Keratin-based biomaterials that mimic the properties of hair, which is tough and strong for its weight, could one day be used in medical devices or cosmetics. But to accomplish that, scientists will need to replicate the internal structure of hair and the way individual structural elements bond to one another. A hair consists of bundles of keratin fibers connected by chemical bonds and surrounded by layers of scales called cuticles. A team led by Robert O. Ritchie at the University of California, Berkeley, and Marc A. Meyers at the University of California San Diego found that these elements interact differently in various types of hair (Matter 2019, DOI: 10.1016/j.matt.2019.09.019). The team collected hair from several mammal species, including humans, and pulled strands until they broke. The researchers examined the broken ends with scanning electron microscopy. They found that thin hair, such as human hair, tends to break in long cracks, with individual keratin fibers coming unstuck. Thicker hair, such as elephant hair, tends to break cleanly straight across. Counterintuitively, the researchers found that thin hair tends to be stronger than thick—a phenomenon they suggest is related to the size and quantity of flaws in the hair strand. One exception was capybara hair, the strands of which have an indent down the middle believed to help shed water. That hair tended to fail along that center line, giving it a lower tensile strength than expected.