes in mechanical response. The `memory' of the forcing history is long-lived, but can be erased by force application in the opposite direction. We further show that the observed mechanical adaptation is consistent with stress-dependent changes in the nematic order of the constituent filaments. Thus, this mechano-memory is a type of nonlinear hysteretic response in which an applied, "training" strain modifies the nonlinear elasticity. This demonstrates that F-actin networks can encode analog read-write mechano-memories, which can be used for adaptation to mechanical stimuli.