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.
Highlights
Emulsion microgel particles are a new class of microgel particle where several oil droplets are trapped within a biopolymer hydrogel particle.[16−18] In this study we demonstrate for the first time how, by employing suitable novel biocompatible emulsion microgel particles, it is possible to provide protection to emulsion droplets combined with the desired lubrication under relevant physiological shear and/or enzymatic conditions
In response to tribological shear and enzyme activity, some oil droplets entrapped in the starch microgel particles are released and improved lubrication
Based on confocal fluorescence microscopy, these oil droplets are still stabilized by an adhering octenyl succinic anhydride (OSA) starch layer and residual microgel material
Summary
Biolubricants are in great demand to reduce friction between soft biological contacting surfaces, such as the eyes, the oral cavity, the gastrointestinal tract, and joints, to prevent discomfort and wear of epithelial tissues and cartilage.[1−5] The use of submicron to micron sized particles (e.g., microgels) as biolubricant additives has recently been a focal point 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.[1,2,6,7] At low sliding speeds and contact pressures, these particles are able to enter the gap and reduce direct contact between the biological surfaces by acting as “true surface separators” by virtue of their rheological properties.[8]. The lubrication properties of emulsions primarily arise from the oil droplets in the boundary regime, which is observed at the lowest entrainment speed where the friction force is independent of the entrainment speed and the load is supported predominantly by the asperity contact and surface adsorbed matter.[7,9] While emulsions demonstrate lubrication properties in both external (in the form of topical creams to create a protective barrier increasing the moisture content or allow the triggered release of a lipophilic bioactive under mechanical shear10) and internal applications (in the form of medicine or food to coat the oral cavity, reduce friction and increase palatability11,12), their instability to physicochemical and biochemical degradation (e.g., light, temperature, shear, ionic and enzymatic activity) can lead to poor release of bioactive compounds and increased physiological oxidation of the oil droplets.[13−15] being able to tailor microstructures that combine high lubrication performances, protection of colloidal droplets against such physicochemical degradation, and controlled release of bioactive compounds under either enzymatic- or mechanical shear-trigger is highly desirable. Most targeted release studies of Received: May 14, 2018 Accepted: July 23, 2018 Published: July 23, 2018
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