Reducing Friction by Control of Isoelectric Point: A Potential Method to Design Artificial Cartilage

Relative motion between artificial cartilages is too slow to form a sufficient lubricating film for hydrodynamic lubrication, and this can result in large friction and wear. Hydration lubrication is effective in reducing friction at low speeds, and the lubricity is mostly determined by surface charges, which can be assessed by isoelectric point (IEP). Therefore, by controlling the IEP of friction pairs, ultralow friction can be achieved even with weak hydrodynamic lubrication. Here, a negatively charged material is synthesized by grafting carboxylate anions into an epoxy resin matrix. Compared with pristine epoxy resin (IEP = 6.0), the modified epoxy resins (IEP = 3.8, IEP = 3.0) show better tribological properties. In particular, at low speeds (4.7 and 1.2 mm s −1 ), the friction coefficients are as low as 0.02–0.04, and the average contact pressures are ≈10 MPa, when a NaCl solutions are used as the lubricants. Furthermore, the addition of hydroxyapatite to the modified epoxy resin not only increases its bioactivity, but also reduces friction. Superlubricity can be achieved for the speeds higher than 23.5 mm s −1 . These results provide a method to design artificial cartilage with good tribological and bearing capacity.