Our civilization depends hugely on energy and materials. Energy gets a lot of attention, especially our use of fossil fuels, which has the unintended consequence of causing climate change.
Materials that underpin our infrastructure get much less attention, undeservedly so in my view, but they are just as critical. After all, stages in civilizational infrastructure have been defined in the past as the stone age, the bronze age, the iron age. I would argue that while we don’t refer to it as such, the last 200-year period has really been the concrete and steel age, while the last 70 years has also been the plastic and silicon age.
The key challenge with any kind of structural or engineering material is to get the properties you want and avoid the properties you don’t want. For structural materials, ideally this means something that is strong, stiff, light, doesn’t fracture easily (a property also known as toughness in the materials world) and can be easily worked into shapes you want. Stone is hard, strong and stiff but fractures easily and it isn’t very light. Bronze is not quite as hard or strong as stone (in compression at least) but is relatively stiff and didn’t fracture as easily as stone and is easy to shape when molten or hot. Iron is hard, stiff, stronger and tougher than bronze and also easily shaped when molten or heated. Concrete is very strong in compression and easily shaped before it cures but is very weak in tension, is fracture-prone and heavy. Steel has a better combination of hardness, strength, stiffness and toughness than all of the materials that preceded it but it corrodes easily, takes a huge amount of energy to produce and is still heavier than would be ideal. Aluminum is very light for its strength, which is why it is used in airplanes but is relatively soft, prone to fatigue failure compared to steel, and it uses even more energy than steel to produce.
These most common human-made engineering materials are generally monolithic, that is uniform throughout. We combine materials to get improved properties we want. The most well-known example is the addition of reinforcing rods of steel added to concrete to give the composite material more useful properties than either would have alone. As this article points out: “However, most engineering materials sacrifice strength for toughness.” By contrast, nature frequently produces materials that are superior in many ways to what humans can produce. The article goes on to say “Biological materials often do not face this tradeoff, thanks to their hierarchical structures and multi-functional abilities optimized over millions of years of evolution. They offer an exciting new paradigm to advance our materials design ability, especially when we can combine distinct material platforms that exist separately in nature, but we as engineers can combine in novel ways.”
One example is the nacre (mother-of-pearl) in seashells. From the same article: “Nacre is a natural material that offers good guidelines for creating high performance composites, being made up of 95 per cent calcite minerals and with 5 per cent biopolymer proteins, and having a toughness around 3,000 times higher than its base material (i.e., the brittle calcite minerals) alone.” Creatures that produce seashells are able to do this with only the materials and energy found in seawater, while being immersed in seawater, a corrosive environment for most man-made structural materials.
Researchers and engineers have long sought to find ways to mimic these properties of natural materials in ways that can be controlled in order to produce materials that are useful to society. They haven’t been broadly successful. Until now.
Qiming Wang and researchers at the Viterbi School of Engineering at the University of Southern California have figured out a way to use bacteria to build engineering materials that are strong, stiff, tough and lightweight. The research was published in the journal Advanced Materials and reported on by Science Daily. Wang says: “We have been amazed by the sophisticated microstructures of natural materials for centuries, especially after microscopes were invented to observe these tiny structures. Now we take an important step forward: We use living bacteria as a tool to directly grow amazing structures that cannot be made on our own.”
How it works is that a 3-D printed lattice structure is made which serves as a scaffold for the bacteria in question. The bacteria latch on to the lattice and secrete urease, an enzyme that triggers formations of calcium carbonate crystals. The clever bit that results in the unusual strength properties comes from the way the lattice is formed, with the layers laid at varying successive angles to form a helical structure.
To understand why and how this is important, think about one material that we are all familiar with, which relies on successive layers oriented differently to produce a material that is stronger than that same material would be if it were monolithic. That material is plywood, which has the fibres of each layer oriented 90 degrees to the one next to it. Wood is a highly anisotropic material – that is, the strength properties are very different, depending on the direction the load is applied. By changing the orientation of alternate layers, plywood has roughly the same strength in all directions, no matter which direction the load is applied.
Back to the helicoidal calcium carbonate material produced by the bacteria growing on the 3-D substrate. How much better is it? While the paper doesn’t give any numbers, according to An Xin, one of Wang’s doctoral students: “We did mechanical testing that demonstrated the strength of such structures to be very high. They also were able to resist crack propagation — fractures — and help dampen or dissipate energy within the material“
There are two other aspects to this that I think are very interesting.
The first, according to Wang, is that “these living materials still possess self-growing properties,” and “When there is damage to these materials, we can introduce bacteria to grow the materials back. For example, if we use them in a bridge, we can repair damages when needed.”
The second is that, presumably, the process will use very much less energy to produce the material compared to just about any other material (aside from wood) and the production of the material may actually extract carbon dioxide out of the environment rather than emit it.
Finally, just as in the past when new materials with improved properties were introduced, new forms of infrastructure, like long span bridges, skyscrapers and very large ships became possible, this new material is likely to generate new uses and forms we haven’t yet imagined.