Role of ions concentration and pH in the tribological operation of phospholipid biomimetic surfaces
Résumé
Since the 50s many studies have tried to understand the remarkable tribological performance of biological rubbing contacts. These performances were initially correlated with hydrodynamic lubrication which was supposed to locate the velocity accommodation within a homogeneous and continuous fluid film. Then the difficulties associated with the rheological characterization of the "biological lubricants" have led to study the molecular composition of these lubricants, especially that of molecular interfaces they create with biological tissues. Therefore, recent studies [Trunfio et al., Langmuir, 2008] have shown the importance of surfactant-type molecules (known as surface active phospholipids - called thereafter SAPL) on obtaining molecular surface layers with high mechanical strength and capable of locating the velocity accommodation, leading to a very low friction coefficient. These molecular layers are indeed made of stacks of 3 to 7 SAPL bilayers, with a thickness of approximately 5nm each and separated by aqueous layers (a few nm thick) with about 150mM ion concentration and the pH around 7 in the healthy case. Recent tribological studies showed that the low friction coefficients are due to the location of the velocity accommodation in the aqueous layers [Klein, Science, 2002]. Thus the changes in ion concentrations or the presence of electrically charged SAPL can significantly influence the friction coefficient. Therefore, this work aims to study the influence of ion concentration and pH on the evolution of friction and the degradation of SAPL bilayers during tests carried on an experimental realistic tribological model. This model uses hydrogels type 2-hydroxyethyl methacrylate (HEMA) to reproduce ex vivo the mechanical and physico-chemical properties of biological tissues. Nanostructural physical techniques such as lipid deposition by co-adsorption of lipid / detergent micelles [Tiberg, BBA, 2005] and atomic force microscopy are used to reproduce and characterize the nano-mechanical resistance of SAPL bilayers forming biological rubbing surfaces. The evolution of SAPL bilayers is visualized in situ, during friction tests (velocity: 0.5 mm/s; contact pressure 0.3 MPa), by fluorescence optical microscopy using specific molecular markers. To study the influence of pH, three tris(hydroxymethyl)aminomethane (TRIS) buffer solutions at pH 3, 7.2 and 9 are used. The influence of ion concentration was tested using pH 7.2 TRIS buffer solutions with ionic concentrations of 0, 50 or 150 mM monovalent NaCl and 20mM divalent CaCl2 (being known the presence of different ions in biological fluids in order to regulate different biological processes). The main experimental results are summarized below: - Using non-buffered solution gives to SAPL bilayers low mechanical strength to the nanoindentation (about 100% of the bilayers do not resist to normal pressures between a few tenths of MPa and few MPa.), which promotes their degradation in friction tests and the increase in the friction coefficient by about 80% after 1 hour of friction. - Using buffered solutions (pH 7.2) increases the mechanical strength of the SAPL bilayers (the bilayers are broken in only 20% of the cases for non-ionic solution and 0% for ionic solution; in these cases they resist to pressures over 20MPa) which prevents their degradation and stabilizes the friction coefficient to a value depending on the pH and ionic concentration. The low pH (~ 3) promotes friction coefficients up to 5 times smaller than the higher pH (7, 9). The ionic concentration decreases by about 1.5 times the friction coefficient compared to a non-ionic solution. There were not noticed significant differences between the use of monovalent (NaCl) or divalent (CaCl2) ions. This study therefore shows that good mechanical resistance of the bilayers (characterized by indentation tests) is essential to obtain low friction and suggests that low friction is ensured by ion hydration layers between adjacent SAPL bilayers. It also suggests an important role of the ions and the buffered medium in maintaining stable bilayers.