Affine versus non-affine deformation in soft biological tissues, measured by the reorientation and stretching of collagen fibres through the thickness of compressed porcine skin

Skin is a complex three-dimensional structure of cells, collagen fibres and other proteins. However most mechanical analyses treat skin as a two-dimen sional membrane, neglecting the through thickness structure. In this paper we investigate through thickness reorientation of collagen fibres. The mode of deformation of skin is also considered. For modelling purposes deformat ion is usually assumed to be affine. This assumption was tested by construc ting a simple geometrical, affine deformation model to predict the through thickness reorientation of collagen fibres, from their initial through thic kness angle and the measured deformations of skin samples during compressio n. The measured reorientation of collagen fibres was found to be very varia ble, however the average reorientations: were consistent with the predictio ns of the model, with the inclusion of a systematic error. The variation in the reorientation of individual fibres call be explained by the variations in the structure at a micrometre scale, The systematic deviation of reorie ntations from the model predictions can be explained by a non-affine relati onship between the collagen fibres and ground substance at a micrometre sca le. However, non-affine deformations at a micrometre scale caused by irregu larities of structure are likely to average out at a millimetre scale, beca use at this level material is evenly distributed. (C) 2001 Elsevier Science Ltd. All rights reserved.