Polymerization-based active matter

Biomimetic active matter is heavily inspired by the cellular cytoskeleton for its ability to self-organize and produce forces necessary for life. Among these, microtubules and kinesin have been used to design experimental systems for study, leading to a great deal of discoveries and insight into active systems. Although these systems have been fruitful, the cytoskeleton can also produce force without the need for molecular motors. Polymerization of another cytoskeletal component, actin, has a hand in a range of cellular behavior, from cell motility, cell division, and maintaining cellular structure. The pathogen Listeria takes advantage of actin’s ability to produce force and uses it as a means of motility to propel around the host cell's cytoplasm for nutrients and even to infect neighboring cells. It does so through polymerization a comet tail of actin filaments behind it which propels it along. 

This phenomenon can be reproduced in vitro using a set of purified proteins and functionalized polystyrene beads. When coated with an actin filament nucleator, filaments polymerize around the bead, leading to a symmetry-breaking event that causes the bead to self-propel through a sample without the need for any molecular motors. With this in mind, we seek to create a new polymerization-based active matter system.

When observing this system, a vast range of phenomenology emerges from the individual bead level to whole groups of beads. Flocking, actin shell symmetry breaking, multiple tail formation, and flagella-like tail beating are some examples of the behavior we see in this system. We seek to understand this behavior and relate it to other systems where collective motion is produced by a number of individual participants, like a flock of birds. Additionally, we aim to realize this system's experimental potential by controlling this behavior through biochemical and physical means to steer us toward new discoveries.