Mechanics of wrinkle formation: micromechanical analysis of skin deformation during wrinkle formation in ultraviolet‐irradiated mice

Background/purpose: The mechanical aspects of wrinkle formation were studied in the dorsal skin of hairless mice. Methods: Wrinkles were induced by irradiating with ultraviolet (UV) B for 10 weeks, while observing skin deformation during wrinkle formation. Changes in skin dimensions were also observed during the specimen excision process. Wrinkle depth and interval were measured before and after removal of the cutaneous muscle layer. Local deformation of wrinkled skin during uniaxial stretch was also measured. Changes in curvature of skin specimens upon muscle layer removal were then observed to determine the force balance in skin layers. Results: The skin showed spontaneous contraction in response to UV irradiation. Wrinkled skin showed a marked decrease in the wrinkle depth and a slight increase in wrinkle interval following muscle layer removal, a peculiar mechanical response that cannot be explained by homogeneous deformation of the skin. This response was due to compressive deformations of dermal tissue caused by the muscle layer and concentrated at valleys of the wrinkles. Curvature measurements indicated that the muscle layer compressed the dermal tissue predominantly in the craniocaudal direction. Morphological observations showed that the wrinkles coincided with rows of pores and sulci cutis, where the structural stiffness of the horny layer was relatively low. The horny layer showed significant thickening. Conclusion: Taken together, we propose the following hypothetical mechanisms of wrinkle formation during UV irradiation: spontaneous contraction of the dermis while maintaining or increasing the epidermal area induces buckling of the epidermis into the dermis at mechanically weak lines, namely, the rows of pores and sulci cutis, and buckling may be amplified by the axial compression of the dermis by the muscle layer.