Block copolymer stabilized nonaqueous biocompatible sub-micron emulsions for topical applications

Unconventional and conventional Pickering emulsions: Perspectives and challenges in skin applications

Pickering emulsions are free from molecular and classical surfactants and are stabilized by solid particles, creating long-term stability against emulsion coalescence. Additionally, these emulsions are both environmentally and skin-friendly, creating new and unexplored sensorial perceptions. Although the literature mostly describes conventional emulsions (oil-in-water), there are unconventional emulsions (multiple, oil-in-oil and water-in-water) with excellent prospects and challenges in skin application as oil-free systems, permeation enhancers and topical drug delivery agents, with various possibilities in pharmaceutical and cosmetic products. However, up to now, these conventional and unconventional Pickering emulsions are not yet available as commercial products. This review brings to the discussion some important aspects such as the use of phases, particles, rheological and sensorial perception, as well as current trends in the development of these emulsions.

Dynamic Covalent Nanoparticles for Acid-Responsive Nonaqueous Pickering Emulsions

Acid-responsive nonaqueous (glycerol in n-decane) Pickering emulsions were prepared using preferentially oil-wetted dynamic covalent silica (SiO₂-pDB) nanoparticles as the Pickering emulsifiers. The acid-responsive Pickering emulsifier SiO₂-pDB was prepared based on a Schiff base reaction between amino silica (SiO₂-NH₂) and p-decanoxybenzaldehyde (pDBA). The formation of SiO₂-pDB was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and elemental analysis. The preferentially oil-wetted character of SiO₂-pDB was indicated by contact angle measurement. Stable nonaqueous Pickering emulsions were prepared using preferentially oil-wetted SiO₂-pDB as the Pickering emulsifier. However, after adjusting the nonaqueous Pickering emulsions to an acidic environment, complete phase separation occurred. In the acidic environment, preferentially oil-wetted SiO₂-pDB decomposed into hydrophilic SiO₂-NH₂ and hydrophobic pDBA due to the decomposition of the dynamic imine bond in the SiO₂-pDB. Then, the hydrophilic SiO₂-NH₂ and hydrophobic pDBA desorbed from the two-phase interface, resulting in complete phase separation of the initially stable nonaqueous Pickering emulsions. The acid-responsive nonaqueous Pickering emulsions show great potential in application in water sensitive systems, such as oil-based drilling fluids.

Partition of Block Copolymers in Phase-Separating Polymer Solutions

The distribution of the AB diblock copolymer in a phase-separating solution composed of immiscible A and B homopolymers in a common solvent has been investigated theoretically. We have utilized the mixing Gibbs energy density for the interfacial phase based on mean-field lattice theory to this four-component system. Distributions of the AB diblock copolymer in the A and B homopolymer-rich bulk phases and the interfacial region between the separating bulk phases are calculated as a function of the B-block content, degrees of polymerization of the copolymer and A and B homopolymers, as well as interaction parameters among the A and B monomer units and the solvent. The copolymer prefers to distribute more in the interfacial region rather than separating bulk phases at a higher copolymer degree of polymerization and a higher interaction parameter between A and B monomer units. The theory is also compared with Asano et al.'s experimental results [ Langmuir 2015, 31, 7488-7495] for polystyrene-b-poly(ethylene glycol) copolymer added to the phase-separating solution of polystyrene and poly(ethylene glycol) homopolymers dissolved in chloroform.

Encapsulation and Characterization of Nanoemulsions Based on an Anti-oxidative Polymeric Amphiphile for Topical Apigenin Delivery

Apigenin (Apig) is used as a model drug due to its many beneficial bio-activities and therapeutic potentials. Nevertheless, its poor water solubility and low storage stability have limited its application feasibility on the pharmaceutical field. To address this issue, this study developed nanoemulsions (NEs) using an anti-oxidative polymeric amphiphile, d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS), hydrogenated soy lecithin (HL), black soldier fly larvae (BSFL) oil, and avocado (AV) oil through pre-homogenization and ultrasonication method. Addition of TPGS (weight ratios 100 and 50% as compared to HL) into NEs effectively reduced particle size and phase transition region area of NEs with pure HL. Incorporation of Apig into NEs made particle size increase and provided a disorder effect on intraparticle molecular packing. Nevertheless, the encapsulation efficiency of NEs for Apig approached to about 99%. The chemical stability of Apig was significantly improved and its antioxidant ability was elevated by incorporation with BSFL oil and AV oil NEs, especially for NEs with single TPGS. NEs with single TPGS also exhibited the best Apig skin deposition. For future application of topical Apig delivery, NEs-gel was formed by the addition of hyaluronic acid (HA) into NEs. Their rheological characteristics were dominated by the surfactant ratios of HL to TPGS.

Advances and Opportunities of Oil-in-Oil Emulsions

Emulsions are mixtures of two immiscible liquids in which droplets of one are dispersed in a continuous phase of the other. The most common emulsions are oil-water systems, which have found widespread use across a number of industries, for example, in the cosmetic and food industries, and are also of advanced scientific interest. In addition, the past decade has seen a significant increase in both the design and application of nonaqueous emulsions. This has been primarily driven by developments in understanding the mechanism of effective stabilization of oil-in-oil (o/o) systems, either using block copolymers (BCPs) or solid (Pickering) particles with appropriate surface functionality. These systems, as highlighted in this review, have enabled emergent applications in areas such as pharmaceutical delivery, energy storage, and materials design (e.g., polymerization, monolith, and porous polymer synthesis). These o/o emulsions complement traditional emulsions that utilize an aqueous phase and allow the use of materials incompatible with water. We assess recent advances in the preparation and stabilization of o/o emulsions, focusing on the identity of the stabilizer (BCP or particle), the interplay between stabilizer and oils, and highlighting applications and opportunities associated with o/o emulsions.

Self-Assembly of Block and Graft Copolymers in Organic Solvents: An Overview of Recent Advances

This review is an attempt to update the recent advances in the self-assembly of amphiphilic block and graft copolymers. Their micellization behavior is highlighted for linear AB, ABC triblock terpolymers, and graft structures in non-aqueous selective polar and non-polar solvents, including solvent mixtures and ionic liquids. The micellar characteristics, such as particle size, aggregation number, and morphology, are examined as a function of the copolymers' architecture and molecular characteristics.

Particles adsorbed at various non-aqueous liquid-liquid interfaces

Particles adsorbed at liquid interfaces are commonly used to stabilise water-oil Pickering emulsions and water-air foams. The fundamental understanding of the physics of particles adsorbed at water-air and water-oil interfaces is improving significantly due to novel techniques that enable the measurement of the contact angle of individual particles at a given interface. The case of non-aqueous interfaces and emulsions is less studied in the literature. Non-aqueous liquid-liquid interfaces in which water is replaced by other polar solvents have properties similar to those of water-oil interfaces. Nanocomposites of non-aqueous immiscible polymer blends containing inorganic particles at the interface are of great interest industrially and consequently more work has been devoted to them. By contrast, the behaviour of particles adsorbed at oil-oil interfaces in which both oils are immiscible and of low dielectric constant (ε<3) is scarcely studied. Hydrophobic particles are required to stabilise these oil-oil emulsions due to their irreversible adsorption, high interfacial activity and elastic shell behaviour.

Potential of Non-aqueous Microemulsions to Improve the Delivery of Lipophilic Drugs to the Skin

In this study, non-aqueous microemulsions were developed because of the challenges associated with finding pharmaceutically acceptable solvents for topical delivery of drugs sparingly soluble in water. The formulation irritation potential and ability to modulate the penetration of lipophilic compounds (progesterone, α-tocopherol, and lycopene) of interest for topical treatment/prevention of skin disorders were evaluated and compared to solutions and aqueous microemulsions of similar composition. The microemulsions (ME) were developed with BRIJ, vitamin E-TPGS, and ethanol as surfactant-co-surfactant blend and tributyrin, isopropyl myristate, and oleic acid as oil phase. As polar phase, propylene glycol (MEPG) or water (MEW) was used (26% w/w). The microemulsions were isotropic and based on viscosity and conductivity assessment, bicontinuous. Compared to drug solutions in lipophilic vehicles, MEPG improved drug delivery into viable skin layers by 2.5-38-fold; the magnitude of penetration enhancement mediated by MEPG into viable skin increased with drug lipophilicity, even though the absolute amount of drug delivered decreased. Delivery of progesterone and tocopherol, but not lycopene (the most lipophilic compound), increased up to 2.5-fold with MEW, and higher amounts of these two drugs were released from MEW (2-2.5-fold). Both microemulsions were considered safe for topical application, but MEPG-mediated decrease in the viability of reconstructed epidermis was more pronounced, suggesting its higher potential for irritation. We conclude that MEPG is a safe and suitable nanocarrier to deliver a variety of lipophilic drugs into viable skin layers, but the use of MEW might be more advantageous for drugs in the lower range of lipophilicity.

A “green” method for preparing ABCBA penta-block elastomers by using RAFT emulsion polymerization

A clean method for preparing ABCBA penta-block terpolymers was developed in a surfactant and organic solvent free emulsion system via one-pot RAFT polymerization.

Non-aqueous Isorefractive Pickering Emulsions

Non-aqueous Pickering emulsions of 16-240 μm diameter have been prepared using diblock copolymer worms with ethylene glycol as the droplet phase and an n-alkane as the continuous phase. Initial studies using n-dodecane resulted in stable emulsions that were significantly less turbid than conventional water-in-oil emulsions. This is attributed to the rather similar refractive indices of the latter two phases. By utilizing n-tetradecane as an alternative oil that almost precisely matches the refractive index of ethylene glycol, almost isorefractive ethylene glycol-in-n-tetradecane Pickering emulsions can be prepared. The droplet diameter and transparency of such emulsions can be systematically varied by adjusting the worm copolymer concentration.