Friday, May 14, 2010

Chemistry of your hair: The hair strand - the cuticle

The cuticle is the part of the hair that probably concerns us the most on a day to day basis. When our hair emerges from our scalp, it has about 6 to 10 layers of cuticle about 50 micrometres on a side and about 0.5 micrometres thick. The cuticle is attached to the cortex and forms a tight sheath around it. The cells of the cuticle overlap -  often times likened to roof tiles - at about a 5˚ angle and point downwards to the ends of the strands. There is a direction or grain to our hair, and this slight angle can cause friction between our hair strands.

The cuticle is covered in a fine layer of covalently bound lipids, which helps mitigate the friction of your hair strands and helps repel water. The lipids include fatty acids (palmitic, stearic, and oleic) and wax esters. These can be degraded by UVA and UVB light, which can lead to fractures in sun damaged hair!

If the mechanical properties of our hair are determined by the cortex, then the optical properties of our hair is determined by the cuticle. Shine is determined by how the light catches your hair strands - if your hair is curly or wavy, it won't shine as much as straight hair - and the state of the 18-methyl eicosanoic acid (18-MEA, which is found in sal butter). Shine can be a determination of the state of your hair - if it's dull looking, you might have build up or might be using the wrong conditioning agents - and sometimes it's not - if you have wavy, frizzy, curly, or generally non-straight hair.

The cuticle protects the cortex from damage, but over time the cuticle wears away. The cells can erode, break, lift, and get ragged edges, all of which can leave the cortex unprotected. When this happens - look out! The cortex is laid bare for all sorts physical and mechanical assaults, and this can lead to even more damage even quicker! Even if you treat your hair well, as it grows longer the cuticle will erode from friction from other hair strands and brushing, leading to split ends and gaps in the cuticle!

There is nothing you can do to repair this damage for the long term, but there are ways we can try to seal over the cuticle layer (with conditioning agents, silicones, and oils). 

If we cut the cuticle into a cross-section, you see a bunch of different layers. This is a fine membrane called the epicuticle, which has three layers (the A-layer, the exocuticle, and the endocuticle). The epicuticle consists of a protein layer covered by a lipid layer, which is covalently bonded by thioester linkages (which are fatty acids with 18-MEA). The epicuticle is thought to play a vital role in determining the surface properties of your hair, both mechanical and chemical. Not only does it help with friction between the hair strands - when your hair touches another hair, or when you comb or brush it - but it acts as a semi-permeable membrane to select what things actually make it through the cuticle.

The A-layer is composed of sulfur rich proteins - like cystine, which makes up about 30% of this layer - and it is considered very strong and resistant. It protects the cuticular cells from mechanical and chemical insults! The exocuticle contains about 15% sulfur rich proteins. These layers are thought to offer mechanical stability to the cuticle scales.

The endocuticle will absorb water and behaves like a cushion to protect our hair from mechanical impact. It's thought the endocuticle is the reason our cuticle lifts after exposure to water - it plumps up and pushes the cuticle out, which makes the little roof shingles lift up for a bit. (And this is one of the reasons you want to treat your hair really well when it's wet - you don't want to tear those little shingles off!)

The cell membrane complex is the strongly adhesive layer that connects the cortex and the cuticle cells.

Wow, that was a lot of information, eh? But it's important to know the biology of our hair to make awesome hair care products! Join me tomorrow for fun with follicles and the growth cycle!

7 comments:

gardeningAngel said...

Hi, I have a question on the cortex: how does your hair turn gray and why do some people (like myself) get gray hair at a very early age? If your hair is regrowing all the time, how does this work?

Thanks!

Kathy

Susan Barclay-Nichols said...

Hi Kathy! I'll be getting to the explanation shortly but the quick answer is the science really isn't clear, meaning we really don't know. I bet you didn't expect that, eh? I know I didn't!

Rocio said...

Hi Susan:
I have a question
What charge does hair strand have?

Thanks

Susan Barclay-Nichols said...

Hi Rocio. I've written about this in this post.

Anonymous said...

Hi susan my hair is thick and black ..... i am 19 years ond but some of my hair is getting thinner wid brownish discoloration ... what can be the reason for it .... nd what is the solution...

Susan Barclay-Nichols said...

Hi anonymous. I don't know if I can help because there are so many reasons that we could have problems with our hair getting thinner or discolouring. For instance, the ends of my hair are lighter than the roots because they've been trashed through years of being exposed to weather, being caught in doors, and other things that cause friction. Can you provide a bit more information?

Ederson Nunes said...

Hi, some people say that keratin is a component of the cuticle, is that true? Is there any Keratin in the cuticle, or just in the cortex?