Rheological behaviour of reconstructed skin.

Reconstructed skins have been developed to replace skin when the integrity of tissue has been compromised following severe injury, and to provide alternative methods validating the innocuousness and effectiveness of dermatological and cosmetic products. However the functional properties of tissue substitutes have not been well characterised, mainly since mechanical measurement devices have not been designed to test cell culture materials in vitro. From the mechanical standpoint, reconstructed skin is a heterogeneous multi-layer viscoelastic material. To characterise the time-dependent behaviour of reconstructed skin, spherical indentation load-relaxation tests were performed with a specific original device adapted to measure small soft tissue samples. Load-relaxation indentation tests were performed on a standard reconstructed skin model and on sub-components of the reconstructed skin (3D-scaffold alone and dermal equivalent). Generalised Maxwell and Kelvin-Voigt rheological models are proposed for analysing the mechanical behaviour of each biological tissue. The results indicated a modification of the rheological behaviour of the samples tested as a function of their biological structure. The 3D-scaffold was modelled using the one-branch Maxwell model, while the dermis equivalent and the reconstructed skin were modeled using a one-branch and a two-branch Kelvin-Voigt model, respectively. Finally, we demonstrated that skin cells contribute to global mechanical behaviour through an increase of the instantaneous relaxation function, while the 3D-scaffold alone influences the mechanical response of long relaxation times.

Dynamic indentation on human skin in vivo: ageing effects.

BACKGROUND/PURPOSE: Knowledge of the mechanical properties of the human skin is very important for cosmetic and clinical research. Objective and quantitative measurements are essential to compare studies performed by different experimenters in different centres. The aim of this paper is to present a method to measure the viscoelastic properties of human skin in vivo using dynamic indentation. METHODS: A complete device to assess the stiffness and damping of skin has been developed. The frequency and strain amplitude range from 10 to 60 Hz and from 1 to 10 mum. Tests on pure elastic inert materials have been performed to validate the device. An in vivo study including dynamic indentation, suction test, hydration measurement and topographic analysis has been performed on 46 subjects aged from 18 to 70 years, divided into three groups. RESULTS: Results on inert materials show the validity of the device developed. The mechanical behaviour of the skin can be described by a Kelvin-Voight model under dynamic indentation. A comparison with a suction test, hydration and topographic measurements shows that the stiffness and the damping measured by dynamic indentation correspond mainly to the natural tense state of the skin on the body due to the dermis. A weak correlation has been found between dynamic indentation and suction parameters. The complex modulus measured by dynamic indentation at 10 Hz frequency stress ranges from 7.2 +/- 2.1 kPa for the oldest group to 10.7 +/- 2.6 kPa for the youngest group. CONCLUSION: The device presented gives convincing results. The measurement of stiffness and damping complements the viscoelastic phenomenological parameters of the suction test.

In vivo characterization of viscoelastic properties of human skin using dynamic micro-indentation.

The human skin, the interface between the body and the outside environment, has a very complex mechanical behaviour. Knowledge of its in vivo mechanical characteristics is essential to characterize the effects of medical or cosmetic products. The aim of this work is to present a non-invasive device using dynamic indentation to quantify the viscoelastic properties of human skin in vivo. The frequency and strain amplitude are in the range of 10 to 60 Hz and 1 to 10 microm. The results on 4 subjects show that a Kelvin Voigt model describes the mechanical behaviour of in vivo human skin with dynamic indentation well. The frequency average values of stiffness and damping have also been used to compare skin properties. We found a stiffness value of 47.3 to 128.3 N/m, and damping of 0.08 to 0.121 N.s/m, corresponding to a complex modulus of 13.2 to 33.4 kPa. These results show the ability of this device to characterize viscoelastic properties of human skin.