Core–shell Particle Embedding into Human Hair: A Novel Approach for Surface Modification and Long-Lasting Hair Coloration

To embed particles into human hair, core-shell (CS) particles were prepared by seeded emulsion polymerization and were arrayed to the surface of natural hair via dip coating. The particle-coated natural hair was treated at various temperatures in a controlled environment with a relative humidity of 5% (5% RH). No particle embedding was observed at 25 °C. However, the degree of particle embedding (DE) slightly increased at 50 °C and substantially at 150 °C. At 100% RH, embedding was effectively achieved at 50 °C, probably because the adsorbed water acted as a plasticizer, softening the hair. Bleached hair exhibited a significant reduction in surface wettability, whereas the DE of CS particles remained comparable to that of natural hair, indicating that surface wettability does not affect the particle embedding process. On the other hand, hair treated with a reducing agent displayed a notable increase in the DE, reaching approximately 20% even at 25 °C and increasing to 35% at 150 °C. At 100% RH, DE values dramatically increased to approximately 35%, even at 25 °C. Sudan Black B was incorporated into the CS particles to produce stained particles (CS-(B) particles). This staining process enabled the coloration of bleached hair by arranging and embedding CS-(B) particles. Visual observation confirmed uniform coloration of the hair samples. The color difference (Δ<i>E</i>*) results strongly indicated that bleached hair could be effectively dyed with CS-(B) particles. Additionally, the color of CS-(B) particle-embedded hair remained stable after ultrasonication, demonstrating that particle embedding ensures long-lasting hair coloration by securely embedding the stained particles within the hair samples.