It will also attempt to correlate the rheological properties to skin sensory attributes. By linking and understanding these properties, formulators can thus easily choose the best rheology modifiers for developing skin care formulations with the right sensory attributes that positively influence the consumers’ perceptions.
Formulations can be tailored to the desired emotive and sensory signals required by consumers to create distinctive consumer product use experiences. Careful design of the sensory properties of a formulation can transform a skin care routine for a consumer into a pleasant, memorable experience that fosters product loyalty and encourages repeat use. Formulating cosmetics to deliver precisely the right feel at exactly the right stage during application to meet specific consumer sensory preferences is more important than ever. That is why recent innovations and studies focus more and more on optimising sensory characteristics of products. There are several ways to optimise sensory attributes of cosmetic formulations. But many formulators do not consider rheology modifiers as a primary option to achieve this goal. Typically, rheology modifiers come to mind only when there is a need for increasing viscosity. Moran1 previously described that the sensory profile of an oil-in-water emulsion can be optimised by intertwining complementary sensory cues from two seemingly unrelated technologies: water dispersible rheology modifiers to impact the texture element of sensory during initial application, and ester emollients to enhance sensory from rub-out through after-feel. This paper will demonstrate how various crosslinked acrylate-based rheology modifiers can affect the sensory attributes of skin care products and how the rheological and inherent properties of these polymers can describe, and even predict, sensory attributes.
Principle
Cosmetic sensory analysis is a scientific discipline that evokes, measures, analyses and interprets human reaction to the characteristics of cosmetic products/ materials as they are perceived by human senses. Rheology is the study of the deformation and flow of material when it is subjected to an applied force. When force is applied on material, such as a cream that is being applied onto the skin, the material may respond in several different ways to relieve the strain.
Limitation of the study
In this paper, we will focus primarily on the concentrations of the crosslinked acrylatebased polymers stated hereafter.
Methodology
Method I: See Table 2.
Method II: viscosity and yield value
Viscosity and yield value of the resulting gels and emulsions were measured using Brookfield DVII+ Pro Viscometer.
Method III: sensory
The sensory evaluations were conducted by 16-20 members of a trained sensory panel in China using the modified ASTM E 1490-03 protocol. Forced Ranking was employed on the gel system and Quantitative Descriptive Analysis Rating was used for the emulsion system. The evaluations were carried out under the following conditions:
• Room humidity: 50%+/–5% (to simulate the average indoor humidity condition in the region).
• Room temperature: 25°C (+/–2°).
• Sample size: 0.03 mL.
• Test area: 45 mm diameter circle inside the inner arm area (above the wrist/2.54 cm below the joint).
Treatment of the data was based on ANOVA Non-parametric Statistics (Wilcoxon & Kruskal-Wallis tests) and the comparison tests by general linear procedure (Tukey).
Method IV: rheology
All the rheology measurements were conducted using a TA AR-550 rheometer. To simulate the rubbing action while applying the product onto the skin, most of the analysis was conducted at high shear rates as suggested by Brummer.5
Results and discussion
Gel system: effect of electrolytes
Studies on isolation of the salt content of human sweat6 support the claim that there are substantial amounts of electrolytes on the skin that can interact with any topically applied products. Since crosslinked polyacrylate polymers are affected by electrolytes, this study attempts to mimic the possible effect of electrolytes on the gel structure by directly adding 0.01 wt%-0.05 wt% NaCl. This enables better understanding of the effect of skin electrolytes on the microgel network. Without the addition of NaCl, the system is expected to maintain its original viscosity. At this point, the system meets the basic requirement for a rheology modifier (i.e., maintains stability and appearance while on the shelf). With the addition of 0.01 wt% and 0.05 wt% NaCl, a decrease in viscosity is observed. This decrease in viscosity is a result of a collapse in the structure, which eases the release of the bound water and other components that are suspended. The degree of collapse is dependent on the type and amount of the rheology modifier used.
Gel system: sensory
Forced ranked sensory data shows that at comparable viscosities, each crosslinked acrylate polymer offers different sensory profiles. For simplicity of the approach in selecting the rheology modifier and interpretation of the data in Figure 1, two opposing systems were identified: light feel/light deformation resistant rheology modifier and heavy feel/heavy deformation resistant rheology modifier. Linking the above results to previous tests conducted on the effect of NaCl, it can be deduced that electrolyte tolerance of the polymer is related to the resulting type of break. Note: In this study, the word “hold” refers to the ability of the polymer to maintain the structure while being exposed to stress, to electrolyte or to a combination of both. The degree of hold is further classified to “soft hold” (less resistance to deformation) and “strong hold” (better resistance to deformation).
Gel system: rheology
Flow behaviour at high shear rate (Fig. 2) shows that all crosslinked polyacrylate polymer-based systems exhibit shearthinning properties both in the presence or absence of electrolytes. Exposing the gels to a low quantity of electrolytes (0.05 wt%) will alter the flow properties of the less electrolyte-tolerant systems such as U10, U21 and 940 polymers. Most likely these polymers will be more affected by skin electrolytes. U20 polymer shows the highest electrolyte resistance, and is less affected by 0.05 wt% NaCl (low shear to high shear rate). These differences in the electrolyte response are best attributed to chemistries involved in the individual polymer and amount used. As shown in Figure 3, certain gels exposed to 0.05 wt% NaCl (Fig. 3) will retain some of their basic structure offering some resistance to oscillatory stress. U20 polymer shows higher critical strain and a long creep plateau, therefore, the system with U20 polymer is more “rigid”, whereas the systems with U10 and 21 polymers exhibit fast transition from high to low modulus. Shorter plateaus signify a quick change in structure from gel to fluid-like texture, signifying quick break sensory.
Gel system: correlating rheology to sensory
Wortel et al7 concluded in their study that many rheological properties are highly correlated to sensory. According to Barry’s8 investigation, however, sensory tests do not correlate with stationary viscosity alone. Using both studies as references, the possible correlation of the rheological properties and sensory attributes of systems containing crosslinked polyacrylate rheology modifiers was investigated. The effect of skin electrolytes in the sensory evaluations was also considered by treating the gels with 0.05 wt% NaCl prior to rheological analysis. A linear relationship is established between critical yield stress and gel cohesiveness. Cohesive gels tend to resist deformation and stick together, requiring higher yield stress to start destabilisation of the gel structure, offering more cushioning feel.
Emulsion system
Emulsion: effect of electrolytes
The addition of electrolytes resulted in a reduction of viscosities and yield values in the emulsion system. This shows that electrolyte sensitivity of the polymers is still being manifested in the emulsion system. The emulsion orientation of the system, however, is somehow mitigating the electrolyte insult as manifested by a lower unit change in viscosity and yield value per amount of polymer used vs. in the gel system.
Emulsion: sensory
The results of the sensory evaluation in Figure 6 shows that polymers can influence the sensory of the resulting emulsions:
• The sensory contribution of polymers in emulsions is felt more from the “jar” and during the primary rub-outs.
• The effect in the after-feel is less intense compared to other attributes, but different polymers will still exhibit recognisable differences (e.g., U20 provides a significant contribution to the after-feel attributes).
Emulsion: rheology
Rogers2 concluded in his study that the rheological behaviour of an emulsion is most often influenced by the rheology of the continuous phase, which is dependent on the nature of the hydrocolloidal network created by water and water swellable/dispersible polymers. Data in Figure 7 shows another factor that will significantly affect the rheology of emulsions containing crosslinked polyacrylate polymers. In the absence of electrolytes, all systems show similar flow behaviours; however, after exposure to 0.05 wt% NaCl, results become more distinctively different. Emulsions stabilised by polymers with good electrolyte tolerance will maintain higher viscosities (internal resistance) from low to high shear rates. The emulsion stress sweep data is different from the gel data. This implies other components of the emulsion are interacting with some of the polymers and causing the differences in its response to 0.05 wt% NaCl. U20 polymer properties are least affected by electrolytes. Both 940 and 980 polymers shifted from high G’ to low G’, meaning these polymers will offer “softer” stabilisation of the emulsions containing or being exposed to electrolytes.
Emulsion: correlating rheology and sensory attributes
Cushion is usually correlated to the perceived thickness (luxurious feel) of the product. It is normally compared to the maximum viscosity at low shear rate.
Conclusion
Carbopol® polymers are renowned for their ability to modify and stabilise emulsions. However, they also have dramatic potential for enhancing other product attributes such as sensorial properties. Crosslinked polyacrylate polymers are highly efficient rheology modifiers, providing viscosity, stability, elegant flow and sensory modification. Owing to the different modifications in the structures, each polymer has its own sensory fingerprint and rheological characteristics. The sensitivity of the polymer to electrolytes opens up an opportunity for developing formulations with a skin electrolyte triggered release mechanism, wherein products are highly stable while in storage, but readily thin-out to ensure efficient release and better absorption of actives. The basic knowledge of the differences in the sensory profiles and rheological properties of crosslinked polyacrylate polymers will help formulators to select the appropriate rheology modifier for their product development. The study also suggests sensory attributes and rheology characteristics of key crosslinked polyacrylate polymers are highly related. This correlation will help formulators in screening formulations faster and may help eliminate unnecessary and costly preliminary sensory evaluations.
Acknowledgment
The authors would like to acknowledge the invaluable support of the following people: Bruce Wang, Christina He, Dan Hasman, Gary Yao, Joanna Wong, Julie Castner, Liu Xin.
References 1 B. Moran et al. SensiMap Formulating Concept: A Formulator’s Tool for Customizing Sensory Properties. in-cosmetics 2008, Amsterdam. 2 Rogers J. A. Means for controlling the rheological behavior of emulsions. Cosmetics and Toiletries, Vol. 93, 1978, 49. 3 Vanzan Brochure, R.T. Vanderbilt, 2000. 4 Steeneken P. Rheological Properties of Aqueous Suspension of Swollen Starch Granules. Carbohydrate Polymers, 11 (1989), p23-42. 5 Brummer R., Godersky S. Rheological studies to objectify sensations occurring when cosmetic emulsions are applied to the skin, Colloids and Surfaces A: Physicochemical and Engineering Aspects. Volume 152, Issues 1-2, 15 July 1999, Pages 89-94. 6 Shirreffs S.M., Maughan R.J. Whole body sweat collection in humans: an improved method with preliminary data on electrolyte content. University Medical School, Foresterhill, Aberdeen AB25 2ZD, United Kingdom. 7 Wortel V., Verboom C., Wiechers J. Linking Sensory and Rheology Characteristics. C&T Magazine, Vol 120, April 2005. 8 Barry B.W., Sensory Testing of Spreadabilityinvestigation of rheological conditions operative during application of topical preparations. J Pharm Sci, 6 (3), 335-341 (1972).