Top Result for FY2021
Three-dimensional microstructure of the robust claw of the coconut crab, one of the largest terrestrial crustaceans -Aiming for the ultimate material-
Tadanobu Inoue
Group Leader, Anisotropic Materials Group, Materials Manufacturing Field, Research Center for Structural Materials
Aiming at developing the ultimate strong and tough structural material, we investigated the microstructure, composition, hardness, and fracture morphology of coconut crab claws, which are covered with a strong and tough exoskeleton, from the viewpoint of biomimetics, and succeeded in visualizing the exoskeleton of coconut crab claws in three dimensions (figures 1 and 2) for the first time in the world. The rigid layer on the surface of the coconut crab exoskeleton has a twisted plywood–pattern structure and a hardness–stiffness balance (an index of abrasion resistance) that exceeds that of other organisms (Figure 3). This is one of the results of deploying the most advanced equipment and technology to organisms, taking hints from interdisciplinary exchanges, and changing the way we look at things. The results are expected to be a major hint for the tissue structure to realize the ultimate material toughening that will break existing material properties, and we aim to create stronger, tougher materials in the three-dimensional (3D) laminated molding process, etc., which will be the main manufacturing process in the near future. 
Q＆A
Q：Why did you decide to do research on coconut crabs?
A：Because I was stuck in my research [laughs]. Materials become more brittle when they become stronger (i.e., when they become harder, they become more fragile). I had been researching strong and tough materials for more than 20 years. When you have been doing research in the same field for a long time, you can usually tell what kind of results you will get if you do something. I realized that this was not sufficient to break out of the current situation, and I felt that I needed a different perspective and interaction with different research fields. After three years of struggling with this, I searched the Internet and came across a paper on coconut crabs published by a biologist. When I read the paper, I had a hunch that I could apply it to my research on strong, tough materials! I realized that airplanes were developed based on birds and cars based on horse-drawn carriages. There are countless organisms in the world. By studying organisms, we should be able to utilize them in other research as well. This kind of concept has been around for a long time and is referred to as biomimetics.
Q：What is the point of this achievement?
A：The coconut crab's pinching force is 90 times its body weight, the strongest in the biological world. It is the largest terrestrial crustacean species and possesses claws that do not break even when pinched with great force. The exoskeleton of the coconut crab's robust claws consists of a composite structure with an iron- and steel-hard exocuticle on the surface and an endocuticle that is one-fifth softer than the exocuticle. The microstructure of the exocuticle is completely different from that of the endocuticle, with the exocuticle having a twisted plywood–pattern structure and the endocuticle having a porous structure. This kind of dense and ultrafine microstructure is not found in artifacts. Perhaps the twisted plywood structure has a mechanism to disperse force, while the porous structure is lightweight and absorbs force. We are aiming to create the ultimate strong, tough material by referring to this structure and microstructure mechanism.
Q：How is 3D visualization of tissue done?
A：We used to mechanically grind the area we wanted to observe and view it with a scanning electron microscope (SEM) or optical microscope. But with a twisted plywood structure, two-dimensional (2D) observation could not show the details well. Therefore, we used an Xe (xenon) plasma focused ion beam scanning electron microscope (PFIB-SEM) system to automatically acquire several thousand 2D images with the SEM while grinding the exoskeleton at a pitch of 15 nm with the focused ion beam, and then used commercially available software to create a 3D image of the microstructure. We succeeded in creating 3D microstructures through commercially available software. In fact, before doing this, I did not think it was possible to obtain such beautiful 3D images from biological specimens. You have to try everything first.
Q：Do you also study organisms other than coconut crabs?
A：When we published the coconut crab results (which created quite a buzz), there were over 400 reviews on Yahoo News. Two of them said, "The best way is this" and mentioned the mud crab, which has claws bigger than those of coconut crabs. I was interested in the huge claws of the mud crab, so I am working on this research as well. If you are interested, please refer to my website. Since organisms are diverse, we may be able to learn from them tissues and structures that we have not yet seen. However, while it is important to observe various microstructures with a microscope, this is a means, not a purpose of research. By examining the microstructure and structure of the strongest organisms, we aim to create the ultimate strong, tough materials. We must remember that the purpose of our research is to create new materials.