Spatial Mechanical Behaviour of Skin

Abstract

Skin is a complex biological composite system that serves as the outermost barrier to the environment and is mechanically robust. Understanding the mechanical properties of skin is important to improve and compare current in vitro experiments to the physiological conditions as the mechanical properties have a crucial role in determining cell behaviour. The mechanical behaviour of skin at the cellular level is expected to be dominated by the collagen fibre network within the dermis, which displays an anisotropic mechanical response to macroscopic loading. However, the three dimensional mechanical properties of skin at the nanoscale are not well understood. The aim of this work is to examine the mechanical properties of skin at the nanoscale in three dimensions and explore the links between the nanoscale and the macroscopic behaviour. Multiple sample preparation techniques are employed to expose the different layers of skin for mechanical testing and the elastic modulus of skin is evaluated by using atomic force microscopy (AFM) nanoindentation. The effect of freezing skin to cryogenic temperatures on the mechanical properties is evaluated and found to have no impact on the mechanical response of skin, indicating that the composition and structure of skin are robust enough to withstand the cryosectioning sample preparation methods used to expose the transverse layers of skin. AFM indentation was used to evaluate the elastic modulus of the dermis depending on the orientation of the sample and found to have an isotropic mechanical response. This result is opposite to anisotropy observed in macroscopic skin due to small scale mechanical testing ignoring collagen fibril orientation during strain. The variations in the elastic modulus of skin are also evaluated by AFM indentation at high spatial resolution to construct a composite model of the mechanical behaviour of skin at the nanoscale to predict the macroscopic response. The AFM nanoindentation technique was extended to evaluate the mechanical properties of a cell derived matrix deposited on an electrospun nanofibre scaffold, where the results indicate increasing the nanofibre diameter produces a cell derived matrix with an increased elastic modulus for more effective scaffolds. This work highlights the use of AFM mechanical testing to evaluate the nanoscale mechanical behaviour of skin, treated as a composite biological system, and determine the influence of the length scale and sample orientation on the observed mechanical response.