Effect of equilibrium pH on the structure and properties of bleach-damaged human hair fibers.

Hair proteins are significantly affected by environmental pH. This impact tends to increase with prior hair damage. To understand how pH affects bleached hair properties, we utilized a number of techniques allowing for the determination of hair thermal properties, swelling and water sorption, and dry and wet tensile properties. At pH 5, hair proteins had the best structural integrity, as determined by differential scanning calorimetry and the highest tensile modulus. At pH 10, protein cross-linking density decreased, water content and hair cross-sectional diameter increased. Alkaline treatment, when compared with pH 5, did not reduce intermediate filament conditions (evaluated via enthalpy measurement) nor mechanical property performance in the wet state. In contrast to alkaline-treated hair, bleached hair equilibrated at pH 3 behaved very differently: it contained two different crosslink density zones, was the least stiff in dry and stiffest in wet conditions. Additionally, it absorbed less water and had the lowest diameter because of reduced water binding by protonated carboxyl groups. The pH 3 to 10 did not affect the mechanical strength of bleached hair in dry or wet conditions.

The effects of water on heat-styling damage.

Heated styling appliances, such as straightening irons, have grown in popularity in recent years, as have hair products such as heat-protection sprays. In this study we investigate whether the water in a heat-protection spray can affect the level of damage caused by heat styling. Tryptophan damage from heat styling was measured using fluorescence spectroscopy, and structural damage was investigated using light microscopy and single-fiber tensile testing. Hair samples were heat treated with straightening irons, following treatment with either a water-based, "wet," heat-protection spray or an ethanol-based, "dry," spray. Results showed that, as expected, tryptophan damage was reduced by repeated applications of both the "wet" and "dry" heat-protection sprays. However, no differences were seen between the "wet" versus the "dry" product. Light microscopy studies showed greater structural damage to hair treated with water and the "wet" spray. Tensile tests confirmed that there was greater damage to hair treated with the "wet" spray. Decreases in Young's modulus were greater in the presence of the "wet" spray. The results of this study suggest that the type of damage caused by heat treatments is different in wet versus dry hair. In dry hair, thermal treatments cause chemical damage and some structural damage. However, in wet hair, thermal treatments cause the same chemical damage, but considerably more structural damage, which causes significant changes in the physical properties of the hair. It is likely that the rapid evaporation of water from the hair is the main causal factor. Our experiments suggest that the effectiveness of commercial heat-protection sprays can be improved by the removal of water and by the use of volatile ingredients, such as ethanol, as base solvents.

Analysis of DNA in hair fibers.

The extraction and identification of deoxyribonucleic acid (DNA) from human hair shafts is described, along with the effects of hair treatments on levels of DNA and suggestions of DNA location within the shaft. DNA was present at low levels in the hair shaft, and was identified using polymerase chain reaction amplification of the human leukocyte antigen (HLA)-DQA1 locus. The use of cleanup columns aided the success of PCR amplification. DNA appears to reside in the cuticle portion of the hair shaft. Levels of DNA were found to be higher at the root-end compared to the tip-end of hair and were also found to be lower after permanent colorant treatment. DNA was found to be lost with surfactant washing, with increased loss occurring with prolonged or an increasing number of washes. These results suggest that small amounts of residual DNA remain after differentiation and add to our knowledge of the constituents of hair.