223 Chapter 7 Mechanical Properties of Hair R. Randall Wickett, PhD James L. Winkle College of Pharmacy, University of Cincinnati Introduction Understanding the mechanical properties of hair is vital for understanding the behavior of hair and its response to routine grooming and cosmetic treatment. Hair is a composite structure built from spindle shaped cortical cells surrounded by a sheath consisting of several layers of flat cuticle cells. The cortical cells that dominate the mechanical properties of hair contain oriented α-helical keratin chains embedded in an amorphous matrix of keratin associated proteins now known as KAPs (see Chapter 1 for details). This highly asymmetric structure leads to complex mechanical behavior. Hair is a viscoelastic solid, as hair that is under stress exhibits phenomena such as creep and stress relaxation exponentially approaching either equilibrium length or stress. Viscous behavior results from flow processes in hair proteins, primarily the KAPs. Much of the early understanding of the physical properties of hair derived from studies of the physical properties of wool. Wool has a very similar structure to hair except that there is only one cuticle layer on wool fibers with the cuticle cells just overlapping each other, and wool fibers are generally finer than hair, with diameters ranging 20–40µ. Early on, Speakman found that the torsional rigidity of wool is dramatically reduced as the water