Photodegradation of human hair

S. Ratnapandian; S. B. Warner; Y. K. Kamath

Table II Loss (%) in Wet Mechanical Properties of Hair Exposed at Various RH Block A Work-to-20%-extension Time/RH 10 20 30 50 70 100 h 26.98 12.56 11.16 17.67 22.79 200 h 38.60 28.84 18.60 22.79 32.09 300 h 46.98 29.30 34.42 25.12 46.51 Stress-at-20%-strain Time/RH 10 20 30 50 70 100 h 22.73 11.36 11.36 13.64 20.45 200 h 38.64 27.27 18.18 18.18 29.55 300 h 47.73 27.27 31.82 22.73 43.18 Block B Initial modulus Time/RH 10 20 30 50 70 100 h 23.62 9.77 12.38 11.31 21.85 200 h 35.23 25.38 33.38 14.54 35.85 300 h 43.31 21.85 26.23 32.23 46.69 Post-yield modulus Time/RH 10 20 30 50 70 100 h 17.42 9.76 42.51 17.77 39.37 200 h 24.04 20.91 50.52 12.89 55.05 300 h 32.06 25.78 47.39 35.54 60.63 Work-to-break Time/RH 10 20 30 50 70 100 h 15.26 13.88 -2.02 19.02 19.09 200 h 31.02 29.21 4.05 13.81 7.66 300 h 41.43 24.15 11.71 13.88 23.14 Stress-to-break Time/RH 10 20 30 50 70 100 h 25.79 19.50 27.67 25.79 33.33 200 h 34.59 32.08 32.08 20.75 44.03 300 h 44.65 29.56 35.85 32.70 49.06 B•& C Turnover point Time/RH 10 20 30 50 70 100 h -11.44 -4.40 -24.04 -5.28 -27.74 200 h -9.94 -4.56 -24.66 -10.07 -40.51 300 h -9.33 -9.12 -21.92 -13.40 -36.54 Strain-to-break Time/RH 10 20 30 50 70 100 h -11.28 -3.86 -23.23 -4.10 -28.49 200 h -10.59 -4.11 -22.82 -9.42 -41.67 300 h -10.67 -8.14 -31.24 -14.08 -43.14

316 JOURNAL OF COSMETIC SCIENCE 5O e 30 • 2O 10 0 i • i 100 hours 200 hours 300 hours 0 20 40 60 80 % RH Figure 4. Reduction in work-to-20%-strain of Piedmont hair exposed to simulated solar radiation. 50 ß 40 • 30 ß • 2O 10 ß 100 hours ß 200 hours ß 300 hours 0 20 40 60 80 % RH Figure 5. Reduction in stress-at-20%-strain of Piedmont hair exposed to simulated solar radiation. ANALYSIS OF THE FTIR SPECTRUM The FTIR/ATR spectra obtained from Piedmont hair exposed for 300 hours at different RH levels are shown in Figure 7. The spectra were normalized using the absorbance at 1513 cm -1 (peak 1), the amide-II band, allowing a semiquantitative comparison of the cystine oxide content. A listing of the different absorbance peaks is given in Table III (2). The absorbances at 1042 cm 1 (peak 5) due to cysteic acid and at 1073 cm -• (peak 4) due to cystine monoxide, the primary degradation products of the disulfide links, indicate that irradiation at 20% RH caused the least amount of degradation. These absorbances for fibers exposed to radiation at other RH levels are essentially invariant, as shown in Figure 7. Since the beam essentially penetrated only the cuticle, the results suggest that damage to the cuticle is nearly identical for irradiation under different conditions over similar duration of exposure. This concurs with the results reported by Hoting eta/. (2). The low absorbances shown for exposure at 20% RH are probably due to experimental error. An important point to be noted is that cuticular damage, unless excessive, does not affect the tensile properties significantly (16). However, such damage is often the cause of crack initiation and fiber fracture. ROLE OF MOISTURE IN HAIR PHOTOLYSIS According to Arnaud (17), photolysis of hair proteins is caused by radiation with

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314 JOURNAL OF COSMETIC SCIENCE Table I Wet Mechanical Properties of Untreated Piedmont Hair Initial modulus Work-to-20%- Work-to-break Stress-at-20%- (Gpa) strain (MJm 2) (MJm 2) strain (Gpa) 1.30 0.22 1.38 0.04 Stress-to-break Turnover strain Strain-to-break Post-yield modulus (Gpa) (% exm) (% exm) (Gpa) 0.16 32.35 61.06 0.29 monotonically with an increase in ambient moisture during irradiation, damage appears to go through a minimum. A second-order polynomial regression indicates that this behavior is characteristic of most mechanical properties, as shown in Figures 4 and 5. Hair fibers exposed at 10% and 70% RH show similar losses--about a 47% loss for a 300-hour exposure period--whereas the loss is only about 30% for exposure at 30% RH for 300 hours. Wool, another keratin fiber, also photodegrades in sunlight, Holt (15) reported that wool undergoes a high degree of photo-tendering and photo-yellowing on being irradiated at low humidity levels or in the presence of moisture. SWELLING RESULTS The extent of transverse swelling in 0.1 N sodium hydroxide solution of Piedmont hair irradiated for 300 hours at various RH levels is shown in Figure 6. Hair fibers exposed at 50% RH show the maximum swelling, 74%. Unlike the decreases in tensile properties, the extent of swelling goes through a maxi- mum. Swelling increases with the loss of crosslinks (8,9). Hence, the results suggest that hair exposed a high and low humidity levels have a larger number of crosslinks compared to hair exposed at intermediate humidity. This interpretation is inconsistent with the trend seen in the wet mechanical properties. This suggests that the swelling is caused by the degraded protein residues in the fiber. At high and low humidity levels, the proteins are extensively degraded and the residues have a molecular weight small enough to let them diffuse out of the fiber during exposure or immersion in 0.1 N sodium hydroxide solution. For exposure at intermediate humidity levels, the extent of degradation is less and the residues are large enough to be retained. This will lead to osmotic swelling of the fiber. This explanation is consistent with the trend seen in the wet mechanical properties. Isolation and identification of the residues that are cleaved and diffuse out of the hair fiber might offer support for the theory suggested. However, the short protein chains might diffuse during irradiation, during immersion in sodium hydroxide solu- tion, or both, thereby making the experiment contrived. An alternative theory may be supported by the work of Wolfram (6), in which exposure caused formation of secondary crosslinks between protein residues, in addition to ex- tensively damaging the fiber. The secondary crosslinks limited swelling without con- tributing significantly to the tensile properties. Exposure at low and high humidity levels favors crosslink formation as compared to exposure at intermediate levels. Hence, swelling passes through a maximum.

Table II Loss (%) in Wet Mechanical Properties of Hair Exposed at Various RH Block A Work-to-20%-extension Time/RH 10 20 30 50 70 100 h 26.98 12.56 11.16 17.67 22.79 200 h 38.60 28.84 18.60 22.79 32.09 300 h 46.98 29.30 34.42 25.12 46.51 Stress-at-20%-strain Time/RH 10 20 30 50 70 100 h 22.73 11.36 11.36 13.64 20.45 200 h 38.64 27.27 18.18 18.18 29.55 300 h 47.73 27.27 31.82 22.73 43.18 Block B Initial modulus Time/RH 10 20 30 50 70 100 h 23.62 9.77 12.38 11.31 21.85 200 h 35.23 25.38 33.38 14.54 35.85 300 h 43.31 21.85 26.23 32.23 46.69 Post-yield modulus Time/RH 10 20 30 50 70 100 h 17.42 9.76 42.51 17.77 39.37 200 h 24.04 20.91 50.52 12.89 55.05 300 h 32.06 25.78 47.39 35.54 60.63 Work-to-break Time/RH 10 20 30 50 70 100 h 15.26 13.88 -2.02 19.02 19.09 200 h 31.02 29.21 4.05 13.81 7.66 300 h 41.43 24.15 11.71 13.88 23.14 Stress-to-break Time/RH 10 20 30 50 70 100 h 25.79 19.50 27.67 25.79 33.33 200 h 34.59 32.08 32.08 20.75 44.03 300 h 44.65 29.56 35.85 32.70 49.06 B•& C Turnover point Time/RH 10 20 30 50 70 100 h -11.44 -4.40 -24.04 -5.28 -27.74 200 h -9.94 -4.56 -24.66 -10.07 -40.51 300 h -9.33 -9.12 -21.92 -13.40 -36.54 Strain-to-break Time/RH 10 20 30 50 70 100 h -11.28 -3.86 -23.23 -4.10 -28.49 200 h -10.59 -4.11 -22.82 -9.42 -41.67 300 h -10.67 -8.14 -31.24 -14.08 -43.14

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PHOTODEGRADATION OF HUMAN HAIR 313 RESULTS AND DISCUSSION TENSILE TEST RESULTS A typical load-elongation curve obtained from the Diastron © tensile tester for an un- treated Piedmont hair fiber, immersed in deionized water, is shown in Figure 3. The wet tensile properties of untreated Piedmont hair fibers are presented in Table 1. Table II shows the percent loss in properties of the hair fibers after exposure to simulated solar radiation at various levels of relative humidity. The data for work-to-20%-strain and stress-at-20%-strain shown in block A of Table II are graphed in Figures 4 and 5. It is important to emphasize that significant differences in the mechanical properties will be observed only in the wet condition, where the contribution of hydrogen bonds to the mechanical properties is eliminated. Ef?•ct of length of exposure. Damage occurs with exposure to simulated solar radiation and increases with duration of exposure, at all humidity levels, in agreement with the results obtained by Ratnapandian (3), Leroy et al. (8) and Dubief (9). As shown in block B of Table II, exposure for 100 hours results in about a 20% decrease in many properties, whereas a decrease approaching 50% is characteristic of fibers exposed for 300 hours. On the other hand, the turnover point, which is the intersection of the yield and post-yield part of the load-elongation curve or the extension at which the [3-keratin begins to play a significant load-bearing role, and extension-to-break data show an increase of about 10% (block C, Table II). This may be due to the increased mobility of molecular chains that results from the loss of disulfide crosslinks. The gains in these two properties do not compound with length of exposure. Exposure to UV radiation of many polymers causes embrittlement and the formation of new crosslinks, which may be the limiting factor, as suggested by Wolfram (6). Efd•ct of change in humidity. The tensile properties of hair fibers exposed to simulated sunlight for any given time period depends on the RH of the atmosphere during exposure. Contrary to the proposed hypothesis that photolyric damage will increase 78.4 5S.S •t k6?"v 39.2 19.õ O.O O.O 16.50 32.S3 49.33 65.S3 % ext Figure 3. Typical load-elongation curve obtained for a single hair fiber on a Diastron © miniature tensile tester.

314 JOURNAL OF COSMETIC SCIENCE Table I Wet Mechanical Properties of Untreated Piedmont Hair Initial modulus Work-to-20%- Work-to-break Stress-at-20%- (Gpa) strain (MJm 2) (MJm 2) strain (Gpa) 1.30 0.22 1.38 0.04 Stress-to-break Turnover strain Strain-to-break Post-yield modulus (Gpa) (% exm) (% exm) (Gpa) 0.16 32.35 61.06 0.29 monotonically with an increase in ambient moisture during irradiation, damage appears to go through a minimum. A second-order polynomial regression indicates that this behavior is characteristic of most mechanical properties, as shown in Figures 4 and 5. Hair fibers exposed at 10% and 70% RH show similar losses--about a 47% loss for a 300-hour exposure period--whereas the loss is only about 30% for exposure at 30% RH for 300 hours. Wool, another keratin fiber, also photodegrades in sunlight, Holt (15) reported that wool undergoes a high degree of photo-tendering and photo-yellowing on being irradiated at low humidity levels or in the presence of moisture. SWELLING RESULTS The extent of transverse swelling in 0.1 N sodium hydroxide solution of Piedmont hair irradiated for 300 hours at various RH levels is shown in Figure 6. Hair fibers exposed at 50% RH show the maximum swelling, 74%. Unlike the decreases in tensile properties, the extent of swelling goes through a maxi- mum. Swelling increases with the loss of crosslinks (8,9). Hence, the results suggest that hair exposed a high and low humidity levels have a larger number of crosslinks compared to hair exposed at intermediate humidity. This interpretation is inconsistent with the trend seen in the wet mechanical properties. This suggests that the swelling is caused by the degraded protein residues in the fiber. At high and low humidity levels, the proteins are extensively degraded and the residues have a molecular weight small enough to let them diffuse out of the fiber during exposure or immersion in 0.1 N sodium hydroxide solution. For exposure at intermediate humidity levels, the extent of degradation is less and the residues are large enough to be retained. This will lead to osmotic swelling of the fiber. This explanation is consistent with the trend seen in the wet mechanical properties. Isolation and identification of the residues that are cleaved and diffuse out of the hair fiber might offer support for the theory suggested. However, the short protein chains might diffuse during irradiation, during immersion in sodium hydroxide solu- tion, or both, thereby making the experiment contrived. An alternative theory may be supported by the work of Wolfram (6), in which exposure caused formation of secondary crosslinks between protein residues, in addition to ex- tensively damaging the fiber. The secondary crosslinks limited swelling without con- tributing significantly to the tensile properties. Exposure at low and high humidity levels favors crosslink formation as compared to exposure at intermediate levels. Hence, swelling passes through a maximum.

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312 JOURNAL OF COSMETIC SCIENCE cross-sectional area is calculated assuming an elliptical cross section. The micrometer was calibrated using standard calibration wires of known cross-sectional area (12). Test specimens for the laser-scan micrometer were prepared using a metal-tube sample- mounting system. This system uses a pair of hollow brass tubes with plastic sheaths on the inside and a stamping press (stock no. RS622-032) to crimp the fiber ends in the holders. The tubes are 1.4-cm long with an inner diameter of 0.1 cm and an outer diameter of 0.2 cm. The base of the press is a stainless steel template with a gauge length of 3.0 cm. The hair was threaded through two tubes, and the assembly was placed on the template. When crimped by mechanical pressure, a hair fiber is held securely by the crimped tubes. After measuring the cross-sectional area, the samples were directly transferred to the tensile tester. Tensile tests were conducted in deionized water using a Diastron © miniature tensile tester. The miniature tensile tester autosampler, MTT 600 series, from Diastron Ltd., UK, has a sample holder with a capacity of one hundred (100) specimens. Each specimen can be wetted i, sit•. The degree of deformation and the rate of extension were preset. Data from each specimen were collected and transferred automatically to a personal computer using the MTTWIN software included with the instrument. The software was also used to calculate the tensile properties of the fibers (13). Forty fibers per sample were tested, a number that earlier work has shown to be statistically acceptable (7). The sample length was 30 mm. The strain rate was 40%/min, that is, a crosshead speed of 12 mm/min was used. Samples were soaked for at least ten minutes to ensure complete wetting and saturation prior to tensile testing. SWELLING TEST Swelling was measured by determining the change in hair diameter in a 0.1 N solution of sodium hydroxide at room temperature (9). The "diameter" both dry and after swelling was measured using a Bausch & Lomb microscope fitted with a Digital Filar © eyepiece connected to a Microcode © meter from Boeckler Instruments. Hair fibers weathered for 300 hours at different humidity levels were tested. FTIR/ATR ANALYSIS The IR spectra of hair fibers exposed for 300 hours at different humidity levels were compared. IR spectra were obtained using a UMA 500 FTIR microscope from Bio-Rad. Samples were pressed between two optically matched diamond surfaces. The ATR (attenuated total reflectance) technique, with a depth of penetration of about 5 pm in hair, was employed. The ATR technique involves bringing the fiber in contact with a germanium crystal designed for total internal reflection of the incident radiation. The reflected beam is altered by absorption by the sample surface in contact with the reflecting medium, and this alteration or attenuation is analyzed. Transmission spectroscopy was not used be- cause the variation in hair sample diameter resulted in spectra unsuitable for comparison (•4).

PHOTODEGRADATION OF HUMAN HAIR 313 RESULTS AND DISCUSSION TENSILE TEST RESULTS A typical load-elongation curve obtained from the Diastron © tensile tester for an un- treated Piedmont hair fiber, immersed in deionized water, is shown in Figure 3. The wet tensile properties of untreated Piedmont hair fibers are presented in Table 1. Table II shows the percent loss in properties of the hair fibers after exposure to simulated solar radiation at various levels of relative humidity. The data for work-to-20%-strain and stress-at-20%-strain shown in block A of Table II are graphed in Figures 4 and 5. It is important to emphasize that significant differences in the mechanical properties will be observed only in the wet condition, where the contribution of hydrogen bonds to the mechanical properties is eliminated. Ef?•ct of length of exposure. Damage occurs with exposure to simulated solar radiation and increases with duration of exposure, at all humidity levels, in agreement with the results obtained by Ratnapandian (3), Leroy et al. (8) and Dubief (9). As shown in block B of Table II, exposure for 100 hours results in about a 20% decrease in many properties, whereas a decrease approaching 50% is characteristic of fibers exposed for 300 hours. On the other hand, the turnover point, which is the intersection of the yield and post-yield part of the load-elongation curve or the extension at which the [3-keratin begins to play a significant load-bearing role, and extension-to-break data show an increase of about 10% (block C, Table II). This may be due to the increased mobility of molecular chains that results from the loss of disulfide crosslinks. The gains in these two properties do not compound with length of exposure. Exposure to UV radiation of many polymers causes embrittlement and the formation of new crosslinks, which may be the limiting factor, as suggested by Wolfram (6). Efd•ct of change in humidity. The tensile properties of hair fibers exposed to simulated sunlight for any given time period depends on the RH of the atmosphere during exposure. Contrary to the proposed hypothesis that photolyric damage will increase 78.4 5S.S •t k6?"v 39.2 19.õ O.O O.O 16.50 32.S3 49.33 65.S3 % ext Figure 3. Typical load-elongation curve obtained for a single hair fiber on a Diastron © miniature tensile tester.

PHOTODEGRADATION OF HUMAN HAIR 317 8O 03 70 '-- 6O '- 40 • 30 ß • 20 y = -0.0616x 2 + 5.2888x - 35.693 R 2 - 0.9982 0 10 20 30 40 50 60 70 RH (%) Figure 6. Diametral swelling of Piedmont hair exposed for 300 hours at different relative humidities. Bio-Rad Win-IR .1- 1 I I 310 I I I 1500 1400 1 0 1200 1100 1000 Wavenumber (cm-1) 20, 30, 50 and 70 indicate %RH during exposure Figure 7. FTIR/ATR spectra for Piedmont hair exposed for 300 hours at different relative humidities. wavelengths in the range of 254 to 350 nm. The absorption spectrum for Piedmont hair, shown in Figure 8, indicates this range to be the primary absorption region of unpig- mented hair. Radiation of this wavelength can also initiate free radicals by Fenton's reaction involving iron, oxygen, and water. This is consistent with the suggestion that photolysis of hair keratin can follow the free-radical pathway as proposed by Tolgyesi (1) and Wei (7). A steady-state analysis suggests that the rate of photolysis of hair should vary with the square root of the concentration of the "initiator." Assuming that hair contains small but uniform amounts of free-radical initiator (im- purities) and that hair photolysis follows the proposed hypothesis, the damage will

318 JOURNAL OF COSMETIC SCIENCE Table III Characteristic Bands in IR Spectra of Hair Fiber Peak Number Molecular group Band (cm -•) 1 Amide-II 1513 2 CH 1449 3 Amide-III 1232 4 Cystine monoxide 1073 5 Cysteic acid 1042 Source: reference (2). 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 220 240 260 280 •00 •20 •40 •60 •80 400 420 Wavelength ( nm ) Figure 8. Absorption spectra of Piedmont hair. increase as the ambient moisture level increases during irradiation. However, the changes in mechanical properties observed in this study, as functions of relative humid- ity, are related to the diffusibility of the radicals. At low humidity levels, the radicals cannot diffuse and terminate. Therefore, the number of radicals increases, causing ex- tensive damage. Low-molecular-weight residues will diffuse out of these fibers when immersed in water. At intermediate RH levels the radicals can diffuse and terminate, thus limiting the number of radicals and the damage caused by them, as shown in Figures 4 and 5. The details associated with the chemical interactions of moisture with hair keratin may be the keys to understanding the role of moisture in hair photolysis. According to Feughelman and Haly (18) the mobility of bound water in keratin increases with increasing moisture content. Water molecules associate more with each other than with keratin as the moisture content increases. Thus, at high RH levels the amount of loosely bound water in hair is large. The loosely bound water may act as a transfer agent for the free radicals as well as residues, thereby actively participating in the photolysis reaction. The removal of degraded protein residues exposes new protein segments to photolysis that accentuates the damage to the fiber.

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