Abstract
Lipids have an important role in the complex lubrication of articulating joints, however changes in lipid phase behavior that occur owing to mechanical confinement are not well understood. Here, a surface force-type apparatus has been combined with neutron reflectometry to measure confinement-induced changes in the structure of lipids, the major surface-active component of the lubricant in articulating joints. The same incompressible state was accessed under low uniaxial stress (1 bar), irrespective of whether the lipids had started out unconfined above or below the Lα phase transition, and irrespective of whether they were fully or partially hydrated. In this incompressible state, the lipid component had thickened indicating extension and rearrangement of the lipid chains in response to the applied stress. The small amount of water remaining between each lipid bilayer was found to be similar for all chain lengths and starting phases. This represents the first structural evidence of the tightly bound water layer at the headgroups, which is required for hydration lubrication under load.
Highlights
In contrast to mechanical devices, which are mostly lubricated with oil, nature lubricates exclusively with water
It has been shown that zwitterionic phosphatidylcholine (PC) lipids (>60% of all lipids) are a significant surface active (∼13%) constituent of synovial fluid [5, 23], forming a stack of approximately 3–7 bilayers at the cartilage surface [39]
We model these surface lipids using stacks of lipid bilayers with PC head groups and a range of saturated chain lengths from dilauryl (12:0) to distearyl (18:0) chains; the lipids are spin coated onto silicon substrates
Summary
In contrast to mechanical devices, which are mostly lubricated with oil, nature lubricates exclusively with water. It has been extensively shown that nature overcomes this with the addition of biological molecules, with the ability to modify surfaces to make them far more lubricating at slow speeds and under the high loads experienced in human joints [1, 2]. A lowering of the friction coefficient was found when all components were included, with the phospholipids interpreted to be located on the outer surface of the lubricin-HA layer. It is still unclear exactly in what form the phospholipids within the synovial fluid exist, they could form vesicles [6, 8], alternatively some authors propose that a single surfactant like layer forms on each cartilage surface
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