Guided wave elastography of human skins with a layered model incorporating the effect of muscle state

In vivo mechanical characterization of skin finds broad applications in understanding skin aging, diagnosis of some skin diseases and assessing the effectiveness of diverse skin care strategies. Skin has a layered structure consisting of the epidermis, dermis and subcutaneous layers. Although much effort has been made towards mechanical characterization of skin, it remains a challenging issue to measure the mechanical properties of an individual layer in vivo. To address this issue, we here report a guided wave elastography method for layered human skin which incorporates the effect of muscle states. Both finite element simulations and phantom experiments have been performed to validate the method. For skin-mimicking phantoms with different fat layer thicknesses, the errors in the identified shear modulus of the skin layers are no more than 11 %. In vivo experiments have been carried out on 6 healthy subjects to demonstrate the potential use of the method in clinics. A statistical analysis indicates the muscle contraction contributes to the stiffening of the skin (p < 0.001). Finally, a phase diagram has been constructed to reveal the extent to which muscle sates (including both passive and active states) affect the measurement of elastic modulus of a skin layer, which may guide the application of the method in practice.