Question

Why would we not be able to successfully evaluate the mechanical properties of skin in the same manner we evaluate metals in tension (uniaxial or 1-D testing such that is done with the Instron 5900R-4206 universal test system)?

Answer 1

Unlike metals skin is composed of multiple layers (typically 3 layers). Each of this layer has different mechanical properties and hence different response to external loading. Skin consist of 3 layers viz. 1. Outermost layer called epidermis, 2. Dermis, 3. Hypodermis.

The stress strain behaviour of skin is studied in 3 phases.

1. When a strain of up to 0.3% is applied, the elastin fibres offer low resistance to the applied strain. The skin exhibits isotropic behaviour and collagen fibres remain tangled and intertwined and do not contribute to the stiffness. This region the stress strain relation is linear.

2. In Phase 2, the collagen fibres offer some resistance to the deformation and the crimped collagen fibres begin to stretch, thus introducing non-linearity into the stress strain relationship.

3. In the final Phase 3, for applied strain above 0.6%, the crimps begin to disappear and a linear stress-strain relationship can be observed. The collagen fibres break after the application of an ultimate tensile strain of 0.7%.

Conventional testing equipment's do not offer the facility of studying a specimen of composite nature having such a switch between linearity and non linearity behaviour. Hence these UTM's cannot be used for testing or evaluating skin specimen.