CAREER: Understanding the Multi-scale Failure Mechanics of Human Skin with Age, Ultraviolet Photodamage and Bacterial Growth

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Human skin acts as the body's first line of defense to the outside world. When ruptured, this barrier function can be lost, leaving living tissue exposed to harmful pathogens. Naturally occurring examples of rupture include cracking, chapping, stretch marks, and the formation of sores, yet the underlying mechanistic processes behind how they occur remain unclear. This Faculty Early Career Development (CAREER) Program award supports fundamental research that will provide biomechanical knowledge of the failure mechanics in skin tissue and reveal how ageing, ultraviolet photodamage and the growth of bacteria on skin weakens the tissue microstructure, causes wrinkles, and increases the risk of skin rupture. The results will impact the US economy and society by providing medicine, biomedical engineering and the cosmetics industry with an improved understanding of the ageing process, the onset of skin diseases and new approaches for transdermal drug delivery. Results will further provide bio-inspired design approaches for controlling fracture and buckling in thin films; exploitable in applications ranging from flexible electronics to energy harvesting. This project will broaden participation of underrepresented groups in research and develop tools to improve public knowledge of skin care. Previous studies characterizing the mechanics of skin use macroscopic testing equipment designed for homogenous materials. Such measurements ignore the essential heterogeneity and complex microstructure of the tissue. Embracing this complexity, this project will complete objectives that will establish changes in the multi-scale mechanics and structure of human skin with age and ultraviolet photodamage, and quantify changes in the structural integrity of the epidermis with bacterial growth. These factors are strongly correlated with the formation of skin wrinkles and an increased propensity of tissue rupture that can lead to infections, however the underlying biomechanical processes that cause them remain largely unexplored. Experimental approaches combining immunostaining, mechanical manipulation, high speed imaging and traction force microscopy will be employed to quantify the mechanical and structural degradation of skin across multiple length scales. These results will provide key insight into: the validity of the prevailing paradigm that macroscopic testing techniques can provide meaningful information about the energy cost of fracture in soft tissues, the impact of ageing and prolonged solar irradiation on the mechanical properties and structure of skin, and the ability of bacteria to permeate into and mechanically degrade epidermal tissue. These studies will provide a first step towards understanding the biomechanical ageing process and the ability of bacteria present in the skin microbiome to cause disease.