Inspired by natural articular cartilage, Tsinghua University research team collaborates to develop self-healing hydrating lubricating surface

  For many delicate and critical mechanical parts, the problems caused by friction and wear cannot be underestimated. It is not only one of the important reasons for the failure of these parts, but also sometimes causes the whole system to fail.
  In the field of implanted interventional medical devices such as artificial joints, because such medical devices will directly contact and rub against human tissue during the working process, once these medical devices do not have good lubricating properties, it is usually very likely to cause patients with In severe cases, it may even lead to further tissue damage and lesions. In addition, if the medical device is worn, the surface is more likely to be contaminated by bacteria, this risk is especially serious for those medical devices that are implanted in the body for a long time, because over time these medical devices can further lead to blood clots, stones, etc. complication.
  Therefore, medical devices with a well lubricated surface undoubtedly have better characteristics, which can not only reduce the damage to human tissue caused by friction, but also bring less discomfort to the patient and less chance of complications. .
  Good lubricating properties are crucial for implanted interventional medical devices such as artificial joints.
  Previously, there have been various studies to improve the lubricating properties of medical devices. For example, the surface coating technology based on covalent bond modification has greatly improved its friction performance, but this coating is still insufficient in the face of long-term friction.
  And “hydrated lubricating surfaces” have the potential to improve the performance of medical devices in the face of long-term friction through ingenious design. We know that natural articular cartilage has very good lubricating properties. The reason for this is that there are charged biological macromolecules on the surface of articular cartilage, which can form a “hydration lubrication” mechanism.
  The so-called “hydration lubrication” refers to the adsorption of water molecules on the surrounding of certain biological macromolecules due to the inherent dipole moment to form a hydration layer. The hydration layer not only has high spatial stability, but also has a large carrying capacity, and the water molecules in the hydration layer can be quickly exchanged with other free water molecules, thereby maintaining rapid relaxation efficiency. In shear, it exhibits better fluid effects and a smaller coefficient of friction. In hydration lubrication, the friction of two surfaces is converted into friction between water molecules on the two surfaces by ion-dipole action. Polymethacryloyloxyethylphosphorylcholine is a good hydrating lubricating material. However, the research group found that the hydrated and lubricated molecular brush structure of the material is damaged by friction over time, leading to wear on the surface of the material.

Schematic diagram of the irreversible wear of the covalently modified surface and the dynamic repair of the self-assembled modified surface

  Recently, a research group from Tsinghua University has designed a repairable hydrated lubricating surface inspired by the lubricating mechanism of natural articular cartilage, which is expected to solve this problem.
  In this study titled “Supramolecular Repair of Hydrated Lubricated Surfaces” published in Chem by Associate Researcher Zhang Hongyu of Tsinghua University and the team of Professor Paul McGonigal of Durham University, UK, the research team achieved The “supramolecular host-guest chemistry” is introduced into the molecular brush structure of the material, which cleverly realizes the repairable function of the hydrated and lubricated surface of the material.
  The repairable function in the study is mainly achieved through “supramolecular action”, that is, combining the main part of the lubricating surface with the initiator of free radicals and further initiating the in situ polymer, so that the host molecule has The molecular brush structure was prepared. Next, the guest molecules are modified on the surface of the substrate, so that the molecular brush structure is “installed” on the surface of the substrate with the interaction of the host and the guest. In this way, when the lubricating layer on the surface is decomposed and separated due to friction, the polymer molecules with lubricating ability will reassemble with the guest molecules on the modified surface, so that the lubricating properties of the surface can be restored.

20μm × 20μm wear area shown by laser confocal microscopy after reciprocating friction on the surface using AFM contact mode

  The hydrated lubricating surface with repairable function has also been confirmed by infrared spectroscopy, XPS spectroscopy and other methods. In performance tests, the coefficient of friction and repairability of this hydrated lubricated surface were also evaluated.
  To show the repairable lubricating properties of the material, the researchers introduced fluorescent molecules into the material, soaked the non-fluorescent polymer material in a solution containing the fluorescent substance, and rubbed it locally for a long time. As can be seen from the figure above, in AFM contact mode, where the tip of the probe is used to bond the bare polystyrene microspheres, the worn surface is self-healing after rubbing the surface and vice versa .