Literature

It may seem surprising that the liquid with which we are most familiar is actually the most unusual. It expands as it changes its state from liquid to solid; it has a high thermal capacity which means it can absorb heat energy without large temperature increases; it is an amazing solvent, etc. These properties, and many more, are exploited to the full by life forms - we cannot envisage life without water. Yet there are surprises:

"Water, together with surfaces containing sugar chains, forms the basis of all biological lubrication systems, from the slithering of a snail to the passage of food along the digestive tract. Yet humans have typically lubricated their machines with oils and fats. [. . .] Water on its own is, however, generally a poor lubricant, and unlike oil, its viscosity does not rise substantially with pressure. This property is essential to the mechanism by which oils can form a lubricating film in high-pressure, nonconformal contacts of hard materials such as gears or ball bearings."

Slugs produce a water-based lubricating film to facilitate movement

Biological lubrication then is something worthy of our attention. "Interest in water lubrication is also high in the food, textile, and pharmaceutical industries, where product contamination by oil is a concern". How does water provide lubrication for organisms?

"The low viscosity of water at high pressures can be overcome by biological lubricant additives, usually glycoproteins, in which large numbers of sugar chains are bound along a protein backbone. For example, mucins are found in most parts of the human body that need lubricating, such as eyes and knees. These molecules probably aid lubrication both via their intrinsic viscoelastic properties in solution and via their behavior when adsorbed on the sliding surfaces. The characteristic bottlebrush structure of the molecules is crucial to this mechanism: The hydrophilic sugars immobilize large amounts of water within the contact region, while the backbone interconnects to other bottlebrushes or to a surface."

These brushlike molecules are being actively studied, and the interfaces that develop between them and soft surfaces.

"One biomimetic approach is to decorate the sliding surfaces with a high density of brush-forming polymer chains. Klein et al. have shown that when two mica surfaces bearing polymer brushes are rubbed past each other under compression in "good solvents," the interfacial friction forces lie below the detection limit. The remarkable lubricating effect of such hairy polymer layers is ascribed to interchain repulsion, which leads to the incorporation of large quantities of solvent. The resulting fluid-like cushioning layer on the surface can sustain the externally applied pressure, thereby lowering the friction forces."

We are still a long way from mastering the technology, but progress is being made. "The practical implementation of biomimetic, aqueous lubrication approaches may become a reality in the not-too-distant future". The key thought here is that water-based lubricants are being inspired by biological systems. The problem is far from trivial: engineered solutions are essential. For us, it is a case of designing suitable brush-forming polymer chains. Some would have us believe that these "remarkable" lubricating effects in living things have come about by Darwinian mechanisms of mutation and natural selection. This is not because there is any evidence of natural selection doing anything remotely like this, but because their dogma demands it.

Sweet, Hairy, Soft, and Slippery
Seunghwan Lee and Nicholas D. Spencer
Science, 319, 1 February 2008: 575-576.

First paragraph: Water, together with surfaces containing sugar chains, forms the basis of all biological lubrication systems, from the slithering of a snail to the passage of food along the digestive tract. Yet humans have typically lubricated their machines with oils and fats. Understanding of biological lubrication has now advanced to the point where these principles can be applied to systems of technological importance using synthetic polymers.