Application Note | ElastoSens™ Bio
Measuring the viscoelasticity of cosmetic creams using ElastoSens™ Bio
by Gloria Pinilla, M.Sc.
Senior Application Specialist, Rheolution Inc.
- Rheological analysis plays an important role in the quality control testing of cosmetic products.
- The rheological characterization of cosmetic products includes exposing samples to thermal stresses, including a profile of temperature or temperature cycles.
- ElastoSens™ Bio was used to characterize the viscoelastic properties of two different Nivea products in response to temperature.
- Results showed that the two different tested products behaved in a different way in response to temperature, and that ElastoSens™ Bio is a good tool to understand the thermal stability of creams.
INTRODUCTION
Cosmetic products pass through a rigorous quality control process to ensure the efficacy and safety of products and its raw-materials. Due to the rapid growth that cosmetic industries have exhibited all over the world, efficient, low cost and rapid methods to assess cosmetics’ quality control are a priority [1].
The main current techniques used in the R&D and quality control of cosmetics are sensory analysis, small angle X-ray scattering (SAXS) and rheology. Whether the consumer perceives the product as being pleasant must first be investigated in sensory tests, to determine the physical features that are part of the consumer experience, including spreadability on the skin, smell, colour, texture, etc. As soon as the ideal formulation of a cream has been found, the product should be quantitatively characterized and documented in order to use this as a reference for later products. Thus, sensory measurements are normally complemented by SAXS, which is a technique used to characterize the formation of crystalline structures to evaluate emulsion breakdown, and rheology, which has been widely used to characterize the physicochemical properties, over-time stability of a product and also, to ensure that the products are standardized.
Rheological tests often include the exposure of the samples to different kinds of environmental stress, including temperature, light, humidity or oxidation. In regard to temperature, the properties of the cream are analyzed within a profile of temperature often going from 20°C to 60°C [1].
In this work, the viscoelastic properties of two different Nivea products, i.e., Nivea Cream (NC) and Nivea Soft (NS) have been characterized. The effect of sample volume and temperature have been investigated with the use of the ElastoSens™ Bio.
MATERIALS AND METHODS
Nivea Cream (NC) and Nivea Soft (NS) were loaded into sample holders with the use of a syringe, to ensure that the cream was homogeneously distributed. (Figure 1). For all tests, the precision balance was used to weight the sample thus ensuring the same volume of cream per sample.
Figure 1. Set up methodology. A syringe was used to load the sample holders with an homogeneous layer of cream.
The effect of the sample volume in the viscoelastic measurements of the ElastoSens™ Bio was first investigated. To that aim, samples containing 1g, 2g, 3g and 5g of NC were prepared and measurements were performed at 25°C during 10 min. Tests were performed in triplicate (n=3) and results are expressed as a mean value ± standard deviation of all replicates.
The thermal stability of NC and NS was then studied. First, a profile of temperature in the measuring chamber of the ElastoSens™ Bio was applied, in order to take measurements of the shear storage modulus (G’) for 30 min at 20°C and gradually increase to 60°C with a temperature step of 10°C. The temperature inside of the sample was precisely measured by installing a temperature probe inside of the sample through the lateral entrances of the thermal chamber and embedding the probe in the sample (Figure 2). Results are expressed as the mean ± standard deviation for each one of the temperature steps (20°C, 30°C, 40°C, 50°C and 60°C) and the temperature represented is the temperature measured with the probe inside of the sample.
Figure 2. Temperature probe to measure the temperature inside of the sample.
Then, the effect of the historicity of temperature on the viscoelastic properties of the creams was studied. To this aim, a cycle of heating and cooling was applied, where measurements were first taken for 20 min at 20°C, followed by 20 min of measurements at 60°C, and others 20 min of measurements at 20°C.
All tests were performed in triplicate (n=3) and results are expressed as a mean value ± standard deviation. Statistical significance was considered if p<0.05 after the performance of One-way ANOVA test.
RESULTS AND DISCUSSION
Figure 3 shows the shear storage modulus (G’) and the loss tangent (tan δ) of NC samples containing 1g, 2g, 3g and 5g of product. G’ represents the elastic component of the material behavior whereas the loss tangent (tan δ) represents the ratio between the elastic and the viscous part of the material (G’/G’’). The statistical analysis showed that there were no significant differences between the different conditions (p value=0.33), which means that the measurement is not dependent on the weight of the sample and supports its physical value. In addition, results proved that 2g per sample was the preferred minimum volume per sample, since samples containing 1g presented higher variability than samples containing 2g or more. Thus, we decided to continue further experiments with an amount of 2g of cream per sample.
Figure 3. G’ and tan δ mean values obtained with different weights (g) of NC.
The evolution of the shear storage modulus (G’) in response to a profile of temperature was different in each of the tested Nivea products (Figure 4). As visually expected, the G’ related to each product at 20°C was really different; NC had a G’ of 27,595 Pa ± 4,093.36 Pa (14.45% SEM) whereas NS had a G’ of 7,913 Pa ± 544.58 Pa (6.88% SEM). This was in coherence with the existing literature for each product [2, 3].
For NC, the elastic properties of the cream decreased with the increase in temperature, fitting into a second order polynomial model (quadratic) model (illustrated as the trendlines of Fig. 4A, Table 1). As for NS, the elastic properties were roughly stable when temperature was under 30°C, and it decreased when the temperature increased over 30°C. The G’ evolution fitted into a third order polynomial model (cubic) model (illustrated as the trendlines of Fig. 4B, Table 2).
Figure 4. Shear storage modulus (G’) of two different types of Nivea products exposed to a profile of temperature. (A) G’ of NC samples decreased following a quadratic polynomial trend whereas (B) G’ of NS samples decreased following a cubic polynomial trend (polynomial trends are represented as trendlines in the image). Orange, sample 1; blue, sample 2; green, sample 3. Each point of the curve represents the mean ± standard deviation for all points corresponding to a specific temperature.
Table 1. Equations and coefficient of determination (R2) of the trendlines corresponding to NC samples (REP) exposed to a profile of temperature (20°C-60°C). T: temperature in the sample (°C). G’: shear storage modulus (Pa).
Table 2. Equations of the trendlines corresponding to NS samples exposed to a profile of temperature (20°C-60°C). T: temperature in the sample (°C). G’: shear storage modulus (Pa).
Finally, a cycle of heating and cooling was applied to the creams (Figure 5). Heating of the NC samples from 20°C to 60°C dropped the shear storage modulus by 5.5-fold (i.e., from 30.94 kPa to 5.6 kPa). Afterwards, samples were cooled down to 20°C and their mechanical properties increased by 7.2-fold (i.e., from 5.6 kPa to 40.2 kPa). In the same way, heating of NS samples dropped by 1.7-fold the shear storage modulus (i.e., from 4.7 kPa to 2.7 kPa), and posterior cool down of samples increased its stiffness by 3.7-fold (i.e., from 2.7 kPa to 9.9 kPa). Both products are sensitive to their historic of temperature; however, NS presents better thermal stability than NC, since the changes in G’ are lower. The differences observed between products can be explained by their different composition, which affects how they behave in response to temperature [4].
Figure 5. Shear storage modulus (G’) evolution of NC (A) and NS (B) in response to a cycle of temperature.
CONCLUSIONS
In this study, the viscoelastic properties of two different commercial creams of Nivea were analyzed with the use of the ElastoSens™ Bio. The mechanical differences of the creams and the effect of the temperature on the materials was studied.
Overall, results indicate that ElastoSens™ Bio can be used to characterize the behavior of cream formulations in response to temperature. This type of characterization can accelerate the process of cream and cosmetics’ development, since it helps to better understand the behaviour of the tested product to thermal stress, as well as to the quality control of commercial products.
INTRODUCTION
Cosmetic products pass through a rigorous quality control process to ensure the efficacy and safety of products and its raw-materials. Due to the rapid growth that cosmetic industries have exhibited all over the world, efficient, low cost and rapid methods to assess cosmetics’ quality control are a priority [1].
The main current techniques used in the R&D and quality control of cosmetics are sensory analysis, small angle X-ray scattering (SAXS) and rheology. Whether the consumer perceives the product as being pleasant must first be investigated in sensory tests, to determine the physical features that are part of the consumer experience, including spreadability on the skin, smell, colour, texture, etc. As soon as the ideal formulation of a cream has been found, the product should be quantitatively characterized and documented in order to use this as a reference for later products. Thus, sensory measurements are normally complemented by SAXS, which is a technique used to characterize the formation of crystalline structures to evaluate emulsion breakdown, and rheology, which has been widely used to characterize the physicochemical properties, over-time stability of a product and also, to ensure that the products are standardized.
Rheological tests often include the exposure of the samples to different kinds of environmental stress, including temperature, light, humidity or oxidation. In regard to temperature, the properties of the cream are analyzed within a profile of temperature often going from 20°C to 60°C [1].
In this work, the viscoelastic properties of two different Nivea products, i.e., Nivea Cream (NC) and Nivea Soft (NS) have been characterized. The effect of sample volume and temperature have been investigated with the use of the ElastoSens™ Bio.
MATERIALS AND METHODS
Nivea Cream (NC) and Nivea Soft (NS) were loaded into sample holders with the use of a syringe, to ensure that the cream was homogeneously distributed. (Figure 1). For all tests, the precision balance was used to weight the sample thus ensuring the same volume of cream per sample.
Figure 1. Set up methodology. A syringe was used to load the sample holders with an homogeneous layer of cream.
The effect of the sample volume in the viscoelastic measurements of the ElastoSens™ Bio was first investigated. To that aim, samples containing 1g, 2g, 3g and 5g of NC were prepared and measurements were performed at 25°C during 10 min. Tests were performed in triplicate (n=3) and results are expressed as a mean value ± standard deviation of all replicates.
The thermal stability of NC and NS was then studied. First, a profile of temperature in the measuring chamber of the ElastoSens™ Bio was applied, in order to take measurements of the shear storage modulus (G’) for 30 min at 20°C and gradually increase to 60°C with a temperature step of 10°C. The temperature inside of the sample was precisely measured by installing a temperature probe inside of the sample through the lateral entrances of the thermal chamber and embedding the probe in the sample (Figure 2). Results are expressed as the mean ± standard deviation for each one of the temperature steps (20°C, 30°C, 40°C, 50°C and 60°C) and the temperature represented is the temperature measured with the probe inside of the sample.
Figure 2. Temperature probe to measure the temperature inside of the sample.
Then, the effect of the historicity of temperature on the viscoelastic properties of the creams was studied. To this aim, a cycle of heating and cooling was applied, where measurements were first taken for 20 min at 20°C, followed by 20 min of measurements at 60°C, and others 20 min of measurements at 20°C.
All tests were performed in triplicate (n=3) and results are expressed as a mean value ± standard deviation. Statistical significance was considered if p<0.05 after the performance of One-way ANOVA test.
RESULTS AND DISCUSSION
Figure 3 shows the shear storage modulus (G’) and the loss tangent (tan δ) of NC samples containing 1g, 2g, 3g and 5g of product. G’ represents the elastic component of the material behavior whereas the loss tangent (tan δ) represents the ratio between the elastic and the viscous part of the material (G’/G’’). The statistical analysis showed that there were no significant differences between the different conditions (p value=0.33), which means that the measurement is not dependent on the weight of the sample and supports its physical value. In addition, results proved that 2g per sample was the preferred minimum volume per sample, since samples containing 1g presented higher variability than samples containing 2g or more. Thus, we decided to continue further experiments with an amount of 2g of cream per sample.
Figure 3. G’ and tan δ mean values obtained with different weights (g) of NC.
The evolution of the shear storage modulus (G’) in response to a profile of temperature was different in each of the tested Nivea products (Figure 4). As visually expected, the G’ related to each product at 20°C was really different; NC had a G’ of 27.595 Pa ± 4.093.36 Pa (14.45% SEM) whereas NS had a G’ of 7.913 Pa ± 544.58 Pa (6.88% SEM). This was in coherence with the existing literature for each product [2, 3].
For NC, the elastic properties of the cream decreased with the increase in temperature, fitting into a second order polynomial model (quadratic) model (illustrated as the trendlines of Fig. 4A, Table 1). As for NS, the elastic properties were roughly stable when temperature was under 30°C, and it decreased when the temperature increased over 30°C. The G’ evolution fitted into a third order polynomial model (cubic) model (illustrated as the trendlines of Fig. 4B, Table 2).
Figure 4. Shear storage modulus (G’) of two different types of Nivea products exposed to a profile of temperature. (A) G’ of NC samples decreased following a quadratic polynomial trend whereas (B) G’ of NS samples decreased following a cubic polynomial trend (polynomial trends are represented as trendlines in the image). Orange, sample 1; blue, sample 2; green, sample 3. Each point of the curve represents the mean ± standard deviation for all points corresponding to a specific temperature.
Table 1. Equations and coefficient of determination (R2) of the trendlines corresponding to NC samples (REP) exposed to a profile of temperature (20°C-60°C). T: temperature in the sample (°C). G’: shear storage modulus (Pa).
Table 2. Equations of the trendlines corresponding to NS samples exposed to a profile of temperature (20°C-60°C). T: temperature in the sample (°C). G’: shear storage modulus (Pa).
Finally, a cycle of heating and cooling was applied to the creams (Figure 5). Heating of the NC samples from 20°C to 60°C dropped the shear storage modulus by 5.5-fold (i.e., from 30.94 kPa to 5.6 kPa). Afterwards, samples were cooled down to 20°C and their mechanical properties increased by 7.2-fold (i.e., from 5.6 kPa to 40.2 kPa). In the same way, heating of NS samples dropped by 1.7-fold the shear storage modulus (i.e., from 4.7 kPa to 2.7 kPa), and posterior cool down of samples increased its stiffness by 3.7-fold (i.e., from 2.7 kPa to 9.9 kPa). Both products are sensitive to their historic of temperature; however, NS presents better thermal stability than NC, since the changes in G’ are lower. The differences observed between products can be explained by their different composition, which affects how they behave in response to temperature [4].
Figure 5. Shear storage modulus (G’) evolution of NC (A) and NS (B) in response to a cycle of temperature.
CONCLUSIONS
In this study, the viscoelastic properties of two different commercial creams of Nivea were analyzed with the use of the ElastoSens™ Bio. The mechanical differences of the creams and the effect of the temperature on the materials was studied.
Overall, results indicate that ElastoSens™ Bio can be used to characterize the behavior of cream formulations in response to temperature. This type of characterization can accelerate the process of cream and cosmetics’ development, since it helps to better understand the behaviour of the tested product to thermal stress, as well as to the quality control of commercial products.
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