Mechanical Behavior of Human Skin: Testing, Modeling and Simulations

Skin is the largest and one of the most complex organs of the human body, representing the first interface between the inner physiology and the external environment. It is not merely a protective envelope but a dynamic and multifaceted structure that plays a crucial role in various physiological functions, including temperature regulation, D vitamin synthesis, and haptic sensation. Its microstructure, mainly composed of elastin and collagen fibers, confers the tissue its characteristic non-linear anisotropic mechanical response, which guarantee the integrity of the membrane while allowing for the mobility of the body. Understanding the mechanical properties of skin is fundamental for enhancing surgical outcomes, improving wound healing, aiding in the design of prosthetics and wearable devices, and advancing dermatological treatments. This knowledge is crucial for both medical innovation and patient care. In this thesis the complex mechanical behavior of skin is modeled by means of advanced constitutive models which can account for the non-uniform dispersion of the collagen fibers. Experimental tests on human skin samples were conducted to determine the parameters required to inform such models. In particular, microstructural parameters related to the collagen fiber dispersion are obtained from Second Harmonic Generation (SHG) images of the collagen fibers using a novel algorithm capable of measuring the the three-dimensional orientation distribution of the fibers. Based on the experimental mechanical and microstructural data, the constitutive models implemented in the commercial Finite Element (FE) software ABAQUS are used to simulate skin corrective surgeries, specifically focusing on the Z-plasty, the triple Z-plasty, and the rhombic flap transposition. These simulations provide mechanical insights into the expected outcomes of surgical interventions, allowing us to define optimized configurations that minimizes deformations and stresses, thus contributing to improved surgical planning and outcomes. The results achieved, beside confirming the effectiveness of mechanical simulations in biomedical applications, represent a step forward in the pursuit of more effective and tailored approaches to skin corrective surgeries. Moreover, the developed algorithms, as well as the experimental results of human skin, are not limited to the scope of the present thesis, but can be exploited in further biomechanical analyses.