Evolution is an amazing thing—take the mantis shrimp, for example. To kill its prey, the colorful crustacean pulls back the hammer-like appendages under its face, called dactyl clubs, and then lets go, unleashing a deadly blow at speeds of 10,000g, or the velocity of a .22 caliber bullet. With that much force, it’s no wonder this creature is also known as the thumb splitter: this unique method of hunting can completely pulverize a clam shell, which is one of nature’s toughest materials, and instead of being merciful, the mantis shrimp will blow off a crab’s claws so it can’t defend itself, and then basically punch it to death. However, this all puts the dactyl clubs under a lot of stress, which is why nature was kind enough to give the material that makes up the hammers a Bouligand shape, wherein the layers are twisted and any structure cracks reproduce in a twisted pattern that spreads the energy throughout the material.
In the past, researchers have been inspired by the mantis shrimp’s club to create strong 3D printable materials, and now a team of engineers from the University of California, Irvine and the University of Southern California are going down a similar path. Using bacteria, they’ve been able to get minerals to grow inside a 3D printed Bouligand structure. They recently published a paper about their work, titled “Growing Living Composites with Ordered Microstructures and Exceptional Mechanical Properties.”
The abstract states, “Living creatures are continuous sources of inspiration for designing synthetic materials. However, living creatures are typically different from synthetic materials because the former consist of living cells to support their growth and regeneration. Although natural systems can grow materials with sophisticated microstructures, how to harness living cells to grow materials with predesigned microstructures in engineering systems remains largely elusive. Here, an attempt to exploit living bacteria and 3D‐printed materials to grow bionic mineralized composites with ordered microstructures is reported. The bionic composites exhibit outstanding specific strength and fracture toughness, which are comparable to natural composites, and exceptional energy absorption capability superior to both natural and artificial counterparts. This report opens the door for 3D‐architectured hybrid synthetic–living materials with living ordered microstructures and exceptional properties.”
The most basic explanation is that the team 3D printed scaffolds and dipped them in a microbe concoction in order to create a stronger, more robust structure, in what could one day lead to self-building roads. They started off by 3D printing a simple lattice structure, which has a lot of empty space inside, kind of like a building’s supporting beams. Then, the lattice was dipped into a solution that contained the Sporosarcina pasteurii bacteria, and then sat out for 12 to 24 hours. During this time, the bacteria attached itself to the 3D printed polymer lattice and began to secrete the urease enzyme.
Once time was up, the 3D printed structure was dipped into another solution, this one containing calcium ions and urea, and the urease that had been secreted during the waiting period began a chemical reaction that led to the creation of calcium carbonate, which is what makes human teeth and bones, as well as clam shells, so strong. Additionally, as you may have guessed, the material is also part of the mantis shrimp’s dactyl club.
As you can see above, the longer the 3D printed scaffold structure remained in this second solution, the more calcium carbonate was created to fill the empty spaces in the lattice. It took ten days to fill in the entirety of the lattice structure, and the result was a really tough material that consisted of minerals inside a polymer skeleton.
Using this method, the team 3D printed lattices that featured multiple internal shapes, such as crosses and wave patterns. Row C in the image below shows where the mineral filled in the polymer skeleton’s gaps, and Row D shows how the lattice structure (green and blue) has a lower stiffness score than the calcium carbonate mineral deposits (red).
This was great work, but not exactly what they wanted, which was of course the Bouligand structure that makes the dactyl club so resilient, and can be seen in the Type IV image on Row A, below. With each layer shifting 45°, this structure is chaos exemplified, but in this case, that’s not necessarily a bad thing.
“This kind of microstructure makes sure that this kind of composite is very tough. When you have a crack, that crack will propagate in the twist pattern to dissipate the energy inside the material,” explained co-author Qiming Wang from USC.
When the lattice strength was tested, the researchers found that the Bouligand structure absorbed twenty times as much energy as the Type 1 lattice.
Wang says that body armor is a good potential application for this research: just like the hammer of a mantis shrimp can absorb the energy of its own punches without breaking, the same could also be the case for any materials created using the research team’s new method. Also, because calcium carbonate is not terribly heavy, other applications could include skins for robots and tougher aircraft panels.
Finally, another possible use could be growing roads, instead of building them, as this nifty material is actually able to regenerate!
“If we have damage, you just introduce bacteria inside, and it can grow it back. These structures are very tough, very strong, and can potentially repair themselves,” Wang said.
This is obviously not happening anytime soon, as scaling all the way up to road construction from growing small amounts of minerals with the bacteria in a lab is a pretty big leap. But the potential does exist.
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