Mechanisms of Mesothelial Tissue Lubrication

In the pleural space, sliding between the lung and chest wall induces shear stress that could damage the delicate mesothelial cells covering the tissue surfaces. Normally, the pleural space, which is filled with fluid, is able to sustain continuous shear loading throughout its lifetime. To understand the mechanisms in preventing frictional damage on mesothelial tissue, we conducted experiments using abdominal tissue excised from a rat. We allowed the tissue to slide against a glass surface, and measured the fluid thickness and shear force between them. We also studied independent variables such as location on the tissue, applied normal load, sliding velocity and direction to determine their effects on mesothelial tissue lubrication. Both thickening and thinning of the fluid layer were observed during sliding. The fluid thickness was found to change with sliding velocity and direction, but invariant with location on tissue surface. In tribological experiments, shear force decreased with increasing velocity until it reached a minimum value varying with different tissue samples. Normal load had a strong effect on shear force, but not on friction coefficient. Overall, the friction curves had similar shape as described by the mixed/elastohydrodynamic regions of the Stribeck curve. Results were consistent within each tissue sample, but varied among samples. The dependency on velocity and direction suggested elasto-hydrodynamic lubrication. Taken together, we conclude that elasto-hydrodynamic lubrication is likely to be an important lubrication mechanism for mesothelial tissue sliding in the pleural region. Our findings support the existence of a continuous fluid layer separating the pleural surfaces. The fluid pressure gradient generated by surface roughness redistributes fluid from thick to thin regions preventing surface contact. Thesis Supervisor: Stephen S. Loring Title: Associate Professor of Anesthesia, Harvard Medical School