Although always on our minds – but not so much on our heads –, there is still plenty to be known about the structure and composition of human hair. Far from a simple scientific curiosity, the proper knowledge about it allow us, for example, to research and develop new and more efficient cosmetic products for the hair treatment and care.

An international team of scientists, composed by Vesna Stanic, researcher at the Brazilian Synchrotron Light Laboratory (LNLS), Jefferson Bettini and Fabiano Emmanuel Montoro from the Brazilian Nanotechnology National Laboratory (LNNano) and Aaron Stein and Kenneth Evans-Lutterodt from the Brookhaven National Laboratory in the USA, investigated the structure of human hair using extremely focalized x-ray beams produced in the synchrotron light source of the American laboratory.

They identified a new intermediate region in the hair cortex, right below the cuticle – the outermost region of the hair. In addition, they verified the presence of beta-keratin in the cuticle, a protein that is usually present in the scales of reptiles and in the feathers of birds.

Hair   

Hair is a disordered and heterogeneous structure, composed of keratinized cells, which grows from organs in the skin called follicles. Keratin is a structural protein that grants protection to the epithelial cells and that, in humans, is component not only of the hair but also of skin, mucosae and nails. The keratinized cells in the hair are organized in different ways depending on the region of the hair they are going to form: medulla, cortex or cuticle.

The cuticle is the outermost region of the hair. It is composed of layers of dead cells that protect the inner regions. The cortex is the intermediate region of the hair. It is formed by the interlacement of keratin fibers of a few nanometers in size in structures called intermediate filaments. These structures are organized in dense packings called macrofibrils, which are oriented along the axis of the hair but transversally disordered. This region also defines the undulation pattern of the hair and holds the melanin, the protein that gives the hair its color.

Finally, the medulla is the innermost region of the hair, but it is not necessarily present in every strand. It is filled by a keratin matrix partially composed of intermediate filaments, disordered in position and orientation.

Figure 1: Electron microscopy image of a strand of hair showing its different regions, such as the outermost cuticle and the macrofibrils that make up the cortex. Although useful for visualizing very small things, electron microscopy is not capable of providing information about their structure or composition. For that, advanced techniques such as x-ray diffraction are necessary.

Diffraction

Each strand of human hair is around 60 to 140 micrometers in diameter and one of the ways of investigating it is using x-ray scattering and diffraction techniques.

General aspects of its structure can be obtained using x-ray beams, such as those produced in the current LNLS synchrotron light source, which is a second-generation machine. For that it is necessary to get a bunch of strands together and to focus x-rays perpendicularly through them, in order to obtain results that represent its average structure.

However, to observe the details of a disordered and heterogeneous material such as hair, it is necessary to use extremely focalized x-ray beams, micron and submicron in size, currently available only on third-generation synchrotrons.

Thus, using a sub-micron x-ray beam obtained using kinoform lenses-optics tested at the X13B beamline (experimental station) at the National Synchrotron Light Source of the Brookhaven National Laboratory, in the USA, the researchers prepared an experiment where the x-ray beam is oriented parallel to the hair axis and incident in the cross-section of a 30 microns thick slice of hair.

With their experiments, the scientist could analyze the structure of a single strand of hair with unprecedented precision and discover an entirely new region in the cortex, in the boundary with the cuticle, formed by the same intermediate filaments of the cortex, but oriented similarly to the layers of the cuticle.

In addition, they observed the presence of an unexpected kind of keratin in the cuticle. This protein can be found in two different secondary structures (the way the protein molecule folds into itself). The alpha-keratin (organized in helices) is usually found in mammals, in which it forms hair, horns and hooves. The beta-keratin (organized in sheets) is usually found in reptiles and birds, forming scales, feathers, beaks and claws.

It was supposed that all the hair was only composed of alpha-keratin. After analyzing the data of their experiment, the scientists noticed that the cuticle did not have the diffraction signal associated with alpha-keratin as present in the cortex and, instead, the data was compatible with the presence of beta-keratin. This is the first direct structural evidence of the presence of this protein in the human hair.

According to the group, the results not only indicate de necessity of reevaluating stablished models for the structure of different regions of the hair, but also highlight the importance of using diffraction techniques with submicron x-ray beams to unravel the structure of disordered and heterogeneous materials.

LNLS, UVX and Sirius

The synchrotron light source UVX, currently in operation at LNLS, is able to produce x-rays dedicated to scattering and diffraction techniques such as the ones used by the researchers. However, for being a second-generation synchrotron, UVX is not capable of producing a beam as small as it would be necessary to investigate details of the structure of the hair.

LNLS is now building Sirius, one of the most advanced synchrotron light sources in the world and one of the first fourth-generation machines. Sirius will open new prospects to the research in several different fields, such as physics, material sciences, nanotechnology, biotechnology, environmental sciences and others.

One of Sirius’ beamlines, called SAPUCAIA, will be able to produce micro- and nanometric beams for x-ray diffraction experiments due to Sirius’ ultralow emittance. By producing much smaller beams than those of third-generation synchrotrons, Sirius will be able to reveal much more details from the structure of heterogeneous materials.

Case in point, there is still speculation over the structure of the intermediate filaments that form the cortex since each filament is about seven nanometers in size. Each filament, on its turn, is composed of several keratin chains and it is not known yet how they are organized. According to Stanic, “it happens because we don’t have the technology necessary to investigate them. Things are changing; we are getting there, to smaller and smaller scales”.

“It was said that that everything was done about hair. I say that is not true for any heterogeneous material. We did not have the tools before, but the third-generation synchrotrons and Sirius will help us discover a completely new science”.

Source: V. Stanić, J. Bettini, F. E. Montoro, A. Stein and K. Evans-Lutterodt, Local structure of human hair spatially resolved by sub-micron X-ray beam, Scientific Reports 5:17347 (2015). doi:10.1038/srep17347