157 Chapter 5 Changing the Shape of Hair R. Randall Wickett, PhD James L. Winkle College of Pharmacy, University of Cincinnati Introduction Consumers’ desire to alter and control the shape of their hair is a major driver of research and development efforts in the hair care industry. People with straight hair often want to add curl and people with wavy or curling hair often want to straighten it. This chapter will focus on changes in shape, originating in the hair fiber itself. Shape can also be altered or altered shapes maintained by styling products, which are discussed later in Chapter 13. The shape of hair can be changed by temporary setting, sometimes called cohesive setting, using water and/or heat, or changed more permanently though chemical reactions. Two steps are required to achieve set in hair. First, the structure must be plasticized while a deformation is applied. Then the plasticizing factor must be removed before the deformation is released. When hair is given a temporary set, either water or heat is used as the plasticizing agent. Hair is wound on a curler while wet and allowed to dry, or is curled and heated by a curling iron. This is known as cohesive set. Cohesive set is primarily stabilized by hydrogen bonds and salt links. It arises because the keratin associated proteins of the matrix can flow into new positions during the deformation process when they are plasticized by heat or water and then resist the tendency of the microfibrils to restore the original shape of the