Protein biomaterials are usually engineered in top-down fashion. For example, chemically synthesized peptides or purified subunits can be assembled in vitro. But that’s not how biology works. Biology works from the bottom up. It is organized in a hierarchy, from macromolecules to macromolecular assemblies to organelles to cells and, finally, to tissues. And at each level of this hierarchy, component materials may be arranged according to built-in directions—or in response to external stimuli—to create structures of diverse scale and function.
What if biology’s bottom-up mechanics could be effectively hijacked? Besides creating variations on natural materials, one might induce natural systems to create hybrid materials, that is, living/nonliving systems. Such systems could combine biological and artificial features and functions.
Engineered hybrid materials could, like living systems, self-assemble, self-heal, and even remodel in response to environmental stimuli. Also, such hybrid materials could incorporate features such as electrical conductivity and the ability to emit light. While living materials consisting of both natural and the artificial components might seem a bit eerie—think of Borg-like cellular consortia—they could prove so functional, so useful, they would be hard to … resist.
As it happens, a team of researchers at MIT has created just this sort of living material. The team, led by Timothy Lu, an assistant professor of electrical engineering and biological engineering, took its inspiration from biological systems such as biofilms, shells, and skeletal tissues. Such systems, the team realized, are able to assemble multifunctional and environmentally responsive multiscale assemblies of living and nonliving components.
Zeroing in on biofilms, the researchers decided to hack genetic and cellular communication circuits in bacteria. In particular, the researchers engineered E. coli to that would manufacture curli amyloid, the substance of biofilms, as needed. In addition, they created hybrid systems capable of studding a coalescing biofilm with artificial components, including inorganic substances, as directed.
The MIT team published its results March 23 in Nature Materials, in an article entitled “Synthesis and patterning of tunable multiscale materials with engineered cells.” In this article, the authors wrote, “We show that E. coli cells can organize self-assembling amyloid fibrils across multiple length scales, producing amyloid-based materials that are either externally controllable or undergo autonomous patterning.”
The researchers also describe how they went even further, interfacing curli fibrils with inorganic materials, such as gold nanoparticles and quantum dots. According to the authors, they were able to create an environmentally responsive biofilm-based electrical switch and produce gold nanowires and nanorods.
The bacteria were programmed to produce different types of curli fibers under certain conditions. They were also engineered to communicate with each other and change the composition of their biofilms over time. Key to the researchers’ ability to control the bacteria (and their biofilms) was the creation of bacteria that would respond to different levels of certain molecules.
“It’s a really simple system but what happens over time is you get curli that’s increasingly labeled by gold particles. It shows that indeed you can make cells that talk to each other and they can change the composition of the material over time,” Lu says. “Ultimately, we hope to emulate how natural systems, like bone, form. No one tells bone what to do, but it generates a material in response to environmental signals.”
These hybrid materials could be worth exploring for use in energy applications such as batteries and solar cells, Lu says. According to a press release issued by MIT, the researchers are also interested in coating the biofilms with enzymes that catalyze the breakdown of cellulose, which could be useful for converting agricultural waste to biofuels. Other potential applications include diagnostic devices and scaffolds for tissue engineering.