Collective Behavior Triggers Chemomechanical Oscillations in Active Hydrogels

Quorum sensing refers to the ability of yeast and other unicellular organisms to switch behavior in response to increasing numbers or density, inducing responses such as oscillating production of chemicals and biofilm formation. There has been much interest in obtaining a quorum sensing behavior in synthetic analogues, employing, for example the enzyme urease or the Belousov Zhabotinsky (BZ) reaction, for novel bioinspired smart material. In this work, we demonstrate that the quorum sensing response of an assembly of soft small BZ hydrogels (smaller than their critical size to oscillate on their own) can trigger their chemomechanical oscillations. We first report that a single spherical BZ gel smaller than a critical size of 350 micrometers in diameter does not chemically oscillate when left alone. In contrast, a 2D layer made of the same spherical BZ gels present chemical oscillations that last for days. To understand the physicochemical cause of this critical size, we experimentally engineer the coupling of single BZ gels to their environment by imposing a flow of reactants around them. By comparing the oscillation patterns obtained in a convective flow set up with those obtained when only diffusion takes place, we establish experimentally that the passive transport of the inhibitor outside the bead produced by the BZ reaction solely controls the pattern of oscillations. We finally discuss why this discovery paves the way to the design of optimized BZ chemomechanical machines.