EFFECTS OF WATER ON HEAT-STYLING DAMAGE 21 The percentage peak intensity correlates to the percentage of remaining tryptophan in the hair. Light microscopy. Hair fi bers were mounted on glass slides and observed using a light mi- croscope (Olympus BX50 System Microscope, Olympus Optical Co UK Ltd, London). Images were taken using a digital video camera (SPOT Insight Camera, Diagnostic Instruments Inc., Sterling Heights, MI). Samples were illuminated from underneath. Polarized light was used to improve contrast (U-Pot fi lter, Olympus Optical Co UK Ltd). Single-fi ber tensile testing. Tensile tests used approximately 60 fi bers from each set of six tresses (ten fi bers per tress). As paired comparisons improve testing sensitivity, heat- treated and control portions of each fi ber were analyzed. Each fi ber, therefore, acted as its own control. The fi bers were permanently mounted with a 10-mm gauge length in PVC-lined brass crimps. The shorter 10-mm gauge length ensured that 100% of the heat-treated hair analyzed had actually been in contact with the irons. The ghd IV® straightening irons had 20-mm-wide plates. The cross-sectional area of each fi ber was measured using the Fiber Dimensional Analysis System (FDAS), which incorporates a Mitituyo laser scanner (Dia-Stron Ltd, Andover, Hampshire, UK). The FDAS takes multiple-diameter measurements from the fi ber and calculates a cross-sectional area based on an ellipse. The laser micrometer has an accuracy of better than 0.1 microns. Crimped fi bers were loaded into the MTT675 cassette (Dia-Stron Ltd) and then equili- brated at 80% R.H. overnight. The fi bers were then extended to break at 12.5 mm/ minute (40% strain rate/minute), using the MTT675 Automated Tensile Tester (Dia- Stron Ltd). The range of the load cell was set at 2N, giving a resolution of 1.0×10−3 N. RESULTS FLUORESCENCE SPECTROSCOPY Table II and Figure 3 show that there is no statistical difference between the percentage of tryptophan remaining in hair treated when wet versus dry (without product applied). In both cases, a cumulative 60-second treatment with the straightening irons reduced the peak intensity at 328 nm by approximately 35%. Such reductions are in good agreement with previous studies (5) that have shown a 40–50% reduction in tryptophan after fi ve minutes treatment with curling irons at 130°–170°C. Application of single doses of prototype heat-protection sprays did not signifi cantly re- duce tryptophan damage versus either wet or dry unprotected controls, although there was a weak trend towards slightly lower damage. There was also no difference in trypto- phan damage in hair treated with a “wet” spray versus a “dry” spray. Treatment with repeated doses of heat-protection sprays signifi cantly reduced tryptophan damage versus the untreated controls (wet versus dry test) in hair treated with single doses of product (ANOVA, p0.05). There was, however, still no difference between hair treated with a “wet” spray versus a “dry” spray.
JOURNAL OF COSMETIC SCIENCE 22 The effectiveness of the repeated doses of spray suggests that a buildup of polymers and conditioning agents on the hair helps insulate the hair from heat-styling damage. These protective effects are in good agreement with previous studies (9). LIGHT MICROSCOPY In order to investigate structural damage, hair was examined by light microscopy. Figure 4 shows typical images of hair fi bers taken by the light microscope. All the images shown have been taken from fi bers used in the fi rst wet-versus-dry test, which did not use heat-protection products. Images A, C, and E show no major differences in the cuticle scale patterns. Images B,D, and F focus on the medulla and cortex. Image B shows how, in untreated fi bers with a medulla, light microscopy reveals a smooth dark band running the length of the hair. However, in hair heat treated while wet (image D), the medulla is less intense in “darkness” or contrast and much more broken in structure. It was possible to run along the length of a single fi ber and observe the change in the medulla as one moved from an untreated area to a treated area and then back into an untreated area. The medulla in human hair is comprised of air-fi lled sacks. It is likely that the medulla fi lls quickly with water when the hair is wetted, and that rapid heating, boiling, and evaporation of this water causes signifi cant damage. It is this type of damage that is clearly visible under the light microscope. In addition to changes in the medulla, heat damage was also sometimes observed in the cortex (image D). Dark spots or elongated strips (parallel to the axis of the fi ber) were seen. One could speculate that these are due to the separation/cracking apart of cortical cells. Table II Summary of Fluorescence Spectroscopy Data Treatment Replicates Normalized peak intensity at 328 nm (a.u.) Percentage peak intensity, treated versus control (%) +/− Standard error ANOVA test Homogenous groups Control Treated Mean +/− Standard error Mean +/− Standard error Wet hair (no product) 32 2.29 0.07 1.45 0.04 64.37 2.20 Dry hair (no product) 33 2.12 0.05 1.40 0.02 67.53 2.03 “Wet” product (single doses) 36 1.72 0.05 1.19 0.03 71.10 2.36 “Dry” product (single doses) 36 1.99 0.04 1.36 0.03 68.81 1.61 “Wet” product (repeat dosing) 29 1.71 0.06 1.48 0.07 88.67 3.94 “Dry” product (repeat dosing) 33 1.76 0.05 1.60 0.05 92.62 3.81 An ANOVA test was used to compare percentage peak intensity (treated versus control) data. Homogenous groups are those which have data with no signifi cant (p 0.05) difference between them.
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