Hydrogels with excellent mechanical performance: An approach to understand the secret of cartilages

J. Jian Ping GONG, Hokkaido University

A hydrogel is a polymer network swollen with large amount of water. It is a solid on the macroscopic scale: having a definite shape and does not flow. At the same time, it behaves like a solution on the molecular scale: water-soluble molecules can diffuse in a hydrogel with various diffusion constants reflecting sizes and shapes of the molecules. Because of its specific structure, a gel exhibits a variety of unique behaviors such as phase-transition, specific adsorption equilibrium, presence of unfrozen water, chemomechanical behavior, etc. Due to the unique properties, a wide range of industrial, medical, pharmaceutical, and prosthetic applications have been proposed.

Application of a hydrogel as a mechanical device is fairly limited due to its lack in mechanical strength. Many gel researchers have thought that the mechanical weakness is unavoidable because of its solution-like nature, i.e., low density of polymer chains and small friction between the chains. Furthermore, it is well known that in synthetic gels are inhomogeneous in structure, which is considered as a factor to decrease the mechanical strength. However, if we pay attention to biological systems, we find some hydrogels, such as a cartilage, with excellent mechanical performances. It is a challenging problem in modern gel science to fill the gap between the man-made gels and the biological gels.

Another interesting problem of a gel is its surface property. Few is known of the surface properties of a gel although we observe fascinating surface behavior of bio-organs. For example, the extracellular mucins, which comprise a family of high molecular weight, extensively glycosylated glycoproteins, are crucial to the biological activity, which relates to lubrication and protection of cell surfaces from damage. Another example is the animal cartilage, which sustains a daily compression of 100kg/cm2 and has an extremely low friction coefficient.

Two topics regarding to the mechanical properties of a gel, as a soft and wet matter, will be addressed in the seminar. The first is how to produce a hydrogel with an excellent mechanical roughness, and the second is what is the friction law that a gel obeys.