Lubricity of surface hydrogel layers

  • Dunn A
  • Urueña J
  • Huo Y
 et al. 
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Many biological interfaces provide low friction aqueous lubrication
through the generation and maintenance of a high water content polymeric
surface gel. The lubricity of such gels is often attributed to their
high water content, high water permeability, low elastic modulus, and
their ability to promote a water film at the sliding interface. Such
biological systems are frequently characterized as ``soft,{''} where the
elastic moduli are on the order of megapascals or even kilopascals. In
an effort to explore the efficacy of such systems to provide lubricity,
a thin and soft hydrogel surface layer (similar to 5 mu m in thickness)
with a water content of over > 80 % was constructed on a silicone
hydrogel contact lens, which has a water content of approximately 33 %.
Nanoindentation measurements with colloidal probes on atomic force
microscopy (AFM) cantilevers revealed an exceedingly soft elastic
modulus of similar to 25 kPa. Microtribological experiments at low
contact pressures (6-30 kPa) and at slow sliding speeds (5-200 mu m/s)
gave average friction coefficients below mu = 0.02. However, at higher
contact pressures, the gel collapsed and friction loops showed a
pronounced stick-slip behavior with breakloose or static friction
coefficient above mu = 0.5. Thus, the ability of the soft surface
hydrogel layers to provide lubricity is dependent on their ability to
support the applied pressure without dehydrating. These transitions were
found to be reversible and experiments with different radii probes
revealed that the transition pressures to be on the order of 10-20 kPa.

Author-supplied keywords

  • Contact lenses
  • Hydrogels
  • Lubricity
  • Microtribology
  • Stick-slip
  • Surface modulus

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