Investigating the factors affecting tensile strength of human hair.

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Investigating the factors affecting tensile strength of human hair

Planning: (Skill A)

Hypothesis

There will be a difference in tensile strength in blonde hair and black hair of similar thickness. Blonde hair will have a higher tensile strength than black hair when at similar thickness. Blonde hair has more sulphur-sulphur covalent bonds than black hair. Hair contains the protein keratin, which contains a large proportion of cysteine with S-S bonds. The disulphide bond is one of the strongest bonds known anywhere in nature. The cross-linking by disulphide linkages between the keratin chains accounts for much of the strength of hair. Blonde hair has more of these bonds therefore blonde hair will have a higher tensile strength and elasticity levels.

Null Hypothesis

There will be no difference in tensile strength between black hair and blonde hair of similar thickness. Blonde hair having more sulphide bridges will not mean that blonde hair has a higher tensile strength than black hair.

Background Knowledge

Hair has a very high tensile strength. It can hold up 60kg of weight before breaking. This high strength is due to its structure.

Hair is made of the fibrous protein keratin. Figure 1 shows keratin molecules are made up of three helices. They are held together by strong covalent bonds called sulphur bonds. Eleven of these molecules group together to form a micro fibril. Then, hundreds of micro fibrils join together to form a single hair.

Hair is made of cells called epithelial cells which are arranged in three layers. The inner most layer is the medulla, the middle layer is the cortex and the out layer is the cuticle. The medulla is mainly soft keratin and the cortex and cuticle are mainly hard keratin. This structure has great strength. The cuticle, the outermost layer, is where you find a lot of the protein keratin. The cortex is the thickest middle layer, providing strength and defining colour of hair. The cortex also gives the hair its elasticity and flexibility. The medulla, central core, gives hair its strength and breadth.

The structure of keratin is maintained by numerous sulphur to sulphur covalent bonds. Keratin contains high concentrations of the amino acid cysteine.

Every Cystine unit contains two cysteine amino acids in different chains which have come to lie near to each other and are linked together by two Sulphur atoms, forming a very strong chemical bond known as a disulphide bridge. Many disulphide bonds form down the length of the keratin chains, joining them together like the rungs of a ladder. The disulphide bond is one of the strongest bonds known anywhere in nature. This cross-linking by disulphide linkages between the keratin chains accounts for much of the strength of hair. A suitable amount of Sulphur Bridge is important in enhancing the elasticity of hair due to the strength of the disulphide bond.

Within each hair bonds of a different kind, called hydrogen bonds also link the keratin chains. There are far more hydrogen bonds than disulphide linkages. The hydrogen bonds are much weaker than the disulphide linkages and more easily broken, and give hair its flexibility. Hydrogen bonds are broken apart when the hair is wetted, and form again when the hair dries.

Hair used for the investigation must not be 'damaged' in any way, i.e; should not be dyed/coloured, permed, straightened, etc. The hair also must not be curly. I will be only looking at naturally straight hair for this investigation.

Permed and dyed hair cause severe damage to hair by reducing and breaking disulphide bonds between protein amino acids (which keep the hair strong) and they change the chemistry of hair by altering the protein rich internal structure of the fibre. In perming, a mild reducing agent is used to break the sulphur bonds. The helices are unwound and the hair is styled. A gentle oxidising agent (usually hydrogen peroxide) is treated to the hair to make the sulphur bonds reform. This results in a 'permanent wave'. (Figure 3 shows the breaking and reforming of sulphur-sulphur bridges can produce permanent changes in the shape of protein molecules). Permed hair has only 90% of the original disulphide bonds, which leaves hair weaker than before it was permed.

Heat (like from hair straighteners) disrupts the structural bonds (particularly weak Hydrogen bonds) enough to give hairs wounded around a roller some temporary curly aspect. However, this effect can be easily abolished by an increase in humidity or contact with water. Longer lasting permanents use chemicals such as thioglycolic acid to disrupt the disulphide bonds.

Hairs to be tested with should not have their disulphide bonds damaged/broken or our hypothesis will become invalid even before the investigation takes place.

Variables

To Control...(keep the same)

To investigate...(measuring/changing)

Hair MUST NOT be:

Tensile Strength (masses applied on hair)

co loured/dyed

Thickness of hair/colour of hair

straightened (by applying heat)

permed (by applying heat)

curly (naturally)

from the same person

Hair MUST be:

black or blonde

straight (naturally)

from the same age group

Equipment

2 X Clamp stand - to hold everything upright

2 X Clamp - to hold paperclip/hair and ruler

00cm ruler - to measure how far hair stretches before it breaks (tensile strength)

2 X paperclip - to hold hair and mass

0kg masses with holder - to put tension on hair

Selotape - to hold loop of hair in paperclip

5 pieces of black hair - to compare

5 pieces of blonde hair - to compare

Micrometer - to measure hair thickness

Method

. I will first take five pieces of hair randomly from six different people. Three of these people should have black hair and three should have blonde hair. My total numbers of hair should be fifteen black and fifteen blonde. Hair samples should be taken from six different people to make sure that a fair and accurate test takes place. For example if all fifteen black hairs were taken from the same person, it could just mean that that person had thicker hair than normal thickness of black hair. This would make my results invalid. I will take all hairs from the same age group (my age group, 17-18), to erase the 'age of hair follicle' variable when comparing its tensile strength.
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2. I will set up the micrometer to measure each hair thickness by placing hair under lens and measuring its thickness using a shown scale. I will record each thickness in an appropriate table.

3. I will place a black hair and a blonde hair of similar thickness side by side. This will be done to compare tensile strength of hairs of similar thickness.

4. Equipment will be set up as shown in Figure 5, (below) to start investigation. The hair will be carefully put in. The meter rule should be touching the bottom of ...

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