Synergistic Microgel-Reinforced Hydrogels as High-Performance Lubricants

The ability to create a superlubricious aqueous lubricant is important for various biological and technological applications. Here, a nonlipid biolubricant with strikingly low friction coefficients is fabricated (patented) by reinforcing a fluid-like hydrogel composed of biopolymeric nanofibrils with proteinaceous microgels, which synergistically provide superlubricity on elastomeric surfaces in comparison to any of the sole components. This two-component lubricant composed of positively charged lactoferrin microgels and negatively charged κ-carrageenan hydrogels is capable of exceeding the high lubricating performance of real human saliva in tribo tests using both smooth and textured surfaces, latter mimicking the human tongue’s wettability, topography, and compliance. The favorable electrostatic attraction between mutually oppositely charged microgels and the hydrogel reinforces the mechanical properties of the hydrogel, allowing friction reduction by combining the benefits of both viscous and hydration lubrication. The superlubricity of these microgel-reinforced hydrogels offers a unique prospect for the fabrication of biocompatible aqueous lubricants for dry-mouth therapy and/or designing of nonobesogenic nutritional technologies

Emulsion Microgel Particles as High Performance Bio-Lubricants

Starch-based emulsion microgel particles with different starch (15 and 20 wt%) and oil content (0 - 15 wt%), were synthesized and their lubrication performance under physiological conditions was investigated. Emulsion microgels were subjected to skin mimicking or oral cavity mimicking, i.e., smooth hydrophobic polydimethylsiloxane ball-on-disc tribological tests, in the absence or presence of salivary enzyme (α-amylase). In the absence of enzyme, emulsion microgel particles (30 - 60 vol% particle content) conserved the lubricating properties of emulsion droplets, providing considerably lower friction coefficients (μ ≤ 0.1) in the mixed lubrication regime compared to plain microgel particles (0 wt% oil). Upon addition of enzyme, the lubrication performance of emulsion microgel particles became strongly dependent on the particles' oil content. Microgel particles encapsulating 5-10 wt% oil showed a double plateau mixed lubrication regime having a lowest friction coefficient μ ~ 0.03 and highest μ ~ 0.1, the latter higher than with plain microgel particles. An oil content of 15 wt% was necessary for the microgel particles to lubricate similarly to the emulsion droplets, where both systems showed a normal mixed lubrication regime with μ ≤ 0.03. The observed trends in tribology, theoretical considerations and the combined results of rheology, light scattering and confocal fluorescence microscopy suggested that the mechanism behind the low friction coefficients was a synergistic enzyme- and shear-triggered release of the emulsion droplets, improving lubrication. The present work thus demonstrates experimentally and theoretically a novel bio-lubricant additive with stimuli-responsive properties capable of providing efficient boundary lubrication between soft polymeric surfaces. At the same time, the additive should provide an effective delivery vehicle for oil soluble ingredients in aqueous media. These findings demonstrate that emulsion microgel particles can be developed into multi-functional bio-lubricant additives for future use in numerous soft matter applications where both lubrication and controlled release of bioactives are essential

Lubrication of soft oral surfaces

Oral lubrication deals with one of the most intricate examples of biotribology, where surfaces under sliding conditions span from the hardest enamel to soft oral tissues in human physiology. Complexity further arises with surfaces being covered by an endogenous biolubricant saliva before exogenous food particles can wet, stick, or slip at the surfaces. In this review, we present a description of soft oral surfaces, comparing them with the recent approaches that have been used to study oral lubrication using in vitro to ex vivo setups. Specifically, lubrication behaviors of saliva and soft microgels are discussed highlighting instances of hydration lubrication. We have structured this information creating a strong link between theoretical concepts and oral lubrication, which has thus far remained elusive in literature. Finally, we highlighted some of the several challenges remaining in this field and discussing how emerging technologies in material science might help overcoming them

Surface adsorption and lubrication properties of plant and dairy proteins: A comparative study

The aim of this work was to compare the surface adsorption and lubrication properties of plant and dairy proteins. Whey protein isolate (WPI) and pea protein isolate (PPI) were chosen as model animal and plant proteins, respectively, and various protein concentrations (0.1–100 mg/mL) were studied with/without heat treatment (90 °C/60 min). Quartz crystal microbalance with dissipation monitoring (QCM-D) experiments were performed on hydrophilic (gold) and hydrophobic polydimethylsiloxane (PDMS) sensors, with or without a mucin coating, latter was used to mimic the oral surface. Soft tribology using PDMS tribopairs in addition to wettability measurements, physicochemical characterization (size, charge, solubility) and gel electrophoresis were performed. Soluble fractions of PPI adsorbed to significantly larger extent on PDMS surfaces, forming more viscous films as compared to WPI regardless of heat treatment. Introducing a mucin coating on a PDMS surface led to a decrease in binding of the subsequent dietary protein layers, with PPI still adsorbing to a larger extent than WPI. Such large hydrated mass of PPI resulted in superior lubrication performance at lower protein concentration (≤10 mg/mL) as compared to WPI. However, at 100 mg/mL, WPI was a better lubricant than PPI, with the former showing the onset of elastohydrodynamic lubrication. Enhanced lubricity upon heat treatment was attributed to the increase in apparent viscosity. Fundamental insights from this study reveal that pea protein at higher concentrations demonstrates inferior lubricity than whey protein and could result in unpleasant mouthfeel, and thus may inform future replacement strategies when designing sustainable food products

Microgels as viscosity modifiers influence lubrication performance of continuum

Biocompatible microgels have been demonstrated to act as excellent lubricants, however, the influence of the continuum on their overall mechanical performance has been neglected so far. In this work, the mechanical performance of colloidal whey protein microgels (hydrodynamic diameter ∼100 nm measured using dynamic light scattering and atomic force microscopy) of different rigidity dispersed in Newtonian (buffer and corn syrup) or complex non-Newtonian fluids (xanthan gum) is investigated for the first time via rheology and soft tribology. Dispersions of both soft microgels (G′ ∼ 100.0 Pa) and hard microgels (G′ ∼ 10.0 kPa) were observed to act as thickeners in buffer as well as in low viscosity corn syrup and correspondingly reduced the friction, latter decreased as a function of the increased rigidity of the microgels. Differently, in high viscosity continuum, the microgels acted as thinning agents and increased the friction. In the lubrication limit, microgels in buffer or corn syrup behaved as Newtonian fluids with effective viscosity corresponding to their second Newtonian plateau value (η∞). However, the lubrication performance of the microgels dispersed in the complex fluid (xanthan gum) could not be described quantitatively by η∞. For the low viscosity xanthan gum, the microgels had no influence on friction. Nevertheless, for the high viscosity counterparts, the soft microgels acted as thinning agents whilst the hard microgels accelerated the onset of elastohydrodynamic regime. This study demonstrates that microgels act as viscosity modifiers directly influencing the tribological performance, depending upon a subtle interplay of rheological properties of the particles and continuum

3D biomimetic tongue-emulating surfaces for tribological applications

Oral friction on the tongue surface plays a pivotal role in mechanics of food transport, speech, sensing and hedonic responses. The highly specialized biophysical features of human tongue such as micro-papillae-dense topology, optimum wettability and deformability present architectural challenges in designing artificial tongue surfaces, and absence of such biomimetic surface impedes fundamental understanding of tongue-food/ fluid interaction. Herein, we fabricate for the first time, a 3D soft biomimetic surface that replicates the topography and wettability of a real human tongue. The 3D-printed fabrication contains a Poisson point process-based (random) papillae distribution, and is employed to micro-mould soft silicone surfaces with wettability modifications. We demonstrate the unprecedented capability of these surfaces to replicate the theoretically-defined and simulated collision probability of papillae, and to closely resemble the tribological performances of human tongue masks. These de novo biomimetic surfaces pave the way for accurate quantification of mechanical interactions in the soft oral mucosa

Dry mouth diagnosis and saliva substitutes ─ A review from a textural perspective

The aim of this review is to assess the objective and subjective diagnosis, as well as symptomatic topical treatment of dry mouth conditions with a clear focus on textural perspective. We critically examine both the current practices as well as outline emerging possibilities in dry mouth diagnosis and treatment, including a patent scan for saliva substitutes. For diagnosis, salivary flow rates and patient‐completed questionnaires have proven to be useful tools in clinical practice. To date, objective measurements of changes in mechanical properties of saliva via rheological, adsorption and tribological measurements and biochemical properties of saliva such as assessing protein, mucins (MUC5B) are seldom incorporated into clinical diagnostics; these robust diagnostic tools have been largely restricted to application in non‐clinical settings. As for symptomatic treatments of dry mouth, four key agents including lubricating, thickening, adhesive and moisturizing agents have been identified covering the overall landscape of commercial saliva substitutes. Although thickening agents such as modified celluloses, polysaccharide gum, polyethylene glycol (PEG) etc. are most commonly employed saliva substitutes, they offer short‐lived relief from dry mouth and generally do not provide boundary lubrication properties of real human saliva. Innovative technologies such as self‐assembly, emulsion, liposomes, microgels are emerging as novel saliva substitutes that hold promise for alternative approaches for efficient moistening and lubrication of the oral mucosa. Their adoption into clinical practice will be dependent on their efficacies, duration of relief, ease of application by the practitioners and patient compliance

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

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

Aqueous Lubrication: A Self-Assembled Binary Protein Model Explains High-Performance Salivary Lubrication from Macro to Nanoscale (Adv. Mater. Interfaces 1/2020)

Salivary pellicle is an outstanding bio-lubricant that coats and protects our tongue, teeth and oral mucosa. Supported by sophisticated multi-scale tribological analyses and real-time adsorption techniques coupled with self-consistent field theory calculations, Anwesha Sarkar and co-workers demonstrate in article number 1901549 for the first time that the unique lubrication performance of salivary pellicle is a result of electrostatic self-assembly between hydrated mucin proteins and positively-charged protein, latter acting as a ‘molecular glue’ between the mucin-mucin and mucin-surface