Human Hair fiber
In a new study, UCSD researchers investigate why hair is incredibly strong and resistant to breaking. The findings could lead to the development of new materials for body armor and help cosmetic manufacturers create better hair care products. Hair has a strength: weight ratio comparable to steel. It can be stretched up to 1.5X its original length before breaking. They examined at the nanoscale level how a strand of human hair behaves when it is deformed, or stretched. Hair behaves differently depending on how fast or slow it is stretched. The faster hair is stretched, the stronger it is.
Hair consists of 2 main parts – the cortex: made up of parallel fibrils, and the matrix, with an amorphous structure. The matrix is sensitive to the speed at which hair is deformed, while the cortex is not. The combination of these two components 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. Stretching further, the structural transformation becomes irreversible. “This is the first time evidence for this transformation has been discovered,” Yang (Daniel) Yu, a nanoengineering Ph.D. student said.
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% 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 degCelsius (140F). Beyond this temperature, hair breaks faster at lower stress and strain.
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. This research is supported by a Multi-University Research Initiative grant through the Air Force Office of Science Research (AFOSR-FA9550-15-1-1-0009). http://jacobsschool.ucsd.edu/news/news_releases/release.sfe?id=2109
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