Tag: properties of hair

Properties of Hair – Part 3

In the third installment of the properties of hair, lipid or fat content of the hair shaft will be our subject. The lipid content of hair is approximately 1-9% .

The lipids found in hair are tightly bound to the proteins via various bonds, and as such, are referred to as Integral Hair Lipids.  They confer resistance  to the hair shaft, protecting it from the environment by forming a lipid envelope. Most of the lipids are found in the the hair cuticle and inner root sheath. Major constituents are fatty acids, phytosphingosine, and ceramide in decreasing order. Minor constituents are cholesterol, cholesterol sulfate and cholesterol oleate. Interestingly the lipid components differ from the skin to hair follicle to the hair shaft.

Table showing lipid constituents in hair and skin compartments

Sebum % Epidermis % Hair mg/g Hair mg/g Follicle %
Free cholesterol 5 26 ± 0.6 0.5 0.5 3.7
Free fatty acid 13.1 ± 1.6 4.0 23.4 29.6
Cholesterol sulfate 3.9 ± 1.4 2.9 0.4 0.3
Triglyceride 57 0.2
Cholesterol esters 5 1.7 ± 1.1
Glucosylceramide 1.0 ± 0.6
Unidentified 5.7 ± 0.9
Sphinghosine 26.3
Wax ester 26 4.1
Squalene 12 <0.2

The major fatty acid, representing 40% of the total fatty acids, is identified as 18-methyleicosanoic acid. The next abundant are palmitic acid and stearic acid. Alkaline treatments (hair colorings, bleaching, perms) or sunlight can lead to a loss of 18-MEA and causes the defatting of hair. As such, the hair becomes dry and brittle and loses its elasticity and luster.

They all serve to decrease the friction between the hair strands. Their role in hair are slowly being delineated. Their presence may indicate something more important with respect to the functioning of the body. Ceramide production has been found to decrease with age. Also, it has been found that errors in lipid metabolism could result in abnormal skin and hair diseases.  As more and more research is done, we shall see the exact role they play in hair.

Properties of Hair- Part 2

Though it may not be that obvious, water is an important component of hair. Its presence helps with the elasticity or pliability of hair. The water content of hair is between 15-35%. The high variability is due to the hairtype and health of the hair fibre. Hair that is damaged is less likely to hold onto water. ‘

Water has the potential to increase not only the weight of the hair fibre but also the diameter. This is referred to as the swelling potential of hair. It can increase to nearly 14% in diameter, but less than 2% in length. Other reagents such as sodium lauryl sulfate, formic acid, and thioglycolic acid have a greater effect on the swelling potential of hair, hence the reason for these products to potentially damage hair.

The more the hair absorbs is the more damaged the hair, indicating that the hair shaft is more porous. This is what is  termed as “porosity”.  Hair that absorbs more water has decreased tensile strength i.e. it easily breaks. The porosity of the hair shaft is about 20%, allowing a weight increase of 12–18% when soaked in water. The absorption rate is very rapid, with 75% of the maximum absorbable water entering the hair shaft within 4 minutes.

However, as quickly as the hair absorbs water, it can quickly lose it. Those with quite porous hair can attest to this. Water has been shown to stay in the hair shaft better if attached to some highly osmotic molecule like sorbitol, or some other hygroscopic (water-loving) substance such as glycerin. These molecules allow water from the atmosphere to stay longer in contact with the hair shaft. This increases the moisture content of the hair shaft and thus allows the hair to not only feel better but to look better as well.

References

Syed AN, Ayoub H. Correlating Porosity and Tensile Strength of Chemically Modified Hair. Cosmet Toil 2002; 117: 57-64.

Draelos MD, Zoe Diana (2007-04-17). Hair Care (Kindle Locations 331-334). Informa Healthcare. Kindle Edition.

Egawa, M., Hagihara, M. and Yanai, M. (2013), Near-infrared imaging of water in human hair. Skin Research and Technology, 19: 35–41