Pickering emulsions with α-cyclodextrin inclusions: Structure and thermal stability

Cyclodextrin-based supramolecular nanogels decorated with mannose for short peptide encapsulation

Nanogels are aqueous dispersions of hydrogel particles formed by physically or chemically cross-linked polymer networks of nanoscale size. Herein, we devised a straightforward technique to fabricate a novel class of physically cross-linked nanogels via a self-assembly process in water involving α-cyclodextrin and a mannose molecule that was hydrophobically modified using an alkyl chain. The alkyl chain-modified mannose was synthesized in five steps, starting with D-mannose. Subsequently, nanogels were formed by subjecting α-cyclodextrin and the hydrophobically modified mannose to magnetic stirring in water. By adjusting the mole ratio between the hydrophobically modified mannose and α-cyclodextrin, nanogels with an average 100-150 nm diameter were obtained. Physicochemical and structural analyses by 1H NMR and X-ray diffraction unveiled a supramolecular and hierarchical mechanism underlying the creation of these nanogels. The proposed mechanism of nanogel formation involves two distinct steps: initial interaction of hydrophobically modified mannose with α-cyclodextrin resulting in the formation of inclusion complexes, followed by supramolecular interactions among these complexes, ultimately leading to nanogel formation after 72 h of stirring. We demonstrated the nanogels' ability to encapsulate a short peptide ([p-tBuF2, R5]SHf) as a water-soluble drug model. This discovery holds promise for potentially utilizing these nanogels in drug delivery applications.

Ejected microcrystals probe jammed states of droplets in cyclodextrin-based emulsions

The cyclodextrin (CD)-based emulsions exhibit complex instability behaviors such as rapid flocculation and creaming, and how to capture droplet dispersion states of the emulsions remains a great challenge. Here we prepare the CD-based emulsions with different oil-water volume ratios and CD concentrations by using high-pressure homogenization, and characterize the emulsion droplets by using optical microscopy and confocal laser scanning microscopy. We evaluate the effects of homogenization pressure on the stability of the emulsions, identify armored droplets with different surface features, measure interfacial concentrations of adsorbed ICs microcrystals, and observe ejection of the oil/CD inclusion complexes (ICs) microcrystals from the droplet surface. The droplet dispersion states are sensitive to the dynamic buildup and evolving morphologies of the interfacial microcrystals, and there are clear correlations between the properties of the ejected microcrystals and the characteristics of the emulsions. We ascribe the subsequent ejection of ICs microcrystals from the droplet surface to consolidation and deformation of the films formed between neighboring droplets. The ejection of the ICs microcrystals affords a simple method to detect the droplet-droplet interactions and phase transitions in the CD-based emulsions, which might be a generic feature in the broader context of the creaming processes of emulsions.

Jamming to unjamming: Phase transition in cyclodextrin-based emulsions mediated by sodium casein

HYPOTHESIS

Cyclodextrin (CD) can spontaneously build up the solid particle membrane with CD-oil inclusion complexes (ICs) by a self-assembly process. Sodium casein (SC) is expected to preferentially adsorb at the interface to transform the type of interfacial film. The high-pressure homogenization can increase interfacial contact opportunities of the components, which promote the phase transition of the interfacial film.

EXPERIMENTS

We added SC by sequential and simultaneous orders to mediate the assembly model of the CD-based films, examined the patterns in which the films adopt phase transitions to retard emulsion flocculation, and studied the physic-chemical properties of the emulsions and films from the structural arrest, interface tension, interfacial rheology, linear rheology, and nonlinear viscoelasticities through Fourier transform (FT)-rheology and Lissajous-Bowditch plots.

FINDINGS

The interfacial and large amplitude oscillatory shear (LAOS) rheological results showed that the films changed from jammed to unjammed. We divide the unjammed films into two types: one is SC dominated liquid-like film, which is fragile and related to droplet coalescence; the other is cohesive SC-CD film, which helps droplet rearrangement and retards droplet flocculation. Our results highlight the potential of mediating phase transformation of interfacial films to improve emulsion stability.

Droplet Dispersion States of Cyclodextrin-Based Emulsions from Nonlinear Rheological Properties

Polymers are desirable to improve emulsion stability by stuffing them into the continuous phase. How to get information on the droplet dispersion states of the emulsions remains a challenge, as the emulsion characteristics are dictated by two intertwining components, the polymer matrix and the droplets. Herein, we use an amphiphilic polymer, gum arabic (GA), to mediate the droplet flocculation of cyclodextrin (CD)-based emulsions and compare them with our previous studies on the stabilization of CD-based emulsions by a nonamphiphilic polymer, methylcellulose (MC). We characterize the emulsions by using optical microscopy, confocal laser scanning microscopy, and laser particle analysis, explore their rheological behavior through large-amplitude oscillatory shear experiments, and analyze the nonlinear viscoelasticities through Fourier transform (FT)-rheology and Lissajous-Bowditch plots. There is a great difference between GA and MC in the viscosity effect and the arrangement around emulsion droplets. GA is not an effective flocculation inhibitor due to a bridging flocculation mechanism rather than a direct viscosity effect. Our analysis highlights the role of the intrinsic nonlinearity parameter (Q₀) extracted by FT analysis in reflecting the droplet dispersion states of the emulsions by decoupling structural contributions from the polymers and the emulsion droplets.

Effect of micro- and nanoparticle shape on biological processes

In the drug delivery field, there is beyond doubt that the shape of micro- and nanoparticles (M&NPs) critically affects their biological fate. Herein, following an introduction describing recent technological advances for designing nonspherical M&NPs, we highlight the role of particle shape in cell capture, subcellular distribution, intracellular drug delivery, and cytotoxicity. Then, we discuss theoretical approaches for understanding the effect of particle shape on internalization by the cell membrane. Subsequently, recent advances on shape-dependent behaviors of M&NPs in the systemic circulation are detailed. In particular, the interaction of M&NPs with blood proteins, biodistribution, and circulation under flow conditions are analyzed. Finally, the hurdles and future directions for developing nonspherical M&NPs are underscored.

Cyclodextrin-based Pickering emulsions: functional properties and drug delivery applications

Cyclodextrins (CDs) are biocompatible, cyclic oligosaccharides that are widely used in various industrial applications and have intriguing interfacial science properties. While CD molecules typically have low surface activity, they are capable of stabilizing emulsions by inclusion complexation of oil-phase components at the oil/water interface, which results in Pickering emulsion formation. Such surfactant-free formulations have gained considerable attention in recent years, owing to their enhanced physical stability, improved tolerability, and superior environmental compatibility compared to conventional, surfactant-based emulsions. In this review, we critically describe the latest insights into the molecular mechanisms involved in CD stabilization of Pickering emulsions, including covering practical aspects such as methods to prepare CD-based Pickering emulsions, lipid encapsulation, and relevant stability issues. In addition, the rheological and textural features of CD-based Pickering emulsions are discussed and particular attention is focused on promising examples for drug delivery, cosmetic, and nutraceutical applications. The functionality of currently developed CD-based Pickering emulsions is also summarised, including examples such as antifungal uses, and we close by discussing emerging possibilities to utilize the molecular encapsulation of CD-based emulsions for translational medicine applications in the antiviral and antibacterial spaces.

Nanotechnology-based Drug Delivery Systems as Potential for Skin Application: A Review

Administration of substances through the skin represents a promising alternative, in relation to other drug administration routes, due to its large body surface area, in order to offer ideal and multiple sites for drug administration. In addition, the administration of drugs through the skin avoids the first-pass metabolism, allowing an increase in the bioavailability of drugs, as well as reducing their side effects. However, the stratum corneum (SC) comprises the main barrier of protection against external agents, mainly due to its structure, composition and physicochemical properties, becoming the main limitation for the administration of substances through the skin. In view of the above, pharmaceutical technology has allowed the development of multiple drug delivery systems (DDS), which include liquid crystals (LC), cubosomes, liposomes, polymeric nanoparticles (PNP), nanoemulsions (NE), as well as cyclodextrins (CD) and dendrimers (DND). It appears that the DDS circumvents the problems of drug absorption through the SC layer of the skin, ensuring the release of the drug, as well as optimizing the therapeutic effect locally. This review aims to highlight the DDS that include LC, cubosomes, lipid systems, PNP, as well as CD and DND, to optimize topical skin therapies.

Methyl-grafted silica nanoparticle stabilized water-in-oil Pickering emulsions with low-temperature stability

HYPOTHESIS

The viscosity of water-in-oil Pickering emulsions may dramatically increase upon cooling. The solvation of the long-chain alkyl groups grafted on the particles stabilizer is the likely cause of the strong dependence of rheological property on temperature. Thus, we hypothesize that silica nanoparticles (NPs) grafted with short-chain alkyl groups can stabilize Pickering emulsions, yielding weakly temperature-dependent rheological property.

EXPERIMENTS

Using alkyl-grafted (methyl, octyl, and octadecyl) silica NPs as emulsifiers, the rheological properties and microstructure of the water-in-oil Pickering, as well as the solvation of the silica NPs, were studied using diffusing-wave spectroscopy microrheology measurements, confocal laser scanning microscopy, and low-field nuclear magnetic resonance measurements.

FINDINGS

The use of methyl- and octadecyl-grafted silica NPs, which have almost identical optimum contact angles, to stabilize emulsions dramatically reduced the effect of cooling on the viscosity. Moreover, the emulsions stabilized by these methyl-grafted silica NPs exhibited nearly constant rheological properties as the temperature decreased from 75 to 5 °C. The nearly constant rheological properties are attributed to the nearly constant solvation in this temperature range. These materials have potential applications in the cosmetics and petroleum industries.

A Novel Solid Nanocrystals Self-Stabilized Pickering Emulsion Prepared by Spray-Drying with Hydroxypropyl-β-cyclodextrin as Carriers

A drug nanocrystals self-stabilized Pickering emulsion (NSSPE) with a unique composition and microstructure has been proven to significantly increase the bioavailability of poorly soluble drugs. This study aimed to develop a new solid NSSPE of puerarin preserving the original microstructure of NSSPE by spray-drying. A series of water-soluble solid carriers were compared and then Box-Behnken design was used to optimize the parameters of spray-drying. The drug release and stability of the optimized solid NSSPE in vitro were also investigated. The results showed that hydroxypropyl-β-cyclodextrin (HP-β-CD), rather than solid carriers commonly used in solidification of traditional Pickering emulsions, was suitable for the solid NSSPE to retain the original appearance and size of emulsion droplets after reconstitution. The amount of HP-β-CD had more influences on the solid NSSPE than the feed rate and the inlet air temperature. Fluorescence microscopy, confocal laser scanning microscopy and scanning electron microscopy showed that the reconstituted emulsion of the solid NSSPE prepared with HP-β-CD had the same core-shell structure with a core of oil and a shell of puerarin nanocrystals as the liquid NSSPE. The particle size of puerarin nanocrystal sand interfacial adsorption rate also did not change significantly. The cumulative amount of released puerarin from the solid NSSPE had no significant difference compared with the liquid NSSPE, which were both significantly higher than that of puerarin crude material. The solid NSSPE was stable for 3 months under the accelerated condition of 75% relative humidity and 40 °C. Thus, it is possible todevelop the solid NSSPE preserving the unique microstructure and the superior properties in vitro of the liquid NSSPE for poorly soluble drugs.

Bulk and Interfacial Contributions to Stabilization of Cyclodextrin-Based Emulsions Mediated by Bacterial Cellulose

Cyclodextrin (CD)-based emulsions have a characteristic of rapid droplet flocculation, which limits their application as functional material templates, so it is very important to improve the stability of CD-based emulsions. In this study, we select bacterial cellulose (BC) as a nonadsorbing inhibitor to prevent flocculation of CD-based emulsions. We map a phase diagram of the aqueous dispersions of CD inclusion complexes (ICs) and BC from morphological observations and investigate the effects of BC on properties of the IC-laden films. We further explore the effects of BC concentration on the stability of the CD-based emulsions and investigate rheological behavior of the emulsions through large-amplitude oscillatory shear experiments. It shows that BC can effectively suppress the flocculation of CD-based emulsion droplets even at a concentration as low as 0.01 wt %. We propose that BC has dual effects from bulk and interfacial contributions on increasing emulsion stability. At low concentrations, BC mainly results in higher packing density of ICs on the emulsion droplet surface through excluded volume repulsion, and at high concentrations, BC creates a network structure that confines the motion of emulsion droplets and retards flocculation.

Main Applications of Cyclodextrins in the Food Industry as the Compounds of Choice to Form Host-Guest Complexes

Cyclodextrins (CDs) are cyclic oligomers broadly used in food manufacturing as food additives for different purposes, e.g., to improve sensorial qualities, shelf life, and sequestration of components. In this review, the latest advancements of their applications along with the characteristics of the uses of the different CDs (α, β, γ and their derivatives) were reviewed. Their beneficial effects can be achieved by mixing small amounts of CDs with the target material to be stabilized. Essentially, they have the capacity to form stable inclusion complexes with sensitive lipophilic nutrients and constituents of flavor and taste. Their toxicity has been also studied, showing that CDs are innocuous in oral administration. A review of the current legislation was also carried out, showing a general trend towards a wider acceptance of CDs as food additives. Suitable and cost-effective procedures for the manufacture of CDs have progressed, and nowadays it is possible to obtain realistic prices and used them in foods. Therefore, CDs have a promising future due to consumer demand for healthy and functional products.

Tomato Oil Encapsulation by α-, β-, and γ-Cyclodextrins: A Comparative Study on the Formation of Supramolecular Structures, Antioxidant Activity, and Carotenoid Stability

Cyclodextrins (CDs) are oligosaccharides, comprising 6 (α), 7 (β), or 8 (γ) glucose residues, used to prepare oil-in-water emulsions and improve oil stability towards degradation. In this research, the aptitude of α-, β-, and γ-CDs to form complexes with a supercritical CO₂ extracted lycopene-rich tomato oil (TO) was comparatively assessed. TO/CD emulsions and the resulting freeze-dried powders were characterized by microscopy, Fourier transform infrared-attenuated total reflection (FTIR-ATR), and differential scanning calorimetry (DSC), as well as for their antioxidant activity. Furthermore, carotenoid stability was monitored for 90 days at 25 and 4 °C. Confocal and SEM microscopy revealed morphological differences among samples. α- and β-CDs spontaneously associated into microcrystals assembling in thin spherical shells (cyclodextrinosomes, Ø ≈ 27 µm) at the oil/water interface. Much smaller (Ø ≈ 9 µm) aggregates were occasionally observed with γ-CDs, but most TO droplets appeared "naked". FTIR and DSC spectra indicated that most CDs did not participate in TO complex formation, nevertheless structurally different interfacial complexes were formed. The trolox equivalent antioxidant capacity (TEAC) activity of emulsions and powders highlighted better performances of α- and β-CDs as hydrophobic antioxidants-dispersing agents across aqueous media. Regardless of CDs type, low temperature slowed down carotenoid degradation in all samples, except all-[E]-lycopene, which does not appear efficiently protected by any CD type in the long storage period.

pH-Responsive Emulsions with β-Cyclodextrin/Vitamin E Assembled Shells for Controlled Delivery of Polyunsaturated Fatty Acids

Lipid-based delivery systems (LBDSs) are widely applied in pharmaceuticals and health care because of the increased bioavailability of lipophilic components when they are coadministered with high-fat meals. However, how to accurately control their in vivo release and stability is still challenging. Here, after introducing the simple esterification and coprecipitation, we created the dual-functional composite ODS-β-CD-VE by the coassembly of β-cyclodextrin (β-CD), octadecenyl succinic anhydride (ODSA), and vitamin E (VE). The resulting dual-functional particle presented a uniform sheetlike shape and nanometer size. In addition, its chemical structure was clarified in detail via nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Benefiting from the antioxygenation of VE, lipid oxidation in the ODS-β-CD-VE-stabilized Pickering emulsion was effectively inhibited. Meanwhile, pH-induced protonation/deprotonation of carboxyl groups guaranteed that the emulsions kept steady at pH ≤4 but were unsteady under neutral conditions. In this way, the lipids contained in the emulsion were protected from gastric juice and then digested and accurately released as n-3 polyunsaturated fatty acids (PUFA) in the simulated intestine environment. This strategy sheds some light on the rational and efficient construction of LBDSs for nutrient supplements and even pharmaceuticals in a living digestive tract.

Complex systems that incorporate cyclodextrins to get materials for some specific applications

Cyclodextrins (CDs) are a family of biodegradable cyclic hydrocarbons composed of α-(1,4) linked glucopyranose subunits, the more common containing 6, 7 or 8 glucose units are named α, β and γ-cyclodextrins respectively. Since the discovery of CDs, they have attracted interest among scientists and the first studies were about the properties of the native compounds and in particular their use as catalysts of organic reactions. Characteristics features of different types of cyclodextrins stimulated investigation in different areas of research, due to its non-toxic and non-inmunogenic properties and also to the development of an improved industrial production. In this way, many materials with important properties have been developed. This mini-review will focus on chemical systems that use cyclodextrins, whatever linked covalently or mediated by the non covalent interactions, to build complex systems developed mainly during the last five years.

Kinetic evolution of DOPC lipid bilayers exposed to α-cyclodextrins

Cyclodextrins are cyclic oligosaccharides capable of forming inclusion complexes with a variety of molecules, and as such have been recognized as a pharmaceutical and biotechnological asset. Cyclodextrins are known to interact with the components of cell membranes, and this correlates with a significant degree of cytotoxicity. In this work, we report on the mechanism of degradation of a model dioleoyl-phosphatidylcholine (DOPC) bilayer exposed to a solution with increasing concentrations of α-cyclodextrins. By combining optical fluorescence microscopy and quartz-crystal microbalance experiments, we study the evolution of supported lipid bilayers (SLBs) and giant unilamellar vesicles (GUVs). The rate of lipid removal is found to display a strong nonlinear dependence on the cyclodextrin concentration. A mechanism involving lipid aggregates is proposed.

Towards Rational Chemosensor Design through Improved Understanding of Experimental Parameter Variation and Tolerance in Cyclodextrin-Promoted Fluorescence Detection

We have previously developed a highly efficient fluorescence-based toxicant-detection method that operates in complex environments to detect aromatic toxicants and toxicant metabolites with high sensitivity and selectivity. This method relies on the ability of γ-cyclodextrin to act as a supramolecular scaffold, and uses a variety of non-covalent interactions between the cyclodextrin, toxicant, and fluorophore to enable efficient detection. Reported herein is an investigation of the effect of various experimental parameters, including host concentration, temperature, pH, salt, and solvent, on the observed energy-transfer efficiencies. These results advance our understanding of γ-cyclodextrin-based association complexes and provide crucial information for the development of fluorescence-based sensors using such complexation and the resultant fluorescence-based detection.