A Self‐Assembled Binary Protein Model Explains High‐Performance Salivary Lubrication from Macro to Nanoscale

Abstract

Salivary pellicle, a spontaneously formed, intricate architecture in the human oral cavity, is a high‐performance bio‐lubricant that coats and protects biological surfaces with varying elastic modulus against frictional damage. Although salivary lubrication underpins the fundamentals of human feeding and speech, the peculiar molecular mechanism behind such lubrication properties remains elusive. For the first time, this work demonstrates a binary model comprised of salivary proteins, mucin, and lactoferrin (LF), forming an electrostatically driven, multilayered self‐assembly that exhibits a lubrication behavior closely resembling that of human saliva, from macro to nanoscale. The multiscale tribological analysis with applied forces ranging from 1 N to 1 nN, supported by real‐time self‐assembly monitoring on hydrophilic and hydrophobic substrates differentially resolves the distinct roles played by the salivary proteins of this proposed lubricating model. Evidences reveal that hydrated mucin controls the macromolecular viscous lubrication entrapping water molecules in the mucinous network and LF acts as a “molecular glue” between mucin–mucin and mucin–surface, latter aiding boundary lubrication. This study puts forward an unprecedented molecular model that explains the synergistic lubrication by salivary components. These results can aid into the design routes for synthesizing highly efficacious nature‐inspired aqueous lubricants for future biomedical applications and nutritional technologies.

Quantify the Protein–Protein Interaction Effects on Adsorption Related Lubricating Behaviors of α-Amylase on a Glass Surface

“…Finally, the comparison between the secondary structure change induced during adsorption treatment at varying bulk protein solution concentrations and the coefficient of friction on the interacting surface was represented with a bar graph in Figure 5. Friction forces were measured using the unidirectional rotating pin-on-disk tribometer (polydimethylsiloxane (PDMS)-glass contact surfaces) between the upper PDMS stage operated in a normal force like the soft tissue in the mouth during mastication (i.e., 0.5N) [41,74] and the lower stage with the proteins-covered glass surface, representing the dental ceramic materials. As shown, we could see clearly that the coefficient of friction between the interacting surface was increasing with the increased secondary structural changes as presented by the reduced β-sheet content (%).…”

“…A friction coefficient is a dimensionless number that can be defined as the ratio of the friction force between two surfaces and the force holding them together [39]. Tribometers measure the friction coefficients in polymeric analogs of soft tissue within the mouth sliding/rolling against the hard surfaces of teeth or artificial dental material [28,40,41], with the friction coefficient reduced by introducing a lubricant film between two solid surfaces at a constant load [39]. In vitro tribological studies further suggested that the friction coefficient of the material surface was reduced when the wear resistance of the dental materials was improved [42][43][44].…”

Probing the frictional properties of soft materials at the nanoscale

“…An electrostatically-driven self-assembly between negativelycharged mucin and non-mucinous positively charged proteins explained the boundary and viscous lubrication properties of the salivary film. 2 A fundamental understanding of the mechanism of salivary lubrication offers the potential for rationally designing an optimally performing salivary substitute that could improve the quality of life of people suffering from xerostomia caused due to age, polypharmacy, immune diseases, etc.…”

“…Friction is the force resisting relative motion between two sliding bodies, spanning many orders of magnitude in length, time and energy scales. It has far-reaching implications in science and engineering, from carbon nanotube bearings 1 to biological lubrication [2][3][4] and geophysical faults 5 in earthquakes, placing it at the forefront of current research. Friction becomes particularly relevant at the nanoscale in a wide range…”

Salivary lubricity (ex vivo) enhances upon moderate exercise: A pilot study

“…sprays, hydrogels) such as carboxymethyl cellulose, xanthan gum ( Nieuw Amerongen & Veerman, 2003 ; Oh, Lee, Kim, & Kho, 2008 ), that are far from the composition of real human saliva bring only limited symptomatic relief and any benefits are often short-lived ( Vinke, Kaper, Vissink, & Sharma, 2018 ). This is in part because, in designing most substitutes, rheology (resistance to flow or viscosity) has been considered as the “gold standard” characteristic with the goal to match the viscosity of human saliva ( Sarkar, Xu, & Lee, 2019 ; Xu et al, 2020 ), but fails to consider the tribological (lubrication) aspects of saliva, which is the main focus of this paper.…”