Alex Zimmerman

Alex Zimmerman

Significant Advance in Materials Biomimicry

 

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.

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2 thoughts on “Significant Advance in Materials Biomimicry”

  1. This is terrific, Alex. I was hoping and kind of expecting that you would make the leap to boatbuilding. Let me try to stimulate you to do that…The planet desperately needs less carbon intensive and much more recyclable materials (one of my Inside Passage Decarbonization Project’s key goals). The current material standard for recreational boats is, of course, a composite, fiberglass. The main constituent, epoxy resins, are predominantly petroleum and consequently FG boats have very high embodied carbon. Like concrete, epoxy requires multidirectional tensile reinforcement, found in the form of glass fibers, which come from silica (sand). Recycling is nearly impossible so old FG boats are generally crushed and moved into landfills, with untold many scuttled into the depths. Around 15 years ago at a Living Future Unconf I had a few conversations with another member of our architects clan, Pliny Fisk III, founder of the Center for Maximum Potential Building Systems outside of Austin. He recommended looking into sugar-based epoxies with basaltic fiber reinforcement. This combination is actually beginning to be experimented with by a few innovative boatbuilders, with its promise still yet to be determined. Concurrently, some innovative builders in Denmark and Norway are experimenting with engineered wooden hulls, where the durability and maintenance issues of wood are enhanced chemically and/or thermally. Not much has been published about either of these two possible innovations to get the industry off of its embodied carbon/non-recyclable habits, but likely will be. I am wondering what your thoughts are on the topic?

  2. Peter, you raise a number of good points about boats and the current problems with them. I am in the midst of building another boat. The main material is wood, but it is held together with epoxy glue. The combination of the two allows a boat to be built at about half the weight (in wood) if it were to be built in traditional solid wood with traditional fastenings. The embodied energy is therefore much lower than a composite glass fibre boat but the epoxy does mean that it will be difficult to recycle at the end of its life, which I hope will be many decades away.
    You mention plant-based epoxy. There is one such epoxy available on the market today, EcoPoxy (https://www.ecopoxy.com/), made in Manitoba. I was excited to learn about it and had intended to built this entire boat out of it. However, in my initial trials with it, I found it to be harder to work with because it is much less viscous than other petroleum-based epoxies I have used, meaning that it required more thixotropics to get it to the point where I could use it as a gap-filling glue. More importantly, it is quite temperature-sensitive so that it takes much, much longer to cure before you can trust it to have enough strength for you to be able to take the next step in building the boat. I found it takes about 3 days for it to cure compared to about 1 day for my previous epoxy. While I’m a fairly patient man I am not patient enough to triple the time it will take to build the boat. A commercial builder will be even less patient. So, from my limited experience, I would say that the plant-based epoxies have a way to go before they are ready for prime time.
    I also have heard of various experiments with wood treated to enhance its properties but I have to say I don’t know much about them. As always, how you fasten the wood together is going to be a challenge.
    I hadn’t elaborated on the potential of this new material I discussed in the blog to be used for boatbuilding, in part because there isn’t yet that much detailed information available on the process of production and the properties of the material. One of the questions I have still is just what properties the calcium carbonate has? Calcium carbonate in its natural state is found in marble, limestone and chalk, each of which has very different mechanical properties and very different corrosion resistance. One of the key phrases in the article was this statement by Wang “. . . we guide the bacteria to grow calcium carbonate minerals to achieve ordered microstructures which are similar to those in the natural mineralized composites.” Similar to which natural mineralized composite? More like marble or more like chalk? If more like marble, then perhaps it is a material that will have good corrosion-resistance and will not degrade in seawater.
    One other potential benefit of this new process and material is that it sounds like it would be possible to set up manufacturing without the massive investments required to build a steel mill or aluminum smelter, for example. One would imagine a boat-builder printing out a boat-hull-shaped scaffold with a large 3-D printer, then immersing the whole thing in a bacteria and ingredient-filled vat and pulling out a completed hull a few days later, shining like the inside of an oyster shell. At the end of the boat’s life, presumably you could grind it up to recover the calcium with some kind of solvent, to feed the bacteria for building the next boat. Now that would be exciting!

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