Salt-mediated chemomechanical actuation in polyelectrolyte complex-stabilized coacervate microdroplets

Regulation of transport is an essential design element in the control of biological networks within living cells and its implementation within coacervate microdroplets where reaction kinetics can be modulated would enable the design of adaptive cell-like compartments. The lack of a membrane limits the potential of coacervate microdroplets as artificial cells. Herein, we illustrate a simple strategy to stabilize coacervate microdroplets via interfacial formation of solid polyelectrolyte complexes mediated by addition of a strong polyelectrolyte. The generality of this method was illustrated using several coacervate microdroplets. The solid polyelectrolyte membrane formed allowed rapid exchange of small molecules and charge-selective permeability in case of macromolecules. The addition of salt triggered network swelling within the membranized coacervate droplets and removal of salt allowed the deswelling of the droplets. This salt-mediated chemomechanical actuation of the membranized droplets was around twice that of native membraneless droplets and reversible actuation for over 20 cycles was illustrated using alternating flows of salt and water. The actuating droplets maintained the coacervate interior in a loosely cross-linked state allowing faster mass transport and enhanced the rate of enzymatic reactions hosted within by a factor of two with respect to non-actuating droplets. The cross linking can be increased simply by flowing water to regulate diffusion within the coacervate droplets. This salt-based active switching of mass transport in membranized coacervate droplets would add another mechanism for control of reaction kinetics in coacervates and add another module to the existing toolbox for design of artificial cells capable of regulation of protometabolism.