Research On Mechanical Response Of Skin Dermis To External Loads

Skin is the largest organ of the human body with multiple physiologic functions.As the physical interface of outside world and our inner body,skin protects the body from complex external mechanical,biological,chemical,and electromagnetic stimuli and controls the exchanges of various signals.Skin is a multi-layer structure consisting of epidermis,dermis,and hypodermis.Each layer has distinct structural composition and exhibits different functionalities,enabling diverse and complex responses of skin tissue.The dermis dominates mechanical behavior of the skin tissue and is largely composed of a dense network of collagen fibers.The investigation of skin mechanics can deepen the understanding of skin tissue and fiber-rich materials,enrich the construction of mechanical models of such materials and broaden theoretic basis of biomechanics.From the perspective of practical application,further investigation on how skin tissue and fibers response to various external stimuli would contribute to provide fundamental basis for clinical and cosmetic applications.The main contents of this paper are as follows:(1)First of all,we introduced theoretical framework of nonlinear continuum mechanics by describing large deformation,the basic conservation equations and the dissipation inequality.Constitutive models for incompressible fiber-reinforced hyperelastic materials are also introduced.(2)To better model the anisotropic mechanical response of skin tissue,a general structure tensor-based constitutive model of skin tissue that incorporates two fiber families with distinct mechanical and structural properties is proposed.The constitutive model also considers the inhomogeneous deformation in the skin as a result of structural heterogeneity,showing good agreement with in vivo measurements of human face skin.(3)A new constitutive model is developed to relate the macroscopic response with the microstructural alteration to investigate the stress softening behavior of skin tissue observed during cyclic loading tests.The model considers microstructural alteration due to fiber uncrimping and incorporates the evolution of microscopic damage as strain increases during loading process.Good agreement has been found between the numerical and ex-vivo experimental data from human skin,indicating that the constitutive model we proposed can capture the typical softening behaviors observed during cyclic loading condition.(4)The influence of fiber remodeling to macroscopic responses of skin tissue is investigated.By studying the mechanical response of skin tissue to external loads at various strain rate,the changes of fiber uncrimping and remodeling mechanism due to strain rate variation is analyzed and a model incorporates the above two mechanisms is proposed to relate the macroscopic response to microstructural alteration.A good agreement has been found between the theoretical results and ex-vivo experimental data from pig skin,indicating that the constitutive model we constructed can capture the microstructural alteration during dynamic loading condition.