Enhancing the sweat resistance of sunscreens

Abstract

Background: While sunbathing of performing outdoor sport activities, sunscreens are important for protection of uncovered skin against ultraviolet (UV) radiation. However, perspiration negatively affects the performance of a sunscreen film by weakening its substantivity and uniformity through the activation of two mechanisms, namely sunscreen wash-off and sunscreen redistribution.

Material and methods: We used a perspiring skin simulator to investigate the effect of sunscreen formulation on its efficiency upon sweating. Specifically, we modified the sunscreen formulation by incorporating a hydrophobic film former and adding water-absorbing particles. Sunscreen performance before and after perspiration is assessed by in vitro sun protection factor measurements, direct detection of changes in the sunscreen distribution using UV reflectance imaging, and by coherent anti-Stokes Raman scattering (CARS) microscopy for microscopic characterization of the UV filter relocation.

Results: The results show that incorporating a hydrophobic film former can decrease sunscreen wash-off due to sweating, while an excessive amount of film former might negatively affect the sunscreen distribution. The addition of water-absorbing particles, on the other hand, had either a negative or positive impact on the sunscreen substantivity, depending on the particle properties. While the addition of large water-absorbing particles appeared to increase sunscreen redistribution, smaller particles that could form a gel-like structure upon contact with water, appeared to change sunscreen wetting and sweat droplet spreading, thereby decreasing sunscreen wash-off and sunscreen redistribution.

Conclusions: We find that using a combination of hydrophobic film formers, which increase water resistance, and small water-absorbing particles, which change the wetting behavior, can make sunscreen formulations more sweat-resistant and less runny.

Keywords: film former; gelatin; perspiring skin simulator; sunscreen; sweat resistance; water-absorbing particles.

FIGURE 1

Schematic illustration of the perspiring skin simulator setup and a picture of the skin‐mimicking substrate during perspiration

FIGURE 2

Schematic illustration of skin‐mimicking substrate in which only the upper side is able to sweat. (A) Untreated substrate, and (B‐D) Substrate with applied sunscreen on the upper side: (B) before onset of perspiration, (C) during perspiration, and (D) after drying. The chamber is kept in a vertical position during perspiration and drying, enabling sweat to flow from the sunscreen‐treated area to the untreated area

FIGURE 3

In vitro SPF measurements for sunscreen formulations with different film former concentrations. (A) Individually measured in vitro SPF values measured on given positions after sweating versus initial SPF values measured on the exact same positions. The data for each formulation are obtained on five individual artificial skin samples and for nine positions on each sample. The dashed lines represent the location of the SPF values if there is no difference before and after sweating. (B) Statistical representation of the difference in measured SPF values before and after sweating. The colored area represents the interquartile range (IQR) showing the middle 50% of the data (25%–75%). The middle line represents the median, and the small white dot is the mean value. The mean value for each set of experiments is also shown. The upper and lower lines represent the maximum and minimum SPF change excluding the outliers. The dots outside the colored region indicate the outliers

FIGURE 4

Ultraviolet (UV) reflectance images of the skin‐mimicking substrates. The images show how sunscreens with different concentrations of hydrophobic film formers are washed off and redistributed during perspiration. (A) After sunscreen application on the area with sweat pores, (B) after sweating for 20 min, (C) 20 min after the sweating was stopped, and (D) after complete drying of the samples

FIGURE 5

In vitro sun protection factor (SPF) measurements for sunscreen formulations with and without water‐absorbing additives before and after sweating. (A) Individually measured in vitro SPF values measured on given positions after sweating versus initial SPF values measured on the exact same positions. The data for each formulation are obtained on five individual artificial skin samples and for nine positions on each sample. The dashed lines represent the location of the SPF values if there is no difference before and after sweating. (B) Statistical representation of the difference in measured SPF values before and after sweating. The colored area represents the interquartile range (IQR) showing the middle 50% of the data (25%–75%). The middle line represents the median, and the small white dot is the mean value. The mean value for each set of experiments is also shown. The upper and lower lines represent the maximum and minimum SPF change excluding the outliers. The dots outside the colored region indicate the outliers

FIGURE 6

Ultraviolet (UV) reflectance images of the skin‐mimicking substrates. The images show how sunscreens with and without water‐absorbing particles are washed off and redistributed during perspiration. (A) After sunscreen application on the area with sweat pores, (B) after sweating for 20 min, (C) 20 min after the sweating was stopped, and (D) after complete drying of the samples

FIGURE 7

1.5 × 1.5 μm² coherent anti‐Stokes Raman scattering (CARS) images showing the growth and merging of sweat droplets on the sunscreen‐treated skin‐mimicking substrate. (A) Sunscreen formulation with no water‐absorbing particles. The z‐axis here shows an elevation up to 250 μm. (B) Sunscreen formulation with 1% wt. PAC‐6 particles. The z‐axis here shows an elevation up to 100 μm