BACKGROUND/AIMS: A new method for the in vivo characterization of the mechanical properties of skin has been developed. This comprises a suction chamber and an ultrasound device to measure both the vertical displacement of the skin's surface, and the skin's thickness. METHODS: A mathematical model of the mechanical behaviour of a taught elastic membrane is used to obtain a set of parameters intrinsic to the skin, such as Young's modulus (E) and the initial stress (sigma0), which reflect the stiffness and the natural tension of the skin, respectively. We also calculated an index of non-elasticity of the skin (unrestored energy ratio, UER), which takes into account the volume of tissue mobilized. It determines a ratio between the energy input to the skin and the energy it dissipates. These parameters were evaluated from the volar forearm of 10 normal male volunteers. RESULTS: The results were: 129+/-88 kPa for E, 13.5+/-5 kPa for sigma0, and 0.42+/-0.04 for UER; with reproducibilities of 9.5%, 12.4% and 6.4%, respectively. CONCLUSIONS: This new suction device was found useful for the study of the behaviour of the skin, and the device may be used for the evaluation of certain skin diseases and their therapy.

We have investigated in vivo the change with age of various parameters that describe the physical properties of skin. The parameters were derived from pressure/displacement curves obtained by applying reduced pressure to a small area of skin and measuring the resulting displacement by 20 MHz scan echography. By fitting the pressure/displacement curves to a theoretical model, the following skin parameters were obtained: E, Young's modulus or stiffness (in Pascals); sigma(0), the initial stress (in Pascals); and the unrestored energy ratio (UER), an index related to cutaneous non-elasticity. These parameters, which are used in mechanics to define the intrinsic physical characteristics of materials, were measured for the first time on volar forearm skin of 206 male and female subjects, aged between 6 months and 90 years. The results showed that skin thickness increases until maturity and decreases for women over 50-60 years old, Young's modulus E increases linearly with age, and ageing is divided into two phases for natural stress, sigma(0) and the non-elasticity index UER. Natural stress sigma(0) increases until maturity and then rapidly decreases. The non-elasticity index decreases until puberty and steadily increases after puberty. This new procedure provides a simple quantitative assessment of the physical properties of the skin, revealing that the skin becomes thinner, stiffer, less tense and elastic with ageing.

This study has clearly demonstrated that irradiation with 1540 nm Er:glass laser can lead to new collagen formation, dermis thickening, reduction of anisotropy of the skin, and clinical improvements. The lack of adverse effects confirmed that this 1540 nm laser emitting in a pulsed mode coupled with an efficient cooling system is safe.

Our study revealed interesting differences in skin color and color heterogeneity with respect to ethnicity and age-related alterations. Data obtained are very useful in improving our knowledge about the skin of people of different origins and helps in the development of specific cosmetic products that are well adapted to all these populations.

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