Mingyue Li
Brittle stars belong to the phylum Echinodermata, class Ophiuroidea. They all have a small rounded disk with five linear arms distinct from it. Each arm is embedded with well-developed ossicles and lots of tube feet that can not only protect them by burrowing themselves into sand but are also helpful for feeding and sensation (Pechenik 2015).
The first time I saw it in the lab, I was fascinated by this little guy. Although brittle stars and starfishes are close relatives, compared to the thick and lovely slow-moving starfish, brittle stars’ sensitivity surprised me. It is no exaggeration to say that, except for the lack of a visible head, they look like an octopus crawling on the seafloor. In order to allow them to stretch their limbs freely, we replaced the small finger bowl with a large plastic frame (Figure 1). We found that brittle stars can not only crawl around quickly but can also use their long arms to turn over in 3 to 4 seconds. Furthermore, after determining the forward direction, it will extend its arm in the corresponding direction, as if to guide itself. Two neighbor arms will swing back and forth like breaststroke (Figure 2). The change of direction in the brittle star can also be done seamlessly.
The oval shaped structure on the left are collagen fibrils with tiny links that made by proteoglycan and cross-linker molecules (this combination is called the interfibrillar matrix) in the flexible status. In contrast, the flatter oval structure on the right are collagen fibrils with higher level cohesion between interfibrillar matrix and collagen fibrils in the stiff status. (from Mo et al. 2016)
During my search for brittle stars, I was lucky to find an interesting article that explains how the brittle star features like physical characteristics can be used on the robot. (Kano et al. 2019) Such a combination can indeed better enhance the performance of the robot and brittle star-like designs will let robots to have a wider range of observation. Because previous robots could only adapt to predictable or computer programmed responses, they were vulnerable to accidental damage in the face of disaster areas, the deep sea, and in outer space. This may be caused by the shortcomings of a central control system. In this system, there is a very long nerve pathway that the nerve cells need to pass through the brain first, and then the brain gives instructions and then transmits them through the nerve network to the corresponding body parts to respond. Since brittle stars are eyeless and brainless animals, they won’t be affected by the shortcomings of the central control system and can respond quickly. (Kano et al. 2019) Another great tool that helps brittle stars move quickly is their internal skeleton: ossicles. (Figure 3) The plates are moved by a type of connective tissue called mutable collagenous tissue (MCT), which is also called catch connective tissue. (Mo et al. 2016) When the star is threatened, the nerve system will tell those tissues near the base of the arm to disintegrate. In another article, scientists talked about the how fibrillar mutable collagenous tissues change in nanoscale. (Mo et al. 2016) They found that by stimulating interfibrillar cohesion into the active state can let a brittle star’s body shift from soft to stiff. (Figure 4) Throughout these two scientific articles, we can not only learn to utilize brittle stars’ body structure in robots but also can take advantage of the flexible fibrillar-hydrogel composites to create dynamic biomaterials!
References
Kano, T., Kanauchi, D., Ono, T., Aonuma, H., & Ishiguro, A. (2019). Flexible coordination of flexible limbs: Decentralized control scheme for inter- and intra-limb coordination in Brittle stars’ locomotion. Frontiers in Neurorobotics, 13. https://doi.org/10.3389/fnbot.2019.00104
Mo, J., Prévost, S. F., Blowes, L. M., Egertová, M., Terrill, N. J., Wang, W., Elphick, M. R., & Gupta, H. S. (2016). Interfibrillar stiffening of echinoderm mutable collagenous tissue demonstrated at the nanoscale. Proceedings of the National Academy of Sciences, 113(42). https://doi.org/10.1073/pnas.1609341113
Pechenik, J. A. (2005). Biology of invertebrates. McGraw-Hill.
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