Measuring Soft Tissue Properties Using Digital Image Correlation and Finite Element Modelling

Abstract

An understanding of the mechanical properties of soft tissues such as skin, muscle and tendon is important in many applications, such as the design of orthopaedic implants, the planning of surgical procedures or the evaluation of cosmetic products or drugs. However, the behaviour of soft tissues is complex, highly nonlinear and poorly understood, and so the measurement of their mechanical properties presents many difficulties. This study focuses on the measurement of the properties of skin, which has the advantage that it can conveniently be tested in situ in a living human subject. In many published studies simple tensile tests have been used, but these require samples to be cut from the tissue and can only characterize the uniaxial tensile properties, whereas in vivo biaxial tension, in- plane compression and shear are also very important. In other studies, tabs have been attached to the skin and loaded in plane, in torsion or by indentation, but the measurements of the resulting deformation have been limited and the boundary conditions are poorly defined, so that it is difficult to properly characterize the resulting state of strain. Full field optical measurements offer the possibility of characterizing the full strain distribution under complex loading, and accurately measuring the displacement boundary conditions, so that the deformation of the skin can be properly characterized. The aim of this study was to use digital image correlation (DIC) in conjunction with a finite element model, using an Arruda- Boyce constitutive law, to identify the properties of the skin in situ in a living human subject. It was found that although the Arruda- Boyce model can represent the uniaxial behaviour of the skin with reasonable accuracy, the strain distribution under more complex loading was very different from that measured using DIC. In particular, the compressive stiffness of the skin is much lower than the Arruda- Boyce model predicts due to wrinkling. A more sophisticated constitutive model that incorporates progressive wrinkling under in- plane compression and correctly represents the shear behaviour of the skin is needed. The DIC technique was highly effective in measuring the deformation of the skin, and the full field strain measurements provided valuable insights into the behaviour of the skin. This technique has great potential for characterizing the properties of the skin and for developing better constitutive models. Introduction