The interior of a mollusk shell always sparkles in the sunlight, but it is often believed that this iridescence is created by colored pigments, which is not true. As a matter of fact, the shimmering is caused by very minute physical structures that are self-assembled from inorganic components and living cells.
Now, at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), researchers have created a new platform to imitate this self-assembly potential by designing living cells to serve as a starting point for constructing composite materials. Engineered living materials, sometimes abbreviated as ELMs, are a new group of materials, which utilize living cells as “materials scaffolds.” These materials can possibly pave the way to many sophisticated applications, including self-healing materials, in biosensing, smart materials, and bioelectronics. Such types of materials can possibly imitate evolving properties present in nature—in which an intricate system possesses properties that are found in individual components, for example, strength or iridescence.
Taking a cue from this complexity existing in nature, the researchers at Berkeley Lab created a bacterium that is capable of attaching an array of nanomaterials to its cell surface. In addition, the team can accurately regulate the makeup and the way the components are closely packed together, thus producing a stable hybrid living material. The results of the study have been recently reported in ACS Synthetic Biology.
Since hierarchical ordering underlies the properties of many biocomposite materials, being able to regulate the spacing of different components in multiple dimensions is the key to designing predictable ELMs. Our new platform offers a versatile starting point that opens a wide range of new possibilities for constructing ELMs.
Caroline Ajo-Franklin, Study Lead and Staff Scientist, The Molecular Foundry, Lawrence Berkeley National Laboratory.
ELMs as well as natural structures they inspire are composed of hierarchical patterns of materials, implying that for a material composed of evenly sized building blocks, every big block is made of tinier blocks, and each of these tinier blocks, in turn, is composed of even smaller pieces. For instance, mollusks construct their shells out of superthin “platelets” that has a thickness of just 500 nm, and every platelet is composed of a countless number of small nanograins, whose diameters are just 30 nm.
In order to regulate the self-assembly of such kinds of structures on the living cells’ surface, Ajo-Franklin and her group created ordered, sheet-like structures on the surface of several microbes by exploiting the surface-layer (S-Layer) proteins.
“It’s the difference between building a foundation out of a solid sheet that conforms to the cell surface versus an unordered set of strings,” stated Ajo-Franklin, who also holds a joint appointment in Berkeley Lab’s Molecular Biophysics and Integrated Bioimaging Division in the Biosciences Area.
For their experiment, the scientists selected the bacterium Caulobacter crescentus because it is capable of enduring low-oxygen and low-nutrient conditions, and also its S-Layer protein called RsaA, has been extensively researched well. Using a biological “lock and key” system, the researchers designed RsaA to accurately control how and where the densely materials fix to the surface of the cell.
We built a set of bacteria that can irreversibly attach a variety of hard or soft materials like biopolymers or semiconducting nanoparticles to the cell surface without damaging the cells. This living construction kit is a fundamental first step toward creating self-assembling, self-healing, hybrid biomaterials.
Marimikel Charrier, Study Lead Author and Scientific Engineering Associate, The Molecular Foundry, Lawrence Berkeley National Laboratory.
The Defense Advanced Research Projects Agency and the National Institutes of Health supported the study. The Molecular Foundry is a DOE Office of Science User Facility.