364 JOURNAL OF COSMETIC SCIENCE 550 500 unmedullated hair 450 400 350 300 en en 250 200 en 150 100 50 0 0 10 20 30 40 50 strain(%) Figure 5. Stress-strain curves of 40 fibers of unmedullated hair. 550 500 medullated hair 450 400 350 300 - en en 250 200 en 150 100 50 0 0 10 20 30 40 50 60 strain (%) Figure 6. Stress-strain curves of 40 fibers of medullated hair. A series zone model has been proposed (25) to explain the behavior of hair in the yield and post-yield region. According to the series zone model, hair can be assumed to consist of two alternating zones along the microfibrils, namely the X zone and the Y zone. 30% of the a-helices in the keratin are capable of exhibiting a reversible folding-unfolding nature without breaking the disulfide bonds and these form the X zone, which charac-

2006 TRI/PRINCETON CONFERENCE 365 120 110- 100- E 90- � � 80- :i.... Q) 70- - $ Q) E 60- «1 :a 50- 40- 30 Figure 7. Diameters data for unmedullated (white box) and medullated hair (gray box). Box plots were obtained from 40 hair fiber measurements from each sample. ca 500- Cl.. � 400- .c - � C) C: Q) 300- :i.... - Cl) C) C: 200- :2 «1 Q) :i.... 100- cc Figure 8. Breaking strength data for unmedullated (white box) and medullated hair (gray box). Box plots were obtained from 40 hair fiber measurements from each sample. 70�------------------, 65 '#- c: 60 0 � 55 C) 5 50 a 45- C) C: :2 40- «1 � 35- .0 30...____________________. Figure 9. Breaking elongation data for unmedullated (white box) and medullated hair (gray box). Box plots were obtained from 40 hair fiber measurements from each sample.