Recent studies suggest that the joints of beetles and other insects comprise micro-structured surfaces in combination with lubricants. Here, we utilize friction force microscopy (FFM) to analyse the tribological properties of the femoro-tibial leg joints by the example of Coelorrhina aurata (metallic green flower beetle) and Otiorhynchus sulcatus (black vine weevil). To preserve the original state of the lubricant as well as the microstructures, the FFM measurements were conducted in silicone oil, which satisfies our requirements of transparency, customizable viscosity, absent health risks and lower density compared to the expected density of the lubricant. Microscopic friction was measured on fresh and air-dried samples to stress the change of the lubricant properties over time. Despite the similarity of the two beetle joints, the FFM measurements reveal different frictional properties of the respective lubricants.

Insects optimize friction in their joints by combining microstructures with a—so far unknown—lubricant. To develop environmentally friendly lubricants, we research the sophisticated tribological system found in the joints of beetles. We characterize the lubricant as well as the microstructure of the joints to gain inspiration for the development of a degradable and—hopefully—superior alternative to mineral-oil-based lubricants. However, restrained by the tiny quantities of beetle lubricant and the compactness of their joints, this tribological analysis is challenging. Therefore, we apply atomic force microscopy (AFM) to record the joints' microstructures and the lubricant's frictional properties. Furthermore, we research the inner structure of the bearing surface in beetle joints by focused ion beam (FIB) tomography. With this approach, we discover a network of channels supplying the lubricant to pores which represent the inlets of the hinged joint system. As a subsequent step, we analyze different types of presently available plant mucilage using AFM friction measurements to compare the suitability of plant mucilage as an alternative lubricant to the tiny quantities of beetle lubricant. Finally, we develop an artificial surface mimicking the microstructure of beetle joints. We determine its frictional properties utilizing colloidal AFM probes in the dry state as well as the lubricated state with plant mucilage as the lubricant.