Cartilage is an extraordinary material, both in terms of its impressive lubricious properties and the fact that it continues to function, without a blood supply, for many decades, providing very low friction coefficients. In the simplest terms, cartilage consists of a hydrogel material with a stiffness gradient that interfaces to bone, attached, at the outer edge, to loose polysaccharide chains, which are thought to provide a lubricating function (1). Polymer brushes, which bear a resemblance to these loose chains, are well known for their lubricious properties, but when coating hard-hard contacts, minor disturbances in tribological conditions or the inclusion of foreign bodies, can rapidly lead to catastrophic failure, as asperities on one hard countersurface encounter the opposing brush. This problem is significantly reduced when the underlying substrate is soft, as in the cartilage case. When imitating cartilage, elastomers can provide this soft base layer, but an even more effective substrate for brushes in tribological applications is a gel. These can be readily tailored to ensure compatibility with the brush, and provide a number of cushioning functions, including elastic, viscoelastic, and porelastic, depending on the loading conditions. In our laboratory, we have explored a variety of systems for imitating cartilage, some of which have actually reached comparable friction coefficients to those observed in cartilage (2), as well as toughness values and wear resistance that render them of interest for medical and industrial applications.
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