When one needs to demonstrate the effectiveness of hair care products, they most often resort to methods on hair strands.
These will therefore highlight a macroscopic effect, often reflecting the effect perceived by the user. It is also possible to use methods on hair fiber to look at the effect of products or treatments on the fiber itself, but these are less varied, more costly and by nature require more measurements to obtain representative results.
If the former are quite successful in quantifying a perceived effect (Frizz control, Ease-of-Combing, Dry Hair Smoothness, Hair Shine), they are difficult to relate to a well-defined state of the hair. This often means that if the effect of a hair care product can be measured, it is not always easy to relate it to its mode of action at the microscopic scale, particularly with regard to the surface condition of the fiber.
It is therefore often necessary to combine the two types of tests: macroscopic and microscopic. Thus, in the case of Scanning Electronic Microscope, one can observe with great precision the condition of the hair fiber but it is impossible to quantify it and one will be limited to a visual scoring, for example, of the damage of the keratin cuticles. It is also not possible to make accurate dimensional measurements of the fiber. For this, a Laser Scanning Micrometer must be used which will allow access to its diameter but will not provide any information on the surface state of the cuticles.
To reproduce the touch of your fingers, the friction test measures the force needed to pull a lock of hair through two rubber cylinders joined together. The result will vary depending on the speed at which the hair is pulled, the pressure between the cylinders and ambient conditions (humidity and temperature), which makes this measurement difficult to reproduce without a large number of measurements.
Another friction measurement, based on hair combing, will also provide information on the surface condition of the hair. We measure the tensile strength created by friction forces when a lock of hair is pulled through a comb at a controlled speed.
However, studies on hair friction are sensitive to curling and therefore subject to great variability.
Let’s now talk about interferometry, applied to the measurement of the surface state of hair fibers. This technique makes it possible to generate topographic coordinates of the surface of the hair with great precision (in the order of a few nanometers in z), and thus to generate 3D visuals in false colors highlighting the relief of the cuticles. We can also obtain contrast images that will provide information on the brightness due to the good condition of the fibers.
Working as an optical profilometer, the device also provides linear roughness measurements in the fiber axis. These values are directly representative of hair damage, whether it is related to age or chemical or thermal stress. Conversely, roughness allows measuring the smoothing or repairing effect of a hair care product.
Compared to previous techniques, this measurement method therefore offers a unique combination of microscopic imaging of the hair and quantification of its surface state. It is reproducible, quite fast, easy to implement since it does not require particular sample preparation, and moreover non-destructive.
There is no doubt that it has a certain future in the field of efficacy testing of hair care products and active ingredients.
CONTACT
Edouard MACE
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