Granular active matter: Mixture of inert particles and living cells

Abstract: 

The concept of “Entangled active matter” has emerged recently to provide a unified understanding of the behavior of swarms of adhesive motile particles. Unlike school of fish and flocks of birds, cells in an aggregate are bound by transient links and behave as active viscoelastic pastes. We derive the mechanical properties of these cellular aggregates using pipette aspiration technique. We observe aggregate reinforcement with pressure, which may lead pulsed contractions or “shivering”. We interpret this reinforcement as a mechano-sensitive active response of the acto-myosin cortex. Deposited on adhesive substrates, aggregates spread by expanding outwards a cell monolayer

                We then describe mixture of inert and living matter and how microparticles play with cells. The size of the particles is varied from nanometers to few microns. Nanoparticles (size 20nm) can be used as a glue “nanostickers” to enable the formation of self-assembled aggregates by promoting cell–cell interactions .Nanostickers by increasing the cohesion of tissues and tumors may have important applications for cellular therapy and cancer treatment. Micro-particles (size ≈ micron) We study the spreading of cell aggregates deposited on adhesive substrates decorated with microparticles. As a cell monolayer expands around the aggregate, cells at the periphery uptake the microparticles “gluttonous cells” by phagocytosis, clearing the substrate and forming an aureole of cells full of particles. Macro-particles (size ≈ten  micron) too large to be eaten by the cells.  We notice two regimes in the spreading of a cell aggregate on a substrate covered with polystyrene or silica macroparticles. Light particles are pushed by the cells and form a ring, which bonds to the substrate by JKR adhesion forces that oppose the spreading. In contrast, for heavy particles, the monolayer spreads above the particle bed. In both cases, cell activity is transmitted to inert beads, leading to the formation of cell-MP aggregates, which flicker and diffuse. We then study the formation and the spreading of hybrid aggregates of microparticles and living cells and observe phase separations and jamming transitions. Our study may have implications on processes such as cancer metastasis and development, and  may guide new cancer therapies based on inert particles.