Bio-lubrication in insect joints: A lesson for designing a low-friction micro-bearing

An insect joint operates, from a mechanical perspective, as a sub-millimetric bearing: remarkable tribological properties are here ensured thanks to relatively soft, elongated-cylindrical filaments secreted through the pores present in the contacting counterparts, and acting as randomly disposed rolling elements. We characterize, by means of atomic-force-microscope (AFM) nanoindentation, the mechanical properties of these filaments and, based on this, develop a numerical discrete-element model. The latter is successfully benchmarked and tuned, at the mesoscale, against normal pressing and frictional tangential tests conducted on a micro-tribometer. Crucially, the numerical model provides a quantitative assessment of the friction in the insect joint, showing how the frictional torque depends on the amount of lubricant. Throughout this combined numerical and experimental multi-scale investigation, we understand how solid lubrication may effectively work on micro-systems by correctly tuning the stiffness, the adhesion and the friction properties of the lubricating particles. Statement of Significance: Ensuring good tribological properties at the micro-scale is particularly challenging, as surface forces - especially adhesion - strongly hinder relative motion. Through a combined experimental, numerical and theoretical analysis, we reveal a response given by Nature to limit stiction and wear in a biological sub-millimetric system: the articular joint in insects. A cross-disciplinary approach, combining entomology, material science and mechanical engineering, is implemented to explain how these filaments support load, reduce friction, and actively clean the joint, thereby limiting wear. This comprehensive assessment of a solid lubrication mechanism operating effectively in a micrometric environment provides a methodological framework for designing artificial biomimetic solutions. These could have significant technological impact across various applications, including micro-sensors and micro-actuators, where stiction is a major cause of failure.