Friction and Lubricants Related to Human Bodies

Dear Colleagues,

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 increases in friction and adhesion between the surfaces in contact, which may lead to the surfaces becoming stuck and wearing, ultimately leading to irritation, pain and trauma. Over the past few decades, major advantage has 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, 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 skin, eye, mouth, longue 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 addition, 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 the contact with clothes, fabrics and solid surfaces, and can lead to an uncomfortable sensation of stickiness, irritation, trauma and wounds such as decubitus. The naturally moist mucosal surfaces of the eyes, mouth, longue, gastrointestinal tract and vulva are kept in lubricated state by specific biolubricants, i.e., glycoproteins (mucins), (phospho-)lipids and water. Disfunctioning lubricating mucosal surface can lead to various discomforting and clinical situations such as the eye ball sticking to the eye lid or to contact lenses, xerostomia (a syndrome due to a educed salivary flow or quality, involving dry mouth syndrome and difficulty in swallowing and speaking), pleuric rub caused by various lung diseases, difficulty in sexual penetration, bowel irritation and trauma that can ultimately lead to cancer development. A special aspect of the bio-lubrication 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).

A final topic that will be addressed is the lubrication of red blood cells in capillaries and of the larger arteries and the way this relates to cardiovascular degeneration and disease.

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, both fundamental studies in the physics of tribology and the biophysics and biochemistry of the involved compounds are conducted and will be reviewed. These will include the in vitro and in vivo measuring tools, and the fundamentals of lubrication theories and polymer biophysics.

Prof. Dr. Ille C. Gebeshuber
Dr. George van Aken
Guest Editors

• biolubrication
• cartilage
• eye
• gastrointestinal tract
• hair
• joint lubrication
• lungs
• mouth
• red blood cells
• skin