Up to date, no worldwide standard in vitro method has been established for the determination of the sun protection factor (SPF), since there are many problems in terms of its repeatability and reliability. Here, we have studied the problems on the in vitro SPF measurements brought about by the phenomenon called viscous fingering. A spatially periodic stripe pattern is usually formed spontaneously when a viscous fluid is applied onto a solid substrate. For the in vitro SPF measurements, the recommended amount of sunscreen is applied onto a substrate, and the intensity of the transmitted UV light through the sunscreen layer is evaluated. Our theoretical analysis indicated that the nonuniformity of the thickness of the sunscreen layer varied the net UV absorbance. Pseudo-sunscreen composites having no phase separation structures were prepared and applied on a quartz plate for the measurements of the UV absorbance. Two types of applicators, a block applicator and a 4-sided applicator were used. The flat surface was always obtained when the 4-sided applicator was used, while the spatially periodic stripe pattern was always generated spontaneously when the block applicator was used. The net UV absorbance of the layer on which the stripe pattern was formed was found to be lower than that of the flat layer having the same average thickness. Theoretical simulations quantitatively reproduced the variation of the net UV absorbance led by the change of the geometry of the layer. The results of this study propose the definite necessity of strict regulations on the coating method of sunscreens for the establishment of the in vitro SPF test method.

Keywords:
Sun protection factor, In vitro measurement, Viscous fingering, Spontaneous pattern formation, Sunscreen application, Ultraviolet absorbance, Simulation
1.
International Standards Organization: Cosmetics - sun protection test methods - in vivo determination of the sun protection factor (SPF) (ISO 24444). Geneva, International Standards Organization, 2010.
2.
Stokes RP, Diffey BL: In vitro assay of high-SPF sunscreens. J Soc Cosmet Chem 1997;48:289-295.
3.
Diffey BL, Tanner PR, Matts PJ, Nash JF: In vitro assessment of the broad-spectrum ultraviolet protection of sunscreen products. J Am Acad Dermatol 2000;43:1024-1035.
4.
Gers-Barlag H, Klette E, Bimczok R, Springob C, Finkel P, Rudolph T, Gonzenbach HU, Schneider PH, Kockott D, Heinrich U, Tronnier H, Bernklau R, Johncock W, Langner R, Driller HJ, Westenfelder H: In vitro testing to assess the UVA protection performance of sun care products. Int J Cosmet Sci 2001;23:3-14.
5.
Heinrich U, Tronnier H, Kockott D, Kuckuk R, Heise HM: Comparison of sun protection factors determined by an in vivo and different in vitro methodologies: a study with 58 different commercially available sunscreen products. Int J Cosmet Sci 2004;26:79-89.
6.
Ferguson J, Brown MW, Hubbard AW, Shaw MI: Determination of sun protection factors. Correlation between in vivo human studies and in vitro skin cast method. Int J Cosmet Sci 1988;10:117-129.
7.
Diffey BL, Robson J: A new substrate to measure sunscreen protection factors throughout the ultraviolet spectrum. J Soc Cosmet Chem 1989;40:127-133.
8.
Kelley KA, Laskar PA, Ewing GD, Dromgoole SH, Lichtin JL, Sakr AA: In vitro sun protection factor evaluation of sunscreen products. J Soc Cosmet Chem 1993;44:139-151.
9.
Soruce SR, Hewitt JP: In vitro SPF, methodology and correlation with in vivo data. Eur Cosmet 1995;6:14-20.
10.
Rohr M, Klette E, Ruppert S, Bimczok R, Klebon B, Heinrich U, Tronnier H, Johncock W, Peters S, Pflucker F, Rudolph T, Flosser-Muller H, Jenni K, Kockott D, Lademann J, Herzog B, Bielfeldt S, Mendrok-Edinger C, Hanay C, Zastrow L: In vitro sun protection factor: still a challenge with no final answer. Skin Pharmacol Physiol 2010;23:201-212.
11.
Prigogine I, Kondepudi D: Modern Thermodynamics: From Heat Engines to Dissipative Structures. Chichester, Wiley & Sons, 1998.
12.
Hele-Shaw H: The flow of water. Nature 1898;58:34-36.
13.
Saffman PG, Taylor G: The penetration of a fluid into a porous medium or Hele-Shaw cell containing a more viscous liquid. Proc R Soc Lond Ser A 1958;245:312-329.
14.
Casademunt J: Viscous fingering as a paradigm of interfacial pattern formation: recent results and new challenges. Chaos 2004;14:809-824.
15.
Paterson L: Radial fingering in a Hele Shaw cell. J Fluid Mech 1981;113:513-529.
16.
Pearson JRA: The instability of uniform viscous flow under rollers and spreaders. J Fluid Mech 1960;7:481-500.
17.
Taylor GI: Cavitation of a viscous fluid in narrow passages. J Fluid Mech 1963;16:595-619.
18.
Hakim V, Rabaud M, Thome H, Couder Y: Directional growth in viscous fingering; in Coullet P, Huerre P (eds): New Trends in Nonlinear Dynamics and Pattern-Forming Phenomena. New York, Springer, 1991, vol 237, pp 327-337.
19.
Pitts E, Greiller J: The flow of thin liquid films between rollers. J Fluid Mech 1961;11:33-50.
20.
Rabaud M, Michalland S, Couder Y: Dynamic regimes of directional viscous fingering - spatiotemporal chaos and wave propagation. Phys Rev Lett 1990;64:184-187.
21.
McEwan AD, Taylor GI: Peeling of a flexible strip attached by a viscous adhesive. J Fluid Mech 1966;26:1-15.
22.
Yamazaki Y, Toda A: Dynamical-morphological property of adhesive tape in peeling. J Phys Soc Jpn 2002;71:1618-1621.
23.
Yamazaki Y, Toda A: Stability of tunnel structure and relationship between peel load and spatiotemporal pattern by deformed adhesive during peeling. J Phys Soc Jpn 2004;73:2342-2346.
24.
Kuroda A, Ishihara T, Takeshige H, Asakura K: Fabrication of spatially periodic double roughness structures by directional viscous fingering and spinodal dewetting for water-repellent surfaces. J Phys Chem B 2008;112:1163-1169.
25.
Joly P, Kuroda A, Asakura K: Preparation of highly water-repellent surface by spontaneous formation of double scale roughness pattern. J Oleo Sci 2010;59:89-94.
26.
COLIPA Guideline: In vitro method for the determination of the UVA protection factor and ‘critical wavelength' values of sunscreen products. 2007.
27.
International Standards Organization: Cosmetics - sun protection test methods - determination of sunscreen UVA photoprotection in vitro (ISO 24443). Geneva, International Standards Organization, 2012.
28.
US Food and Drug Administration: Sunscreen drug products for over-the-counter human use. Final rules and proposed rules. Register 2011;76:35620-35665.
29.
Asakura K, Kuroda A: Japan Patent No 063180. 2012.
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2014
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