The joints in human elbows, knees and the like exhibit very little friction even at moderately high pressure — man-made materials can offer nothing as good. Zwitterions might put that right.
Zwitterions are molecules with discrete positive and negative charges in different places. Jacob Klein of the University of Oxford, UK, and his colleagues have created polymer 'brushes' made of zwitterionic phosphorylcholine, in which the multiple positive and negative charges strongly attract water molecules, and attached them firmly to mica surfaces. The result is a system with very low friction when the surfaces move against each other, probably because the water molecules clinging to the phosphorylcholines prevent the brushes becoming entangled. The bound water can exchange freely with other water molecules, which also reduces friction.
This work might have application in biomedical devices where friction is often a problem.