Stimuli-Responsive Hydrogels Enable Reversible Shape Change in Engineered Living Materials
The overarching goal of this research is to engineer shape-morphing engineered living materials (ELMs) that deform reversibly in response to biochemical stimuli transduced by living organisms. The innovation of this approach arises from the use of living cells to sense and transduce a specific environmental stimulus into a controllable, reversible mechanical response of the ELM.
ELMs are an emerging class of stimuli-responsive hybrid materials that leverage the sensitivity of living organisms to environmental stimuli. ELMs make attractive platforms for biosensing and drug delivery, as they can change shape in response to stimuli including biochemicals, pollutants, and light. To date, however, this response is coupled strictly with bacterial colony growth and is challenging to reverse; i.e., volume changes result from proliferating cells pushing outward on the matrix, rather than from conformational changes of the matrix itself.
Incorporating stimuli-responsive hydrogels into ELMs thus presents an attractive opportunity to enable reversible mechanical responses. This is achievable by harnessing extracellular chemical changes that arise from cellular metabolic products to trigger shape morphing in a responsive hydrogel matrix. Stimuli-responsive hydrogels can be designed to i) swell in response to external stimuli including pH changes, ii) to produce volume changes over several orders of magnitude, and iii) to easily reverse their response. However, the stimuli required to trigger large conformational changes are often substantial, e.g., the required pH change to actuate a hydrogel can range in magnitude from 1 to 3 logs. Some common bacteria species, like nonpathogenic E. coli, can be engineered to alter the extracellular pH, or to exhibit auxotrophy, i.e., the inability to synthesize a biomolecule necessary for growth.
We are designing an ELM in which bacteria embedded in a stimulus-responsive hydrogel matrix can trigger a swelling/deswelling response in the hydrogel. This technology has the potential to enable mechanically active hybrid materials with applications ranging from drug delivery and biosensing to environmental remediation and soft robotics.