Melanin, a ubiquitous biological pigment produced by melanocytes in most organisms, is an important component of animal pigmentary systems. Naturally occurring pigmentation that determines hair, eye and skin coloration is attributed to two types of melanin: eumelanin and pheomelanin. Eumelanin is the dominant component of brown and black pigments in dark skin and black hair, while pheomelanin is more common in yellow and red pigments in hair. Melanin has not been well understood, as it is an insoluble polymer without a well-defined structure, which makes it difficult to isolate and study.
Melanin is related to many skin diseases, such as malignant melanoma, the most aggressive skin cancer; vitiligo, a disease characterized by the loss of melanin pigment; melasma, an acquired brown hypermelanosis of the face; solar lentigines and ephelides, benign pigmented spots that are associated with an increased risk of skin cancer; and nevus of Ota, a syndrome comprising a grayish-blue, macular discoloration affecting the sclera of an eye and the ipsilateral facial skin in the area of the distribution of the corresponding trigeminal nerve.
Selective photothermolysis has been widely used for treatments of melanin-related skin diseases. This technique utilizes nanosecond-domain laser pulses to selectively ablate melanin and melanin-related cells. Despite its effectiveness, photothermolysis lacks the ability to target individual melanin particles, which limits its usage in treating skin diseases in some sensitive regions, such as nevus of Ota around the eye.