Microbially driven reversible size- and color-changing materials

Features of natural living systems underexplored in engineered living materials (ELMs) are macroscale appearance changes driven by active cellular processes. To overcome this technological gap, we demonstrate an ELM wherein the natural metabolism of Escherichia coli is used to drive reversible changes in pH-responsive hydrogels through the production or consumption of acidic metabolites. A color-changing function of the hydrogels relies on the custom design, synthesis, and coupling of a synthetic pH indicator dye into the polymer network. Manipulation of the starting pH conditions and the identity of the primary carbon source leads E. coli to alter pH, resulting in reversible size and color changes in the gels. Arrayed arrangements of multiple responsive hydrogels can mimic dynamic pixels that respond to changes in cell metabolism. Here, we expand the tool kit of ELMs to include size and color change as functional performance features that can be driven by active cellular processes.