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.

Keywords: biological enzymes; biolubricant; colloidal stability; encapsulation; friction reduction; lubricant additives; microgel; soft tribology.

Figure 1

(a) Cryo-SEM micrograph of the external structure and (b) internal structure of starch emulsion microgel particles (15 wt % starch–10 wt % oil), respectively. Scale bar represents 5 μm.

Figure 2

(a) Viscosity versus shear rate of the oil-in-water emulsion at 37 °C before and after tribological shear in the absence or presence of buffer and α-amylase, viscosity versus shear rate of the emulsion microgel particles (15 wt % starch) at different concentrations of oil at 37 °C, before and after tribological shear in the absence of buffer (b), in the presence of buffer (c), and α-amylase (d), respectively.

Figure 3

(a) Coefficient of friction as a function of entrainment speed for sunflower oil, buffer with α-amylase and OSA starch stabilized emulsion in absence or presence of buffer and/or α-amylase subjected to a normal load of 2 N and at 37 °C; (b) Particle size distribution of the OSA starch stabilized-emulsion (40 wt % oil) with and without α-amylase before and after being subjected to tribological shear.

Figure 4

Confocal fluorescence images of the emulsion 0 s (a) and 60 s (b) after the addition of α-amylase before tribological shear (λ = 488 nm, oil droplets); (c) photographs of the emulsions in the absence or presence of buffer and/or α-amylase in the tribometer.

Figure 5

Coefficient of friction as a function of entrainment speed of starch microgel particles encapsulating different oil content measured at 2 N and 37 °C in absence of buffer and α-amylase (a,b); in the presence of buffer (50:50 w/w) without α-amylase (c,d); in the presence of buffer (50:50 w/w) with α-amylase (e,f). Controls are the OSA-stabilized emulsion under the same conditions.

Figure 6

Particle size distribution of 15 wt % starch particles encapsulating different oil concentrations before or after being sheared by the tribometer being in the absence (a) or presence of α-amylase (b) and (c) photographs of the emulsions after tribology in the absence or presence of buffer and/or α-amylase.

Figure 7

Confocal fluorescence images of the emulsion microgel particles 0 s (a) and 60 s (b) after the addition of α-amylase (λ = 488 nm, oil droplets and 639 nm, starch shell), scale bar represents 20 μm.

Scheme 1. Schematic Representation of OSA Starch Stabilized Oil-in-Water Emulsion in the Mixed Regime of a Tribometer in the Absence (a) or Presence of Buffer (b) with and without α-Amylase (c)

Scheme 2. Schematic Representation of Native Starch Particles in the Boundary Regime of a Tribometer at Low (a) and High (b) Starch Concentration in the Absence or Presence of α-Amylase (c); Schematic Representation of Starch Based Emulsion Microgel Particles at Low (d) or High (e) Particle Volume Fraction without or with α-Amylase (f)