Well lubricated for animals: Researchers report on a substance in the leg joints of beetles that, according to tests, reduces friction as well as Teflon. In addition to its biological significance, the discovery may also have technical potential: If the lubricant can be artificially manufactured, it could be used in robotics, say the scientists.
Movements should run as smoothly as possible: In the joints of vertebrates like humans, fluids ensure that limbs can be moved painlessly and wear and tear remains low. In the case of insects with their exoskeleton, on the other hand, the joints have a completely different structure: They are unencapsulated and therefore partially directly exposed to environmental influences. They are therefore more similar to elements of robotic systems. Obviously the joints of the insects are very efficient – the innumerable species have produced an impressive ability to move. “But research knows almost nothing about how insect joints are built to cope with these movements and what materials they are made of,” says Stanislav Gorb from the University of Kiel.
How insect joints work without protective capsules, Gorb and his colleagues have examined using the example of the approximately two centimeter large black beetle (Zophobas morio), which can be easily reproduced for laboratory studies. First of all, through scanning electron microscopic examinations, they discovered innumerable pores on the contact surfaces of the beetle joints that are only a thousandth of a millimeter in size. They have been shown to produce thin strands of a substance with an unusual, waxy consistency. The secretions partially break up and form small lumps. The exact composition remains unclear, but investigations using infrared spectroscopy have at least shown that the substance consists largely of proteins.
Powerful synovial fluid
To investigate the assumption that it was a lubricant, the scientists first extracted the material from numerous beetle joints. They then used the yield for experiments in which they placed the substance between two glass surfaces. Then it was recorded which frictional forces act when the two surfaces are moved against each other. For comparison, they carried out the tests with the plastic polytetrafluoroethylene (PTFE) – better known under the trade name Teflon. “Our experiments showed that the sliding effect of the beetle substance is similar to that of Teflon, which is known to have the best anti-friction effect that can currently be produced artificially,” reports first author Konstantin Nadein from the University of Kiel.
As the researchers explain, the special consistency of the substance is probably decisive for its function in wear protection: The fine lumps that form from the strands probably act like a cushion that prevents direct contact between the joint surfaces. In addition, the strength of the lubricant is important in the exposed joints of the insects. Because a liquid material like in the joint capsules of vertebrates would dry out in the insects, explain the scientists.
Biological and technical importance
They suspect that the discovered lubricant is a general functional element of insect joints. “This is supported by the fact that we have already found it in the knee joints of other insect species, such as the Argentine forest cockroach, which is not closely related to the beetles,” the researchers say. However, further studies are necessary to clarify the distribution of the substance in the insect kingdom. In addition, the researchers now also want to investigate the question of how insects protect their open joints from contamination.
As they emphasize, the significance of the results could go beyond biology: Since insect joints are basically similar to artificial micro-joints, the powerful beetle lubricant could also be of interest for technical applications. In robotics, for example, it could be used to reduce wear on joints and thus increase the service life of components. But there is still a problem along the way: The scientists still have to master the challenge of artificially producing the substance in larger quantities. This would also be important for a more detailed study of their characteristics. “It is difficult to analyze the substance chemically because it hardly dissolves in any liquid. We also need a lot more test material for this, but collecting it by hand is very time-consuming, ”says Nadein.
Source: University of Kiel, specialist article: Proc. R. Soc., Doi: 10.1098 / rspb.2021.1065