Joints should function optimally, neither become stiff nor wear out prematurely. Where bones and cartilage form a joint, a viscous fluid called synovial fluid ensures that movement is as smooth as possible. The situation is different with insects that do not have an internal skeleton, but an external one. They are completely enveloped in a complex composite material made of chitin fibers and proteins, which comes partly as a hard armor, partly as a stretchable and foldable membrane. As part of this so-called cuticle, the joints are also open to the outside, instead of being stuck in a sealed capsule with lubricant. Zoologists working with Konstantin Nadein from the University of Kiel have investigated how insects still manage to travel fairly smoothly.

The scientists used the large black beetle (

Zophobas morio

)

as a handy research object

. This roughly two centimeter tall relative of the meal beetle comes from South and Central America. In this country, its larvae are in great demand as food for pets from geckos to gerbils, similar to the mealworm larvae known as “mealworms”. Contemporaries who like to experiment even put larvae of the black beetle on their own menu.

Nadein and his colleagues were interested in the knee joint of the adult beetle in their studies.

Using micro-computed tomography, the researchers reconstructed how the upper end of the lower leg protrudes into a depression in the thigh: Two hemispherical articular processes there engage the concave counterparts of the lower leg.

The lower leg rotates around the axis formed in this way as soon as muscles in the thigh are activated accordingly.

Using a scanning electron microscope, the researchers were able to make out a large number of pores on the contact surfaces of the knee joint, the average diameter of which is a thousandth of a millimeter.

Micrometer-thick strands of protein synovial fluid

It is known that the cuticle is traversed by wafer-thin canals at various points. Thanks to such connections between the skin and the surface of their body, insects can protect themselves from dehydration not only with waxy secretions. They also secrete pheromones through pores in their cuticle, which attract the opposite sex, and antibiotics, which keep fungi and bacteria in check. What wells out of the pores in the knee joint of the big black beetle, on the other hand, is reminiscent of grout that was pressed out of a cartridge. However, in miniature format: strands that were no more than one micrometer thick and one hundred micrometers long were visible under the scanning electron microscope. Most, however, were broken into cylindrical fragments that formed clumps in places.

Together with Jan Thøgersen and Tobias Weidner from the chemistry department at Aarhus University, the Kiel zoologists also examined the physical and chemical properties of the suspected synovial fluid. Analysis using infrared spectroscopy showed that this substance consists mainly of proteins. In contrast to other secretions that penetrate the cuticle, it does not seem to contain any fatty components. Although mainly composed of protein, the presumed synovial fluid of the black beetle is remarkably heat-resistant with a melting temperature of more than 100 degrees Celsius. After several days at room temperature, no signs of evaporation or decomposition were discernible; and because the secretion from the beetle's knee joint is not soluble in water,even high humidity cannot harm it.

Lubricant and shock absorber at the same time

The filigree protein strands, it turned out, reduce the sliding friction between two glass surfaces just as much as Teflon does.

This suggests that they actually act as synovial fluid in a biological context.

According to Nadein and colleagues, when the load is low, the cylindrical strands likely act like rollers, over which the joint head and the joint socket slide in opposite directions.

If the joint space narrows due to particularly strong pressure, the protein strands are likely to deform plastically and act as shock absorbers.

In any case, they prevent direct contact between the convex and concave part of the joint, which arguably reduces the wear and tear on these surfaces considerably.

As the protein strands swell out of the pores, they tend to break up into shorter sections. Therefore, very small amounts are probably sufficient to supply the entire joint surface with this not-so-greasy lubricant. In addition, small snippets are more evenly distributed and can penetrate narrow gaps better than long strands of the same material. How well such synovial fluid has proven itself in the course of evolution is testified not only by various types of beetles, including water beetles, in whose joints cylindrical protein strands have also been discovered. Also cockroaches, as a primitive insect order only very extensively related to the beetles, use synovial fluid of this type.

What insects have been using successfully for more than 300 million years may serve as a stimulus for engineers who develop microsystem technology components. When mechanical components of such systems become microscopic, there is often little that can be done with conventional lubricants: New types of means are required that also work in the micro range. Always inspiring for the construction of mobile robots, insects could serve as a model. After all, most of them are not just a flash in the pan. To crawl around for months or to jump around requires correspondingly hard-wearing, long-lasting joints.

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