Some dermatologists and sunscreen users have expressed concerns that EBS sprays may have a drying effect on skin. This perception likely is based on the fact that ethanol is a volatile solvent and could compromise the skin’s protective lipid barrier. Also, recreational activities related to the use of these sprays, such as swimming, can affect the skin's ability to retain moisture by depleting the natural moisturization factor (NMF). In the most widely appreciated context, skin barrier function refers to the stratum corneum’s (SC’s) ability to regulate trans-epidermal water loss (TEWL).
Practically anything that assaults or insults the SC also increases TEWL. It is known that moisturizing the skin renders it less vulnerable to barrier abrogation and can be achieved by emollients, occlusive agents and lipid fluidizing agents.
Some of the ingredients with such functions are often present in the non-volatile phase of EBS sprays, including emollients, sunscreen actives with emollient properties, occlusive and lipid fluidizing agents; i.e., glycerin, and polymers that help to impart water resistant properties.
To evaluate the impact of EBS sprays on skin barrier function, confirm their safety and efficacy, and to disprove the assumption that anhydrous sunscreen sprays dry the skin, several
studies included contact angle measurements (static and dynamic).
studies refer to TEWL and SPF Very Water Resistance (VWR, 80 minutes according to the FDA test protocol) of EBS sprays.
Contact angle (CA) is the angle formed by a liquid droplet on the surface at the three-phase boundary where a liquid, air and surface intersect; CA is often indicated by symbol Theta (q
). The hydrophobic or hydrophilic tendency of the substrate surface can be characterized by the value of CA formed by the water droplet on the liquid-surface interface. On a hydrophobic or lower energy surface, water forms droplets with CA values of 90°and higher, whereas on hydrophilic or higher energy surfaces, it forms droplets with CA less than 90°.
Clean, degreased human skin produces a high CA with water (> 100°) and behaves like a hydrophobic surface. The human skin - water CA was found to be sensitive to pretreatments on skin and could be reduced by the presence of minute quantities of surface active agents on skin, particularly by hydrophilic substances.5
It also indicates how readily the skin repels water; CAs above 90° indicate that skin is sufﬁciently hydrophobic to repel water, while a lower angle indicates that the skin attracts water.6
Human skin - water CA measurements were found to be a reliable screening and predictive tool to identify water-resistant formulations with their efficacy confirmed by in vivo testing according to a standardized water resistance COLIPA protocol.7
The correlation between CAs of a sessile water drop on artificial skin-like substrate after application of the leave-on daily moisturizers and their skin feel and a moisturization property was previously established. Products that generate relatively low angles of 50-60° tend to make more sensory claims related to “Light” and “Non-Greasy” feel, while products that produce relatively high angles of 80° and higher tend to make more claims related to long-term moisturization.8
A link between CAs in vitro and Water Resistant (WR) or Very Water Resistant (VWR) properties in vivo measured according to the FDA testing methodology was also found.9
Overall, the CA in vitro techniques that correlate with in vivo attributes of finished goods formulations are useful tools for the competitive benchmarking, screening of prospective active ingredients and polymers, optimization of product development and minimization of the costs associated with in vivo testing.
To conduct CAs measurements, an artificial skin-like substrate Vitro Skin (N-19)10 containing protein and lipid components and designed with the topography, pH, critical surface tension and ionic strength to effectively mimic human skin was chosen. This substrate is often used to evaluate the SPF/UVA in vitro, water resistance and photostability of sunscreens, to measure water contact angles to assess the probability of moisturization, sensory and water resistant claims, as well as the performance of adhesive bandages and spreading of emollients. As it absorbs water, the substrate swells and plasticizes in a manner similar to the SC. It is hydrophobic; i.e., water droplets are not fully absorbed and do not spread easily on it, and the static water contact angle on the substrate is >100°. 8-10
This is similar to clean, degreased human skin, which was previously reported to produce a CA with water (> 100°) and behave like a hydrophobic surface.5
Throughout the studies, the substrate was pre-hydrated according to a procedure previously described. 4,9 After pre-hydration, the substrate was equilibrated at ambient conditions until its evaporative water loss measured by VapoMeter11 reached a value of approximately 5g/m2/hr which is close to the TEWL range of untreated human skin. Reported TEWL values of untreated human forearm skin were in the range of 6.3 +/- 2.1g/m2/hr, when measured under similar conditions.12
Test products and model systems were applied using a 2mg/cm2 application dose on the skin topography-side of the substrate using “rub-on” technique also previously described.8-9
A contact angle measuring instrument, the DSA20E EasyDrop system13 was employed to determine the static contact angles of sessile drops and to conduct dynamic contact angle observations. De-ionized water was used as a probe solution.
Measurements of static CAs were taken within approximately 45 seconds, and dynamic CAs were measured continuously within approximately 11.5 minutes, recorded and analyzed.
Acrylates/octylacrylamide copolymer, which will be referred to as Polymer A, is a hydrophobic, carboxylated acrylic copolymer soluble in ethanol, isopropanol and fatty alcohols. The film-forming properties of this polymer help to maintain active sunscreen ingredients on the site of application by imparting resistance to abrasion or rub-off and entrapping actives within its film matrix to reduce their penetration into skin and sustain their release. To support the safe use of Polymer A in inhalable sunscreen products, an acute inhalation toxicity study14 and a 13-week sub-chronic inhalation toxicity study15 were previously conducted. To fully assess the risk potentialof an inhalable ingredient, the No Observable Adverse Effect Level (NOAEL) was determined. In the safety assessment, the exposure of the consumer is generally compared to a concentration or dose causing no effect in a relevant in vivo experiment. From these studies, it was determined that even at concentrations 100X the normal use level, there were no significant adverse effects.
VA/butyl maleate/isobornyl acrylate copolymer, which will be referred to as Polymer B, is a terpolymer of vinyl acetate, mono n-butyl maleate and isobornyl acrylate. It is soluble in ethanol and can achieve solubility in water by controlling the degree of neutralization of its free acid groups.
It was originally developed as a hair styling polymer but also has found application in anhydrous continuous spray sunscreen formulationsto boost water-resistant properties of anhydrous continuous sunscreen sprays.16
First, the impact of polymers A and B on substrate surface properties were evaluated in vitro in simple model systems representing 1%, 3% and 5% polymer solutions in anhydrous ethanol. Petrolatum was used as a well-known skin occlusive benchmark that reduces TEWL and provides a hydrophobic protective barrier to the skin surface.
Test results are presented in Figure 1.
Figure 1. Static water contact angles measured in vitro on untreated substrate and after application of model systems and petrolatum
Static water CA on untreated substrate was about 101.7°, which confirmed its hydrophobic properties. After application of petrolatum, a known occlusive benchmark, the CA was 103.9°. Polymer A has demonstrated the ability to maintain the hydrophobic properties of the untreated artificial skin substrate at all concentrations tested by generating static CAs ranging from 99.1 to 102.9°, which are similar to the value of untreated substrate of 101.7°.
Polymer B produced lower static CAs of 83.8 to 92.7°, all of which are lower than those on the untreated substrate. Overall, Polymer A demonstrated the similar to petrolatum ability to maintain the hydrophobic properties of the substrate, while Polymer B generated lower static CAs.
Next polymers A and B were incorporated in EBS sprays and tested in a similar manner.
Compositions of EBS sprays were as follows: avobenzone, 3%; octocrylene, 2%; homosalate, 12%; octisalate, 5%; oxybenzone, 4%; glycerin, 2%; polymer A or B at 0% (none) or at 3% (as active), and alcohol (ethanol) denaturated anhydrous, qs to 100%.
Results are presented in Figure 2.
Figure 2. Static water contact angles measured in vitro on untreated substrate and after application of EBS sprays and petrolatum
The EBS spray containing 3% Polymer A generated a higher static water CA of 96.2°, compared with the spray base without the polymer producing 83.6°. Petrolatum (occlusive benchmark) generated the highest CA of 103.9°, and the EBS spray with 3% Polymer B generated the lowest value of 70.8°. A comparison of these values with published data regarding in vitro/in vivo correlations among water contact angles and moisturization properties of lotions suggested that some of the EBS sprays tested could impart beneficial moisturizing properties by forming a hydrophobic film.9
The next step was to compare the barrier properties of the films formed by EBS sprays.
We evaluated if dynamic contact angle measurements can be used to determine and compare to what extent the petrolatum (occlusive benchmark), the EBS spray base (without polymer), EBS spray with 3% Polymer A or with 3% Polymer B can prevent the penetration of water through the film and the consequent interaction of the substrate with water.
The dynamic contact angles of water droplets applied to treated—i.e., with the EBS sprays including or excluding polymers A or B, or petrolatum—and untreated test substrates were measured continuously, within about 11.5 minutes after the water droplet contacted the surface, and recorded as movies.
From these movies, final pictures were taken (see Figure 3a-e),
which show the resulting contact angles after 11.5 minutes and the changes in the substrate due to the surface wetting, water absorption and swelling.
Figure 3. Dynamic contact angle observations:
a) on test skin substrate
b) with petrolatum
c) with EBS spray base omitting polymer
d) with EBS spray including 3% polymer A
and e) with EBS spray including 3% polymer B
Figure 4. TEWL, % change from the baseline
Fig. 5: Commercially available EBS sprays with highest SPF: 100-110 (top); high SPF: 50-85 (middle); and low-to-medium SPF: 15-30 (bottom)
The dynamic contact angle observations suggest that the untreated substrate interacts with water during prolonged contact by slightly swelling; however, the water did not spread noticeably, which again confirms hydrophobic properties of the substrate. Petrolatum (occlusive benchmark) prevented water from reaching the substrate beneath the occlusive film. The EBS spray base (without polymer) prevented the interaction of the substrate with water to some extent. The EBS spray with 3% Polymer A more effectively preventedthe outside water from penetrating the film, reaching the artificial skin substrate and interacting with the substrate to cause swellingcompared to the spray base.
Surprisingly, the EBS spray with 3% Polymer B made the substrate surface more wettable by water, compared with the EBS spray base (without polymer) and with untreated substrate.
This is a surprising observation because according to the in vivo study (n = 5), the EBS spray with 3% Polymer A had an SPF “Very Water Resistant” value (VWR or 80 min) of 33, and the EBS spray with 3% Polymer B had an SPF VWR value of 31.These data show that EBS with Polymer A and Polymer B impart somewhat similar very water resistant properties with Polymer A being slightly more effective; this means that both polymers helped to prevent sunscreen actives from being washed off from the skin and kept them on the skin surface.
Polymer A also prevented water from penetrating the sunscreen film and interacting with the substrate.
At the same time Polymer B did not prevent outside water from penetrating the sunscreen film and causing its wetting and swelling.
Thus, dynamic contact angle measurement technique helped to differentiate polymers A and B and to discover the differences in their respective barrier properties.
Both polymers tested impart water resistant properties to EBS sprays; however, their skin surface modifying properties are markedly different. Whereas Polymer A helped to prevent outside water from spreading on and reacting with the substrate under the film, Polymer B did not. The static and dynamic contact angle measurements also indicated that the EBS spray base and EBS with Polymer A could form a physical barrier film to potentially retain stratum corneum water content, prevent outside water from reaching the surface under the film and reduce TEWL.
This hypothesis was confirmed in a follow-up in vivo TEWL study.
TEWL studies were conducted on 10 panelists using EBS spray base (without polymer) and EBS spray with 3% Polymer A. The respective compositions of test articles were previously described.
The samples were applied via the “rub-on” application technique using a 2mg/cm2 dose.
“Rub-on” application technique is a standard technique used in the industry in human efficacy studies (TEWL, SPF static and VWR, etc.) and in many in vitro methods. Size of each test site was 25cm2. A Vapometer11 was used to measure the initial TEWL (baseline) and at 1 hour and 5 hours after application.
Both EBS sprays demonstrated a statistically significant reduction in TEWL 1 hour after application in comparison with the baseline values of -20.8% for the EBS spray with 3% Polymer A, and -16.4% for the EBS spray base. This decrease was still noticeable, although to a lesser extent, for both sprays five hours after application. These results suggest the positive impact of EBS sprays on skin barrier function. The durability of the film was indicated by the fact that a decrease in TEWL was still noticeable after 5 hours after EBS spray application. The EBS spray with 3% Polymer A produced a larger decrease in TEWL, in comparison with the EBS spray base (without polymer).
Additionally static water contact angles were measured on the substrate after application of numerous commercially available EBS sprays with SPF levels of 15-30 (low-medium), 50-85 (high) and 100-110 (highest).
See Figure 5 with photos of commercial EBS sprays.
According to the inactive ingredients lists, the majority of the commercially available EBS sprays contained polymer A, although their concentrations may have varied. The percent amounts of non-volatile ingredients in each product were determined by comparing the weight of liquid EBS spray sample with the weight of residual non-volatile matter, after anhydrous ethanol, a volatile carrier has been fully evaporated.
The experimental data shows there is a correlation between the static contact angle (CA) formed by water droplet on the skin-like substrate surface and the amount of non-volatile components deposited on the substrate, which at identical application doses, also corresponds to their concentration in the formulation. Consequently, the composition of the non-volatile phase can also contribute to this equation.
This may be illustrated by the sigmoid relation below where Y is water CA in degrees and X is the percentage of non-volatile components by weight, given the fixed application dose of 2mg/cm2.
Figure 6: Static water contact angles measured in vitro on test skin substrate after application of commercial EBS sprays.
R2 = 0.9*
*R2 coefficient is a measure of how accurate the regression line approximates the data points; when R2 is about 0.9, the average accuracy is above 96%.
Based on their CA values, it is plausible that some commercially available EBS sprays containing polymer A could potentially benefit skin barrier function by a TEWL-reduction mechanism.
Static and dynamic in vitro contact angle measurements and observations can be effectively utilized to determine the ability of the sunscreen film to prevent outside water from reaching the skin-like substrate surface, to assess the potential of the formulation to reduce TEWL, and to conduct comprehensive evaluations of polymers and commercial benchmarks. There is a correlation between static CA of water formed on skin-like substrate and the amount of non-volatile components deposited on substrate, which at identical application doses also correspond to their concentration in the formulation.
Acrylates/octylacrylamide copolymer was shown to enhance the occlusive properties of continuous ethanol-based sunscreen (EBS) sprays, which was demonstrated via the in vitro
static and dynamic contact angle measurements and confirmed by the in vivo
TEWL studies; additionally the safe use of this polymer in inhalable sunscreen products was confirmed. Thus, the findings presented effectively address some concerns associated with EBS sprays by demonstrating that their use can actually benefit skin barrier function and not dry out the skin. The work presented here can assist in the development of multifunctional EBS sprays with moisturizing and protective properties. Based on their ability to generate certain CA
values, it is plausible that some commercially available EBS sprays containing acrylates/octylacrylamide copolymer could potentially benefit skin barrier function by TEWL-reduction mechanism.
More info: Olga Dueva-Koganov, email: Olga.Dueva-Koganov@akzonobel.com or http://sc.akzonobel.com/en/personalcare/Pages/product-detail.aspx?prodID=6638
Acknowledgments: The authors wish to thank Damani Parran, Jeff Rogers, Katherine Spetrino, Maria Tolchinsky and Heidi Lebel.
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