Effect of Liquid Crystals on Biological Mechanisms of Reducing Joint Friction

Abstract

The results of studying the friction of joint cartilages lubricated by synovia and lubricants without and with additives of liquid crystal compounds are presented. Based on the experimental data, the structural-mechanical and antifriction properties of joint synovia are compared. It is shown that, having in mind the regular structure of surface layers and deformation behavior of cartilages, the present results confirm the assumption on liquid crystal compounds as a cause of low friction in joints. The best tribological characteristics for the combinations with blood serum are demonstrated by drugs based on chondroitin sulfate, which display characteristics close to natural synovial fluid during frictional interaction of cartilage.The liquid crystal state seems to be an ideal medium for the occurrence of many biochemical reactions and the provision of the reproduction of biological structures. It is characteristic of cell membranes and organelles, photoreceptors of retina, microfibrils, collagen albumens, myelin nerve fibers, nucleic acids, saliva, and other natural objects [1][2][3][4]. The joint synovia also belongs to these objects. Its mesomorphic state may play an important role in reducing joint friction. Effect of Friction Surfaces and Lubricant on Joint Cartilage FrictionProceeding from modern ideas on anatomical, physiological, and biochemical features of members of the human and animal locomotor system including mechanisms of joint lubrication it is important to study the boundary lubrication of cartilages which occurs under heavy loads because collagen fibers are brought together. According to the generally accepted concept, low friction under these conditions should result from the formation of adsorbed films on friction surfaces. The latter are effectively separated by the films acting like boundary films of fatty acids, i.e. surfactants whose molecules are elongated and produce polymolecular adsorbed films normal to the friction surfaces [5][6][7]. Such structures are known to be characteristic of smectic liquid crystals. Since synovia contains acid components [8] it can be assumed that low friction in the boundary lubrication of cartilages is due to the mechanism of the action of fatty acids, i.e. the substances capable of forming smectic liquid crystal structures on bearing surfaces.Results of these studies are of interest for the following reasons. The lubricating behavior of the systems containing fatty acids is known to be governed by features of the structure of lubricating films in the zone of dynamic contact between solids rather than the lubricant rheological behavior [9,10]. Fatty acids of the biological origin belong to smectic liquid crystals characterized by a low shear resistance because shear in friction is localized in adsorbed lubricant films [10][11][12][13][14][15]. The use of fatty acids as surfactants to reduce friction is most effective when the nature of the friction surfaces provides chemical reactions between them and the acids [10,16].The assumption that synovia dem...