2023 Spring Physics Colloquium Series: Prof. José Alvarado

The actomyosin cytoskeleton is a naturally occurring active gel found in virtually all mammalian cells. Its ability to contract allows cells to move, change shape, exert force, sense stiffness, and maintain constant tension. In order for the "hardware" of actomyosin gels to support such a diverse set of mechanical tasks, it is tightly coupled to information flows from the "software" of cellular biochemical signaling networks, which sense, process, and control contractility. However, the response of actomyosin active gels to biochemical control signals remains poorly understood. One reason for this knowledge gap is that active gels are currently characterized with quantities that express response to external stresses, rather than the enzymatic motor activity that drives contraction. Here we experimentally measure enzymatic activity and determine the response of actomyosin gels to step input signals. We find a slip-dominated nonlinearity as the gel contracts and densifies. We also subject gels to small, pulsatile bursts of activity to determine the response to time-varying control inputs in the linear regime. Our novel characterization is essential to understanding how actomyosin active gels interact with a network of signaling proteins to orchestrate complex mechanical function, and can also be used to design optimal microrobotic applications.