Emulsion Microgel Particles as High-Performance Bio-Lubricants

Abstract

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.

Gellan gum: A new member in the dysphagia thickener family

“…'Artificial saliva buffer' used in this study composed of 0.16 g/L NaCl, 0.33 g/L NH 4 [31] and has been referred as 'artificial saliva'. Artificial saliva in the absence or presence of α-amylase was added to the different thickener solutions at 1:1 w/w ratio at 37 ºC and mixed for 2 h based on standardised static in vitro digestion method [32].…”

Microgels as viscosity modifiers influence lubrication performance of continuum

“…Hydrated layers formed near the surfaces by this mechanism are capable of supporting high pressure such as those developed in synovial joints. 11 The aqueous lubrication performance of colloidal microgels has recently attracted research attention, such as in the case of particles made of thermo-responsive hydrogels 12 or biopolymers including starch, 13 whey protein 14,15 as well as non-starch polysaccharides (alginate, 16 agarose, 17 k-carrageenan 18 ). The capacity of these soft particles to reduce friction in soft mechanical contacts has been attributed to their action as surface separators in combination to a roll bearing mechanism, the latter phenomenon is still under debate.…”

“…The capacity of these soft particles to reduce friction in soft mechanical contacts has been attributed to their action as surface separators in combination to a roll bearing mechanism, the latter phenomenon is still under debate. 13 An important aspect which has been as-of-yet neglected in the literature, whilst possibly having an exceptional relevance in the mechanical performance of the colloidal dispersions, is the role of the continuous phase i.e., the continuum in which the microgels are dispersed. 19 Farres et al carried out an elegant study where the role of co-solutes, such as glucose and glycerol on the lubrication properties of agar fluid gels was investigated.…”

Overcoming in vitro gastric destabilisation of emulsion droplets using emulsion microgel particles for targeted intestinal release of fatty acids

“…Emulsion microgel particles are a relatively new class of "smart" soft solid vehicles where several emulsion droplets are encapsulated within a biopolymer hydrogel particle (Torres, Murray, & Sarkar, 2016, 2017Torres, Reyes, Murray, & Sarkar, 2018;Torres, Tena, Murray, & Sarkar, 2017). This structure offers several advantages over conventional O/W emulsions.…”