In vivo mechanical characterization of human facial skin combining curved surface imaging and indentation techniques

BACKGROUND: The mechanical properties of human facial skin are of considerable importance for clinical research and cosmetic industry. As a result of their susceptibility to the individual difference and complex surroundings of human beings, the in vivo mechanical characterization by objective and quantitative devices is challenging. METHODS: In this study, an experimental setup was custom-designed for the mechanical characterization combining curved surface optical imaging and indentation techniques. By means of an independently developed transparent indenter, the contact area and topography of facial skin can be in vivo and in situ captured in real time. Especially, the perpendicularity between indenter and facial skin can be adjusted and guaranteed by imaging analyses. RESULTS: A modified formula for the contact area calculation of silica gel, one of the most common materials used to simulate human facial skin, has been proposed. The highly improved agreement with the indentation tests shows its reliability and better applicability compared to the classical Hertz theory. Furthermore, we perform the in vivo indentation tests on human facial skin to evaluate the Young's modulus, which shows a potential for better understanding of their mechanical properties. CONCLUSION: The device presented could give convincing results. The in vivo mechanical properties of human facial skin obtained by our modified formula agree well with open literature, and a better reliability than classical Hertz theory is evidenced.