Aqueous Lubrication, Structure and Rheological Properties of Whey Protein Microgel Particles

Abstract

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, 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, R < 50 nm, ball-on-disk set up). The WPM particles (D = 380 nm) displayed shear-thinning behavior and good lubricating performance in the plateau boundary as well as the mixed lubrication regimes. The WPM particles facilitated lubrication between bare hydrophobic PDMS surfaces (water contact angle 108°), leading to a 10-fold reduction in boundary friction force with increased volume fraction (ϕ ≥ 65%), largely attributed to the close packing-mediated layer of particles between the asperity contacts acting as "true surface-separators", hydrophobic moieties of WPM binding to the nonpolar surfaces, and particles employing 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 O plasma-treated hydrophilic PDMS contacts coated with bovine submaxillary mucin (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 interpenetration of particles that possibly impeded effective rolling, explaining the slight increase in friction. These effects are reflected in the highly shear thinning nature of the WPM dispersions themselves plus the tendency for the apparent viscosity to fall as dispersions are forced to very high volume fractions. 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. These findings hold great potential for designing biocompatible particles for aqueous lubrication in numerous soft matter applications.

Gellan gum: A new member in the dysphagia thickener family

“…This confirms that water showed practically no lubrication performance at low entrainment speed ( < 100 mm s -1 ), as compared to that of commercial thickeners [26].…”

“…Due to the complex composition of saliva (ions, mucin, enzyme) it is highly likely that some sensitive thickeners could alter their mechanical properties when in contact of saliva. Additionally, viscosity alone might not allow a thorough understanding of the mechanical properties of a food product undergoing oral processing [2,[24][25][26][27][28]. Therefore, studying the lubrication properties of these thickeners, via oral tribology measurements which is currently gaining momentum in oral processing research, is required to provide understanding on the interaction of these thickeners with oral surfaces and therefore, indirect insights on the stickiness or slipperiness of different thickeners.…”

“…The surface used was hydrophobic with a water contact angle of 108 ○ [26]. A normal load of 2 N was used in all tests.…”

Design principles of food gels

“…This means that a particular gel design may reduce fat, salt or sugar intake without affecting the sensory perception. For example, incorporating microgels in semi-solid foods could mimic the fat-related perception (creaminess) due to the effective reduction in the friction coefficient via the mechanism of ball-bearing 100,101 and enzymatic 102 reactions for whey protein and starch microgels, respectively. Mouth-melting gels could also increase surface lubrication to mimic fat perception 103 : gelatin gels are widely applied to achieve such a mouthfeel as they are in a gel state at room temperature (~25 °C) but melt at mouth temperature (~37 °C).…”

Oral Lubrication Matters: Effects on Satiety

“…The smooth steel surfaces in this device, commonly used in engineering disciplines, were replaced by a polydimethylsiloxane (PDMS) ball and disc set-up at 37 °C to mimic the oral surfaces (surface roughness, Ra < 50 nm). 33,34 The rolling speed was reduced from 1000 to 1 mm/s at a load of 2N, using a slide-to-roll ratio (SRR) of 50 %, and the coefficient of friction in the mixed lubrication regime (50 mm/s) was measured in triplicate.…”