LubSat is a European Research Council (Starting Grant) funded research project for 5 years (2017-22) with a value of €1.5 million to understand the role of oral lubricity on satiety. In particular, our quantitative multi-scale understanding of lubrication of the human salivary film when exposed to stimuli from food biomolecules, which in turn can have significant appetite suppression consequences, remains poorly understood. The key limitation to accurately measure oral lubrication is the lack of availability of tribo-contact surfaces that effectively emulate the oral surfaces (i.e. the soft, slippery mucous-coated human tongue and the upper palate). The project will apply classical theories from Physics and tools from Mechanical Engineering to design novel soft lubricious surfaces emulating our saliva-coated human tongue. This will be then used to create fundamental understanding of how food molecules lubricate the oral surfaces and the implications this has on the satisfaction of the food and perceived satiety.
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New results on Microgels acting as viscosity modifiers and lubricants published in Soft Matter
Microgels are soft colloids made of cross-linked polymers that are capable of entrapping significant proportions of solvent or water. In this work published in Soft Matter, Volume 15, Pages 9599-9599, the mechanical performance of colloidal whey protein microgels (hydrodynamic diameter ∼100 nm measured using dynamic light scattering and atomic force microscopy (AFM) (see artwork (top) and experimental AFM image, (bottom)) of different rigidity (soft microgels (G′ ∼ 100.0 Pa) and hard microgels (G′ ∼ 10.0 kPa) ) dispersed in Newtonian or complex non-Newtonian fluids was investigated for the first time via rheology and soft tribology complemented with theoretical considerations. Dispersions of both soft microgels (G′ ∼ 100.0 Pa) and hard microgels (G′ ∼ 10.0 kPa) were observed to act as thickeners in low viscosity Newtonian fluids 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. Variation of mechanical characteristics of microgels and continuum allowed establishing a relationship between the high shear rheology of the dispersions and their lubrication properties, which to the best of our knowledge has never been reported for this kind of complex continuum before. Thus, in order to obtain a benefit from microgels on the tribology performance of a dispersion, simplistically, it is necessary to have harder particles when the continuum viscosity is increased until the microgels become too hard that they can increase the abrasiveness in biological contacts. To read the full article, click here: https://doi.org/10.1039/C9SM01802F
An Invited Review on Saliva published in Advances in Colloid and Interface Science
Human saliva, a seemingly simple aqueous fluid, is, in fact, an extraordinarily complex biocolloid that is not fully understood, despite many decades of study. In the last few decades, colloid scientists have attempted designing model (i.e. ‘saliva mimicking fluid’) salivary formulations to understand saliva-food colloid interactions in an in vitro set up and its contribution on microstructural aspects, lubrication properties and sensory perception. We present the first review published in Advances in Colloid and Interface Science 27 Article No. 102034 on the the current state of knowledge on bulk and interfacial properties of model saliva in comparison to real human saliva and highlight how far such model salivary formulations can match the properties of real human saliva. Real human saliva has an extremely complex architecture with multiple proteins and consequently specialized properties, understanding of which is far from complete. Hence, designing one standardized formulation of real human saliva is not straightforward. Indeed, model saliva and real human saliva show similarities in food colloid-saliva interaction outcomes when the model saliva emulates the biochemical composition (e.g. ions, negatively charged commercially available mucin) and the electrostatic charge, to a certain extent. Such understanding has helped to decipher the physico-chemical mechanisms behind sensory perceptions. However, considering the growing research interests on oral lubrication, model saliva studied mainly using less expensive and crude pig gastric mucin (PGM) is particularly problematic in replicating the lubrication and adsorption properties of saliva. Based on surface adsorption and lubrication studies from a wide pool of studies, we recommend that bovine submaxillary mucin (BSM) as the most optimal choice for the commercially available mucin source for model saliva preparation in terms of lubrication properties. Despite that mucin is recognized as the chief contributor to the lubricating properties of real saliva, it is evident that aqueous solution of mucin alone cannot fully represent various physicochemical and biophysical properties of saliva. Systematic studies on designing mucin-multilayers with targeted tribological properties have to be investigated in future, particularly with respect to examining its potential to replicate human salivary pellicle. Tribological measurements in the presence of mucins and mucin-polycationic additive systems (multilayers) in comparison to ex vivo human salivary conditioning films are needed in first place to warrant its use in food tribology experiments. To read this comprehensive review paper, please click in the open access link: https://doi.org/10.1016/j.cis.2019.102034
A Systematic Review on Oral Tribology-Sensory Relationship published in Current Opinion in Food Science
Oral tribology is rapidly entering into the food scientists’ toolbox because of its promises to predict surface-related mouthfeel perception. We present the first systematic review published in Current Opinion in Colloid Food Science 27 Pages 64-73 on the relationships between instrumental tribology measurements and sensory perception. In recent years, an impressive suite of commercially available and bespoke tribometers have surfaced to quantify the friction in presence of model and real food systems that allow the plotting of Stribeck curve. We discussed the advantages and disadvantages of these tribometers that are currently used across various food material science laboratories. It is exciting times that food scientists have already moved on from using steel-steel to steel-elastomers (such as polydemthylsiloxane (PDMS)) or PDMS–PDMS surfaces to mimic tongue-palate contacts, which have enabled developing empirical relationships with some sensory attributes that are either fat-related or non-fat related. We narrowed down the recent studies involving model and real food systems from 2016 onwards from 4857 to only 16 articles that found correlations between friction coefficients (μ) and specific sensory attributes, such as ‘smoothness’, ‘slipperiness’, ‘pastiness’. However, considering lubrication is a system property and not an intrinsic material property of the lubricant, generalizability of such relationship can be questioned. It is highly likely that such existing relationship is only valid within the remits of those specific experimental conditions, and such tribology–sensory relationship might not hold well with other equipment or experimental conditions. Hence, it is crucial to build mechanistic hypotheses before trying to examine tribology–sensory relationships. Finally, to marry oral tribology to sensory, we have listed challenges and opportunities on the road to identify generalized relationships between instrument and sensory perception. To read the full paper, please click in the open access link: https://doi.org/10.1016/j.cofs.2019.05.007
New results on Rheology and Tribology of Polysaccharides published in Biotribology
Combination of rheology and tribology can be a promising tool to characterize polysaccharides and understand their potential for acting as promising thickeners for dysphagia (swallowing disorder) patients. In this study, which has been recently published in Biotribology 33 Pages 14699-14708, gellan gum (GG) (0.075–0.3 wt%), a, bacterial exopolysaccharide from Sphingomonas elodea was compared against commercial starch-based thickeners (modified starch with or without gums, 5 wt%) and xanthan gum (XG, 0.5–1.0 wt%) for rheological and oral tribological properties. Gellan gum is a linear tetrasaccharide, composed of 4)-L-rhamnopyranosyl-(α-1 to 3)-d-glucopyranosyl-(β-1 to 4)-D-glucuronopyranosyl-(β-1 to 4)-d-glucopyranosyl-(β-1 to 4) containing acetyl substituents on the 3-linked glucose. Gelation occurs upon cooling of a heat-treated gellan gum solutions under quiescent conditions through the aggregation of its double helices. We compared the polysaccharides using apparent viscosity, oral tribology using polydimethylsiloxane (PDMS) ball-on-disc set up and ζ-potential measurements. The measurements were conducted in presence of artificial saliva containing mucin with or without α-amylase at 37 °C to mimic oral conditions. Viscosity results suggested that the commercial starch-based thickeners behaved like water in orally relevant shear, largely associated with the hydrolysis of modified starch by α-amylase, whereas, XG and GG showed no responsiveness to α-amylase. In the case of oral tribology, artificial saliva containing mucin adsorbed to the PDMS surfaces reducing friction as compared to water. From a tribological perspective, the lubrication profile of all thickeners was found to be controlled mainly by their viscous component. Hence, samples with higher viscosities at orally relevant shear rates (>0.1 Pa s at 100 s−1 shear rates i.e. two orders of magnitude higher viscosity than water) showed promising lubrication performances (i.e. μ ≤ 0.07 in the mixed regimes) as compared to that of water (μ ~ 1.0 in the mixed regimes) due to their larger drag force to allow entrainment as compared to water. The increase in boundary friction coefficients in commercial starch-based thickeners was likely associated with α-amylase-induced hydrolysis, increasing the PDMS-PDMS asperity contacts. In simulated oral conditions, the increase in friction coefficients in presence of XG and GG was influenced by the exclusion of mucin-rich artificial saliva from the PDMS surfaces due to electrostatic interaction between the gums and mucin. A combination of rheological and tribological techniques in orally relevant conditions appears as a reliable approach to understand the potential of GG (0.3 wt%) to act as a dysphagia thickener that offers similar mechanical properties as XG (1.0 wt%) at a lower concentration. Further studies on extensional viscosity measurement of gellan gum in simulated oral processing conditions and relationship of instrumental data with sensory measurements (e.g. thickness, cohesiveness, stickiness) are needed to warrant its applications in dysphagia management. You can read the paper at https://doi.org/10.1016/j.biotri.2019.02.002
An Invited Review on Oral Lubrication published in Current Opinion in Colloid & Interface Science
Oral lubrication deals with one of the most intricate examples of bio-lubrication, where surfaces under sliding conditions span from the hardest enamel to soft oral tissues in human physiology. The situation is more complex as these surfaces are covered by an endogenous biolubricant ‘saliva‘ before the exogenous food particles can wet, stick, or slip at the surfaces. In this Invited Review published in the Special Issue of “Outstanding Young Researchers in Colloid and Interface Science” in Current Opinion in Colloid and Interface Science 39 Pages 61-75, we covered the latest advances in tribology research in soft oral contacts and theoretical developments relevant for this field. We have briefly covered the material physics and chemistry (roughness, wettability and modulus) of human soft oral architectures under shear fields, which influence oral lubrication and highlight the transition from rheological to tribological limits. Talking about surface roughness of human tongue, the human filiform papillae can contain 6–12 protruding hairs, each of which are 34–50 μm wide contributing to overall 420–500 μm diameter in the root and height of 250 μm, whereas fungiform papillae almost doubling the diameter of the filiform ones. Overall, all these surface topographic analyses of tongue point out that engineering of these filiform papillae on tongue surface by nature provides it a more coarse texture than even a 100-grit sandpaper. However, the tongue does not feel as coarse as represented by its roughness largely due its reduced “stiffness” (Young’s modulus).
In terms of lubricants, we discussed one endogenous lubricant (saliva) and one exogenous lubricant (food-based microgels/hydrogel particles) to capture the recent knowledge gathered in how they reduce friction in sliding soft contacts in an in vitro set up. We discussed the legacy of materials used for oral lubrication studies in the last decade covering latest experiments conducted mainly using polydimethylsiloxane (PDMS) tribopairs in in vitro and pig tongue surfaces in ex vivo experiments. For example, according to classical Hertz contact theory, in the case of the popular PDMS ball on disc tribological set-up (E* ∼ 2 MPa, R ∼ 0.01 m) with a typical load of 2 N, the maximum contact pressure is ∼200 kPa. This pressure is about one order of magnitude higher than that measured in oral-palate contact of healthy adults (30–50 kPa). For future, we highlight the window of opportunity offered by a range of polymeric surfaces together with the advent of 3D Printing technology that can be used to emulate oral surfaces with accurate roughness and modulus to enable colloid scientists to underpin principles that govern oral lubrication at colloidal scale. To read the full paper, please click in the link: https://doi.org/10.1016/j.cocis.2019.01.008
New results (with Slide Presentation) on Lubrication of Emulsion Microgel Particles published in ACS Applied Materials & Interfaces
Biolubricants reduce friction between soft biological contacting surfaces, such as the tear in eyes, saliva in the oral cavity, synovial fluid in the articular joints, to prevent discomfort and wear of epithelial tissues and cartilage. The use of submicron to micron sized particles (e.g., microgels) as biolubricant additives has attracted recent attention in biomaterial science research due to their ability to reduce friction and wear in the boundary regime of sliding contact in biological areas, such as tongue-oral palate contacts, cartilage, or eye blinking. In this paper published in ACS Applied Materials & Interfaces 10 (32), Pages 26893–26905, starch-based emulsion microgel particles with different starch (15 and 20 wt %) and oil contents (0–15 wt %) were synthesized, and their lubrication performance under physiologically relevant conditions was investigated. Emulsion microgels were subjected to skin mimicking or oral cavity mimicking conditions, 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 emulsion microgel particle as a novel biolubricant 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 multifunctional biolubricant additives for future use in numerous soft matter applications where both lubrication and controlled release of bioactives are essential.. To read the full paper, please click in the link https://pubs.acs.org/doi/10.1021/acsami.8b07883. To hear an audio presentation on this paper by Ophelie Torres, please click below:
New results on Aqueous Lubrication using Microgel Particles published in Langmuir
Aqueous lubrication has emerged as an active research area in recent years due to its prevalence in nature in biotribological contacts and its enormous technological soft-matter applications. In this study, which has been recently published in Langmuir 33 Pages 14699-14708, we designed aqueous dispersions of biocompatible whey-protein microgel particles (WPM) (10–80 vol %) cross-linked via disulfide bonding and focused on understanding their rheological, structural and biotribological properties (smooth polydimethylsiloxane (PDMS) contacts, Ra < 50 nm, ball-on-disk set up). The WPM particles (Dh = 380 nm) displayed shear-thinning behavior and facilitated lubrication between bare hydrophobic PDMS surfaces (HB) (static water contact angle 108°), leading to a 10-fold reduction in boundary friction force at increased volume fraction (ϕ ≥ 65%), largely attributed to the close packing-mediated layer of particles between the asperity contacts acting as “true surface-separators”. In addition, WPM particles employed a rolling mechanism analogous to “ball bearings”, the latter supported by negligible change in size and microstructure of the WPM particles after tribology. An ultralow boundary friction coefficient, μ ≤ 0.03 was achieved using WPM between O2 plasma-treated hydrophilic PDMS contacts coated with bovine submaxillary mucin (HL+BSM) (static water contact angle 47°), and electron micrographs revealed that the WPM particles spread effectively as a layer of particles even at low ϕ∼ 10%, forming a lubricating load-bearing film that prevented the two surfaces from true adhesive contact. However, above an optimum volume fraction, μ increased in HL+BSM surfaces due to the packing and interpenetration of particles that possibly impeded effective rolling, explaining the increase in friction. The present work demonstrates a novel approach for providing ultralow friction in soft polymeric surfaces using proteinaceous microgel particles that satisfy both load bearing and kinematic requirements. To read the full paper, please click in the link https://doi.org/10.1021/acs.langmuir.7b03627