Application of Polymer Physics to tissue viscoelasticity: Entangled active matter

We first introduce the field of “Entangled Active Matter”. Active systems span an enormous range of length scales, from the cytoskeleton of individual living cells to tissues and animal groups such as fish or insect swarms. We will focus on “entangled active matter”, where the building blocks are transiently bound. We will point out strong similarities of mechanical properties between aggregates of cells, swarm of ants and inert viscous pastes.
We characterize the tissue mechanical properties (surface tension, elasticity, viscosity) using pipette aspiration technique. Cellular aggregates exhibit a viscoelastic response in analogy to ultra-viscous polymer melts. When we apply oscillating pressures, we observe a reinforcement of the tissue at high frequencies. We developed a model to determine the main mechanical properties of the system as a function of the oscillation frequencies. Unlike inert polymer melts, we observe aggregate’s reinforcement with pressure, which gives rise to pulsed contractions or “shivering”, interpreted as a mechano-sensitive active response of the acto-myosin cortex.
We then describe the spreading of aggregates on rigid and soft substrates, varying both intercellular and substrate adhesion. We find both partial and complete wetting, with a precursor film forming a dense cellular monolayer in analogy with the stratified spreading of viscous polymer melts. On soft substrate, the precursor film is unstable, leading to a symmetry breaking of cells polarity causing the aggregate to move spontaneously as a giant keratocytes.
References:
David Gonzalez-Rodriguez et al. (2012) Soft Matter Models of developing Tissues and Tumors. Science 338, 910; 1226418
F Brochard-Wyart et al, Entangled Active Matter: from ants to living cells EPJE 2015