Chapter 13 441 Stiffness and flexibility: These properties of the styling formulation-hair fiber composite can be measured by a tensile meter such as a texture analyzer under various temperature and humidity conditions.28-30 Various mechanical parameters can then be assessed such as the initial stiffness, durability of stiffness, flexibility, and plasticity. Tack profile may also be measured using a texture analyzer by measuring adhesive forces. For styling products, profiles should be measured for the magnitude and duration of tack during drying, and after drying with various humidity exposures. Hold—High humidity curl retention: Although this is an old method, it is still used today for styling efficacy measurements to determine the humidity resistivity of the styling formulation. An example of an early reference was presented by Micchelli and Koehler in 1967.37 The method can be adapted for products added to wet hair such as hair gels, dry hair such as hair sprays, and to determine the initial curl droop from especially low VOC formulations. Curl retention measurements are taken at periodic intervals up to 24 hours for high hold claims. Figure 8. Styling product development evaluation process