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South Wales Materials Association presents:

The Mechanical Properties of Skin by Prof Sam Evans

19th October, 19:00 in the Civil and Computational Lecture Theatre, College of Engineering, Swansea University

Skin is literally all around us but it is strangely invisible in engineering and materials science.  It has very unusual and interesting properties, with a very low stiffness, a very high strain to failure and tear resistance, a greater toughness than high strength steel, and a Poisson’s ratio that can be negative or extremely large.  It is highly nonlinear and undergoes very large deformations; this makes it very difficult to analyse and this is why it has been largely ignored in engineering.  Because of its mathematical complexity, it was not possible to analyse in the past except for very simple cases such as uniaxial tension, but the availability of fast computers and modern imaging techniques now make it possible to measure and predict its mechanical behaviour.  There are many applications in engineering, for example in the design of razors or anything else where touch and feel are important, in medicine, for example to plan or simulate plastic surgery, and in other areas such as animation.  There are many situations where it would be useful to measure the mechanical properties of skin, for example to quantify the severity of skin diseases or the effect of drugs or cosmetics, but in the absence of a satisfactory model it is difficult to know what parameters to measure.

Because of its highly nonlinear behaviour, it is generally necessary to make a computational model of an experiment and adjust the properties by trial and error to match the experimental results.  We have used a variety of tests, including tensile and indentation tests on isolated skin and living people.  The skin has been modelled as a membrane and as a layered solid and we have developed optimisation algorithms to identify the material parameters.  These methods give a reasonable approximation to the elastic behaviour of skin, but many questions remain, including its viscoelastic behaviour and the mechanisms of damage and failure, and the uncertainty and uniqueness of the measured material parameters.