UC Berkeley

Knowledge of the mechanical response and deformation behavior of individual skin layers during microprobe penetration is of high clinical and societal importance. In this thesis, the elastic behavior of stratum corneum, viable epidermis, dermis, and whole multilayer skin were investigated by combining micro/nanoindentation and microscratching techniques. Statistical analysis shows insignificant differences in reduced elastic modulus of skin samples obtained from three different porcine breeds. The reduced elastic modulus of stratum corneum is shown to be about three orders of magnitude higher than that of dermis. For relatively shallow and deep indentation depths, skin elasticity is controlled by that of stratum corneum and dermis, respectively. Skin indentation mechanics are interpreted in the context of a layered structure model consisting of a stiff and hard layer supported by a compliant and soft substrate, derived on the basis of microscopy observations and indentation measurements.

Time-dependent deformation of porcine skin was also studied in vitro. The deformation behavior of stratum corneum, dermis, and whole skin tissue are examined in the context of measurements of creep strain, elastic stiffness, and viscoelastic constants obtained for different values of hold time, loading/unloading rate, and maximum indentation depth (load). It is shown that dermis viscoelasticity significantly affects the time-dependent deformation of skin up to a critical indentation depth (load) beyond which, the viscoelastic behavior of skin is controlled by the outermost hard epidermis, particularly stratum corneum. A conceptual deformation model that explains skin viscoelastic behavior under constant load (creep) and zero load (stress relaxation) conditions is developed on the basis of the phenomenological observations and experimental trends of this study.

Representative friction and wear results of skin subjected to unidirectional and reciprocal (cyclic) scratching are interpreted in terms of sliding speed, normal load, and scratch cycles to illustrate the effects of stratum corneum, cellular epidermis, and dermis on the skin friction and wear characteristics. Depending on the applied normal load and scratch time (cycles) various friction mechanisms (adhesion, plowing, and squeeze film lubrication) and wear processes (surface plasticity/plowing, bulk shearing, cohesive failure, tearing, and delamination) control shear-induced skin damage. The obtained results provide insight into microscale friction and wear processes influencing the mechanical response of skin to normal and shear surface tractions.