Solid lipid microparticles (SLMs), combined with the advantages of double emulsions (W/O/W) and water-in-oil micro emulsions, are developed for transdermal administration. A modified one-step emulsification method was introduced, which made the industrial production of SLMs easier. Variables involved in preparation that may affect the solubility, stability, and entrapment efficiency of SLMs were investigated. With vitamin C as a model active, the advantages of SLMs in transdermal administration were studied further. Vitamin C, encapsulated by SLMs, exhibited an enhanced storage stability up to 1 month under 25°C and a sustained releasing profile over 24 h. Furthermore, the SLMs was very helpful in delivering vitamin C molecules into skin through stratum corneum. The result showed 5.52-fold (p < 0.05) increased absorption of vitamin C, compared to the vitamin C solution. These studies demonstrate that SLMs could be a promising method for facilitating transdermal penetration of active ingredients with high solubility and low permeability.Practical applications: SLMs could be used as a drug delivery system for hydrophilic active ingredients with low permeability. Experiments have shown better stability and skin permeability of vitamin C incorporated in SLMs when compared with pure vitamin C. Furthermore, the method of producing SLMs is simple and can be realized in a way of industrial production. Since SLMs can dissolve in water, SLMs can be directly used on wet skin with excellent skin-touch feel. As a result, SLMs could be a practical delivery system for hydrophilic compounds with low permeability in many applications such as pharmaceuticals and cosmetics.

The present study investigated a self-double-emulsifying drug delivery system loaded with epigallocatechin-3-gallate to improve epigallocatechin-3-gallate skin retention. The long chain solid lipids (cetostearyl alcohol) and macadamia oil were utilized as a carrier to deliver the bioactive ingredient. Response surface methodology was used to optimize the formulation, and the solid lipid to total lipid weight ratio, concentration of epigallocatechin-3-gallate and hydrophilic surfactant on skin retention were found to be the principal factors. The optimum formulation with high encapsulation efficiency (95.75%), self-double-emulsification performance (99.58%) and skin retention (87.24%) were derived from the fitted models and experimentally examined, demonstrating a reasonable agreement between experimental and predicted values. Epigallocatechin-3-gallate-self-double-emulsifying drug delivery system was found to be stable for 3 months. Transdermal studies could explain a higher skin diffusion of epigallocatechin-3-gallate from the self-double-emulsifying drug delivery system compared with EGCG aqueous solution. In vitro cytotoxicity showed that epigallocatechin-3-gallate-self-double-emulsifying drug delivery system did not exert hazardous effect on L929 cells up to 1:10.