Hair has a strength to weight ratio comparable to steel. It can be stretched
up to one and a half times its original length before breaking. "We wanted to
understand the mechanism behind this extraordinary property," said Yang (Daniel)
Yu, a nanoengineering Ph.D. student at UC San Diego and the first author of the
"Nature creates a variety of interesting materials and architectures in very
ingenious ways. 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," said Marc Meyers, a professor of mechanical engineering at the UC San
Diego Jacobs School of Engineering and the lead author of the study.
In a study published online in Dec. in the journal Materials Science and
Engineering C, researchers examined at the nanoscale level how a strand of
human hair behaves when it is deformed, or stretched. The team found that hair behaves
differently depending on how fast or slow it is stretched. The faster hair is
stretched, the stronger it is. "Think of a highly viscous substance like honey,"
Meyers explained. "If you deform it fast it becomes stiff, but if you deform it
slowly it readily pours."
Hair consists of two main parts—the cortex, which is made up of parallel
fibrils, and the matrix, which has an amorphous (random) structure. The matrix
is sensitive to the speed at which hair is deformed, while the cortex is not.
The combination of these two components, Yu explained, is what gives hair the
ability to withstand high stress and strain.
And as hair is stretched, its structure changes in a particular way. At the
nanoscale, the cortex fibrils in hair are each made up of thousands of coiled
spiral-shaped chains of molecules called alpha helix chains. As hair is
deformed, the alpha helix chains uncoil and become pleated sheet structures
known as beta sheets. This structural change allows hair to handle up a large
amount deformation without breaking.
This structural transformation is partially reversible. When hair is
stretched under a small amount of strain, it can recover its original shape.
Stretch it further, the structural transformation becomes irreversible. "This is
the first time evidence for this transformation has been discovered," Yu
"Hair is such a common material with many fascinating properties," said Bin
Wang, a UC San Diego PhD alumna and co-author on the paper. Wang is now at the
Shenzhen Institutes of Advanced Technology in China continuing research on
The team also conducted stretching tests on hair at different humidity levels
and temperatures. At higher humidity levels, hair can withstand up to 70 to 80
percent deformation before breaking. Water essentially "softens" hair—it enters
the matrix and breaks the sulfur bonds connecting the filaments inside a strand
of hair. Researchers also found that hair starts to undergo permanent damage at
60 degrees Celsius (140 degrees Fahrenheit). Beyond this temperature, hair
breaks faster at lower stress and strain.
"Since I was a child I always wondered why hair is so strong. Now I know
why," said Wen Yang, a former postdoctoral researcher in Meyers' research group
and co-author on the paper.
The team is currently conducting further studies on the effects of water on
the properties of human hair. Moving forward, the team is investigating the detailed
mechanism of how washing hair causes it to return to its original shape.