Experimental tests and constitutive analysis of plantar skin

Background: The human body is covered by skin, whose mechanical behaviour is due to ground substance, and elastic and collagen fibres. These components give the skin a non -linear, time -dependent and anisotropic response [1]. The plantar skin is highly specialized, protects the body from injuries and adapts to external stresses during daily activities. Methods: Plantar skin was collected from male human donors (aged 56±18 years) who underwent amputation due to cancer at the Orthopedics and Orthopedic Oncology Unit of the University Hospital of Padova. M echanical testing was carried out using the Biomomentum testing machine (Model Mach -1 v500csst, ©Biomomentum Inc.). Uniaxial tensile tests were performed on dog-shaped samples cut in the posterior-anterior (PA) and lateral-medial (LM) directions, while compression tests along the craniocaudal (CC) direction were carried out on cylindrical samples. Preconditioning was carried out before the stress relaxation tests, performed following 5 ramps of 7% strain at 10%/s strain rate and a resting period of 600s for the tensile tests and 5 ramps of 1% strain at 10%/s strain rate and a resting period of 600s for the compression tests. Results from experimentations suggested describ ing the mechanical behaviour of the skin by using a visco-hyperelastic fiber -reinforced constitutive model [2]. The identification of constitutive parameters required the inverse analysis of experimental results, considering both equilibrium response and viscous behaviour. Results: Tensile and compression tests demonstrate that plantar skin exhibits non -linear anisotropic behaviour. Stress- relaxation curves confirm the tissue's time -dependent response. Experimental results allow obtaining constitutive parameters to characterize the fiber-reinforced visco-hyperelastic behaviour (Figure 1). Conclusions: According to the experimental findings, the skin shows visco-elastic mechanical characteristics that differ depending on the directions. The visco-hyperelastic fiber -reinforced constitu tive model correctly describes the mechanical response of the plantar skin. These data can be used in developing in silico tools for the foot, which can assist in evaluating the effects of various conditions, e.g., diabetes, or exploring the interactions between the foot and footwear.