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.