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| Category | Title | Author | ||
| Guest Article | The Piezoelectric Stick-Slip Technique | Wolf Eisfeld |
The way we perceive a cosmetic product depends on a complex interplay of various conscious and unconscious perceptions. To evaluate the performance of a product, mechanical parameters which are perceived by the consumer in a tactile way, i.e. by touch or palpation, are of major importance. These properties of a product may include parameters such as softness, smoothness, roughness, tackiness, elasticity and flexibility.
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Although these parameters are of crucial importance for the consumer's well-being, they are usually assessed by purely subjective means, and the respective margins of interpretation are often highly variable and diffuse. It is certainly possible to quantify single aspects by laboratory methods. However, due to a strictly defined set of boundary conditions, these methods only describe very few dimensions of reality and usually can only partly be correlated with subjective perception. Due to current legislation, a cosmetic product can only be successful if the claims made about it are clearly perceivable by the customer. And this is top priority for cosmetic producers.
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Figure 2: Setup of the stick-slip measurement — A sensor, glued onto a brass plate for purpose of stabilization and fixed to a spring, is moved over the skin surface. The sensor is moved from below to the volar forearm until a defined indentation depth is achieved. Within a few seconds a 20 mm long section of the volar forearm skin is then scanned by the sensor. The resulting voltage signal is recorded by an A/D converter and subsequently analyzed via powerful software tools. For every product and each volunteer five data traces are recorded. As a rule, more than five data traces are recorded for each product and each volunteer. It could be shown that these subsequently measured signals are reproducible with utmost accuracy. As is typical of experiments involving the human skin the inter-subject variability between different volunteers is considerable, depending on the respective individual skin state. Therefore, average values from ten volunteers were calculated. |
The piezoelectric stick-slip technique (from Greek piézein = to press) is a new technique used to evaluate tactile skin properties. This technique allows us to measure a full spectrum of consumer-relevant skin properties in an objective manner. The stroking movement of a finger when we pass our hand over the skin, especially after applying a cosmetic product, is imitated by a sensor. This sensor is based on the physical phenomenon of piezoelectricity, where appropriate materials generate a measurable voltage signal already at minimal deformations (Figures 1, 2 and 3). To evaluate the signals, a so-called "effective value" (Ueff) is determined from the signals, which is a direct measure of the level of the voltage gauge (high or low signal amplitudes signify high or low effective values, respectively).
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Figure 3: Analysis of stick-slip measurements
— Improvement of skin smoothness by a skin care product, as
deduced from a 7 per cent reduction of Ueff. |
When evaluating the skin, the deformation of the sensor required to generate a voltage signal is achieved by the movement over the skin surface: Due to obstacles like wrinkles or humps, the sensor temporarily adheres to the skin, tears off again, slides for a short distance, sticks to the skin again and so on. The result is a stuttering, alternately sticking and slipping movement and the termed used for this is "stick-slip" effect.
Stick-slip motions are usually connected with acoustic signals. The signals from the piezoelectric sensor can be directed towards a loudspeaker so that they are directly audible. Dependent on the skin state, a more or less enjoyable scratching can be heard.
The piezoelectric stick-slip technique opens up completely new possibilities for claim support and advertising. Thanks to its proximity to daily practice, it provides a bridge between objective laboratory techniques and subjective experience. Another distinct advantage of the new technique is that it can be safely be carried out in vivo, without using any skin models.
Characterization of cosmetic oils (emollients)
In the stick-slip measurements carried out so far, the characterization of the effects achieved by emollients was of particular interest. This method is well-suited to monitor the temporal evaluation of the effects achieved by emollients. 20 µl of pure oil were distributed on a volar forearm skin area and measuring values were recorded at certain intervals within 30 min. Figure 4 shows the effective values for five emollients from various substance classes measured 10 min after the treatment:
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Figure 4: Stick-slip data of emollients, measured 10 min after product application and represented as relative change of effective value compared with reference. |
The figure shows that the different emollients can very well be distinguished, with maximum changes between +15 percent and -7 percent by comparison with the reference (untreated skin). Dicaprylyl ether (Cetiol OE) displays a particularly low and thus advantageous value. The complete time-dependent behavior over 30 min results in the same graduation between the oils measured: the effective values decrease continuously and for all products in an almost parallel manner. This behavior may be interpreted as follows: Immediately after the treatment, the emollients have not yet spread over the skin surface or have not penetrated into the uppermost skin layer. This leads to a good adherence between sensor and skin surface and thus to an increased signal level (high effective value). In the course of time, the effective value decreases due to spreading and absorption and thus shows an increasing skin smoothness, which improves by approximately 12 percent, e.g. after 30 min, for particularly suitable emollients such as dicaprylyl ether (Cetiol OE).
With the aid of data like these it is possible to rank further emollients within this framework. The cascade-like graduation of the stick-slip values, which has also been confirmed for further emollients, suggests the proximity to concepts like the "spreading cascade" developed by Ansmann and coworkers. In this concept, emollients are classified according to their skin feel by means of sensory evaluation. It requires further investigation, however, to answer the question whether correlations exist between the spreading cascade concept and stick-slip measurements. In any case, it is possible to develop new formulation concepts where oils with low, intermediate and high stick-slip values may be combined to achieve a particularly favorable and permanent feeling of skin smoothness.
Another major question is which significance the stick-slip parameters, e.g. the Ueff values, have and if they can be correlated with physicochemical material constants such as viscosity, spreading value or molecular weight. Without going too much into detail, we can state that some interdependencies exist (e.g. spreading values decrease at increasing effective values and low/high viscosities cause low/high effective values). They are, however, not unequivocal and only allow a coarse classification.
These findings demonstrate the benefits of the piezoelectric stick-slip technique because in vivo experiments with comparably high expenditures would be needless if a simple measurement of a material constant was sufficient for a product characterization. The real benefit of the technique becomes clear when we return to the demand from the introductory section that a relevant test method should also deliver data that describe consumer relevant product properties.
After completion of each measuring period the panel volunteers were asked to report on their subjective assessment of the different emollients. The evaluation was mainly tactile and consisted in passing the right hand fingers over the treated area of the left volar forearm, and subsequent description of the experience in free words. It was found that the subjective impressions could be categorized according to the main terms "skin feel" and "residues on the skin surface". For dicaprylyl ether (Cetiol OE) e.g. an agreeable skin feel and excellent absorption properties were found, while oleylerucate (Cetiol J 600) left significant residues on the skin which were partly evaluated as tacky and oily. On the basis of these subjective evaluations the emollients tested were put in an order, namely in a combination of the following criteria "decreasing residues" and "improved skin feel". At first glance, the result may be surprising, as the ranking is in very good agreement with the sequence from the stick-slip experiments. As was to be expected, due to the "soft" subjective data, the graduation is not as sharp. Hence, it can be stated that the stick-slip technique yields quantitative and objective data which apparently are very closely linked to the subjective consumer perception. The objective evaluation of such a complicated term as "skin feel" thus may be viewed from a totally new perspective. The correlations thus may be of high value for future testing of cosmetic products and formulations and end-products, especially for skin care applications.
The piezoelectric stick-slip method is of course not only well suited for testing cosmetic raw materials, but also for the evaluation of the product effect of all kinds of cosmetic formulations. Thus, dicaprylyl ether (Cetiol OE) or oleylerucate (Cetiol J600) were added to a very simple emulsion formulation (18% respective emollient; 2% ceteareth-20 (Eumulgin B2); 4% cetearyl alcohol (Lanette O); 76% water). As Figure 5 illustrates, both formulations show a very low and thus favorable Ueff value. Due to absorption of the emulsion in the upper skin layer the effect is still more pronounced after 15 min than after 5 min (improvement of up to 15%). By comparison with the values for the pure raw materials (Figure 4) the effective values found for both emollients in the formulation are significantly lower which consequently means that the skin smoothing effect is significantly higher.
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Figure 5:Stick-slip data of basic formulations with addition of emollients (18 %), measured 5 or 15 min after product application and represented as relative change of effective value compared with reference. |
Outlook
As was demonstrated with the preceding examples, the stick-slip technique makes it possible to obtain objective data on subjectively experienced tactile properties of human skin. It could be shown that the degree of accordance with subjective data from a sensory evaluation is impressively high. Thus, the concept to imitate the motion of a human finger or hand with a piezoelectric sensor while assessing one's own skin status was successful. As a consequence, it is possible to prove the effects of a targeted cosmetic skin treatment with a likewise specific method. The possibilities for testing cosmetic raw materials and formulations for skin care are, without exaggeration, virtually unlimited. A broad spectrum of skin properties which can be perceived by tactile means obviously influences the stick-slip voltage signals, such as smoothness, roughness, softness, elasticity, flexibility, tackiness, waxiness and absorption behavior. The effective value Ueff is not specific enough to differentiate between these often hardly separable properties. For future examination, it will thus be of considerable importance to extract separation sharp parameters from the measured signals which can reliably be attributed to the skin properties listed above. A good chance of success lies in a frequency component analysis of the voltage signals via Fourier transformation. Thus, a tacky skin surface with distinctive stick-slip signals displays a much more complex frequence spectrum than untreated skin.
References
Eisfeld, W.; Busch, P.; Das Stick-Slip-Verfahren - eine neue Methode zur sensorischen Bewertung taktiler Hauteigenschaften, Conference Proceedings 46th Congress of the SEPAWA, Bad Dürkheim, 116-123 (1999)
Eisfeld, W.; Vienenkötter, T.; Kara, Y.; Busch, P. Evaluation of Tactile Skin Properties by Piezoelectric Sensors; SÖFW-Journal, 125, 9 (1999) 2-12
Author
Dr. Wolf Eisfeld
Dr. Wolf Eisfeld studied physics at Freiburg and Göttingen universities and obtained his doctorate at Max-Planck-Institute for Biophysical Chemistry [on the subject of laser-spectrographic investigations into bacteria proteins]. Since 1996 he has been working as laboratory head [in the "Biophysics/Sensorics/Hair Chemistry" Department] within Henkel Düsseldorf's Chemical Research Department. His duties include the development of measuring techniques for demonstrating the effects of cosmetic products [on hair and skin and of detergents and cleansers]. The emphasis is on perceived phenomena and contrasts between objective laboratory methods and subjective consumer experience.
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