Evaporation of Particle-Stabilized Emulsion Sunscreen Films

Topical pickering emulsion versus classical excipients: A study of the residual film on the human skin

The interest in Pickering emulsions is based on the possibility of replacing classical emulsifiers with solid particles. These emulsions are very attractive in the pharmaceutical field for their stability virtues and as a vehicle to deliver active ingredients. The study aimed to analyze the properties of the residual film of the Pickering emulsions on the human skin compared to conventional systems. For this project, three types of solid particles were used: titanium dioxide, zinc oxide and silicon dioxide. All of them are capable of stabilizing the oil/water interface and thus forming totally emulsified systems. To create an emulsion of reference, a classical surfactant was used as an excipient. Complementary systems containing both particles and the emulsifier were also analyzed. Then, a combined approach between physicochemical and biometrological in vivo analysis was employed. The study proved that Pickering emulsions stabilized by the metal oxides were distinct from the reference emulsion in terms of droplet sizes and organization, rheological and textural responses. Consequently, it impacted the properties of the residual film once the product was applied to the skin. The particle-stabilized emulsions formed a hydrophobic film counter to conventional excipients. Also, the Friction parameter (or the roughness of the film) was directly linked to the quantity of the particles used in the formulation and their perception on the skin surface. The use of the particles blurs the glossy effect of the oil phase. Finally, it was observed that the appearance of the residual film was impacted by the type of the particle, namely TiO2 and ZnO particles.

Glycols: The ubiquitous solvent for dermal formulations

Glycols stand out as one of the most commonly employed safe and effective excipients for pharmaceutical and cosmeceutical products. Their widespread adoption can be attributed to their exceptional solvency characteristics and their ability to interact effectively with skin lipids and keratin for permeation enhancement. Notably, propylene glycol enjoys significant popularity in this regard. Ongoing research endeavours have been dedicated to scrutinising the impact of glycols on dermal drug delivery and shedding light on the intricate mechanisms by which glycols enhance skin permeation. This review aims to mitigate the discordance within the existing literature, assemble a holistic understanding of the impact of glycols on the percutaneous absorption of active compounds and furnish the reader with a profound comprehension of the foundational facets pertaining to their skin permeation enhancement mechanisms, while simultaneously delving deeper into the intricacies of these processes.

In vitro Evaluation Method of UV Protecting Ability of Sunscreens: Clarifying and Overcoming Problems to Develop New Method

Factors influencing on in vitro evaluation of UV protecting ability of sunscreens were analyzed. It was found that any factors making the sunscreen layer spatially inhomogeneous, such as directional viscous fingering during the sunscreen application, dewetting of applied sunscreen layer, and the surface roughness of the standard PMMA plate, alter the UV transmittance. New application procedure and new type of flat hydrophilic plate were developed for inhibiting the generation of spatial inhomogeneity in applied sunscreen layer. The method created by the combination of these newly developed technologies succeeded in providing reliable and reproducible in vitro evaluation of UV protecting ability.

New in vitro SPF Evaluation Method for Hydrophilic Sunscreen Samples

A new method was developed for the in vitro sun protection factor (SPF) evaluation of sunscreen samples. A new type of substrate, a hydroxyalkyl cellulose-coated plate, was also prepared specifically for hydrophilic samples. This new substrate was required because hydrophilic samples would be unlikely to wet the surface of the standard cosmetic PMMA UV evaluation plate. A super-hydrophilic quartz plate was prepared by corona-discharge treatment before an aqueous solution of hydroxyalkyl cellulose was spread on it. A flat and uniform hydroxyalkyl cellulose film was subsequently formed through the evaporation of water. Special care was taken to inhibit the generation of spatial non-uniformity. Six hydrophilic sunscreen samples with in vivo SPF values of 56, 55, 52, 25, 15, and 4, were then applied to the prepared hydroxyalkyl cellulose-coated plate, as well as a super-hydrophilic quartz plate and a flat hydrophobic PMMA plate. The thicknesses of the applied layers were determined using a wheel-shaped wet film thickness gauge immediately after the application, and UV transmission was measured using an SPF analyzer. The value of in vitro SPF was calculated from the UV absorbance and the thickness of the layer. For two out of the six samples, PMMA plate could not be available, as the samples were unable to wet the PMMA surface. Relatively small differences were observed between the in vitro SPF values when the super-hydrophilic and hydroxyalkyl cellulose-coated plates were used. Samples exhibiting higher in vivo SPF were also associated with higher in vitro SPF values, although a linear relationship was not observed. In contrast to the super-hydrophilic plate whose half-life of the super-hydrophilicity is only approximately five days, the hydrophilicity of the hydroxyalkyl cellulose-coated plate scarcely varied during six months of storage. Finally, a simplified evaluation method was also proposed. The validity of the method was verified through a ring test where three researchers employed this method in different laboratories at three independent organizations.

Patterning of a Drying Emulsion Film

Stabilizing layers of colloidal dispersions or emulsions to obtain homogeneous films is a real challenge. We describe here a new kind of instability in drying films of emulsions: during evaporation of the internal phase, cracks appear between the droplets that create aggregates according to a regular pattern. We show that this pattern only appears if the emulsion is adhesive, i.e., if droplets stick together. The pattern exhibits a characteristic length which depends on the adhesion strength and film thickness. These experimental results support a model where this instability is due to the gel structure and elastic properties of adhesive emulsions. Understanding this phenomenon will allow us to get a homogeneous film or to control it to get structured materials.

Buckling and Drying Kinetics of Particle-Stabilized Water Droplets Fully or Partially Immersed in an Oil Layer

We have investigated the effect of buckling of particle-stabilized water droplets on the drying kinetics. Particle-stabilized water droplets in an oil phase were prepared and the shrinking modes of the droplets during drying were controlled by the wettability of the particles. We obtained water droplets with and without buckling and used them in drying experiments. The drying times were comparable when the droplets were fully immersed in a thick oil layer. However, when the thickness of the oil layer was smaller than the droplet diameter, the buckled droplets showed faster drying. Observation of the reflection images around the droplets suggested that the buckled droplets preferentially shrank in the height direction, while the droplets without buckling isotropically shrank. Mathematical models that assumed diffusion of dissolved water molecules in the oil layer showed good agreement with the experimental data. The effective water-oil interfacial area was constant in the buckled droplets, whereas it shrank in the droplets without buckling. This would be a reason for the faster drying of the partially immersed buckled droplets. Particulate shells on liquid droplets could be used to enhance droplet drying.

Evaporation kinetics of continuous water and dispersed oil droplets

Drying of volatile oil droplets immersed in a continuous water phase was observed and analysed. Drying sample solutions were sandwiched between two glass plates and the water and oil phases were observed by confocal microscopy. In the initial stage of drying, evaporation of water was dominant and drying of the oil droplets was negligible. However, the rate of water evaporation decreased when the oil droplets were compressed. Comparison of experimental data with a diffusion model of water vapour showed that the decline in drying rates occurred earlier in the experiment than in the theoretical prediction. This implies that compression and narrowing of water paths caused the decline in the rate of water evaporation. After most water had evaporated, evaporation of the oil droplets occurred. The oil droplets did not shrink isotropically and the air-liquid interface invaded into the drying oil droplets. Cross-sectional observation by z-scanning revealed direct exposure of the oil droplets and they were pinned by the residual water phase. The water network between the oil droplets collapsed after the oil droplets had evaporated. The correlation between changes in structures and drying kinetics in both liquid phases was discussed.

Lipo-Dipeptide as an Emulsifier: Performance and Possible Mechanism

A lipo-dipeptide (C₁₃-lysine-arginine, C₁₃-KR) was designed as a potential emulsifier with good emulsifying properties under acidic condition. Compared with two traditional emulsifiers (whey protein isolate and Tween 80), C₁₃-KR emulsion had the minimum mean size but the highest zeta potential (around +100 mV). Moreover, C₁₃-KR emulsion showed better stability against environmental stresses, such as high salt concentrations and high temperature. The C₁₃-KR particles had the fastest move rate around 400 Hz when it attained an equilibrium state. Furthermore, C₁₃-KR emulsifier could sharply reduce the interfacial tension and had the lowest tension value at the oil/water interface. The interfacial tension of C₁₃-KR emulsifier was only 3.6 mN/m (0.5% w/v). In conclusion, the lipo-dipeptide C₁₃-KR could be considered as an emulsifier to produce emulsion under acidic condition.

Efficient UV Filter Solubilizers Prevent Recrystallization Favoring Accurate and Safe Sun Protection

Sun protection is a global concern, and maximizing sunscreen stability and efficacy depends partially on the prevention of UV filters recrystallization. We aimed to study the efficacy of hydrophobic solubilizers in preventing the recrystallization of solid hydrophobic UV filters in predissolutions, sunscreen formulations, and during simulated human use. Recrystallization of UV filters induced by ultrasonication, temperature variation, or simulated human application was analyzed by different methods. Maximum solubility of UV filters in solubilizers was determined. Surprisingly, the best solubilizer was not necessarily the best solvent to prevent recrystallization, suggesting there are different forces controlling these phenomena. Hydrophobic solubilizers tend to perform better than ethanol in predissolutions, but the presence of other components in final products may change their performance. Results suggest that some UV filters tend to form liquid clusters, which may behave as crystals and affect the desired even distribution of UV filters on the skin. UV filters were also found to respond differently to Hansen Solubility Parameters. Scanning electron microscopy supports the fact that recrystallization upon sunscreen application is an issue to be tested during development. A timesaving method to predict recrystallization of UV filters in clear systems was developed and is presented as a tool to enhance the efficacy of sunscreens.

Drying kinetics of water droplets stabilized by surfactant molecules or solid particles in a thin non-volatile oil layer

We have investigated drying of water droplets stabilized by solid particles or surfactant molecules in a thin oil layer. The surfactant-stabilized droplets isotropically shrink, whereas the droplets stabilized by spherical particles severely deform during drying because of buckling of the particulate shells. However, buckling of the shells hardly affects droplet drying. The drying times for complete evaporation are almost the same for water droplets with the same initial diameter and the drying time is independent of the type of surface stabilizer (particles or surfactant). The drying kinetics of the water droplets is well described by mathematical models, in which diffusion of water molecules in the oil phase to the oil-air interface is proposed as the rate-determining process. Droplets with a diameter comparable with the thickness of the oil layer shrink faster than small droplets because of the short diffusion length from the water droplets to the oil-air interface. We also investigated drying of water droplets stabilized by plate-like mica particles. The droplets also buckled but larger shells of mica particles remained compared with those of spherical particles. In addition, a longer drying time is necessary for some droplets stabilized by mica particles. These results indicate the possible effect of the particle morphology on the buckling and drying kinetics of particle-stabilized water droplets.

Silk Nanofibers as Robust and Versatile Emulsifiers

Peptides have been extensively studied as emulsifiers due to their sequence and size control, biocompatibility, versatility, and stabilizing capacity. However, cost and mass production remain the challenges for broader utility for these emulsifiers. Here we demonstrate the utility of silk fibroin nanofibers as emulsifiers, with superior functions to the more traditional peptide emulsifiers. This silk nanofiber system is universal for different oil phases with various polarities and demonstrates control of microcapsule size through tuning the ratio of silk fibroin nanofiber solutions to oils. Besides the improved stabilizing capacity to peptides, these silk fibroin nanofibers endow additional stability to the emulsions formed under high salt concentration and low pH. Highly efficient encapsulation of biomarkers through interfacial networks suggests potential applications in therapeutics, food, and cosmetics. Compared to peptide emulsifiers, these silk fibroin nanofibers offer advantages in terms of cost, purification, and production scale, without compromising biocompatibility, stabilizing capacity, and versatility.

Spectrophotometry of Thin Films of Light-Absorbing Particles

Thin films of dispersions of light-absorbing solid particles or emulsions containing a light-absorbing solute all have a nonuniform distribution of light-absorbing species throughout the sample volume. This results in nonuniform light absorption over the illuminated area, which causes the optical absorbance, as measured using a conventional specular UV-vis spectrophotometer, to deviate from the Beer-Lambert relationship. We have developed a theoretical model to account for the absorbance properties of such films, which are shown to depend on the size and volume fraction of the light-absorbing particles plus other sample variables. We have compared model predictions with measured spectra for samples consisting of emulsions containing a dissolved light-absorbing solute. Using no adjustable parameters, the model successfully predicts the behavior of nonuniform, light-absorbing emulsion films with varying values of droplet size, volume fraction, and other parameters.