Relationship Between Wettability and Lubrication Characteristics of the Surfaces of Contacting Phospholipid-Based Membranes

Abstract

The wettability of the articular surface of cartilage depends on the condition of its surface active phospholipid overlay, which is structured as multi-bilayer. Based on a hypothesis that the surface of cartilage facilitates the almost frictionless lubrication of the joint, we examined the characteristics of this membrane surface entity in both its normal and degenerated conditions using a combination of atomic force microscopy, contact angle measurement, and friction test methods. The observations have led to the conclusions that (1) the acid–base equilibrium condition influences the lubrication effectiveness of the surface of cartilage and (2) the friction coefficient is significantly dependent on the hydrophobicity of the surface of the tissue, thereby confirming the hypothesis tested in this paper. Both wettability angle and interfacial energy were obtained for varying conditions of the cartilage surface both in its wet, dry and lipid-depleted conditions. The interfacial energy also increased with mole fraction of the lipid species reaching an asymptotic value after 0.6. Also, the friction coefficient was found to decrease to an asymptotic level as the wettability angle increased. The result reveal that the interfacial energy increased with pH till pH = 4.0, and then decreased from pH = 4.0 to reach equilibrium at pH = 7.0.

The Probable Explanation for the Low Friction of Natural Joints

“…The microelectrophoretic apparatus and the method are described in [36,37]. The interfacial energy was measured 12-15 times on a freshly formed lipid bilayer membrane with about 10 instrument readings of the lipid spherical cap.…”

“…3. [36], at an ambient temperature at 295 K and a relative humidity (HR) of approximately 45 %. Five tests were performed on each specimen and each set-up.…”

“…The tests were conducted at room temperature, at a speed of 1 mm/s during 15 min under a load of 15 N (1.2 MPa) [36], which correspond to physiological lubrication condition. Before each test, the cartilage samples were left for 1 h in a saline solution.…”

“…3 Phospholipidic bilayers of articular cartilage under the: a wet and b air-dry conditions. Variations in surface energy lead to conformational transformations in the surface phospholipids from bilayer (hydrophilic) to monolayer (hydrophobic) ones [36] total number of five tests were conducted using fresh samples for each experimental set-up.…”

“…Nature essentially employs the same lubrication system in many parts of the body, where tissues slide over each other with their surfaces coated by surface-active phospholipids (SAPLs) [8,9,15,16,19,20,43]. These SAPLs have attracted considerable scientific interest and further research [2,6,14,27,33,35,36,47,48,52].…”