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Soft, Slimy, Sliding Interfaces: Biotribological Properties of Mucins and Mucus gels

Final Report Summary - 3S-BTMUC (Soft, Slimy, Sliding Interfaces: Biotribological Properties of Mucins and Mucus gels)

Mucins are a family of high-molecular-weight glycoproteins and a major macromolecular constituent in slimy mucus gels that are covering the surfaces of internal biological tissues. One of the most often commented, yet poorly understood characteristics of mucus gels is probably its slipperiness. Detailed and systematic understanding of the lubrication mechanisms of mucins and mucus gels is very important from many angles, such as understanding the fundamental functions of various mucus-covered internal organs (e.g. eye blinking, mastication in oral cavity), proper use of tissue-contacting devices or personal care products (e.g. catheters, endoscopes, contact lenses), and potential biomimicry of its remarkable lubricating performance based on water. This project started with multiple aims of (1) to establish the lubrication mechanisms of mucins and mucus gels in accordance to origin, structure, composition, and environment (2) to understand the interaction of mucins and mucus gels with other bio- and synthetic molecules as well as biomedical devices, (3) to develop artificial soft biological tissues, i.e. soft substrates covered with slimy and slippery mucus gel layers so that they can be used in biomedical simulations, and finally (4) to apply the lubrication principles of mucins to man-made systems to utilize water as eco- and environment-friendly, “green” lubricant.
The central achievement of the project is establishment of fundamental understanding on the lubrication mechanisms of mucins and mucus, as planned. It is important to stress that not all mucins were observed to be slippery; for example, salivary mucin is slippery nearly at all conditions in aqueous solutions whereas gastric mucin is not slippery unless the environmental pH becomes acidic (pH 3 or less). This is a typical example where the lubricity of mucins can be tailored by environmental parameters. Other parameters that affect the lubricity of mucins are temperature and ionic strength of aqueous solution, type and size of mucins, biochemical treatments of mucins, and lastly measurement parameters such as contact scale, pressure, speed and substrates. As expected, the lubricity of mucins is greatly influenced when other substances are present together in the mucin solution. In the case of proteins, such as albumin, beta-lactoglobulin, and lysozyme, the lubricating behavior is mainly ‘competitive’ in the sense that which molecules between mucins and these proteins dominate the sliding interface determines the lubricity. Meanwhile, the lubricating behavior of mucin-polymer systems was often ‘associative’ in the sense that the magnitude of associative interaction between mucin and polymers is chiefly responsible for the modified lubricity of mucins. It was most interesting to observe that mucoadhesion mechanism, i.e. well-known specific interaction between mucin and some hydrophilic polymers can be exploited to improve the lubricity of mucins. Overall, the lubricating properties of molecular mucins can be fairly well described in terms of polymer physics when its structure, conformation, and surface adsorption behavior are clearly understood. Meanwhile, the lubricating properties of mucus were much more complex. Mucus is a non-Newtonian fluid but its lubricating behavior cannot be described as any other widely studied lubricant fluids, such as oils, emulsions, or greases. For instance, wettability (hydrophilicity) of the substrate (tribopair) was observed to be very important to ensure effective lubricating properties of mucus, and this may contrast its effective lubricity at biological tissues. An excellent mimicry of biological lubrication systems was also achieved in this project by formulating silicone matrix that can segregate slippery hydrophilic polymer chains in aqueous environment from internal source. For this reason, it can self-restore the structure and functionality of slippery polymer chains nearly indefinitely in case of any damage occurring from the operation.
Along with the established knowledge on the lubrication mechanisms of mucin and mucus, the project provided various technological templates that, we believe, are very useful for a broader range of scientific disciplines related to mucin, polymer, and soft matter sciences. Representative examples include: (a) formulation of artificial mucus (or mucus models) based on aggregation between mucin and polymers that are mimicking mucus not only its rheological but also tribological properties, (b) formulation of mucin coatings that show excellent non-fouling properties, which is superior to many synthetic polymer based coatings, (c) application of mucins for prosthetic joint implants as added lubricants for both fundamental science and biomedical application points of view, (d) development of in-vitro slipperiness tester of urinary catheters, (e) elucidation of the interaction between mucin and dairy proteins in the context of tribological behavior, (f) development of electrospun microfibers for drug delivery modeling and use of mucins as components (g) development of silicone matrix that self-restores hydrophilic polymer brushes under tribological stress and potential damages.