Issue 25 — June 2001

Issue 25
Category		Title
Newsletter		Evaluation of Hair Cuticle Properties with Piezoelectric Sensors		Wolf Eisfeld and Peter Busch

Introduction — The Era of Haptics

While vision is still our most significant sense for perceiving the world around us, the importance of tactile sensations is more and more increasing. Trend researchers even designate the new century as the era of haptics. However, very little is still known about how a surface has to be structured to achieve a certain sensation or even emotion, and there are no objective methods to quantify surface properties like for example "silky" or "velvety", which are based on a subjective linguistic assessment.

In hair cosmetics, the relevant surface properties are related of course to the hair itself, and are determined on the one hand by the cuticle properties of a single hair fiber and on the other hand by the three-dimensional arrangement of a hair collective. As a large variety of hair care product effects are perceived by the consumer via tactile means, usually by passing our hand over the hair or by stroking it with a finger (for example hair softness, smoothness, flexibility, body or volume), we developed a method for objectively measuring hair surface properties by imitating this movement with an artificial sensor.

Figure 1: Experiments showed that for untreated hair the effective value for tip-root orientation is always significantly higher than for root-tip orientation (approximately 20 percent).

The Stick-Slip Technique

The principle of the sensor is based on the piezoelectric effect: When a suitable crystal material is mechanically deformed, a charge displacement is induced, which can be detected as a voltage signal. In our concrete case, the required submicroscopically small deformation of the sensor is achieved by the movement over the hair cuticle surface: Due to the very special step-like scale structure of the cuticle, the sensor temporarily is stopped, e.g. by a scale edge, tears off again, slides for a short distance, sticks again, and so on. This results in a stuttering, alternately sticking and slipping motion of the sensor which consequently is denominated as "stick-slip" behavior. Thus, it should be expected that movement against the direction of hair growth, i.e. tip-root orientation, yields basically different signals compared to root-tip orientation, which indeed is clearly confirmed by our measurements.

The piezoelectric stick-slip technique recently has been established to monitor consumer-relevant tactile skin properties [1] (see also Wolf Eisfeld, The Piezoelectric Stick-Slip Technique, Skin Care Forum 23). As will be demonstrated in this paper, the method is by far not limited to this single application, but is also highly suited for evaluation of fiber surfaces, especially human hair.

Experimental Setup

All experiments were carried out in an air-conditioned laboratory with constant temperature and humidity (23°C, 50% r.h.). Though it is in principle possible to measure single hair fibers, we generally used hair strands with parallel fiber arrangement mounted on a metal surface. In order to enhance the sensitivity, compared with the human skin measurement setup, we fixed a microporous sponge to the sensor edge.

The piezoelectric voltage signals are recorded via an A/D converter and finally analyzed by a tailor-made software tool. Within the scope of this paper, we focused on the so-called "effective value" or r.m.s. value which is a direct measure for the magnitude of the detected voltage signals (for exact definition and more details of data analysis see [1]). The following basic rule of thumb holds: Rough or tacky hair surfaces render high effective values, whereas low effective values can be correlated with smooth or slippery surfaces.

Figure 2: Experimental data from a consecutive application (1x, 3x, 5x) of a permanent wave (thioglycolate, pH 8.0) and bleaching procedure (H2O2 + ammonium peroxodisulfate, pH 9.4).

Results and Discussion

As already mentioned, it can be expected from the special "tiled roof" scale structure of the cuticle, which is governed by the hair growth process, that movement of the sensor along the two different orientations root-tip and tip-root should render different voltage signals. We found in all experiments carried out so far, that for untreated hair the effective value for tip-root orientation is always significantly higher by approximately 20% than for root-tip orientation (Figure 1).

It has to be noted that in most experiments carried out with the current setup, the differentiation in root-tip orientation was fairly poor, whereas in tip-root direction, we found a prominent distinction between different hair treatment procedures. Thus, the data which shall be discussed in the following are mainly measured against growth direction.

In the next step, we evaluated the influence of various hair damaging procedures on the stick-slip values, e.g. perming, bleaching, combing or UV stress.

Figure 2 shows experimental data from a consecutive application (1x, 3x, 5x) of a permanent wave (thioglycolate, pH 8.0) and bleaching procedure (H2O2 + ammonium peroxodisulfate, pH 9.4). For the reason mentioned above, only the data in tip-root direction will be discussed.

Obviously, a permanent wave even after a single application raises the effective value by a factor of approximately three. Subsequent applications of further permanent waves still increase the values, however not as strong as the first one. This behavior can be explained by a considerable influence of the perming procedure on the cuticle structure: Evidently, the cuticle surface roughness increases, which is directly reflected by the stick-slip values.

Application of a single bleaching step also increases the effective value, however only by a factor of 1.7. Considering the fact that the underlying chemistry of bleaching and perming is completely different, it is clear that the resulting surface effects don't have to be alike. The impact of the permanent wave on the inner hair structure generally is stronger and is connected with a higher swelling of the fiber, which can easily explain the higher effective values compared to bleaching. It is more interesting to look at the evolution which becomes evident after subsequent application of further bleaching steps: The stick-slip values decrease (instead of increase, which logically could have been expected) and even fall below the value for the untreated hair. How can this be explained?

A microscopic evaluation of the corresponding hair fibers allows for the correct interpretation: Comparison of two images of the hair surface from fibers bleached 5 times (Figure 3 A) and 10 times (Figure 3 B) shows that the bleaching process leads to a strong cuticle abrasion and major clipping of scales, which finally results in a plaindowned surface of the hair (magnification: 1000x). These findings recently could be confirmed by atomic force microscopy (AFM).

This "naked" cortex structure is smoother than before and thus leads to lower effective values. Whereas this effect under the microscope is well perceivable only after 10 bleaching steps, apparently the stick-slip method is more sensitive, revealing a value decrease already after 3 bleaching steps.

Figure 3: Comparison of two images of the hair surface from fibres bleached 5 times (Figure 3 A) and 10 times (Figure 3 B). The bleaching process leads to a strong cuticle abrasion and major clipping of scales, which finally results in a plaindowned surface of the hair (magnification: 1000x).

A) 5 x bleached

B) 10 x bleached

Another example for detection of hair surface damage is shown in Figure 4, where hair strands have been subjected to consecutive combing strokes (up to 10.000 times). The resulting clear cut increase of stick-slip values can be directly correlated with the increasing level of cuticle damage inflicted by the combing. The relationship, however, is nonlinear, as the initial damage (up to 1000 strokes) is higher than the damage occurring later after many more combings. The diagram shown in Figure 5, where the results from 10 different hair strands measured in tip-root-orientation are shown, also illustrates the favorable reproducibility of the method.

Figure 4: Example of detection of hair surface damage: Hair strands subjected to consecutive combing strokes (up to 10.000 times). The resulting clear cut increase of stick-slip values can be directly correlated with the increasing level of cuticle damage inflicted by the combing. The relationship is nonlinear, as the initial damage (up to 1000 strokes) is higher than the damage occurring later after many more combings.

With respect to UV damage, we carried out an experiment under extensive irradiation conditions. However, no significant effects with respect to stick-slip values could be found. Thus we have to conclude that UV irradiation, which is well known to affect hair structure rather strongly, has a much less pronounced effect on the cuticle.

Interestingly, even a water-only treatment (swelling in water with subsequent drying procedure) slightly increases the stick-slip values. This behavior can be explained by a loosening of the cuticle scale association as a consequence of the swelling procedure, which becomes obvious when the sensor moves against the growth direction of the hair fibers. Hence, the piezoelectric stick-slip method seems to be a highly sensitive tool which is well suited for monitoring even the smallest changes in hair surface structure.

As could be demonstrated, there is a very good correlation between hair damage and stick-slip experimental data. Thus, the piezoelectric sensor should be highly suitable for evaluation of the manifold of cosmetic hair fiber treatments. As an illustrative example, Figure 5 shows - again measured in tip-root direction - the dramatic damage inflicted by a treatment of hair strands with chlorinated salt water (0.4 ppm active chlorine, 33g/l sea salt, 10 consecutive treatment cycles) and heat which is typical for what might happen to hair during holidays (swimming pool, sea water and sun). The effective value which rises by a factor of more than 3.5 indicates that the cuticle roughness strongly increases due to this treatment, which is experienced by the consumer as highly blunt hair. However, prophylactic application of a suitable shampoo containing a blend of care additives after each of the 10 treatment cycles leads to a distinctly decreased level of damage (factor 2.3 compared to the untreated reference). Thus, thanks to the favorable properties of the "holiday shampoo", the hair damage is lower by approximately 36%.

Conclusions

To summarize, it can be stated that the stick-slip method is a both powerful and sensitive method for evaluation of hair surface properties. Especially the smoothness or roughness of the cuticular envelope is monitored. Thus on the one hand, distinct modifications of the cuticle by environmental stress, grooming practice or chemical treatment are directly mirrored by the stick-slip values. On the other hand, beneficial cosmetic effects can be verified on a highly differentiated level, allowing for a likewise differentiated claim substantiation.

Of course, a lot of work still has to be done: Obviously, the number of cosmetic formulations to be tested is virtually unlimited, only to mention shampoos, conditioners or styling products. Another important item will be the correlation of stick-slip values with the perception of the consumer: Evidently, as was pointed out in the introduction to this paper, cuticle properties are not only measurable by physical devices, but can also be sensed by tactile means. The experiments we carried out so far clearly indicate that such a correlation holds indeed, but without doubt, more elaborate investigations are required to come to final conclusions. Finally it should be mentioned that the method is by far not restricted to the evaluation of hair or skin properties, but also can be used to quantitatively determine surface characteristics of textile fibers, fabrics and hard surfaces in general.

Figure 5: Dramatic damage inflicted by a treatment of hair strands (measured in tip-root direction) with chlorinated salt water (0.4 ppm active chlorine, 33 g/l sea salt, 10 consecutive treatment cycles) and heat. The effective value rises by a factor of more than 3.5. Application of a suitable shampoo strongly decreases the level of damage.

Reference

(1) W. Eisfeld, T. Vienenkötter, Y. Kara, P. Busch; Evaluation of Tactile Skin Properties by Piezoelectric Sensors; SÖFW-Journal 125(9), 2-12 (1999)

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. In 1996 he assumed the position of laboratory head in the Hair Physics/ Sensorics Department within Henkel Düsseldorf's Chemical Research. Since the end of 2000 he has been responsible for building up a biophysical-sensorical research team at Cognis Research&Technology, emphasizing hair performance properties as well as consumer perception of raw materials and formulations.