Formulation and Characterization of Structured Lipid Microparticles Prepared Using Microchannel Emulsification and Rotor-stator Homogenization

概要

Emulsions, as delivery systems, have drawn attention from many researchers of different fields, such as food, chemical and pharmaceutical industries. Among the many types of droplets and particles in emulsions, structured-lipid particles which contain the solid and liquid lipids at room temperature, are expected to present better encapsulation efficiency and storage stability. The conventional solid lipid particles were proved that they tend to expulse encapsulated ingredients out due to the condensed internal structure after crystallization. However, the existence of liquid oil in the structured lipid particles could ensure a better encapsulation capability of ingredients.

　Different types of structured lipid particles have been developed a lot since the last decade, especially the structured lipid nanoparticles for drug delivery. Among the techniques which could develop monodispersed system, microchannel emulsification (MCE) is able to prepare particles with a very narrow size distribution and coefficient of variation (CV) values lower than 5%. MCE involves a very mild droplet generation process with a very low energy requirement that is driven owing to interfacial tension difference at the micrometer scale. In addition, to observe the internal structure and preparation characteristics of particles, microparticles with the diameter from 0.1 to 1000 m are preferred instead of nanoparticles with the diameter less than 0.1 m. To the best of the author’s knowledge, MCE had not been used to fabricate structured lipid microparticles (StLMs) based on monodisperse emulsion. However, it is necessary to study formulation of StLMs by MCE, aiming for potential practical applications.

　On the other hand, one of the most popular method of structured lipid particles is rotor-stator homogenization (RSH). This method is especially common for large-scale production of lipid carriers and has many advantages, such as easy scale up and short production time. The disadvantages of the RSH method include the high energy input and wide particle size distribution.

　The first objective of this thesis was to investigate the formulation of StLMs using MCE at high temperature and their characterization. In addition, the comparative study of morphological property, storage stability and encapsulation efficiency of StLMs fabricated by MCE and RSH was performed.

　In Chapter 2, monodisperse StLMs with PA or tripalmitin (TP) mixed with soybean oil (SO) at different ratios were formulated using MCE at approximately 70 °C, which was followed by crystallization via cooling. Based on the physiochemical properties of the different dispersed phases, the formulation characteristics were observed and investigated by coupling a microscope video system to the MCE system. For each ratio and composition, the lipid droplets presented average diameter and CV in the respective ranges of 15.5–19.0 μm and 3.3– 8.4%, at approximately 70 °C. Following droplet crystallization via cooling, the StLMs exhibited notable morphological changes, but did not exhibit significant changes in the average size and CV. From the internal structure of each lipid molecule and the fundamental information supplied by differential scanning calorimetry (DSC) experiments, the assumptions for morphology variation were inferred. This information can be used as reference for research on the encapsulation abilities of StLMs.

　In Chapter 3, StLMs were formulated via MCE and RSH at approximately 70 °C. The samples were stored at room temperature, and crystallized. The collected samples were stored at 5 and 25 °C for 4 weeks. During this period, the appearance and particle size were observed and measured each week. StLMs with different lipid ratios exhibited different aggregation behaviors. After 4 weeks, the average diameter (d3,2) of StLMs (TP50:SO50) changed from 50.2 μm to 57.9 μm using MCE and from 57.9 μm to 68.4 μm using RSH. StLMs with TP ratios of 25 and 75 wt% aggregated into many large particles at room temperature. Although the TP microparticles formulated by MCE demonstrated some aggregation after crystallization, the formulation continued to maintain independent particles after 4 weeks. In contrast, the TP microparticles formulated by RSH aggregated as one large tablet after one week. The StLMs formulated by MCE demonstrated a more uniform size than those formulated by RSH. Microscopic observations revealed superior storage stability for StLMs and TP microparticles in terms of agglomeration.

　In Chapter 4, encapsulating bioactive compounds in StLMs has been demonstrated to be vital for the stability of the compounds. In this study, the StLMs loaded with α-Tocophenol (α-TC) formulated using MCE agglomerated more significantly than those using RSH, which contradicted the general results obtained for StLMs without bioactive compounds. Although the StLMs formulated using MCE were re-coalesced after natural crystallization at room temperature, the recovery ratios of freshly produced StLMs were 93.6% and 96.2% for MCE and RSH, respectively. The results of the encapsulation efficiency EE test demonstrated that, over 3 weeks, the α-TC concentrations in particles formulated by MCE stored at 5 and 25 °C decreased by 9.4% and 10.6%, respectively. The concentrations of particles formulated by RSH decreased by 26.7% and 12.1% at 5 and 25 °C, respectively.

　In conclusion, this study verified that monodisperse StLMs composed of solid and liquid lipids have been formulated successfully and stably using MCE with an appropriate range of lipid compositions. The physical properties and storage stabilities of StLMs formulated by MCE are superior to those of the StLMs formulated by RSH in terms of particle size distribution, aggregation behavior, and EE.