Sustainable materials—powered by sunlight and living microbes—that remove pollutants from water, release oxygen into a wound or heal themselves after damage could become simpler to create thanks to new research by a team of biologists and engineers at the University of California San Diego. The team developed a method to make engineered living materials (ELMs)—a class of materials that combines synthetic polymers with living microbes—out of a much wider range of ingredients than is currently possible.
The advance was published in Proceedings of the National Academy of Sciences by researchers at the UC San Diego Materials Research Science and Engineering Center (MRSEC) . The team was led by Jinhye Bae, professor in the Aiiso Yufeng Li Family Department of Chemical and Nano Engineering, and Susan Golden, professor in the Department of Molecular Biology, both at UC San Diego.
Traditionally, ELMs are made by mixing live cells into a polymer before it hardens. This limits researchers to using only biocompatible starting materials so that the cells can survive the polymerization process. The new method flips that approach: it introduces live cells after the polymer is formed. Here, researchers showed that by using some "shape-shifting" features of a polymer that has toxic precursors, photosynthetic cyanobacteria could diffuse into the finished, non-toxic polymer as it swelled in the culture solution. Moreover, the bacteria caused the ELM to change shape as the cells grew.
"We have shown for the first time that diffusion is a viable method of creating ELMs, enabling others to use a wider variety of polymers to create ELMs in the future," said study co-first author Lisa Tang, a chemical engineering Ph.D. student in Bae's research group. This includes polymers that were previously off-limits due to their toxicity to live cells.
To demonstrate the concept, the researchers used a temperature-responsive polymer called poly(N-isopropylacrylamide). This material can be made to expel water, like squeezing a sponge, by elevating the temperature to 37 degrees Celsius (body temperature). Once returned to room temperature, the matrix absorbs water and expands, which makes it ideal for soaking up a suspension of cyanobacteria. Once embedded in the material, the microbes remained active and even changed the material's properties. Over time, the researchers observed that the incorporation of cyanobacteria softened the polymer and caused it to permanently change shape. This observation led to discovery of a previously unknown secreted enzyme that partially degrades the material.
"Such surprising findings highlight the value of studying dynamic, non-equilibrium systems like engineered living materials (ELMs)," said study co-first author Nathan Soulier, postdoctoral scholar in Golden's lab.
The team envisions that this approach could work with other materials, such as polymers that respond to pH changes or conduct electricity—scaffolds that were previously too harsh for cell survival. Cyanobacteria, in particular, offer exciting possibilities and are particularly promising for ELMs. These microbes can be genetically engineered to produce specific chemicals or carry out specialized tasks, such as cleaning environmental pollutants , which co-authors on this study had previously demonstrated. And because they are powered by sunlight, cyanobacteria can serve as sustainable ingredients for a new generation of ELMs.
"By integrating photosynthetic organisms into materials science, we can harness the sun's renewable energy to create valuable materials," said Bae. "There is a great need for sustainable alternatives to current practices that rely on finite resources, and ELMs may offer a path forward."
Building on this work, the team will continue fundamental studies on how cyanobacteria interact with various polymers. In addition, they are working on developing an ELM that can respond to multiple environmental cues.
Full study: " A responsive living material prepared by diffusion reveals extracellular enzyme activity of cyanobacteria "
This work was primarily supported by the National Science Foundation through the UC San Diego Materials Research Science and Engineering Center (UC San Diego MRSEC, grant DMR-2011924).
