Skin Research and Technology 2016; 22: 46–54 Printed in Singapore All rights reserved doi: 10.1111/srt.12227
© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Skin Research and Technology
Tactile friction of topical formulations L. Skedung1,2, I. Buraczewska-Norin3, N. Dawood3,4, M. W. Rutland1,2 and L. Ringstad1 1 SP Technical Research Institute of Sweden, Stockholm, Sweden, Surface and Corrosion Science, KTH Royal Institute of Technology, Stockholm, Sweden, 3 Omega Pharma Nordic, Kista, Sweden and 4Department of Pharmacy, Uppsala University, Uppsala, Sweden 2
Background: The tactile perception is essential for all types of topical formulations (cosmetic, pharmaceutical, medical device) and the possibility to predict the sensorial response by using instrumental methods instead of sensory testing would save time and cost at an early stage product development. Here, we report on an instrumental evaluation method using tactile friction measurements to estimate perceptual attributes of topical formulations. Methods: Friction was measured between an index finger and an artificial skin substrate after application of formulations using a force sensor. Both model formulations of liquid crystalline phase structures with significantly different tactile properties, as well as commercial pharmaceutical moisturizing creams being more tactile-similar, were investigated. Friction coefficients were calculated as the ratio of the friction force to the applied load. The structures of the model formulations and phase transitions as a result of water evaporation were identified using optical microscopy. Results: The friction device could distinguish friction coefficients between the phase structures, as well as the
commercial creams after spreading and absorption into the substrate. In addition, phase transitions resulting in alterations in the feel of the formulations could be detected. A correlation was established between skin hydration and friction coefficient, where hydrated skin gave rise to higher friction. Also a link between skin smoothening and finger friction was established for the commercial moisturizing creams, although further investigations are needed to analyse this and correlations with other sensorial attributes in more detail. Conclusion: The present investigation shows that tactile friction measurements have potential as an alternative or complement in the evaluation of perception of topical formulations.
topical formulations with high consumer acceptance is central in the cosmetics industry, and substantial efforts and resources are in general placed on perceptionrelated issues for cosmetic formulations. Within the pharmaceutical area and in the treatment of various types of dry skin disorders, for example psoriasis or atopic dermatitis, where continuous and often life-long treatment is required, perception aspects are not as central although they are highly important for patient compliance (1). In fact, compliance difficulties due to the formulations being non-appealing from a cosmetic perspective are one of the main reasons why patients do not adhere to the prescribed therapies, even though the formulations have evident therapeutic effects (2–4). Perception is thus a critical element of all types of topical formulations and the possibility to include simplified methods for perception analysis at an early
stage would facilitate the development of formulations with improved acceptance. The tactile feel in product use is normally evaluated by means of human panels; however, these panel tests are costly and time-consuming. In addition, human panels often provide a subjective opinion based on comparison with reference products which is not always beneficial. Use of instrumental techniques for these types of evaluations, with demonstrated predictive capacity of perceptual response, would represent savings in both time and cost and also generate objective results which can be compared without using reference products. In regard to topical formulations, both rheology and texture (5–10), as well as friction (11–13) measurements have been evaluated as instrumental methods to predict various sensory attributes such as stickiness (7), slipperiness (13), greasiness (11), and skin smoothness (12) associated
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EVELOPMENT OF
Key words: skin friction – finger/tactile friction – tactile perception – hydration – topical formulations – moisturizers
Ó 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Accepted for publication 15 February 2015
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with the skin feel after treatment. It has been proposed that friction between finger and skin is the dominant interaction contributing to perception, and thus the measurement of this property should be a straightforward means of quantifying the experience (14). Skin friction is a system variable and depends on many factors, for example the probe material, environmental conditions, anatomical site, hydration, and roughness of the skin and counter surface, and has been recently reviewed (15–17). Moisture or an increase in skin hydration has shown to increase skin friction (18–25). This is believed to be due to a decrease in the Young’s modulus of the skin, resulting in more compliant skin and an increase in contact area during contact with a probe (26, 27). Since human skin is elastic, the skin can deform over surface asperities, and the real contact area may approach the apparent contact area, which is often discussed in combination with an observed decrease in friction with increasing surface roughness (28–33). The majority of skin friction studies are measured in dry contacts, but when a formulation is applied different lubrication regimes apply. During the initial application of a topical formulation the formed film is rather thick and the contact between the two surfaces (i.e. finger and skin) is dominated by hydrodynamic or fullfilm lubrication (34). At this stage the friction is highly affected by the viscosity of the formulation, and the friction properties are related to the perceived slipperiness of the product, where a low friction is related to a more slippery feel (35). After spreading of the formulation the surfaces in contact are separated by a thin film and mixed lubrication followed by boundary lubrication takes place (34, 36). During this stage, effects on the skin after product application can be detected. For example, hydrated and soft skin gives rise to a higher friction and adhesion compared to dry skin, initially resulting in a stickier feeling which gradually transforms to being smooth or moist depending on the properties of the formulation, resulting in a reduction of friction and adhesion over time (35). Considerable research has been devoted to measuring the effect of various cosmetic skin treatments on friction in vivo using a rotating or laterally moving probe on the inner forearm (11–13, 17, 37). Furthermore, the nanotribological properties of skin have been investigated using atomic force microscopy (38) and Timm
et al. (39) studied the lubrication effects of various particles in cosmetic powders by measuring friction using the mini traction machine and poly-dimethoxysiloxane as skin mimicking surfaces. In the latter case two model substrates in sliding contact were used and correlations were established between friction measurements and panel assessments of skin feel after powder application. However, rather less attention has been paid to measuring friction by moving fingers over a skin treated area, in which the friction encountered would resemble the friction perceived, i.e. tactile friction. Thus, the aims of this article are to evaluate the feasibility of measuring differences in tactile friction after application of topical formulations using the human finger and a model skin substrate, to study the change in friction over time and finally how this is related to formulation properties. Firstly, model formulations containing different liquid crystalline phase structures are evaluated to investigate if different structures exhibiting very different tactile properties would give different friction responses. Also the time dependent water evaporation from these formulations, which results in phase transitions, is examined in terms of friction properties. Secondly, commercial pharmaceutical skin creams containing different moisturizing components are evaluated to study the effect of formulation composition on friction properties as well as sensorial characteristics and to investigate if the finger friction method can discriminate between more tactile-similar formulations.
Experimental Materials Monoolein, Rylo Mg 19-glycerol monooleate of Pharma grade, was purchased from Danisco (Brabrand, Denmark) and sodiumoleate (purity >82%) was from Riedel-de Ha€en (Seelze, Germany). The water used was purified by means of a Milli-Q Plus Unit (Millipore, Billerica, MS, USA). Vitro-Skinâ (IMS Inc., Portland, ME, USA) was used as a model skin substrate and mimics human skin surface properties in terms of topography, pH, elasticity, surface tension, and ionic strength. This artificial skin was cut into pieces of 2.5 mm 9 5 mm and placed in a desiccator on the internal shelf above a beaker with a mixture of 85 wt% Milli-Q water and 15
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wt% glycerin (Sigma-Aldrich, St. Louis, MO, USA) for 16–24 h before use. This allowed for reproducible hydration of the skin samples prior to friction measurement.
Formulations Three formulations were prepared to represent different liquid crystalline phases from the ternary monoolein-sodiumoleate-water (MONaO-W) system (40), a lamellar phase (A, 28% : 27% : 45% MO : NaO : W, w/w/w), a cubic phase (B, 48% : 7% : 45% MO : NaO : W, w/w/w), and a reversed hexagonal phase (C, 64% : 16% : 20% MO : NaO : W, w/w/w), as shown in Fig. 1. The liquid crystalline phases were prepared by mixing the three components and heating the samples to 45°C in a water bath for 3 h while stirring from time to time, whereupon the samples were centrifuged at 2500 g for 10 min followed by manually stirring and heating to 45°C. The centrifugation-mixing-heating procedure was repeated six times until visually homogenous samples were obtained. Three moisturizing skin creams, denoted X, Y, and Z, were used to investigate whether differences in friction could be measured as a consequence of formulation composition and type of hydration mechanism. X and Z are commercial pharmaceutical creams available in Swedish pharmacies, and Y is a placebo formulation of X, with different composition in terms of specific key components. The compositions of the creams are presented in Table 1. The active
(a) (b)
(c)
Fig. 1. The ternary phase diagram of the monoolein-sodiumoleatewater system (40) where the approximate positions of formulation A, B, and C are indicated (triangles). Adapted with permission from (40), copyright (2001) American Chemical Society.
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moisturizing component in moisturizer X and Z is urea (3% in X and 5% in Z, respectively). Formulation Y is a placebo formulation of X, i.e. it does not contain urea. Furthermore, formulation X and Y contains several additional moisturizing agents such as propylene glycol, glycerin, dimethicone, and canola oil that are not present in Z.
Finger friction measurements Finger friction measurements were performed using a ForceBoardâ (41) (Industrial Dynamics Sweden AB, J€arf€alla, Sweden), equipped with both a horizontal and tangential load cell, consisting of strain gauges in a Wheatstone bridge configuration. A mechanical load results in voltage changes that are proportional to the applied load. The friction force (F) and applied load (L) were continuously recorded as a finger interrogated the model skin surface by moving the index finger back and forth, and the friction coefficients (l) were calculated as the ratio of the friction force and load according to: l¼
F L
ð1Þ
Approximately 3–4 mg/cm2 of the model formulations (formulation A, B, and C) were applied to the model skin substrate and spread using a gloved finger to obtain an even film thickness. The time schedule for the measurements were decided based on prior evaporation experiments (decrease in weight of formulation spread on a glass slide). Evaporation experiments showed that about 68% of the water evaporated within 20 min in formulation B and about 52% and 58% in formulation A and formulation C, respectively, with most water evaporating the first 5 min. Upon water evaporation, phase-transitions can occur, depending on the starting position in the ternary phase diagram (Fig. 1). The respective amount of water loss indicates that formulation B and C would undergo phase transitions within 20 min, whereas formulation A most probably would not. Due to the observed fast water evaporation, friction was decided to be measured at 0 min (i.e. immediately after spreading) and after 2.5, 6, and 20 min. When studying the commercial pharmaceutical moisturizing skin creams (moisturizer X, Y, and Z) 15 mg cream was applied to the model
Tactile friction of topical formulations TABLE 1. Composition of pharmaceutical moisturizing creams investigated Cream id
Composition (INCI)
X
Aqua, Caprylic/Capric Triglyceride, Canola Oil, Urea, Propylene Glycol, Cetearyl Alcohol, Glycerin, Dimethicone, Paraffin, Sodium Lactate, Carbomer, Polysorbate 60, Glyceryl Stearate, PEG-100 Stearate, Methylparaben, Propylparaben, Glyceryl Acrylate/Acrylic Acid Copolymer, Lactic Acid, Citric Acid Aqua, Caprylic/Capric Triglyceride, Canola Oil, Propylene Glycol, Cetearyl Alcohol, Glycerin, Dimethicone, Paraffin, Sodium Lactate, Carbomer, Polysorbate 60, Glyceryl Stearate, PEG-100 Stearate, Methylparaben, Propylparaben, Glyceryl Acrylate/Acrylic Acid Copolymer, Lactic Acid, Citric Acid Aqua, Paraffinum Liquidum, PEG-5 Glyceryl Stearate, Sodium Chloride, Urea, Cetearyl Alcohol, Palmitic Acid, Stearic Acid, Tromethamine, Hydrochloric Acid, Methylparaben, Propylparaben
Y
Z
INCI, The International Nomenclature of Cosmetic Ingredients.
skin and spread using a gloved finger to obtain 3 mg/cm2 evenly distributed across the surface. Friction was measured at 0 min (i.e. immediately after spreading) and after 10 and 60 min. The fingers used for measurements are affected by the formulation and were therefore cleaned by soap and water and dried between each measurement. As a control of the status of the model skin, influenced by exposure to ambient conditions which affect its hydration level, a reference measurement at an untreated part of the model skin was always performed prior to the actual measurement. For each friction experiment the index finger, inclined at about 30°, was stroked forward (away from the body) and back (toward the body) 13 times over the sample area (3–4 cm2) at a sliding speed of approximately 14 mm/s (31). The friction force and the applied load were recorded with a sampling rate of 100 Hz. The load was 0.5–1 N, which is in agreement with the reported optimum load when detecting tactile stimulus (42). All experiments were performed in triplicate and in controlled environment (T = 23°C and RH = 50%). Optical microscopy Formulation phase behavior was studied by an optical microscope using polarizing light (Zeiss Axioplan, Oberkochen, Germany). About 3–4 mg/cm2 formulation was spread with a gloved finger onto a microscope glass. The formulations A, B, and C were characterized directly after spreading and after 2.5, 6, and 20 min exposure to the surrounding environment allowing water to evaporate which may result in phase transitions. Prior to analysis after the elapsed time, a cover glass was placed on the formulation and further water evaporation was thus prohibited.
Sensory analysis of commercial pharmaceutical moisturizing creams X and Z by a trained panel The moisturizing creams X and Z were also examined for their sensory parameters. 15 volunteers (12 women and three men, aged 28–52) with experience of assessing cosmetic products recruited in-house at Omega Pharma Nordic AB participated in the study. The participants were trained using reference products selected from Omega Pharma Nordic products available on the market in addition to laboratory products designed to represent different levels of the attributes investigated. The evaluated attributes were the following; whitening effect of the moisturizer, absorption of moisturizer into the skin, moisturizing/hydrating ability, and smoothening of the skin surface. The assessments were made 5 min after application of the creams and graded on a visual analogue scale from 1 to 10. The whitening effect was graded from 1-no whitening effect to 10- highest whitening effect. The absorption of moisturizer into the skin was graded from 1-fast absorbing to 10-slow absorbing. The moisturizing/hydrating ability of test moisturizers was graded from 1-no moisturizing/hydrating effect to 10-highest moisturizing/ hydrating effect. The smoothening of the skin surface was graded from 1-rough surface to 10very smooth surface.
Results and Discussion Tactile friction of liquid crystalline phases Figure 2 displays optical microscopy images of the three different liquid crystalline phases A, B, and C indicated in Fig. 1. Optical microscopy directly after spreading on a microscope glass (Fig. 2a) verified a lamellar structure in formulation A, a cubic (isotropic) phase in formulation B and predominately reversed hexagonal structures in formulation C. Friction coefficients
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Skedung et al. A
B
C
a
b
Fig. 2. Optical microscopy images of formulation A, B, and C directly after spreading on a cover glass (a) and after exposure to the surrounding environment for 6 min (formulation B) and 20 min (formulation A and C) (b); The magnification is 109 or 209 and the scale bar is 100 lm.
Fig. 3. Average friction coefficients measured between an index finger and model skin after application of formulations of different liquid crystalline phases; lamellar phase (A), Cubic phase (B) and mainly reversed hexagonal phase (C), at different times; immediately after application and spreading (0 min), as well as after 2.5, 6, and 20 min. The error is presented as one standard deviation. Changes in friction over time are associated with phase transitions upon water evaporation.
for the three formulations at different times are displayed in Fig. 3. As shown in Fig. 2b, Formulation A remained in the lamellar phase also 20 min after spreading on the surface, and no significant changes in friction were detected over this time period (Fig. 3). For formulation B, on the other hand, a dramatic reduction in friction coefficient was already observed 2.5 min after application (Fig. 3), accompanied by a phase transition from cubic (isotropic) to a mixture of reversed hexagonal, lamellar, and isotropic phases (Fig. 2b). For Formulation C, the reversed hexagonal phase became more uniform within a minute after application, followed by a transition
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to a mixed phase with isotropic regions, where the isotropic regions increased with time (Fig. 2b). In case of formulation C the rate of water evaporation is lower compared to formulation B due to the structures being inverse, with the polar regions encased by hydrophobic regions, which hinders release of water from the formulation. This is mirrored in the reduction of the friction coefficient, which in the case of formulation C takes place between 6 and 20 min (Fig. 3), i.e. after a considerably longer time than for formulation B. Thus, it appears that different liquid crystalline phases give different friction responses with the friction coefficient decreasing in the following order; cubic > reversed hexagonal > lamellar, and that phase changes upon water evaporation can be followed by friction measurements. The phases, in turn, have considerably different tactile properties. For example, a cubic phase feels sticky, a reversed hexagonal phase greasy whereas a lamellar phase, which has a similar structure to the outer layer of skin, has a more soft and pleasant feel. Reversed hexagonal and cubic phases are not traditionally used in skin creams, however, they have shown interesting penetration enhancement properties for topical delivery of pharmaceutical components to the skin and mucosa (43–46).
Tactile friction of commercial pharmaceutical moisturizing creams The primary role of moisturizers is to increase the water content in stratum corneum and smoothen the skin surface. This can be achieved
Tactile friction of topical formulations
by including humectants such as urea, propylene glycol, and glycerin in the formulation, which act by attracting water from the surrounding to hydrate the skin. A different, complementary approach is to include emollients/lipids and other components that act occlusively as a protective barrier on the skin and hinder water loss. Often a combination of these two strategies is employed. Hydration and resulting smoothening of the skin (filling of fractures) will result in an increased friction as a result of increased contact area between skin and contacting material (4, 15, 25, 26). In the present investigation friction properties of model skin after application of three different pharmaceutical moisturizing creams for dry skin (denoted moisturizer X, Y, and Z) were studied by finger friction measurements to investigate whether differences in friction could be measured as a consequence of different moisturizing components. The average friction coefficients during application as well as 10 and 60 min after application, respectively, are displayed in Fig. 4, where the friction of untreated model skin is also included as a reference. The results for untreated model skin show that water evaporation over time results in a reduction in the friction coefficient, which in turn verifies that hydrated skin display higher friction. This was also observed during studies of liquid crystalline phases (reference bars in Fig. 3), and has been shown in many previous studies (25, 27–29, 33, 47).
Fig. 4. Average friction coefficients measured between an index finger and model skin after application of commercial moisturizing creams during spreading (0 min) and after 10 or 60 min. The error is presented as one standard deviation. The reduction in friction coefficient for the reference (untreated VitroSkin) is a result of water evaporation from the model skin substrate.
The friction properties detected during spreading, where hydrodynamic or full-film lubrication is dominating (34), is highly influence by formulation viscosity and related to the ease of application and the perceived slipperiness of the formulation (35). The creams investigated all give rise to a reduction in friction coefficient during application compared to the reference, where no significant differences between the creams are observed. 10 min after application, on the other hand, where boundary lubrication dominates (34), the friction coefficients for moisturizer X and Y are similar to the untreated model skin whereas a lower increase compared to the initial value is observed for moisturizer Z. This shows that the differences observed between the three creams are not a result of the urea (humectant and active moisturizing agent, present in X and Z but not in Y) but rather of other formulation components in X and Y. Furthermore, the sensory analysis performed by the test panel (Fig. 5) showed that that the skin is perceived as smoother after use of moisturizer X compared to moisturizer Z which likely is an effect of skin hydration, and it also indicated that the perceived moisturizing/hydrating ability is somewhat higher for moisturizer X. After 1 h similar friction coefficients are detected for all three moisturizing creams. This may be related to the fact that the hydrating effect of urea is achieved after a somewhat longer period in contact with the skin compared to the occlusive agents. It could also of course simply indicate that insufficient formulation is left on the model skin to have any effect 60 min after application, since similar friction coefficients are observed for both the cream-treated and the untreated substrates. The results from the sensory analysis (Fig. 5) show that moisturizer X was rated higher than moisturizer Z for all attributes assessed, although the differences were rather small. In addition, the variability in the sensory data is high (for example, the absorption into the skin of moisturizer X was graded from 2 to 7). This indicates that assessment of the skin feel properties absorption, moisturizing/hydrating ability, skin smoothening and whitening is a difficult task even for a trained panel, which has been observed also in previous studies (5). By use of reliable instrumental studies instead of panel testing these types of difficulties would be overcome. The present investigation show that finger friction measurements have potential for such purpose
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Fig. 5. Result of sensory analysis of moisturizers X and Z performed by the volunteers of the test panel. The graph presents the number of volunteers that gave a specific value to an attribute. The absorption of moisturizer into the skin was graded from 1-fast absorbing to 10-slow absorbing. The moisturizing/hydrating ability of the moisturizers was graded from 1-no moisturizing/hydrating effect to 10-highest moisturizing/hydrating effect. The smoothening of the skin as a result of moisturizers was graded from 1-no smoothening to 10-highest smoothening. The whitening effect of the moisturizers was graded from 1-no whitening effect to 10-highest whitening effect.
although more thorough correlation studies between finger friction measurements and perception of topical formulations are required in order to fully understand how friction is related to various sensory attributes.
Conclusion The results indicate that with the use of tactile friction measurements the friction response of various topical formations applied on a model skin substrate can be detected. Phase-transitions in model formulations as a result of water evaporation, confirmed by optical microscopy, result in changes in the friction coefficient between the finger and lubricated model skin substrate. The friction coefficient is increased for hydrated skin due to an increased contact area between finger and skin, as verified by measurements on untreated model skin during dehydration. During application of a cream where full-film lubrication dominates, the friction coefficient is reduced and no
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difference is observed between the creams, whereas a difference in friction related to the presence/absence of occlusive components in the creams are detected 10 min after application. By the tactile friction approach evaluated in this article it is feasible to detect small changes in skin friction due to either different formulation structure and composition or hydration states of the skin substrate. Further studies are needed to link friction measurements with various sensorial attributes to see whether measurable differences in friction actually correspond to different perception attributes of the skin, both during and after application.
Acknowledgments We thank Irena Blute for skillful experimental assistance with the Optical microscopy measurements. We would also like to acknowledge Dr. Adam Feiler and Dr. Anna Fureby for valuable discussions.
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Address: Lovisa Ringstad SP Technical Research Institute of Sweden Chemistry, Materials and Surfaces Box 5607 SE-114 86 Stockholm Sweden Tel: +46-10-5166017 Fax: +46-8-208998 e-mail: [email protected]
© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Skin Research and Technology
Tactile friction of topical formulations L. Skedung1,2, I. Buraczewska-Norin3, N. Dawood3,4, M. W. Rutland1,2 and L. Ringstad1 1 SP Technical Research Institute of Sweden, Stockholm, Sweden, Surface and Corrosion Science, KTH Royal Institute of Technology, Stockholm, Sweden, 3 Omega Pharma Nordic, Kista, Sweden and 4Department of Pharmacy, Uppsala University, Uppsala, Sweden 2
Background: The tactile perception is essential for all types of topical formulations (cosmetic, pharmaceutical, medical device) and the possibility to predict the sensorial response by using instrumental methods instead of sensory testing would save time and cost at an early stage product development. Here, we report on an instrumental evaluation method using tactile friction measurements to estimate perceptual attributes of topical formulations. Methods: Friction was measured between an index finger and an artificial skin substrate after application of formulations using a force sensor. Both model formulations of liquid crystalline phase structures with significantly different tactile properties, as well as commercial pharmaceutical moisturizing creams being more tactile-similar, were investigated. Friction coefficients were calculated as the ratio of the friction force to the applied load. The structures of the model formulations and phase transitions as a result of water evaporation were identified using optical microscopy. Results: The friction device could distinguish friction coefficients between the phase structures, as well as the
commercial creams after spreading and absorption into the substrate. In addition, phase transitions resulting in alterations in the feel of the formulations could be detected. A correlation was established between skin hydration and friction coefficient, where hydrated skin gave rise to higher friction. Also a link between skin smoothening and finger friction was established for the commercial moisturizing creams, although further investigations are needed to analyse this and correlations with other sensorial attributes in more detail. Conclusion: The present investigation shows that tactile friction measurements have potential as an alternative or complement in the evaluation of perception of topical formulations.
topical formulations with high consumer acceptance is central in the cosmetics industry, and substantial efforts and resources are in general placed on perceptionrelated issues for cosmetic formulations. Within the pharmaceutical area and in the treatment of various types of dry skin disorders, for example psoriasis or atopic dermatitis, where continuous and often life-long treatment is required, perception aspects are not as central although they are highly important for patient compliance (1). In fact, compliance difficulties due to the formulations being non-appealing from a cosmetic perspective are one of the main reasons why patients do not adhere to the prescribed therapies, even though the formulations have evident therapeutic effects (2–4). Perception is thus a critical element of all types of topical formulations and the possibility to include simplified methods for perception analysis at an early
stage would facilitate the development of formulations with improved acceptance. The tactile feel in product use is normally evaluated by means of human panels; however, these panel tests are costly and time-consuming. In addition, human panels often provide a subjective opinion based on comparison with reference products which is not always beneficial. Use of instrumental techniques for these types of evaluations, with demonstrated predictive capacity of perceptual response, would represent savings in both time and cost and also generate objective results which can be compared without using reference products. In regard to topical formulations, both rheology and texture (5–10), as well as friction (11–13) measurements have been evaluated as instrumental methods to predict various sensory attributes such as stickiness (7), slipperiness (13), greasiness (11), and skin smoothness (12) associated
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EVELOPMENT OF
Key words: skin friction – finger/tactile friction – tactile perception – hydration – topical formulations – moisturizers
Ó 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Accepted for publication 15 February 2015
Tactile friction of topical formulations
with the skin feel after treatment. It has been proposed that friction between finger and skin is the dominant interaction contributing to perception, and thus the measurement of this property should be a straightforward means of quantifying the experience (14). Skin friction is a system variable and depends on many factors, for example the probe material, environmental conditions, anatomical site, hydration, and roughness of the skin and counter surface, and has been recently reviewed (15–17). Moisture or an increase in skin hydration has shown to increase skin friction (18–25). This is believed to be due to a decrease in the Young’s modulus of the skin, resulting in more compliant skin and an increase in contact area during contact with a probe (26, 27). Since human skin is elastic, the skin can deform over surface asperities, and the real contact area may approach the apparent contact area, which is often discussed in combination with an observed decrease in friction with increasing surface roughness (28–33). The majority of skin friction studies are measured in dry contacts, but when a formulation is applied different lubrication regimes apply. During the initial application of a topical formulation the formed film is rather thick and the contact between the two surfaces (i.e. finger and skin) is dominated by hydrodynamic or fullfilm lubrication (34). At this stage the friction is highly affected by the viscosity of the formulation, and the friction properties are related to the perceived slipperiness of the product, where a low friction is related to a more slippery feel (35). After spreading of the formulation the surfaces in contact are separated by a thin film and mixed lubrication followed by boundary lubrication takes place (34, 36). During this stage, effects on the skin after product application can be detected. For example, hydrated and soft skin gives rise to a higher friction and adhesion compared to dry skin, initially resulting in a stickier feeling which gradually transforms to being smooth or moist depending on the properties of the formulation, resulting in a reduction of friction and adhesion over time (35). Considerable research has been devoted to measuring the effect of various cosmetic skin treatments on friction in vivo using a rotating or laterally moving probe on the inner forearm (11–13, 17, 37). Furthermore, the nanotribological properties of skin have been investigated using atomic force microscopy (38) and Timm
et al. (39) studied the lubrication effects of various particles in cosmetic powders by measuring friction using the mini traction machine and poly-dimethoxysiloxane as skin mimicking surfaces. In the latter case two model substrates in sliding contact were used and correlations were established between friction measurements and panel assessments of skin feel after powder application. However, rather less attention has been paid to measuring friction by moving fingers over a skin treated area, in which the friction encountered would resemble the friction perceived, i.e. tactile friction. Thus, the aims of this article are to evaluate the feasibility of measuring differences in tactile friction after application of topical formulations using the human finger and a model skin substrate, to study the change in friction over time and finally how this is related to formulation properties. Firstly, model formulations containing different liquid crystalline phase structures are evaluated to investigate if different structures exhibiting very different tactile properties would give different friction responses. Also the time dependent water evaporation from these formulations, which results in phase transitions, is examined in terms of friction properties. Secondly, commercial pharmaceutical skin creams containing different moisturizing components are evaluated to study the effect of formulation composition on friction properties as well as sensorial characteristics and to investigate if the finger friction method can discriminate between more tactile-similar formulations.
Experimental Materials Monoolein, Rylo Mg 19-glycerol monooleate of Pharma grade, was purchased from Danisco (Brabrand, Denmark) and sodiumoleate (purity >82%) was from Riedel-de Ha€en (Seelze, Germany). The water used was purified by means of a Milli-Q Plus Unit (Millipore, Billerica, MS, USA). Vitro-Skinâ (IMS Inc., Portland, ME, USA) was used as a model skin substrate and mimics human skin surface properties in terms of topography, pH, elasticity, surface tension, and ionic strength. This artificial skin was cut into pieces of 2.5 mm 9 5 mm and placed in a desiccator on the internal shelf above a beaker with a mixture of 85 wt% Milli-Q water and 15
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wt% glycerin (Sigma-Aldrich, St. Louis, MO, USA) for 16–24 h before use. This allowed for reproducible hydration of the skin samples prior to friction measurement.
Formulations Three formulations were prepared to represent different liquid crystalline phases from the ternary monoolein-sodiumoleate-water (MONaO-W) system (40), a lamellar phase (A, 28% : 27% : 45% MO : NaO : W, w/w/w), a cubic phase (B, 48% : 7% : 45% MO : NaO : W, w/w/w), and a reversed hexagonal phase (C, 64% : 16% : 20% MO : NaO : W, w/w/w), as shown in Fig. 1. The liquid crystalline phases were prepared by mixing the three components and heating the samples to 45°C in a water bath for 3 h while stirring from time to time, whereupon the samples were centrifuged at 2500 g for 10 min followed by manually stirring and heating to 45°C. The centrifugation-mixing-heating procedure was repeated six times until visually homogenous samples were obtained. Three moisturizing skin creams, denoted X, Y, and Z, were used to investigate whether differences in friction could be measured as a consequence of formulation composition and type of hydration mechanism. X and Z are commercial pharmaceutical creams available in Swedish pharmacies, and Y is a placebo formulation of X, with different composition in terms of specific key components. The compositions of the creams are presented in Table 1. The active
(a) (b)
(c)
Fig. 1. The ternary phase diagram of the monoolein-sodiumoleatewater system (40) where the approximate positions of formulation A, B, and C are indicated (triangles). Adapted with permission from (40), copyright (2001) American Chemical Society.
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moisturizing component in moisturizer X and Z is urea (3% in X and 5% in Z, respectively). Formulation Y is a placebo formulation of X, i.e. it does not contain urea. Furthermore, formulation X and Y contains several additional moisturizing agents such as propylene glycol, glycerin, dimethicone, and canola oil that are not present in Z.
Finger friction measurements Finger friction measurements were performed using a ForceBoardâ (41) (Industrial Dynamics Sweden AB, J€arf€alla, Sweden), equipped with both a horizontal and tangential load cell, consisting of strain gauges in a Wheatstone bridge configuration. A mechanical load results in voltage changes that are proportional to the applied load. The friction force (F) and applied load (L) were continuously recorded as a finger interrogated the model skin surface by moving the index finger back and forth, and the friction coefficients (l) were calculated as the ratio of the friction force and load according to: l¼
F L
ð1Þ
Approximately 3–4 mg/cm2 of the model formulations (formulation A, B, and C) were applied to the model skin substrate and spread using a gloved finger to obtain an even film thickness. The time schedule for the measurements were decided based on prior evaporation experiments (decrease in weight of formulation spread on a glass slide). Evaporation experiments showed that about 68% of the water evaporated within 20 min in formulation B and about 52% and 58% in formulation A and formulation C, respectively, with most water evaporating the first 5 min. Upon water evaporation, phase-transitions can occur, depending on the starting position in the ternary phase diagram (Fig. 1). The respective amount of water loss indicates that formulation B and C would undergo phase transitions within 20 min, whereas formulation A most probably would not. Due to the observed fast water evaporation, friction was decided to be measured at 0 min (i.e. immediately after spreading) and after 2.5, 6, and 20 min. When studying the commercial pharmaceutical moisturizing skin creams (moisturizer X, Y, and Z) 15 mg cream was applied to the model
Tactile friction of topical formulations TABLE 1. Composition of pharmaceutical moisturizing creams investigated Cream id
Composition (INCI)
X
Aqua, Caprylic/Capric Triglyceride, Canola Oil, Urea, Propylene Glycol, Cetearyl Alcohol, Glycerin, Dimethicone, Paraffin, Sodium Lactate, Carbomer, Polysorbate 60, Glyceryl Stearate, PEG-100 Stearate, Methylparaben, Propylparaben, Glyceryl Acrylate/Acrylic Acid Copolymer, Lactic Acid, Citric Acid Aqua, Caprylic/Capric Triglyceride, Canola Oil, Propylene Glycol, Cetearyl Alcohol, Glycerin, Dimethicone, Paraffin, Sodium Lactate, Carbomer, Polysorbate 60, Glyceryl Stearate, PEG-100 Stearate, Methylparaben, Propylparaben, Glyceryl Acrylate/Acrylic Acid Copolymer, Lactic Acid, Citric Acid Aqua, Paraffinum Liquidum, PEG-5 Glyceryl Stearate, Sodium Chloride, Urea, Cetearyl Alcohol, Palmitic Acid, Stearic Acid, Tromethamine, Hydrochloric Acid, Methylparaben, Propylparaben
Y
Z
INCI, The International Nomenclature of Cosmetic Ingredients.
skin and spread using a gloved finger to obtain 3 mg/cm2 evenly distributed across the surface. Friction was measured at 0 min (i.e. immediately after spreading) and after 10 and 60 min. The fingers used for measurements are affected by the formulation and were therefore cleaned by soap and water and dried between each measurement. As a control of the status of the model skin, influenced by exposure to ambient conditions which affect its hydration level, a reference measurement at an untreated part of the model skin was always performed prior to the actual measurement. For each friction experiment the index finger, inclined at about 30°, was stroked forward (away from the body) and back (toward the body) 13 times over the sample area (3–4 cm2) at a sliding speed of approximately 14 mm/s (31). The friction force and the applied load were recorded with a sampling rate of 100 Hz. The load was 0.5–1 N, which is in agreement with the reported optimum load when detecting tactile stimulus (42). All experiments were performed in triplicate and in controlled environment (T = 23°C and RH = 50%). Optical microscopy Formulation phase behavior was studied by an optical microscope using polarizing light (Zeiss Axioplan, Oberkochen, Germany). About 3–4 mg/cm2 formulation was spread with a gloved finger onto a microscope glass. The formulations A, B, and C were characterized directly after spreading and after 2.5, 6, and 20 min exposure to the surrounding environment allowing water to evaporate which may result in phase transitions. Prior to analysis after the elapsed time, a cover glass was placed on the formulation and further water evaporation was thus prohibited.
Sensory analysis of commercial pharmaceutical moisturizing creams X and Z by a trained panel The moisturizing creams X and Z were also examined for their sensory parameters. 15 volunteers (12 women and three men, aged 28–52) with experience of assessing cosmetic products recruited in-house at Omega Pharma Nordic AB participated in the study. The participants were trained using reference products selected from Omega Pharma Nordic products available on the market in addition to laboratory products designed to represent different levels of the attributes investigated. The evaluated attributes were the following; whitening effect of the moisturizer, absorption of moisturizer into the skin, moisturizing/hydrating ability, and smoothening of the skin surface. The assessments were made 5 min after application of the creams and graded on a visual analogue scale from 1 to 10. The whitening effect was graded from 1-no whitening effect to 10- highest whitening effect. The absorption of moisturizer into the skin was graded from 1-fast absorbing to 10-slow absorbing. The moisturizing/hydrating ability of test moisturizers was graded from 1-no moisturizing/hydrating effect to 10-highest moisturizing/ hydrating effect. The smoothening of the skin surface was graded from 1-rough surface to 10very smooth surface.
Results and Discussion Tactile friction of liquid crystalline phases Figure 2 displays optical microscopy images of the three different liquid crystalline phases A, B, and C indicated in Fig. 1. Optical microscopy directly after spreading on a microscope glass (Fig. 2a) verified a lamellar structure in formulation A, a cubic (isotropic) phase in formulation B and predominately reversed hexagonal structures in formulation C. Friction coefficients
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Skedung et al. A
B
C
a
b
Fig. 2. Optical microscopy images of formulation A, B, and C directly after spreading on a cover glass (a) and after exposure to the surrounding environment for 6 min (formulation B) and 20 min (formulation A and C) (b); The magnification is 109 or 209 and the scale bar is 100 lm.
Fig. 3. Average friction coefficients measured between an index finger and model skin after application of formulations of different liquid crystalline phases; lamellar phase (A), Cubic phase (B) and mainly reversed hexagonal phase (C), at different times; immediately after application and spreading (0 min), as well as after 2.5, 6, and 20 min. The error is presented as one standard deviation. Changes in friction over time are associated with phase transitions upon water evaporation.
for the three formulations at different times are displayed in Fig. 3. As shown in Fig. 2b, Formulation A remained in the lamellar phase also 20 min after spreading on the surface, and no significant changes in friction were detected over this time period (Fig. 3). For formulation B, on the other hand, a dramatic reduction in friction coefficient was already observed 2.5 min after application (Fig. 3), accompanied by a phase transition from cubic (isotropic) to a mixture of reversed hexagonal, lamellar, and isotropic phases (Fig. 2b). For Formulation C, the reversed hexagonal phase became more uniform within a minute after application, followed by a transition
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to a mixed phase with isotropic regions, where the isotropic regions increased with time (Fig. 2b). In case of formulation C the rate of water evaporation is lower compared to formulation B due to the structures being inverse, with the polar regions encased by hydrophobic regions, which hinders release of water from the formulation. This is mirrored in the reduction of the friction coefficient, which in the case of formulation C takes place between 6 and 20 min (Fig. 3), i.e. after a considerably longer time than for formulation B. Thus, it appears that different liquid crystalline phases give different friction responses with the friction coefficient decreasing in the following order; cubic > reversed hexagonal > lamellar, and that phase changes upon water evaporation can be followed by friction measurements. The phases, in turn, have considerably different tactile properties. For example, a cubic phase feels sticky, a reversed hexagonal phase greasy whereas a lamellar phase, which has a similar structure to the outer layer of skin, has a more soft and pleasant feel. Reversed hexagonal and cubic phases are not traditionally used in skin creams, however, they have shown interesting penetration enhancement properties for topical delivery of pharmaceutical components to the skin and mucosa (43–46).
Tactile friction of commercial pharmaceutical moisturizing creams The primary role of moisturizers is to increase the water content in stratum corneum and smoothen the skin surface. This can be achieved
Tactile friction of topical formulations
by including humectants such as urea, propylene glycol, and glycerin in the formulation, which act by attracting water from the surrounding to hydrate the skin. A different, complementary approach is to include emollients/lipids and other components that act occlusively as a protective barrier on the skin and hinder water loss. Often a combination of these two strategies is employed. Hydration and resulting smoothening of the skin (filling of fractures) will result in an increased friction as a result of increased contact area between skin and contacting material (4, 15, 25, 26). In the present investigation friction properties of model skin after application of three different pharmaceutical moisturizing creams for dry skin (denoted moisturizer X, Y, and Z) were studied by finger friction measurements to investigate whether differences in friction could be measured as a consequence of different moisturizing components. The average friction coefficients during application as well as 10 and 60 min after application, respectively, are displayed in Fig. 4, where the friction of untreated model skin is also included as a reference. The results for untreated model skin show that water evaporation over time results in a reduction in the friction coefficient, which in turn verifies that hydrated skin display higher friction. This was also observed during studies of liquid crystalline phases (reference bars in Fig. 3), and has been shown in many previous studies (25, 27–29, 33, 47).
Fig. 4. Average friction coefficients measured between an index finger and model skin after application of commercial moisturizing creams during spreading (0 min) and after 10 or 60 min. The error is presented as one standard deviation. The reduction in friction coefficient for the reference (untreated VitroSkin) is a result of water evaporation from the model skin substrate.
The friction properties detected during spreading, where hydrodynamic or full-film lubrication is dominating (34), is highly influence by formulation viscosity and related to the ease of application and the perceived slipperiness of the formulation (35). The creams investigated all give rise to a reduction in friction coefficient during application compared to the reference, where no significant differences between the creams are observed. 10 min after application, on the other hand, where boundary lubrication dominates (34), the friction coefficients for moisturizer X and Y are similar to the untreated model skin whereas a lower increase compared to the initial value is observed for moisturizer Z. This shows that the differences observed between the three creams are not a result of the urea (humectant and active moisturizing agent, present in X and Z but not in Y) but rather of other formulation components in X and Y. Furthermore, the sensory analysis performed by the test panel (Fig. 5) showed that that the skin is perceived as smoother after use of moisturizer X compared to moisturizer Z which likely is an effect of skin hydration, and it also indicated that the perceived moisturizing/hydrating ability is somewhat higher for moisturizer X. After 1 h similar friction coefficients are detected for all three moisturizing creams. This may be related to the fact that the hydrating effect of urea is achieved after a somewhat longer period in contact with the skin compared to the occlusive agents. It could also of course simply indicate that insufficient formulation is left on the model skin to have any effect 60 min after application, since similar friction coefficients are observed for both the cream-treated and the untreated substrates. The results from the sensory analysis (Fig. 5) show that moisturizer X was rated higher than moisturizer Z for all attributes assessed, although the differences were rather small. In addition, the variability in the sensory data is high (for example, the absorption into the skin of moisturizer X was graded from 2 to 7). This indicates that assessment of the skin feel properties absorption, moisturizing/hydrating ability, skin smoothening and whitening is a difficult task even for a trained panel, which has been observed also in previous studies (5). By use of reliable instrumental studies instead of panel testing these types of difficulties would be overcome. The present investigation show that finger friction measurements have potential for such purpose
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Fig. 5. Result of sensory analysis of moisturizers X and Z performed by the volunteers of the test panel. The graph presents the number of volunteers that gave a specific value to an attribute. The absorption of moisturizer into the skin was graded from 1-fast absorbing to 10-slow absorbing. The moisturizing/hydrating ability of the moisturizers was graded from 1-no moisturizing/hydrating effect to 10-highest moisturizing/hydrating effect. The smoothening of the skin as a result of moisturizers was graded from 1-no smoothening to 10-highest smoothening. The whitening effect of the moisturizers was graded from 1-no whitening effect to 10-highest whitening effect.
although more thorough correlation studies between finger friction measurements and perception of topical formulations are required in order to fully understand how friction is related to various sensory attributes.
Conclusion The results indicate that with the use of tactile friction measurements the friction response of various topical formations applied on a model skin substrate can be detected. Phase-transitions in model formulations as a result of water evaporation, confirmed by optical microscopy, result in changes in the friction coefficient between the finger and lubricated model skin substrate. The friction coefficient is increased for hydrated skin due to an increased contact area between finger and skin, as verified by measurements on untreated model skin during dehydration. During application of a cream where full-film lubrication dominates, the friction coefficient is reduced and no
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difference is observed between the creams, whereas a difference in friction related to the presence/absence of occlusive components in the creams are detected 10 min after application. By the tactile friction approach evaluated in this article it is feasible to detect small changes in skin friction due to either different formulation structure and composition or hydration states of the skin substrate. Further studies are needed to link friction measurements with various sensorial attributes to see whether measurable differences in friction actually correspond to different perception attributes of the skin, both during and after application.
Acknowledgments We thank Irena Blute for skillful experimental assistance with the Optical microscopy measurements. We would also like to acknowledge Dr. Adam Feiler and Dr. Anna Fureby for valuable discussions.
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Address: Lovisa Ringstad SP Technical Research Institute of Sweden Chemistry, Materials and Surfaces Box 5607 SE-114 86 Stockholm Sweden Tel: +46-10-5166017 Fax: +46-8-208998 e-mail: [email protected]
