Pickering emulsions stabilized by colloidal gel particles complexed or conjugated with biopolymers to enhance bioaccessibility and cellular uptake of curcumin

Abstract

The aim of this study was to investigate the fate of curcumin (CUR)-loaded Pickering emulsions with complex interfaces during in vitro gastrointestinal transit and test the efficacy of such emulsions on improving the bioaccessibility and cellular uptake of CUR. CUR-loaded Pickering emulsions tested were whey protein nanogel particle-stabilized Pickering emulsions (CUR-E_WPN) and emulsions displaying complex interfaces included 1) layer-by-layer dextran sulphate-coated nanogel-stabilized Pickering emulsions (CUR-DxS+E_WPN) and 2) protein+dextran-conjugated microgel-stabilized Pickering emulsions (CUR-E_WPDxM). The hypothesis was that the presence of complex interfacial material at the droplet surface would provide better protection to the droplets against physiological degradation, particularly under gastric conditions and thus, improve the delivery of CUR to Caco-2 intestinal cells. The emulsions were characterized using droplet sizing, apparent viscosity, confocal and cryo-scanning electron microscopy, zeta-potential, lipid digestion kinetics, bioaccessibility of CUR as well as cell viability and uptake by Caco-2 cells. Emulsion droplets with modified to complex interfacial composition (i.e. CUR-DxS+E_WPN and CUR-E_WPDxM) provided enhanced kinetic stability to the Pickering emulsion droplets against coalescence in the gastric regime as compared to droplets having unmodified interface (i.e. CUR-E_WPN), whereas droplet coalescence occurred in intestinal conditions irrespective of the initial interfacial materials. A similar rate and extent of free fatty acid release occurred in all the emulsions during intestinal digestion (p > 0.05), which correlated with the bioaccessibility of CUR. Striking, CUR-DxS+E_WPN and CUR-E_WPDxM significantly improved cellular CUR uptake as compared to CUR-E_WPN (p < 0.05). These results highlight a promising new strategy of designing gastric-stable Pickering emulsions with complex interfaces to improve the delivery of lipophilic bioactive compounds to the cells for the future design of functional foods.

Keywords: Caco-2 cells; Cellular uptake; Curcumin; Microgel; Pickering emulsion; in vitro digestion.

Graphical abstract

Fig. 1

Cryo-SEM images of the three Pickering emulsions used for delivering curcumin i.e. a) CUR-EWPN emulsion (magnification of 15,000 × (a1), and 50,000 × (a2), respectively), b) CUR-DxS+EWPN (magnification of 25,000 × (b1), and 50,000 × (b2), respectively) and c) CUR-EWPDxM (magnification of 10,000 × (c1), and 20,000 × (c2), respectively). Arrows indicate the nanogel particle in a2, nanogel particle aggregated with dextran sulphate in b2, and conjugate microgel particles in c2.

Fig. 2

Confocal images with superimposed droplet size distribution of the freshly-prepared Pickering emulsions i.e. CUR-E_WPN, CUR-DxS+E_WPN and CUR-E_WPDxM and after their exposure to 120 min of in vitro gastric or 180 min of in vitro sequential gastrointestinal digestion conditions.

Fig. 3

Flow curves of freshly prepared Pickering emulsions i.e. CUR-E_WPN (black squares) CUR-DxS+E_WPN (blue triangles) and CUR-E_WPDxM (red circles) at 37 °C. Data points represent the average of at least three measurements on triplicate sample. Error bars indicate the standard deviations. Solid lines are the best fits to the experimental data predicted using the Ostwald de Waele model (Eq. (1)). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

Average droplet diameter (d₄₃) (1) and ζ-potential value (2) of CUR-EWPN (black bars), CUR-DxS+E_WPN (blue bars with diagonal lines) and CUR-E_WPDxM (red bars with horizontal lines) after in vitro gastric digestion (a) and in vitro intestinal digestion (b). 0 min in each case indicates the behaviour of the emulsions in presence of SGF (a) and SIF (b) buffer without any added enzymes. Error bars represent the standard deviations. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5

Percentage of free fatty acid (% FFA) released (a) from CUR-E_WPN (black squares), CUR-DxS+E_WPN (blue triangles) and CUR-E_WPDxM (red circles) with insets representing maximum FFA release (Φmax, %), lipolysis rate constant (k, μmol s⁻¹ m⁻²) and the time to achieve 50% digestion (t_1/2, s), and bioaccessibility (b) of CUR after in vitro gastrointestinal digestion from the micellar phase of the aforementioned emulsions. The solid lines connecting the data points in the %FFA cuves (a) are the best fits to the experimental data predicted using mathematical model (Eq. (2). Data presented are mean with standard deviation of three independent experiments. Different letters indicate significant differences. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6

Cell viability (a) of MCT-dissolved CUR (non-emulsified) systems and the three Pickering emulsion-based delivery vehicles (CUR-E_WPN, CUR-DxS+E_WPN and CUR-E_WPDxM) at different concentrations of CUR against Caco-2 cells incubated for 2 h along with the digested blank emulsions without any curcumin (blank bars with diagonal lines), and cellular uptake (b) of CUR by the Caco-2 cells in the three Pickering emulsion-based delivery vehicles. Data presented are mean with standard deviation of three independent experiments. Different letters indicate mean significant differences between CUR concentrations (0.5 μM - μM) for cell viability, and significant diferences between emulsion types (CUR-E_WPN, CUR-DxS-E_WPN and CUR-E_WPDxM) for cellular uptake.