211 Polyphenol-metal Complex With Dopamine
for the complex molecules to penetrate the hair cortex layer (Figure 7B). By unfolding
the hair scales using ethanolamine emulsion, the maximum critical molecular size that
the protective layer of hair scales can allow to pass through increases, and large-size dye
molecules can also easily penetrate the hair cortex layer to achieve natural white hair
dyeing. Dopamine, which is free in the hair dye, also penetrates the cortical layer of hair
and is deposited and autoxidized to polymeric polydopamine, which increases dramatically
in size and cannot penetrate from the cortical layer to the hair surface through the hair
scales. Its adhesive properties effectively secure the dye molecules within the cortical layer
of the hair, accomplishing color fixation, enhancing the hair’s resistance to washing, and
resulting in a permanent dyeing effect.
In our study, we observed that natural white hair dyed solely with GA complexed with
Fe(II) dye molecules exhibited poor resistance to washing, with a significant color difference
occurring even after just 40 washes. Dopamine can adhere to the surface of the hair and
form an adhesive film that acts as a color fixation.31 Therefore, we introduced dopamine
to prepare a composite hair dye for improving the problem of poor washing resistance and
achieving permanent hair dyeing. However, in our study, we found that dopamine did not
adhere to the hair surface, but rather entered the hair cortical layer through the hair scales.
Based on the property that dopamine is prone to spontaneous oxidation and polymerization
into polydopamine,32 we hypothesized that dopamine accumulates in the cortical layer
and undergoes autoxidation and polymerization into polydopamine, which adheres to fix
Figure 6. UV-Vis absorption spectra of (A) DA, (B) GA, and (C) GA-Fe(II)+DA. (D) FT-IR spectra of DA,
GA, Fe(II), GA-Fe(II)+DA.

212 JOURNAL OF COSMETIC SCIENCE
the dye molecules, making them less susceptible to washing and enhancing the washing
resistance of dyeing (Figure 7C). We determined that permanent hair dyeing could be
achieved using GA-Fe(II)+DA hair dye. In addition, the introduction of DA resulted in
a subsequent increase in the color difference value of the hair, i.e., a better dyeing effect.
In addition, we did a tensile test on dyed hair versus hair without dyeing treatment, and
according to the results in Figure 7D, the dyed hair was able to withstand greater force
within a period of strain value (approximately 32%), and the hair broke when this strain
value was exceeded. However, both dyed and undyed hair can withstand similar stress
values, which means that the dyed hair can withstand the same force as undyed natural
white hair. However, it can withstand less strain in the stress range, i.e., the dyed hair
has slightly poorer tensile resistance, which we speculate is caused by the unfolded hair
scales not closing in time by ethanolamine. Nevertheless, over time, the unfolded hair
scales gradually close, and through oxidation with the deposited dopamine in the cortical
layer, they self-polymerize into polymeric polydopamine. This polymeric substance forms
an adhesive film within the cortical layer, encapsulating and securing the materials within
the hair to prevent loss. Moreover, it enhances the tensile properties of the hair.
CONCLUSIONS
In this study, we developed a method for dyeing natural white hair by creating a complex
formed through the interaction of polyphenols with metals, followed by the addition of
dopamine to formulate a complex hair dye. With the help of the alkaline environment of
Figure 7. (A) Van der Waals ball model of DA, GA, and GA-Fe(II). (B) Ball-and-stick model of DA, GA with
GA-Fe(II). (C) ΔE values of GA-Fe(II) and GA-Fe(II)+DA for washing resistance test. (D) Stress-strain curves
of natural white hair (untreated) and GA-Fe(II)+DA dyed hair.