A Critical Review of the Structural Mechanics of Wool and Hair Fibres

The potential of genetic engineering gives current importance to the need to clarify the relations between structure, properties and performance of wool. There are at present three different models to explain the tensile stress-strain properties of alpha-keratin fibres: a development by Wortmann and Zahn (W/Z) of Feughelman's series-zone model based on the structure of intermediate filaments and a gel-->sol matrix; a new model by Feughelman (F94) based on a matrix of protein globules surrounded by water; and Chapman's model based on the composite mechanics of microfibrils, characterised by critical and equilibrium stresses for the alpha<-->beta transition, in an elastomeric matrix. This paper examines the arguments related to these theories, with additional attention to the Chapman/Hearle (C/H) treatment. The main area of uncertainty, on which more information is much needed, concerns the chemical and physical structure and properties of the matrix, which is composed of keratin-associated proteins. The conclusion is that the C/H model, which is based on reasonable values of input parameters and gives good agreement with a wide range of experimental results, is most likely to be valid, though it needs some additional refinement and incorporation in a total model, which includes larger-scale structural features.