Biomimetics in thin film design – Wrinkling and fracture of pulsed laser deposited films in comparison to human skin

The goal of the current work is the biomimetic comparison of wrinkling effects on the nano- and micrometer scale in thin films and on the micro- and millimeter scale of human skin. Ti, Ag, Au, W, C, and TiN thin films with 10 to 500nm thickness were deposited by pulsed laser deposition (PLD) on smooth polyurethane, polycarbonate, polyamide 6, and polyimide polymer substrates. Such high energetic conditions like in PLD plasma lead to stiffening of the polymer surface layers by pseudodiffusion during the initial stages of film growth. Both the high intrinsic film stresses due to high energetic film growth and the huge difference in elastic properties of films and polymer substrates result in hierarchical and self-adapting nanowrinkling phenomena. Intrinsic stresses are a reason for the hierarchical and self-adapting wrinkling of human skin too, being mechanically treated quite similar to the coated polymers as a hard layer (stratum corneum) on a soft foundation (viable epidermis and dermis). Mechanically, the wrinkles have high impact on the elongation behavior: In the elastic region of the stress–strain curve, they are smoothing out at increasing strain. In fracturing, the wrinkles are an origin of cracking (channeling cracks) and afterwards fragmentation of both thin film and stratum corneum islands, respectively, as proofed with in-situ tensile straining experiments of the thin films in scanning electron microscopy.