A puzzle in the porous microstructures of sea urchin exoskeletons has been solved by Ling Li, assistant professor in the Department of Mechanical Engineering at Virginia Tech, and it may help in the development of lightweight synthetic ceramics.
Because of their exceptional heat resistance, ceramics are a preferred material for meeting the extreme thermal requirements of above-the-speed-of-sound vehicles. When moving at extreme speeds, compressed air generates a lot of friction with the car, which quickly raises the temperature it encounters.
Ceramics’ ability to withstand heat may be its greatest asset, yet damage tolerance is a problem. A single precise hit on a ceramic plate can cause a crack to propagate quickly and completely destroy the structure. When ceramics are made porous to reduce weight, they become even more vulnerable to damage. Nonetheless, reducing weight is an essential requirement for many structural applications, including high-speed vehicles.
Improved mechanical performance of ceramic materials has long been a goal for the U.S. Air Force, one of Li’s funders for his research. In addition to help from the Air Force Office of Scientific Research, the National Science Foundation gave money to Li’s team.
These combined grants, which the lab received in 2018, have enabled researchers to investigate new design principles hidden in organic cellular solids, such as sea urchin ceramics. The exoskeleton of a sea urchin is a particular kind of cellular solid, or “foam,” so named because its microstructure consists of a collection of open cells with solid edges or faces that are tightly packed together to fill space. Because there is space between the cells, they become porous, which makes them stronger than dense structures.
How to handle the damage like a sea urchin
According to Li, with its porous microstructure, the sea urchin is able to be resistant and strong while also offering weight reduction. “In this work, we think we uncovered some of the fundamental solutions,” Li added. This Nature Communications paper talks about what we found inside and how we found it.
Sea urchins have brittle, robust, and light spines. Although the calcium carbonate that makes up these spines is brittle and resembles manmade ceramics, sea urchins have a considerably higher tolerance for damage when subjected to weight or force. By manually pressing the spines, Li’s team evaluated this idea by imitating the conditions that an engineered ceramic could have to withstand. The sea urchin spines bent beautifully under the strain applied, as opposed to the modern synthetic ceramic cellular solids, which break catastrophically. Because of this “graceful failure” feature, the sea urchin’s spines can take damage well and absorb a lot of energy.
The research done by Li’s team showed that the urchin uses a number of ways to keep its structure strong when it is being moved.
Secrets of the deep
“The structural characteristics of sea urchin spines hold a few surprises. One has to do with how branches are connected, “said Li. The size of the pores comes second.
Under a microscope, Li’s team discovered a network of interconnected small branches. These branches are connected by a network of nodes, and the balance between the number of nodes and branches is one of the keys to the urchin’s damage tolerance. The reason that number is crucial is that nodes with too many connected branches would weaken and break more easily. Because the nodes in the porous structure of sea urchin spines are often connected to three branches, the network of branches will break from bending rather than from stretching, which would be much worse.
The size of the pores, or spaces between branches, holds the second secret. The scientists found that the gaps in sea urchin spines’ porous structures are just slightly less than the width of the branches. Because of these smaller apertures, the branches can be promptly secured in place after they break. On the surface, broken branches pile up on top of one another, forming a thick area that may still support a load.
Additionally, the surface shape of sea urchins differs from that of artificial lightweight synthetic ceramics. Manufactured cellular ceramics are more prone to failure because of the numerous minute flaws present both internally and on their surfaces. The sea urchin spine, on the other hand, has a nearly glass-like surface that is incredibly smooth down to the nanoscale scale. A lack of faults signifies a lack of vulnerable areas because defects are where damage can begin.
Li illustrated this concept using some paper. “An intact piece of paper is difficult to tear when you attempt. However, the tear will continue from the damaged place if you make a little tear at the edge of the paper. ”
Lightweight sea urchin spines are very strong and can take a lot of damage. This is because branches, pores, and a smooth surface help spread stress throughout the structure and absorb energy more effectively.
Making the next generation of lightweight synthetic ceramics
Can we reproduce the smoothness, lack of flaws, and precise branch and node architectures required to profit from the sea urchin’s secrets now that we know these things? We are unable to do so at the moment since ceramic processing techniques are still in need of improvement.
Synthetic ceramics are typically created in a two-step procedure. The shaping process takes place first, and then the object is fired to cause the ceramic to harden and acquire its renowned strength. When a pot is made and heated in a kiln, potters use this technique. Lightweight synthetic ceramics that are printed with a 3D printer follow similar steps. After the 3D printing step, the final ceramic parts must be fired to finish the shape.
As a result of the production of microscopic flaws during the sintering process, which results in low strength, the fire, or sintering, stage is the most challenging for replicating the sea urchin’s microstructure.
“We are also curious about how creatures like sea urchins create these organic ceramic cellular solids in my lab,” Li said. “Hopefully, one day we will be able to incorporate not only the material design principles but also the material processing strategies learnt from natural systems into bio-inspired lightweight ceramic materials.”
