How Do Heat and Hydration Affect the Tensile Properties of Human Hair Fibers?

Overview

Objectives/Goals (1) Study a full spectrum of tensile properties of keratin-based fibers (human hair), beyond simple tensile strength (e.g., generate full stress/strain curves). (2) Explore how heat & hydration, which alter protein structure, act to affect the tensile properties of individual keratin-based fibers. (3) Key is to successfully design and build a practical, inexpensive, and accurate mechanism that can generate full stress/strain curves for single fibers, and to make a time-effective methodology that overcomes the problem of high variability between different hairs. Methods/Materials An inexpensive stress/strain assessing mechanism (SSAM) was designed to precisely apply stresses to single fibers and measure resulting strains. It uses a Hookes law ideal spring & a camcorder to capture data on-the-fly for later analysis. A fiber has each end threaded in a needle, tied, and glued. Needles are clamped into the SSAM. Tensile properties of 32 normal hair fibers were studied to refine and prove the SSAM. Stress/strain curve analysis successfully gave 7 measures for each: Youngs modulus (stiffness) pre-yield, yield stress & strain, post-yield modulus, pre-breakage modulus, and breakage stress & strain. Results Tensile properties varied greatly between hairs, so paired internal controls were developed. A hair is divided, one half used as control for its twin, reducing variability. Heat, 95-100 °C, 2-3 hrs, was applied to one half of each of 15 hairs. Heated fibers showed less elasticity over a longer range of stresses than their unheated twins. Many yield points nearly disappeared. Hydration at 40-45 °C, 2-3 hrs, was also studied on 21 fibers. Water increased elasticity & reduced yield stress, compared to their dry twins. Finally, stretching & releasing fibers showed fibers return to their original length if strain is low, but they lose an ability to recover (even overnight) if strained past the yield point. Conclusions/Discussion My new SSAM & procedures worked well to make single fiber stress/strain curves, and internal controls overcame variability problems. High heat makes keratin fibers stiffer and hydration raises elasticity & eases yield. Interestingly, keratin fiber stress/strain curves seem counterintuitive: When stretched, they are first stiff, then loose, then stiff again, before breaking. A model with a spring-like structure having cross-links is proposed to account for this behavior.

Awards (1)

• Category Award

Competition history

• CSEF 2003 Structural Capability & Strength of Materials (Junior Division Only) · Entry J1816

Resources

• CSEF project summary ↗

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