Editorial: Friction and Lubricants Related to Human Bodies

Biolubrication plays a crucial role in assisting the sliding contacts in many organs in the human body. Notable examples are the joints, the skin surface, the eye, the mouth and gastrointestinal tract, the lungs and the red blood cells. In all cases, reduced lubrication leads to increased friction and adhesion between the surfaces in contact, which may lead to the surfaces becoming stuck and damaged, ultimately leading to irritation, pain and trauma. Over the past few decades, major advancements have been achieved in the accumulation of knowledge on the subject. Joint lubrication has been shown to involve a complex and probably adaptive set of lubrication mechanisms involving the smoothness of the cartilage lining of the bone surfaces, the properties of the synovial fluid, which is a lubricating hydrogel of collagen fibrils, hyaluronic acid, glycoproteins (lubricin) and water, and a weeping mechanism by which joint pressure releases synovial fluid into the interspacing liquid film separating the cartilage surfaces. The lubrication of epidermal surfaces such as the skin, eye, mouth, lung and gastrointestinal tract follows different mechanisms. The relatively dry surface of the skin and hair is protected and lubricated by a thin coating of sebum in which moisture plays a critical role. On the one hand, moisture reduces the sliding resistance by providing suppleness, elasticity, plasticity, flexibility and softness to the deeper layers of the skin, but on the other, it increases the adhesive properties, friction and microbial proliferation of the outer surface. Loss of skin surface lubricity can lead to unpleasant sensations of contact with clothes, fabrics and solid surfaces, and can lead to uncomfortable stickiness, irritation, trauma and wounds such as decubitus. The naturally moist mucosal surfaces of the eyes, mouth, lung, gastrointestinal tract and vulva are kept in a lubricated state by specific biolubricants, i.e., glycoproteins (mucins), (phospho-)lipids and water. Insufficient lubrication can lead to various discomforting and clinical conditions such as the eyeball sticking to the eyelid or to contact lenses, xerostomia (dry mouth syndrome, due to a reduced salivary flow or quality, resulting in difficulty in swallowing and speaking), pleuric rub caused by various lung diseases, difficulty in sexual penetration, bowel irritation and trauma which can ultimately lead to cancer development. A special aspect of the biolubrication of the mouth is in the way it affects sensory and texture perception. The effect is strongly related to the perception of smoothness, creaminess and the opposite of roughness and astringency, which probably have the biological function to test the quality of the food material for the way it affects the lubrication of the mucosal lining of the alimentary tract, giving feedback to food preference and eating behavior (speed, subsequent food selection during a meal). In relation to the various symptoms of imparted lubrication, various studies are focused on specific lubricants and moisturizers that can correct the malfunctioning of the various organs. To measure and interpret the function of these products, fundamental studies in the physics of tribology and the biophysics and biochemistry of the involved compounds are being conducted.