Development of a stiffness‐angle law for simplifying the measurement of human hair stiffness

In a study from 2018, researchers developed the Stiffness-Angle Law to measure human hair stiffness in a cost-effective way. They applied a common shampoo with 4% caffeine to hair strands 10 times and confirmed caffeine absorption using Fourier Transform Infrared Spectroscopy (FTIR). The stiffness was measured by analyzing the bending angle, weight, and distance the hair was pulled. The study, which included over 150 hair strands and 50 measurements on three randomly selected strands, showed that hair stiffness increased by 13.2% after caffeine treatment. The increase in stiffness was attributed to the absorption of small molecules and moisture, not caffeine itself. The Stiffness-Angle Law was validated as an effective model for hair stiffness, suggesting that caffeine could have a lasting effect on hair stiffness.