The evolution of skin’s biomechanical and optical properties as a function of aging and/or photoaging is one of the main targets of cosmetic and dermatological research. Many noninvasive devices to measure skin’s biomechanical properties have been developed using alternative methods such as stretching, torsion, indentation and suction. Measurements of skin deformation after suction or torsion are the most widely used techniques in cosmetic research.1, 2
The skin’s optical properties play an important role as well, and devices measuring these characteristics assess reflected light after illumination of the skin surface. Different noninvasive methods have been proposed for evaluating skin complexion in vivo. These include quantitative measurements of skin color, using colorimetry—i.e., L*a*b* and Individual Typological Angle (ITA°);3 or of the intensity of specular reflection and the back-scattering of light from the skin.4, 5 The purpose of this study was to demonstrate the evolution of the measured parameters with aging, and to find the correlation between measured mechanical and optical properties of the skin.
Test population and body sites: A total of 113 female volunteers ages 18–76 participated in the study. All participants were Caucasian, with no apparent signs of skin disease. The volunteers applied no cosmetic products for 24 hr before measurements were taken. Measurements were conducted on two different anatomical regions: the inner forearms and/or the face, i.e., the forehead, temples or cheeks.
Biomechanics: The biomechanical properties of the skin were measured using commercially available standard devicesa, b as well as an internally developed device, referred to as a corneovacumeter.6 The corneovacumeter measures capacitance between skin and a conductive plate, whereby capacitance is proportional to skin deformation. With this type of device, 12 simultaneous suctions are realized and the result provided is the mean of the measurements. While such suction-based devices measure vertical skin deformation after suction, torque-based devices measure deformation after torsion. For the present study, both standard and surface methods were used for calculating biomechanical skin parameters.7
Skin color, brightness: Skin color was measured by colorimeterc in the L*a*b* colorimetric system. In addition, skin brightness was evaluated by another internally developed device, referred to as a brillanometer. This device enables the in vivo determination of both specular and diffuse light reflection, continuously and in numerous directions, in a contactless manner.8 The studied parameters were: peak height in specular reflection, peak width of specular reflection in the middle height, their ratio, and peak height in diffuse reflection.
Fluorescence: To acquire fluorescence spectra of the two fluorophores present in skin, a spectrofluorimeterd was used; the fluorophores of interest were: tryptophan (exc. 295 nm), which reflects epidermal proliferation; and collagenase digestible cross-links (exc. 370 nm), which reflect the accumulation of Advanced Glycation End-products (AGEs) in the dermis.9
Additional parameters, analysis: Auxiliary parameters, such as skin pH, sebum levels and hydration state, were measured using a specialized metere. The data was analyzed using descriptive statistics and correlation software, and the correlation between the different methods also was evaluated. This approach allowed the authors to not only determine the relation of each tested property with aging, but also determine which properties were statistically dependent and correlated.
The obtained skin deformation curves were similar for all devices (see Figure 1, Figure 2 and Figure 3), and on the basis of the deformation curves, the biomechanical parameters were calculated. Here, the authors present only the ratio parameters that are independent of skin thickness, i.e.: skin elasticity (ratio Ur/Ue = [R5]), skin visco-elasticity (ratio Uv/Ue = [R6]) and skin firmness (Ratio Ur/Uf = [R7]). The correlation of biomechanical properties with age was more significant on the measurements taken on the forearms in comparison with the face, where the combination of intrinsic and photoaging increased the variability of the measurements. This was observed with all devices used to measure biomechanical properties. In addition, a strong correlation was found between the expression of biomechanical parameters, standard versus surface parameters (data not shown).
Further, a significant negative correlation with age for skin elasticity [R5] and firmness [R7] was observed by all three biomechanical evaluation devices. The viscoelastic component of skin, described by parameter [R6] and represented by the ratio of “viscoelastic” to “elastic distension,” i.e., Uv/Ue, increased with age (see Figure 1, Figure 2 and Figure 3).
Table 1 illustrates the correlation coefficients between the three devices for the biomechanical parameters measured as a function of age. The best and highly significant correlation was obtained on the skin firmness parameter, Ur/Uf. The correlation between devices was lower for the viscoelastic component, as illustrated here on the coefficient of correlation for skin viscoelasticity, especially between the dermal torquemeter and corneovacumeter. The higher variability of the viscoelastic part of skin deformation may explain this lower correlation. Significant correlation between the three devices was observed for skin elasticity.
Results in Skin Color and Brightness
Colorimetry measurements of luminosity and ITA° revealed a statistically significant evolution of parameters with age, and the diminution observed related to both UV exposed and non-exposed skin on the cheeks and forearms (see Figure 4). The evaluation of colorimetric parameters on macrophotographs confirmed skin color modification with aging.
The diminution of luminosity and ITA°, as well as an increase of the yellow and red component of color, significantly changed. This increase of yellowish color has been reported in the literature and linked to the glycation and carbonylation of dermal proteins.10 In relation, an increase in red color may be linked to the unevenness of microcirculation and appearance of the small vessels closer to the skin surface (see Figure 5).
The skin complexion parameters measured by the brillanometer were also significantly modified with aging. A diminution of the peak height of specular reflection and increase of the peak width of the peak and concomitant decrease of the peak height (D/2) in cross light, i.e., diffuse reflection, corresponds to lower, duller skin luminosity with less radiance (see Figure 6). In addition to the decrease of specular reflection, a decrease in diffuse reflection was observed, meaning the skin was less radiant with age.
A strong positive correlation was found between the diffuse reflection and colorimetric parameters L* and ITA° (see Table 2). The correlation with specular reflection was positive, but with a lower correlation coefficient and limited significance (p = 0.1). These results confirmed that the diffuse reflection had a strong impact on skin luminance and radiance.
Tryptophan fluorescence is a good marker for noninvasive evaluation of the epidermal cell proliferation rate. Since this fluorescence is associated with the cell proliferation, its decline with age may indicate the reduced replicative capacity of epidermal cells; i.e., replicative senescence.9 From this study, the authors could conclude that photoaging as well as intrinsic aging reduced cell renewal (see Figure 7). In contrast, the fluorescence of collagen and elastin cross-links increased with age on the forearms and face (temples), indicating an accumulation of AGEs (see Figure 8). The correlation between different optical endpoints is illustrated in Table 3.
A positive correlation was observed between the skin complexion, evaluated by L* and ITA° measurements, and epidermal turnover. Higher epidermal turnover resulted in higher values for skin complexion. On the contrary, the accumulation of AGEs with age correlated negatively with skin complexion values. AGEs have been shown to be responsible for the yellow discoloration of skin with aging. Indeed, the negative correlation of AGEs and ITA°, which takes into consideration the L* and b* colorimetric parameters, confirms this finding.10, 11
Epidermal proliferation, i.e., tryptophan fluorescence, was also positively correlated with specular and diffuse reflection, again indicating its importance in skin radiance. Finally, no correlation was observed between AGEs and skin specular reflection; on the contrary, a strong negative correlation was found between diffuse refection and AGEs. Diffuse reflection is diminished with aging due to a higher absorption of light by chromophores present in the skin.
In regard to additional skin parameters, no statistically significant variation of pH with age was observed in any of the tested zones. Also, no statistically significant evolution in hydration rate was observed on the cheeks, although a significant increase was observed on the forearms. Sebum excretion was strongly dependent on the age of volunteers, with a significant decrease in the group of post-menopausal volunteers observed (see Figure 9).
Mechanical vs. Optical Properties
To compare the mechanical and optical properties of skin, the authors used the data obtained with the cutometer as representative of skin’s biomechanical properties, since this device is generally used. Table 4 shows the correlation between mechanical and optical properties. The statistical comparison demonstrates there are significant correlations between the skin firmness parameter, R7, and optical properties linked to radiance, such as color and brightness. A negative correlation was found between the viscoelasticity of skin, R6, and skin radiance, characterized by luminance and specular and diffuse reflection. The correlation between elasticity of the skin, R5, and optical parameters was weaker and less significant.
The results of the present studies confirm previously published data on the evolution of skin elasticity and firmness with age.12-17 Skin’s optical properties and their evolution throughout the aging process were evaluated, and statistical correlation between the different methodologies was performed. A strong correlation was observed between all devices used for the determination of biomechanical properties. Optical properties including color and brightness were strongly linked to age, as was the formation of AGEs, as measured by spectrofluorimetry.
The parameters that correlated positively with aging were the viscous components of skin’s biomechanical properties, shown via an increase in skin fluorescence due to the glycation of proteins, as well as an increase of redness and yellowness. The negative correlation with aging was observed for skin elasticity, firmness, radiance and turnover, and these parameters were positively correlated with one another. Skin firmness and elasticity was also negatively correlated with the appearance of collagenase digestible cross-links of collagen.
Interestingly, there was a significant negative correlation between skin radiance, L* and ITA°, and the formation of AGEs, with a simultaneous positive correlation of AGE and yellow color. Negative correlations also were found for epidermal skin turnover and increased red and yellow colors.
Finally, a decrease in skin luminance and radiance was positively correlated with the decrease in epidermal turnover. Thus, this overview of the evolution of different parameters with aging demonstrates how all these parameters are tightly linked together and modified during the aging process.
Acknowledgements: The authors wish to acknowledge Catherine Bonnaud-Rosaye and Nadine Duc-Sikora for their skilled technical contributions.
- BC Murray and RR Wickett, Correlation between Dermal Torque Meter, Cutometer and Dermal Phase Meter measurements of human skin, Skin Res Tech 3, 2 101–106 (1997)
- N Krueger, S Luebberding, M Oltmer, M Streker and M Kerscher, Age-related changes in skin mechanical properties: A quantitative evaluation of 120 female subjects, Skin ResTechnol 17, 2 141–148 (2011)
- GE Piérard, EEMCO Guidance to the assessment of skin color, J Eur Acad Dermatol Venereol 10, 1, 1–11 (1998)
- JC Pittet, C Musnier, SC Jacobs, M Lebel, JM Baret and P Beau, Evaluation of the property of the skin to absorb and reflect the light, New instrumental method and visual sensory evaluation, Proceedings of the 22nd IFSCC Congress, Edinburgh 485–490 (2002)
- A Petitjean, JM Sainthillier, S MacMary, P Muret, B Closs, T Gharbi and P Humbert, Skin radiance: How to quantify? Validation of an optical method, Skin Res Technol 13, 2–8 (2007)
- M Sabadotto, O Freis, G Perie and A Rathjens, The Corneovacumeter: A new device for evaluation of biomechanical properties of the skin, J Cosmet Sci 5, 354–355 (2011)
- D Qu et al, Correlation age and quantifying product efficacy on human skin using novel viscoelastic parameters, Proceedings IFSCC 1–6 (2007)
- V Gillon, G Perie, S Schnebert and G Pauly, A new method for contactless in vivo quantitative measurement of stratum corneum gloss attributes: Influence of natural active ingredients, in The Essential Stratum Corneum, R Marks, JC Lévêque and R Voegeli, eds, Taylor & Francis, Philadelphia (2002) 331–334
- GN Stamatas, RB Estanislao, M Suero, ZS Rivera, A Khaiat and N Kollias, Facial skin fluorescence as a marker of the skin’s response to chronic environmental insults and its dependence on age, Br J Dermatol 154, 125–132 (2006)
- H Ohshima et al, Melanin and facial skin fluorescence as markers of yellowish discoloration with aging, Skin Res Technol 15, 496–502 (2009)
- Y Ogura et al, Dermal carbonyl modification is related to the yellowish color change of photo-aged Japanese facial skin, J Dermatol Sci 64, 45–52 (2011)
- T Ezure and S Amano, Influence of subcutaneous adipose tissue mass on dermal elasticity and sagging severity in lower cheek, Skin Res Technol 16, 332–338 (2010)
- T Ezure and S Amano, Involvement of upper cheek sagging in nasolabial fold formation, Skin Res Technol 18 259–264 (2012)
- N Krueger, S Luebberding, M Oltmer, M Streker and M Kerscher, Age-related changes in skin mechanical properties: A quantitative evaluation of 120 female subjects, Skin Res Technol 17 141–148 (2011)
- H Ohshima et al, Use of Cutometer area parameters in evaluating age-related changes in the skin elasticity of the cheek, Skin Res Technol 19 e238-242 (2013)
- A Firooz et al, Variation of biophysical parameters of the skin with age, gender and body region, Scientific World J 1–5 (2012)
- PA Wendling and G Dell’Acqua, Skin biophysical properties of a population living in Valais, Switzerland, Skin Res Technol 9 331–338 (2003)
- C Arce-Lopera, T Igarashi, K Nakao and K Okajioma, Effects of diffuse and specular reflections on the perceived age of facial skin, Optical Rev 19 167–173 (2012)