ABSTRACT
Biological connective tissues, such as cartilage and corneal stroma, are essentially hydrogels consisting of brous collagen and proteoglycans. Few facts are known of the surface properties of the hydrogels, although we observe fascinating tribological behavior of biological soft tissues, such as the extremely low friction between animal cartilages. We consider that the role of a solvated polymer network existing in extracellular matrix as a gel state is critically important in the specic frictional behavior of cartilages. In order to elucidate the general tribological features of solvated polymer matrix, friction of various kinds of hydrogels have been investigated, and very rich and complex frictional behaviors are observed. The friction force and its dependencies on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition, which are totally different from those of solids. Most importantly, the coefcient of friction of gels, μ, changes in a wide range and exhibits very low values (μ ≈ 10-3-10-4), which cannot be obtained from the friction between two solid materials. A repulsion-adsorption model has been proposed to explain the gel friction, which says that the friction is due to lubrication of a hydrated layer of polymer chain when the polymer chain of the gel is non-adhesive (repulsive) to the substrate, and the friction is due to elastic deformation of the adsorbed polymer chain when it is adhesive to the substrate.
