The major synovial joints such as hips and knees are uniquely efficient tribological systems, able to articulate over a wide range of shear rates with a friction coefficient between the sliding cartilage surfaces as low as 0.001 up to pressures of more than 100 atm. No human-made material can match this. The means by which such surfaces maintain their very low friction has been intensively studied for decades and has been attributed to fluid-film and boundary lubrication. Here, we focus especially on the latter: the reduction of friction by molecular layers at the sliding cartilage surfaces. In particular, we discuss such lubrication in the light of very recent advances in our understanding of boundary effects in aqueous media based on the paradigms of hydration lubrication and of the synergism between different molecular components of the synovial joints (namely hyaluronan, lubricin, and phospholipids) in enabling this lubrication.

Macromolecules, which adsorb or intrinsically form boundary layers at surfaces sliding past each other in aqueous media, are ubiquitous both in technology and in biological systems and can form effective boundary lubricants. Over the past decade or so, hydration layersrobustly bound water molecules that surround charges or zwitterionic groups of different macromolecular specieshave been identified as remarkable lubricating elements, sustaining high loads while exhibiting a fluid-like response to shear with extremely low friction. This modification of frictional forces in aqueous systems, based on the behavior of water molecules confined to hydration shells, is the central idea behind the hydration lubrication mechanism, which is presented and discussed in detail in the current Perspective. We describe the nature of hydration under confinement and the underlying experiments revealing this mechanism, focusing in particular on synthetic and biological macromolecules attached to surfaces and on phospholipid assemblies. We also emphasize these recent findings in relation to physiological environments and functions of the human body, such as cartilage lubrication, in which hydration lubrication is believed to play an important role.

A number of catechol-terminated copolymers of di(ethylene glycol) methyl ether methacrylate (MEO 2 MA) and poly(ethylene glycol) methyl ether methacrylate (OEGMA) with varied MEO 2 MA-to-OEGMA molar ratios were synthesized via atom transfer radical polymerization triggered by dopaminederived initiators. They were grafted on magnetite nanoparticles (NPs) via ligand exchange, thus imparting the NPs with robust colloidal stability against salt and excellent biocompatibility. Of importance is that similar to the copolymers of MEO 2 MA and OEGMA, their coated magnetic NPs showed a lower critical solution temperature. This leads to a reversible agglomeration of the resulting composite NPs in buffer and physiological solution in response to the environment temperature. This reversible and thermosensitive agglomeration were also observed within red blood cells after loading the resulting composite NPs into the cells. The agglomeration of magnetite NPs in red blood cells endowed the NP-loaded composite cells with a better magnetic response, for example, contrast enhancement for magnetic resonance imaging.

Percutaneous collagen induction therapy offers a modality with which to rejuvenate and improve skin appearance and quality without risk of dyspigmentation.

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