Science Literature

Bone joints are remarkable for their physical properties. They are associated with very low coefficients of friction and can work effectively under high pressures. For several years, researchers have tried to emulate biological systems, with advances in understanding coming steadily. Previous blogs related to this topic are here and here. Recent research has, for the first time, matched and exceeded the performance of natural synovial joints, both in terms of the coefficient of friction achieved and in the ability to operate under pressure. The success has come through the use of zwitterionic molecules attached in specific ways to the surfaces. Zwitterions are molecules that are neutral overall but carry both positively and negatively charged groups.

Zwitterionic phosphorylcholine groups on the polymer bristles trap water which lubricates the surfaces (Source here)

Jacob Klein from the Weizmann Institute in Rehovot, Israel, and his group have made polymer 'brushes' by attaching one end of polymer chains to a host surface, leaving the other end sticking out. The polymer chain 'bristles' tend to repel each other and so stand up from the surface to form a brush-like structure. 'When two surfaces coated with polymer brushes compress against each other, they tend to squeeze within themselves rather than intermingling with each other,' explains Klein. 'This allows them to be pressed together quite hard without entangling, so the friction between them is low.' (Source here)

The reason the bristles repel each other is that they have side chains carrying zwitterions and these interact with each other and with water.

'By introducing charges to the brushes you get a hydration layer of water molecules around the charges,' explains Klein. 'These water molecules are tightly bound, in the sense that it's hard to remove them all at once, but individual molecules are able to rapidly exchange with water in the surrounding solvent or the hydration layer of another charge. This gives them the properties of molecular ball-bearings - you can press hard on them and they won't release their water, but when you come to shear them [move the surfaces over each other] they behave in a fluid way, which gives the excellent lubrication properties.'

The researchers say that their system does not have a clear analogue with cartilage surfaces, but they do point to a mechanism (highly hydrated macromolecules) that can achieve the performance of natural systems. A surface chemist and tribologist has commented:

"It's an interesting observation from the point of view of natural lubrication. There are lots of zwitterions in nature, but there are also lots of systems that don't have them and lubricate as well - it's not universal, but it might contribute to our understanding of how natural lubrication systems work."

Although this research leaves the nature of cartilage lubrication in need of further clarification, it does point to significant elements of the explanation: complex macromolecules, strong attachment to substrates, ordered side chains with bipolar molecules that interact with each other and with water, together with some distinctive physical properties of water (it is bipolar and incompressible). This is a highly ordered system, but it is not self-assembling. It needs another system to put it together - just as the research team was needed to put together the artificial system. The common element here is Intelligent Design.

Lubrication at Physiological Pressures by Polyzwitterionic Brushes
Meng Chen, Wuge H. Briscoe, Steven P. Armes, and Jacob Klein
Science, 324, 27 March 2009: 1698-1701.

Abstract: The very low sliding friction at natural synovial joints, which have friction coefficients of mu less than 0.002 at pressures up to 5 megapascals or more, has to date not been attained in any human-made joints or between model surfaces in aqueous environments. We found that surfaces in water bearing polyzwitterionic brushes that were polymerized directly from the surface can have mu values as low as 0.0004 at pressures as high as 7.5 megapascals. This extreme lubrication is attributed primarily to the strong hydration of the phosphorylcholine-like monomers that make up the robustly attached brushes, and may have relevance to a wide range of human-made aqueous lubrication situations.

See also:

Broadwith, P. 'Molecular ball-bearings' for artificial joints, Chemistry World, 26 March 2009