Date Published:
9 November 2006 |
A better way of lubricating human joints and implants
Researchers at the University of Oxford have discovered that certain lubricants
reduce friction much more effectively in water or water-based solutions than
in machine oil or air, which may be how the process works in biological systems
as well.
" Boundary lubrication is common in machines but is also thought
to act between joints and other living organs in the form of phospholipid
films,"
said Professor Jacob Klein.
" This new mechanism could lead to better
lubricated artificial implants, as well as to more effective treatments for
joint problems
like osteoarthritis."
Professor Klein and his colleagues at the Physical and Theoretical Chemistry
Laboratory at Oxford reported their discoveries in the 9 November issue of
the journal Nature.
For more than fifty years, films or layers which are one molecule thick have
been used in air or oil to lubricate surfaces which rub together, reducing
friction and wear. These layers have usually belonged to the class of amphiphilic
surfactants, whose head is water-loving, while their tail is water-repelling.
" Each of the rubbing surfaces is coated by a “boundary” layer
of surfactant molecules, with charged heads that stick to the surface while
their hydrocarbon tails dangle out," explained Professor Klein.
" In
classical boundary lubrication in air or oil, the rubbing occurs between
these protective tails and greatly reduces friction and wear. "
The Oxford researchers studied the friction between mica surfaces in the different
environments, with and without overlaying surfactants. They have shown that
the friction stress between two sliding surface coated by surfactant monolayers
can decrease much more in water than in air or oil, falling to one percent
or less of its value for the latter environments.
" We believe this happens because the charged head groups then become
hydrated, that is, coated with water molecules," said Professor Klein.
" This
enables them to slide much more easily past the substrate than the hydrocarbon
tails can slide past each other. As a result, the slip occurs at the substrate,
rather than between the surfactant tails as in the classical mechanism."
The researchers proved that the hydration of the anchors must be largely responsible
for the reduction in friction by testing surfactants which were homologous
to the original but could not be fully hydrated at the surface because of their
structures, which resulted in greater friction. They also eliminated the possibility
of this occurring due to the flipping of the surfactants’ anchors when
they came into contact with water by performing the same experiments on surfaces
which were brought into adhesive contact before being immersed, so that the
anchors could not flip.
Reference: The paper 'Boundary lubrication under water' by Wuge H Briscoe,
Simon Titmuss, Fredrik Tiberg, Robert K Thomas, Duncan J McGillivray and Jacob
Klein
is published
in Nature on 9 November 2006.
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