The various biocompatible artificial materials such as metallic, inorganic, polymeric and composite ones have been used for clinical application of medical devices. Furthermore, biological tissues and regenerated tissues have been applied. In this review paper, the importance of tribology in biomedical materials and biomaterials is discussed including the viewpoint of benefit and risk in medical devices with rubbing surfaces and interfaces.

Dry eyes have been recently epidemic with the number of contact lenses user or LASIK (laser-assisted in situ keratomileusis) operations. Basic treatment for the dry eye is eye drops. The lubricating effect by eye drops is important because the high friction causes dry eye complaint. In the present review, tribology on lachrymal fluid and eye drops is explained in relation to the pathology of dry eyes and the clinical treatment. The tribological conditions on ocular surfaces are shown. Wedge film effect during a blink motion is estimated on the lid wiper. The friction measurements using for the assessment of the eye drop are explained.

The bearing surfaces of a natural synovial joint are covered with a specialized type of hyaline cartilage, termed articular cartilage, which protects the joint interface from mechanical wear and facilitates a smooth motion of joints during daily activity. Articular cartilage consists of chondrocytes surrounded by extracellular matrix macromolecules and surface active phospholipids. Owing to the charge on these molecules, they can trap water to maintain the water-fluid and electrolyte balance within the articular cartilage tissue, making it highly hydrophilic and providing an effective boundary lubricant. Based on an inspiration from nature, biomimetic technology has been a highly successful approach to producing artificial tissues and implants. Therefore, the strategy of investigating and then reproducing the natural bearing surfaces in artificial joints by using a phospholipid polymer in order to mimic the role of cartilage has great potential. In this review, we introduce the development and clinical application of the cross-linked polyethylene modified by grafting with the poly(2-methacryloyloxyethyl phosphorylcholine) for joint replacement. Such works are of great importance in the design of lubricated surfaces for artificial joints, and in better understanding the lubrication mechanisms of both natural and artificial joints.

Low friction and low leakage of blood are simultaneously required for mechanical seal for blood. Friction property of mechanical seal was investigated by means of specialized test apparatus for mechanical seal of actual ventricular assist device. It was found that formed protein film on sealing surfaces contributes to generate high and unstable friction property with unique periodic frictional peak. Fundamental analysis of protein film clarified that denatured protein film is formed on friction region and high shear strength induces high friction coefficient in blood. Therefore, running-in was introduced to SiC/SiC-made mechanical seal to form hydrophilic layer, which prevents protein adsorption, on sealing property and low and stable friction in blood was realized. Furthermore, to realize selective protein adsorption on sealing surface, DLC coating was introduced on textured mechanical seal and stabilized friction coefficient was also realized.

To clarify the low friction mechanism of CNx coating in PAO oil lubrication, we measured reflectance spectrum at friction area in-situ by reflectance spectroscopy during friction test against a sapphire hemi-sphere pin in PAO oil. We observed and estimated thickness of PAO oil film, surface roughness of CNx coating, thicknesses and polarizability of transformed layer formed on bulk CNx coating during friction test by analyzing reflectance spectrum. We proposed that the decreasing of load ratio of boundary lubrication mode to total load is the one of governing factor to reduce friction on the basis of the increasing of Λ value. Also we proposed the increasing of adsorption of PAO oil to the CNx transformed layer is the second governing factor on the basis of the increasing of polarizability of PAO oil and transformed layer, that leads to the increasing of van der Waals force between transformed layer and PAO oil as low friction mechanism of CNx in PAO oil lubrication.

The high-pressure viscosity of 23 kinds of VII-blended oil was measured. Using those high-pressure viscosity, pressure-viscosity coefficient α_ot-Bl-obs under the atmospheric pressure and α_B-Bl-obs under the pressure were decided using Sargent equation and Barus equation, respectively. Then, correlations between α_Bl-obs and pressure-viscosity coefficient α_Bf-obs of the base oil were investigated. Obvious quantitative relations were found in those two α_Bl-obs - α_Bf-obs relations. Each equation to calculate α_ot-Bl-obs and α_B-Bl-obs was derived. Those equations consist of three parameters, pressure-viscosity coefficient α_Pm of polymer coil itself, each α_Bf-obs and polymer concentration w_Pm (wt%). Furthermore, the following knowledge and results were provided. The polymer pressure-viscosity coefficient α_Pm was considered to be a universal numerical characteristic not to be affected by difference in compound type and molecular weight of the polymer, pressure and oil temperature. It was interpreted that the polymer coil performed two functions at the same time. One is a function as the formation nucleus of the hydrodynamic rigid sphere on viscosity increase, and other one is a function to produce α_Pm mentioned above.

Using high-pressure viscosity measurements of 23 kinds of VII-blended oil reported in Part 1, a high-pressure viscosity prediction formula for VII-blended oil was derived. This equation is expressed in a so-called Barus formula, and the pressure-viscosity coefficient is the secant pressure-viscosity coefficient α_B(p)-Bl (= ln(η_pt / η_ot)/P) in each ln(η_pt)‒P relation curve. The calculation formula of α_B(p)-Bl is composed of the corresponding base oil value α_B(p)-Bf, the polymer coil itself pressure-viscosity coefficient α_Pm and the polymer concentration w_Pm (wt%). In relation to mentioned above, the volume fraction of one polymer molecule in hydrodynamically equivalent sphere, and the volume fraction of the hydrodynamic volume occupied in 100cm³ of VII-blended oil at critical concentration c* of polymer coil were investigated. It was also found that the viscosity index VI_o-Bl in the atmospheric pressure of the VII-blended oil drops with pressure. For this reason, the effect of temperature on the ln(η_pt)‒P relation curve, the influence of polymer type and base oil in η_i‒P‒t relationship, and the influence of polymer type on atmospheric pressure viscosity η_ot ‒t relationship were discussed.