STABILITY AND RELEASE KINETICS OF NATURAL OIL MICROEMULSIONS 33 lauric acid and 19.140% saturated myristic acid as exhibited in Table I. This fi nding was in good agreement with a previous report that higher degrees of unsaturation of triglyceride oils in ME systems resulted in weaker interfacial interactions (29). Release profi les of both MEC1-N and MEO1-N were the best fi tted with the Higuchi model as illustrated in Table VII. Hence, the release of nicotinamide from both MEs was dependent on the diffu- sion mechanism (30). CONCLUSION Our fi ndings indicated that among four developed nicotinamide-loaded MEs (MEC1-N, MEC2-N, MEO1-N, and MEO2-N), only MEO2-N was not stable. Other three formula- tions were physically and chemically stable when kept at 4°C and RT during the period of 3 mo. However, storage at 45°C should be avoided for all samples. MEC1-N and Table VII Release Parameters of MEC1-N and MEO1-N Formulation Zero order First order Higuchi model r2 k0a (μg/cm2/h) r2 kfa (1/h) r2 kHa (μg/cm2/h1/2) MEC1-N 0.9175 106.29 ± 6.94 0.7129 0.08 ± 0.00 0.9977 615.87 ± 41.04 MEO1-N 0.9104 132.36 ± 4.97 0.7044 0.08 ± 0.00 0.9956 769.09 ± 31.71 a The k0, kf, and kH are release rate constants of zero order, fi rst order, and Higuchi model, respectively. Figure 4. In vitro release profi les of MEC1-N and MEO1-N through dialysis membrane.

JOURNAL OF COSMETIC SCIENCE 34 MEO1-N provided slow release profi les and their release kinetics were the best fi tted with the Higuchi model. The current study presented that natural oil MEs were possibly promising nano-carriers for topical delivery of nicotinamide. AC KNOWLEDGMENTS Th e study was fi nancially supported by the Nanotec-PSU Center of Excellence on Drug Delivery System, Thailand. Nang Hnin Ei Hlaing’s M.Sc. study was supported by the Higher Education Research Promotion and the Thailand’s Education Hub for Southern Region of ASEAN Countries Project Offi ce of the Higher Education Commission (TEH- AC 020/2017). The authors also thank Ms. Maria Mullet for assistance with the English. RE FERENCES (1) T. Hakozaki, L. Minwalla, J. Zhuang, M. Chhoa, A. Matsubara, K. Miyamoto, A. Greatens, G. G. Hillebrand, D. L. Bissett, and R. E. Boissy, The effect of niacinamide on reducing cutaneous pig- mentation and suppression of melanosome transfer, Br. J. Dermatol., 147, 20–31 (2002). (2) N. Otte, C. Borelli, and H. C. Korting, Nicotinamide–biologic actions of an emerging cosmetic ingredi- ent, Int. J. Cosmet. Sci., 27, 255–261 (2005). (3) E. Forbat, F. Al-Niaimi, and F. R. Ali, Use of nicotinamide in dermatology, Clin. Exp. Dermatol., 42, 137–144 (2017). (4) M. Venus, J. Waterman, and I. McNab, Basic physiology of the skin, Surgery, 29, 471–474 (2011). (5) P. Boonme, Applications of microemulsions in cosmetics, J. Cosmet. Dermatol., 6, 223–228 (2007). (6) A. Azeem, M. Rizwan, F. J. Ahmad, Z. I. Khan, R. K. Khar, M. Aqil, and S. Talegaonkar, Emerging role of microemulsions in cosmetics, Recent Pat. Drug Deliv. Formul., 2, 275–289 (2008). (7) P. Boonme, V. B. Junyaprasert, N. Suksawad, and S. Songkro, Microemulsions and nanoemulsions: novel vehicles for whitening cosmeceuticals, J. Biomed. Nanotechnol., 5, 373–383 (2009). (8) L. B. Lopes, Overcoming the cutaneous barrier with microemulsions, Pharmaceutics, 6, 52–77 (2014). (9) F. T. Vicentini, T. R. Simi, J. O. Del Ciampo, N. O. Wolga, D. L. Pitol, M. M. Iyomasa, M. V. Bentley, and M. J. Fonseca, Quercetin in w/o microemulsion: in vitro and in vivo skin penetration and ef- fi cacy against UVB-induced skin damages evaluated in vivo, Eur. J. Pharm. Biopharm., 69, 948–957 (2008). (10) S. Songkro, N. L. Lo, N. Tanmanee, D. Maneenuan, and P. Boonme, In vitro release, skin permeation and retention of benzophenone-3 from microemulsions (o/w and w/o), J. Drug Del. Sci. Tech., 24, 703–711 (2014). (11) P. Boonme, C. Boonthongchuay, W. Wongpoowarak, and T. Amnuaikit, Evaluation of nicotinamide microemulsion on the skin penetration enhancement, Pharm. Dev. Technol., 21, 116–120 (2016). (12) A. Pandey, A. Mittal, N. Chauhan, and S. Alam, Role of surfactants as penetration enhancer in transder- mal drug delivery system, J. Mol. Pharm. Org. Process Res., 2, 1000113 (2014). (13) M. T. Evangelista, F. Abad-Casintahan, and L. Lopez-Villafuerte, The effect of topical virgin coconut oil on SCORAD index, transepidermal water loss, and skin capacitance in mild to moderate pediatric atopic dermatitis: a randomized, double-blind, clinical trial, Int. J. Dermatol., 53, 100–108 (2014). (14) A. H. Mota, C. O. Silva, M. Nicolai, A. Baby, L. Palma, P. Rijo, L. Ascensão, and C. P. Reis, Design and evaluation of novel topical formulation with olive oil as natural functional active, Pharm. Dev. Technol., 23, 794–805 (2018). (15) K. Wuttikul and P. Boonme, Formation of microemulsions for using as cosmeceutical delivery systems: effects of various components and characteristics of some formulations, Drug Deliv. Transl. Res., 6, 254– 262 (2016). (16) J. Leanpolchareanchai, K. Padois, F. Falson, R. Bavovada, and P. Pithayanukul, Microemulsion system for topical delivery of Thai mango seed kernel extract: development, physicochemical characterisation and ex vivo skin permeation studies, Molecules, 19, 17107–17129 (2014). (17) P. Boonme, N. Suksawad, and S. Songkro, Characterization and release kinetics of nicotinamide microemulsion-based gels, J. Cosmet. Sci., 63, 397–406 (2012).