| Biological mucus is a type of complex fluid widely distributing on tissue surfaces of organisms and providing multi functions. Mucus is natural weak hydrogel, that is, it can flow, but possesses network structure, which determines mucus functions. In situ, mucus is often forced to flow and deform by mechanical stimuli, so rheological behaviors are important for mucus. Mucus can also effectively reduce the friction that is imposed on tissues, possessing excellent lubricating abilities. To elucidate the relationship between functions and structures of mucus, this thesis investigated constructive material, rheological behavior, supramolecular structure, and lubricating ability of loach skin mucus.Loach skin mucus, as a weak hydrogel, showed special rheological responses during steady and oscillatory shear. With increased oscillatory strain, the breakdown process of loach skin mucus could be divided into two regions. During steady flow tests, shear thickening and three-region shear thinning were observed with increased shear rate (stress), and one-region shear thinning and two hystereses were observed with decreased shear rate (stress). With increased shear time under certain shear rates (stresses), relatively low shear rate (stress) led to rheopexy, while relatively high shear rate (stress) led to thixotropy. Interestingly, the rheopexy-thixotropy transition coincided with the first-second shear thinning region transition.This study developed a convenient method to isolate the constructive material of loach skin mucus, that is, loach skin mucin (LSM). Amino acids and sialic acid accounted for 30 wt% and 5 wt% of LSM, respectively. Sialic acid contains carboxyl, which rendered LSM negatively charged in water. LSM showed linear shapes, with lengths ranging from 100 to 1000 nm and widths around 15 nm. LSM could form hydrophobic associations in water, which led to the formation of a weak hydrogel like crude mucus under physiological concentrations. The molecular structure and properties of LSM were almost same with those of mammalian mucins, which indicated that LSM and mammalian mucins were of the same category.In water, LSM could self-assemble into micron-scale fibrils along molecular axes. Fibrils could hydrophobically aggregate along lateral direction, which led to branched fibrils and fibrillar network with increased LSM concentration. Within fibrillar network, lateral hydrophobic aggregations existed as fibrillar bundles, with diameters ranging from several ten to several hundred nanometers. Bundles connected different fibrils, and thus acted as crosslinking regions. Between bundles, loose LSM could be visualized, which did not or only slightly laterally aggregated. Similar with crude mucus, LSM fibrillar network exhibited two rheological responses, which corresponded well with the two structures (i.e., bundles and loose fibrils between bundles) within the network. Given that LSM was same with most mucins in both properties and molecular structure, the fibrillar network reported herein should be the gel structure of most biological mucus.Dissolved in water, LSM exhibited excellent lubricating abilities, and effectively reduced boundary friction coefficients of both hydrophobic and hydrophilic polydimethylsiloxane (PDMS) rubber surfaces. Boundary lubricating ability of LSM gradually worsened with increased rubbing time between hydrophilic PDMS, but kept constant between hydrophobic PDMS. Different with traditional polymer lubricants, the boundary lubricating ability of LSM solution could not attribute to the tightly adsorbed layer on PDMS. Only at concentration high enough for the formation of intermolecular associations in bulk solution, LSM solution could reduce boundary friction of PDMS. After destroying the inter-LSM associations with surfactant, the boundary lubricating ability of LSM solution disappeared. This study disclosed that boundary lubricating ability of biological mucus originates from intermolecular associations of mucin. |
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