# Contractility in an Extensile System

## TL;DR

This study demonstrates a cytoskeletal system capable of switching between extensile, contractile, or static states by adjusting filament or crosslinker concentrations, supported by a simple model that mimics observed behaviors.

## Contribution

It introduces a controllable cytoskeletal system with tunable dynamics and a minimal model that explains the local filament interactions driving these behaviors.

## Key findings

- System can switch between extensile, contractile, and static states.
- A simple one-dimensional model reproduces experimental dynamics.
- Contractile phases can produce autonomous, cell-like motile networks.

## Abstract

Essentially all biology is active and dynamic. Biological entities autonomously sense, com- pute, and respond using energy-coupled ratchets that can produce force and do work. The cytoskeleton, along with its associated proteins and motors, is a canonical example of biological active matter, which is responsible for cargo transport, cell motility, division, and morphol- ogy. Prior work on cytoskeletal active matter systems showed either extensile or contractile dynamics. Here, we demonstrate a cytoskeletal system that can control the direction of the network dynamics to be either extensile, contractile, or static depending on the concentration of filaments or transient crosslinkers through systematic variation of the crosslinker or micro- tubule concentrations. Based off these new observations and our previously published results, we created a simple one-dimensional model of the interaction of filaments within a bundle. Despite its simplicity, our model recapitulates the observed activities of our experimental sys- tem, implying that the dynamics of our finite networks of bundles are driven by the local filament-filament interactions within the bundle. Finally, we show that contractile phases can result in autonomously motile networks that resemble cells. Our experiments and model allow us to gain a deeper understanding of cytoskeletal dynamics and provide a stepping stone for designing active, autonomous systems that could potentially dynamically switch states.