Metabolic Activity to Animate Coacervate Materials

We describe the creation of "metabolic" droplets in which chemical activity serves to alter and control material properties such as droplet composition, size, and internal structure. The droplets are based on the complex coacervation of oppositely charged macromolecules: the enzyme glucose oxidase (GOx) and the cationic polymer DEAE-dextran. The phase behavior of these liquid-like drops is coupled to their catalytic activity via the local pH. Enzymatic oxidation and hydrolysis of glucose "fuel" lowers the pH, triggering a variety of responses that include changes in drop composition, formation of internal compartments (vacuoles), osmotic swelling and rupture, and size-dependent dissolution. These responses depend on the rate of reaction-induced pH changes relative to relaxation mechanisms inside the drops. Slow changes are controlled by equilibrium thermodynamics; faster pH changes couple to macromolecule transport on the drop scale. We describe how spatially uniform, time-varying pH profiles can produce structured droplets with internal compartments. We discuss how size-dependent dissolution can enable self-regulation of droplet size.