The design and validation of a multi-layer model of human skin

Abstract

Human skin is a complex multi-layered material consisting of the epidermis, dermis and underlying hypodermis. For several decades, much effort has gone into the development of mathematical and computer models of skin. A physically accurate and realistic model of skin has applications in several diverse areas from artificial skin design to computer animation. There are many numerical models of skin in existence, which accurately simulate several of the skin’s complex mechanical characteristics. However, many of these models assume skin to be a homogeneous material and thus ignore the individual contribution of each layer. While these single-layer models may be adequate when simulating skin under homogeneous loading conditions, a multi-layer model is needed to simulate skin under more complex deformations such as wrinkling - a phenomenon common to all human skin. A multi-layer finite element model of skin, consisting of the stratum comeum, dermis and hypodermis, has been proposed to simulate skin under deformations that cause it to wrinkle. The stratum comeum is represented by a neo-Hookean function, the dermis by an orthotropic-viscoelastic function and the hypodermis by a Yeoh strain energy function with a Prony series to model viscoelasticity. In vivo wrinkling experiments on volar forearm skin were conducted to validate the skin model. Results from this study show that a model representing the stratum comeum, dermis and hypodermis more accurately simulates the wrinkling of skin than singlelayer models. The three-layer model represents a significant improvement over existing single and two-layer models. The model shows that the natural tension or prestress in living skin plays a significant role in the formation of wrinkles - a larger prestress delays the initiation and reduces the size of the wrinkles. The model predicts that a stiffer stratum comeum and a greater collagen fibre density increase the size of wrinkles - occurrences that are observed in aging skin. The model also predicts that larger wrinkles occur when they are parallel to the Langer’s lines in the skin. The model has also been used to explore the wrinkle formation around contracting healing scars. Comparison between the results of finite element analyses and simplified experiments of contracting scars show that the pre-stress and the orthotropic nature of skin play a significant role in the orientation of the wrinkles.