Biological systems show fascinating tribological behaviors,as can be seen from the extremely low friction between animal cartilages,and between eye and eyelid,etc.It is believed that the soft and wet nature of soft bio-tissues is responsible for these low friction behaviors but the true mechanism is not fully clarified.Hydrogels are a class of soft and wet materials,similar to the bio-tissues.Recently,due to the invention of tough hydrogels,hydrogels are drawing great attention as model substances or substitutes of load-bearing bio-tissues.