Non-destructive monitoring of creaming of oil-in-water emulsion-based formulations using magnetic resonance imaging.

A non-destructive method for monitoring creaming of emulsion-based formulations is in great demand because it allows us to understand fully their instability mechanisms. This study was aimed at demonstrating the usefulness of magnetic resonance (MR) techniques, including MR imaging (MRI) and MR spectroscopy (MRS), for evaluating the physicochemical stability of emulsion-based formulations. Emulsions that are applicable as the base of practical skin creams were used as test samples. Substantial creaming was developed by centrifugation, which was then monitored by MRI. The creaming oil droplet layer and aqueous phase were clearly distinguished by quantitative MRI by measuring T1 and the apparent diffusion coefficient. Components in a selected volume in the emulsions could be analyzed using MRS. Then, model emulsions having different hydrophilic-lipophilic balance (HLB) values were tested, and the optimal HLB value for a stable dispersion was determined. In addition, the MRI examination enables the detection of creaming occurring in a polyethylene tube, which is commonly used for commercial products, without losing any image quality. These findings strongly indicate that MR techniques are powerful tools to evaluate the physicochemical stability of emulsion-based formulations. This study will make a great contribution to the development and quality control of emulsion-based formulations.

Impact of the state of water on the dispersion stability of a skin cream formulation elucidated by magnetic resonance techniques.

This study investigated the relationship between the state of water and the dispersion stability of a skin cream formulation. Hydrophilic ointments treated with a high-pressure wet-type jet mill were used as model formulations. Spin-lattice relaxation times (T(1)) were measured by magnetic resonance techniques to estimate the state of water in samples. A shorter T(1) relaxation time was obtained from samples with higher surfactant content, whereas the processing pressure of the jet mill and 1-week storage at 40 °C did not influence the T(1) relaxation time. Observations using scanning electron microscopy (SEM) showed that coalescence occurred in samples with lower surfactant contents (1.0% by weight) following 1-week storage at 40 °C. We also investigated samples prepared using a hydrophilic surfactant with a short polyethylene glycol (PEG) chain and with PEG-4000. From the change in T(1) relaxation times after removing the oil phase from samples by centrifugation, it was clarified that most of the surfactant was located on the surface of oil droplets. Furthermore, SEM observations showed that phase separation was facilitated as the PEG chain length of the surfactant shortened. Thus, a thin water layer over oil droplets is the most important factor for stabilizing their dispersion. This study provides proof-of-principle results on the contribution of the state of water to the dispersion stability of a skin cream formulation.

Improving rheological characteristics of hydrophilic ointment base by treatment with a high-pressure wet-type jet mill.

BACKGROUND: A high-pressure wet-type jet mill is a powerful equipment used for the dispersion and emulsification of substances. In this study, we investigated its usefulness in the preparation of skin cream formulations. METHOD: We prepared a hydrophilic ointment base as a typical skin cream base, and then treated it with the wet-type jet mill under different conditions. Controllable factors of the wet-type jet mill included processing pressure, treatment cycle, and temperature of the treatment. RESULT: Treatment with the wet-type jet mill had a great impact on the rheological characteristics of the hydrophilic ointment base. The hysteresis areas and yield values of the treated ointments were significantly increased by increasing the processing pressure and temperature during the treatment. From scanning electron microscopic observations, the oil droplet size of the hydrophilic ointments decreased after treatment with the wet-type jet mill, suggesting that a decrease in oil droplet size mediates changes in the rheological characteristics. CONCLUSION: Because we can expect the wet-type jet mill to control the rheological characteristics of the ointment, it is a promising tool for the preparation of skin cream formulations.