NEW YORK – At once deeply personal and publicly scrutinised, hair sits at the boundary between biology and identity. Few attributes are as tied up in culture, beauty and expression as the strands that frame people’s faces – and dot their skin.

You have about 100,000 hair follicles on your scalp and many more across your body, said Dr Maksim Plikus, a cell biologist at University of California, Irvine. Each is like a 3D printer, he said, home to a growing strand of hair and pigment that gives it colour.