Abstract
Mucus is a biological gel protecting several tissues. Its key properties result from a crucial balance between solid-like and fluid-like behavior, ensured by the nonpermanent nature of the bonds between its macromolecular constituents. Our understanding of the micrometer-scale response of mucus to an applied stress is still rudimentary, although in living organisms, stresses acting on mucus are ubiquitous, from bacterial penetration to coughing and peristalsis. We show that under a modest applied stress, in the mechanical linear regime, the microscopic dynamics of pig gastric mucus transiently accelerate by up to 2 orders of magnitude. A simple model rationalizes this previously unrecognized fluidization mechanism stemming from elastic recoil following bond breaking and generalizes our findings to networks with reversible bonds.Mucus is a biological gel covering the surface of several tissues and ensuring key biological functions, including as a protective barrier against dehydration, pathogen penetration, or gastric acids. Mucus biological functioning requires a finely tuned balance between solid-like and fluid-like mechanical response, ensured by reversible bonds between mucins, the glycoproteins that form the gel. In living organisms, mucus is subject to various kinds of mechanical stresses, e.g., due to osmosis, bacterial penetration, coughing, and gastric peristalsis. However, our knowledge of the effects of stress on mucus is still rudimentary and mostly limited to macroscopic rheological measurements, with no insight into the relevant microscopic mechanisms. Here, we run mechanical tests simultaneously to measurements of the microscopic dynamics of pig gastric mucus. Strikingly, we find that a modest shear stress, within the macroscopic rheological linear regime, dramatically enhances mucus reorganization at the microscopic level, as signaled by a transient acceleration of the microscopic dynamics, by up to 2 orders of magnitude. We rationalize these findings by proposing a simple, yet general, model for the dynamics of physical gels under strain and validate its assumptions through numerical simulations of spring networks. These results shed light on the rearrangement dynamics of mucus at the microscopic scale, with potential implications in phenomena ranging from mucus clearance to bacterial and drug penetration.ASCII and Excel files for all the datasets shown in the figures of the main text and SI Appendix have been deposited in Zenodo (DOI: 10.5281/zenodo.5533877) (74).