The bricks in Wil Srubar’s lab at the University of Colorado, Boulder, aren’t just alive, they’re reproducing. They are churned out by bacteria that convert sand, nutrients, and other feedstocks into a form of biocement, much the way corals synthesize reefs. Split one brick, and in a matter of hours you will have two.
Engineered living materials (ELM) are designed to blur boundaries. They use cells, mostly microbes, to build inert structural materials such as hardened cement or woodlike replacements for everything from construction materials to furniture. Some, like Srubar’s bricks, even incorporate living cells into the final mix. The result is materials with striking new capabilities, as the innovations on view last week at the Living Materials 2020 conference in Saarbrüken, Germany, showed: airport runways that build themselves and living bandages that grow within the body. “Cells are amazing fabrication plants,” says Neel Joshi, an ELM expert at Northeastern University. “We’re trying to use them to construct things we want.”
Humanity has long harvested chemicals from microbes, such as alcohol and medicines. But ELM researchers are enlisting microbes to build things. Take bricks, normally made from clay, sand, lime, and water, which are mixed, molded, and fired to over 1000°C. That takes lots of energy and generates hundreds of millions of tons of carbon emissions annually. A Raleigh, North Carolina, company called bioMASON was among the first to explore using bacteria instead of heat, relying on the microbes to convert nutrients into calcium carbonate, which hardens sand into a sturdy construction material at room temperature.
Now, several groups are taking the idea further. “Could you grow a temporary runway somewhere by seeding bacteria in sand and gelatin?” asks Sarah Glaven, a microbiologist and ELM expert at the U.S. Naval Research Laboratory. In June 2019, researchers at Wright-Patterson Air Force Base in Ohio did just that to create a 232-square-meter runway prototype. The hope, says Blake Bextine, who runs an ELM program for the U.S. Defense Advanced Research Projects Agency, is that rather than ferrying tons of materials to set up expeditionary air fields, military engineers could use local sand, gravel, and water, and apply a few drums of cementmaking bacteria to create new runways in days.
The bricks and runway cement don’t retain living cells in the final structure. But Srubar’s team is taking that next step. In their self-reproducing bricks, researchers mix a nutrient-based gel with sand and inoculate it with bacteria that form calcium carbonate. They then control the temperature and humidity to keep the bacteria viable. The researchers could split their original brick in half, add extra sand, hydrogel, and nutrients, and watch as bacteria grew two full-size bricks in 6 hours. After three generations, they wound up with eight bricks, they reported in the 15 January issue of Matter. (Once the bacteria are done growing new bricks, the team can turn off the temperature and humidity controls.) Srubar calls it “exponential material manufacturing.”
ELM makers are also harnessing microbes to make biomaterials for use in the human body. Microbes naturally exude proteins that bind to one another to form a physical scaffold. More bacteria can adhere to it, forming communal microbial mats known as biofilms, found on surfaces from teeth to ship hulls. Joshi’s team is developing biofilms that could protect the gut lining, which erodes in people with inflammatory bowel disease, creating painful ulcers. In the 6 December 2019 issue of Nature Communications, they reported that an engineered Escherichia coli in the guts of mice produced proteins that formed a protective matrix, which shielded the tissue from chemicals that normally induce ulcers. If the approach works in people, physicians could inoculate patients with an engineered form of a microbe that normally makes its home in the gut.
In another medical use, bacteria could turn conventional materials into drug factories. In the 2 December 2019 issue of Nature Chemical Biology, for example, Christopher Voigt of the Massachusetts Institute of Technology and his colleagues describe seeding a plastic with bacterial spores that continuously generate bacteria. The microbes synthesize an antibacterial compound effective against Staphylococcus aureus, a dangerous infectious bacterium.
A team of researchers led by Chao Zhong of ShanghaiTech University engineered biofilms for a different purpose: detoxifying the environment. They started with the bacterium Bacillus subtilis, which secretes a matrix-forming protein called TasA. Other researchers had shown that TasA was easy to genetically engineer to bind to other proteins. The team tweaked TasA to get it to bind an enzyme that degrades a toxic industrial compound called mono (2-hydroxyethyl terephthalic acid), or MHET. They then showed that biofilms created by the engineered bacterium could break down MHET—and that biofilms made by a mix of two engineered strains of B. subtilis could carry out a two-step degradation of an organophosphate pesticide called paraoxon. The results, which the team reported in the January 2019 issue of Nature Chemical Biology, raise the prospect of living walls that purify the air.
Regulatory issues could slow progress, however. Many of the bacteria that ELM researchers have harnessed occur in nature and should not trigger regulatory scrutiny. But genetically engineered organisms will—and the prospect of engineered microbes embedded in, say, living walls might unsettle regulators. Still, Voigt predicts, “I think in 10 years, we’re going to find living cells in a whole range of living products.”