For material scientists scouring the natural world for inspiration, there appears to be plenty of tougher customers than a strand of human hair. But with a steel-like strength-to-weight ratio and an ability to endure stretching up to one and a half times its original length, our luscious locks have their own unique offering for efforts to develop tough new materials like futuristic body armor. Such ventures have just become a little more enlightened, with scientists studying the secrets of human hair observing some of the key mechanisms that allow it such strength and durability.
"Nature creates a variety of interesting materials and architectures in very ingenious ways," says Marc Meyers, a professor of mechanical engineering at UC San Diego and the lead author of the new study. "We're interested in understanding the correlation between the structure and the properties of biological materials to develop synthetic materials and designs — based on nature — that have better performance than existing ones."
So Meyers and his team set out to study the structures within a strand of human hair at the nanoscale and how they behave mechanically under different conditions, specifically, when they are deformed or stretched. The work examined the activity in the two main components of the human hair: the cortex, which is made up of tiny parallel fibers that each consist of thousands of spiraling links of molecules called alpha helix chains, and the matrix which has an amorphous or random structure.
The team found that when a hair is stretched, the alpha helix chains uncoil and convert into folded sheet structures, which enables the hair to endure the strain without breaking. Whether it returns to its normal shape thereafter depends on the degree of strain, with the researchers finding that anything beyond a small amount makes the transformation irreversible. They also found that the speed of the stretch made a difference, with a faster tug resulting in a stronger strand.
"Think of a highly viscous substance like honey," Meyers explains. "If you deform it fast it becomes stiff, but if you deform it slowly it readily pours."
Another area the team set out to explore was how hair behaved at different temperatures and humidity levels. It discovered that at higher humidity levels, hair can endure up to around 70 or 80 percent deformation, a marked improvement on the 50 percent observed in dry hair. The reason for this is that water seems to soften the hair by entering the matrix and breaking down sulfur bonds connecting the filaments within. Conversely, the researchers found that at temperatures beyond 60° C (140° F), hair breaks faster at lower stresses and strain and begins to suffer permanent damage.
The team is now carrying out further studies exploring how water influences the behavior of human hair, and will next look to investigate how washing washing hair causes it to return to its original shape. It says the findings of its work can help scientists develop tough new materials for body armor, and also help cosmetics companies produce advanced hair care products.