Figures Figure 1 Novel hair follicle plugging method. Novel hair follicle plugging method to evaluate follicular contribution towards magnesium permeation. A) Hair follicle. B) Add 0.2 μL acriflavine dissolved in water (1 mg/mL) around the hair. C) Add equal amount of cyanoacrylate. D) All hair follicles plugged. Figure 1

Figure 1 Figure 2 Validation for hair follicle plugging method. Test of the effectiveness of hair follicle plugging method by topical application of 3% caffeine solution on both normal skin (open hair follicle) and plugged hair follicle over 8 hours using a Franz cell setup. The resulting transdermal permeation of caffeine was sampled every hour from the receptor chamber of the Franz cell and evaluated using HPLC. Figure 2

Figure 2 Figure 3 Imaging Mag-fura-2 fluorescence using multiphoton microscopy. A) Excitation (320 nm) and emission spectra (520 nm) of mag-fura-2, tetra potassium salt. B) We used a LaVision Biotec Nikon multiphoton system with a tunable titanium Sapphire laser to visualize the skin sections. Excitation wavelength was 740 nm and emission was detected using three non-descanned filters: 447 to 460 nm, 485 to 550 nm, and 593 to 600 nm. Figure 3

Figure 3 Figure 4 Magnesium ions penetrate through skin but the extent depends on stratum corneum thickness. A) Images from three donors showed increased fluorescence in tape stripped (TS) and SC intact skin treated with 5 mM MgCl 2 solution for 30 min compared to skin untreated with MgCl 2 . B) Histograms showing normalized fluorescence intensity in viable epidermis in each donor. Scale bar = 50 μm. Figure 4

Figure 4 Figure 5 Magnesium ion permeation at varying times and concentrations. A) Representative images indicating penetration of magnesium ions at varying times and concentrations. B) The skin was treated with 52 mM and 1.9 M MgCl2 solutions for 5, 15 and 60 minutes. (* p<0.05, ** p<0.01). Figure 5

Figure 5 Figure 6 Hair follicles significantly contribute to magnesium permeation. A) Plugged skin (plugged region shown in blue). B) Unplugged skin sections allow penetration of magnesium ions. C) Increased penetration of magnesium ions in unplugged skin, (* p<0.05). Figure 6

Authors

1 School of Chemistry and Molecular Biosciences, Australian Infectious Diseases Research Centre, The University of Queensland, Brisbane, Queensland, 4072 Australia

2 Therapeutics Research Centre, School of Medicine, Translational Research Institute, The University of Queensland, 37 Kent St. Woolloongabba, Queensland, 76131, Australia

3 School of Pharmacy and Medical Sciences, University of South Australia, City East Campus, North Terrace, Adelaide, South Australia, 5001, Australia

* Correspondence: Prof. Ross T. Barnard, School of Chemistry and Molecular Biosciences, Australian Infectious Diseases Research Centre, The University of Queensland, Brisbane, Queensland, 4072 Australia.

Magnesium is an important micronutrient essential for various biological processes and its deficiency has been linked to several inflammatory disorders in humans. Topical magnesium delivery is one of the oldest forms of therapy for skin diseases, for example Dead Sea therapy and Epsom salt baths. Some anecdotal evidence and a few published reports have attributed amelioration of inflammatory skin conditions to the topical application of magnesium. On the other hand, transport of magnesium ions across the protective barrier of skin, the stratum corneum, is contentious. Our primary aim in this study was to estimate the extent of magnesium ion permeation through human skin and the role of hair follicles in facilitating the permeation. Upon topical application of magnesium solution, we found that magnesium penetrates through human stratum corneum and it depends on concentration and time of exposure. We also found that hair follicles make a significant contribution to magnesium penetration.