No abstract

Biocompatible microgels modify the rheology of continuum acting as thickening or thinning agents. Here, the influence of high shear rheology of microgels on tribological performance of the continuum is demonstrated.

Novel starch-based emulsion microgel particles were designed using a facile top-down shear-induced approach. The emulsion droplets were stabilized using octenyl succinic anhydride (OSA) modified starch and incorporated into heat-treated and sheared native starch gels, forming emulsion gels. Using gelation kinetics and small deformation rheological measurements of sheared native starch gels and emulsion gels, OSA starch-stabilized emulsion droplets were demonstrated to act as "active fillers". By varying native starch concentrations (15-20wt%) and oil fractions (5-20wt%), optimal concentrations for the formation of emulsion microgel particles were identified. Microscopy at various length scales (transmission confocal laser scanning and cryo-scanning electron microscopy) and static light scattering measurements revealed emulsion microgel particles of 5-50μm diameter. These novel emulsion microgel particles created via careful combination of gelatinized native starch and OSA stabilised-emulsion droplets acting as active fillers may find applications in food and personal care industries for delivery of lipophillic molecules.

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 physiological 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 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.

Please cite this article as: O. Torres, A. Yamada, N.M. Rigby, et al., Gellan gum: A new member in the dysphagia thickener family, Biotribology, https://doi. AbstractIn this study, gellan gum (GG) (0.075-0.3 wt%) is proposed as a new dysphagia thickener and compared against commercial starch-based thickeners (modified starch with or without gums, 5 wt%) and xanthan gum (XG, 0.5-1.0 wt%) 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. 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, while, 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. The increase in boundary friction coefficients in commercial starch-based thickeners was likely associated with α-amylaseinduced hydrolysis, increasing the PDMS-PDMS asperity contacts. Interestingly, the tribological behaviour of XG and GG was dictated mainly by viscous lubrication. However, in simulated oral conditions, the increase in friction coefficients in presence of XG and GG was influenced by depletion of 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. Extensional viscosity measurement of GG is needed to understand its applications in dysphagia management.