Using a surface forces apparatus, we have measured the normal and shear forces between surfaces containing physisorbed hyaluronic acid (HA), which is a major component of synovial fluid and which is believed to play a role in joint lubrication. The adsorption mechanisms involved the ionic binding of negatively charged HA (i) to positively charged surfactant bilayers supported on mica substrates and (ii) to negatively charged mica surfaces via Ca2+ bridges. Our results show that physisorbed HA fails as a lubricant when surfaces are pressed strongly together and/or sheared due to the squeezing out of the HA from between the surfaces. On the basis of these and previous studies, we conjecture that HA might function as an effective boundary biolubricant, exhibiting both low friction and low wear, but only if it is chemically or specifically bound exclusively to each surface, but not to both surfaces.
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