Physical and Chemical Structure of Wool Fiber

By Eman Abdallah Kamel

Eman is a writer and textile engineer. She has her bachelor’s degree in textile science from the Faculty of Applied Arts, Helwan University, Egypt.

This article covers the wool’s physical structure, including the cuticle, cortex, cell membrane complex, matrix, helical coil, and chemical composition, and its effect on wool properties.

Wool is a versatile natural fiber with a complex physical and chemical composition that sets it apart from other fibres.

Wool

Wool is the hair of sheep and other animals, such as camels, goats, and musk oxen. Wool belongs to the group of hard structures in mammals, including nails, claws, and hooves. These structures are tissues, and their cells are different from epithelial cells and contain proteins called keratins. Mammalian keratin is different from the keratin found in non-mammalian species, such as the avian claws, the skin of reptiles, the beaks, and the birds’ feathers.

The keratin of wool fibre is a strong, fibrous protein that maintains its rigid structure by disulfide bridges, a characteristic feature of the amino acid cysteine.

Sheep wool and other animal hair are composed of 90% cortical cells by weight, surrounded by about 10% flat, scale-like epidermal cells.

Of all the textile fibres, wool is the most intricate. It can be used to make a variety of fabric products. Wool products are comfortable to wear because of the fiber’s insulating and moisture-wicking qualities. Additionally, wool’s chemical structure makes dyeing it simple.

Did You Know?

Wool knowledge has been applied to research into the structure and growth mechanisms of human hair to understand and treat hair diseases and to develop new products for the cosmetic/social role that hair plays.

An illustration of the structure of wool fibres. Image source: wikimedia.org

1. Physical Structure

Wool fibre consists of three main components:

Cuticle,
Cortex,
Cell Membrane Complex (CMC),
Matrix,
Helical Coil.

Many species have medulla, and within a species, the medulla cells can have different packing patterns. For example, the medulla in rabbit fur can vary along the fibres due to cellular changes during growth. The proteins within the medulla have no visible fine structure and are lumps of amorphous protein material when examined under a transmission electron microscope. The main protein is a specific protein called trichohyalin, which has a matrix-like role in the cells of the inner root sheath of the follicle.

1. Cuticle

The physical structure of the cuticle cells is complex. One important function of the cuticle cells is to anchor the wool fibres to the sheep’s skin. Each cuticle cell’s exposed edge points from the fiber’s root toward the tip. This results in a greater surface friction value when the fibre is drawn in the direction opposite the scale than in the direction with the scale. The benefits of friction are,

Removing dirt from wool.
The felting property of wool when it is moved in water. This property enables the production of fabrics with dense structures, such as felt, blankets, and coats.

2. Cortex

The cortex is made up of long, thin cells with finger-like tips. There are four types of cortical cells: orthocortex, paracortex, mesocortex, and metacortex. The main types of cortical cells in the wool are the orthocortex and paracortical. Paracortex cells have a more homogeneous/integrated keratin structure and are more resistant to chemical and mechanical attacks (such as swelling) than the orthocortex, explaining why wool fibres are curly.

Different types of wool vary in softness due to differences in the cortical cells; the softest wool fibres have two distinct halves of ortho- and paracortical cells, which swell to varying degrees when exposed to moisture and cause the fibres to bend. This crimping, which retains air between the fibres, gives wool its insulating properties.

3. Cell membrane complex (CMC)

The CMC makes up about 5% of the total fibre mass. The cell membrane complex is a dense layer about 15 nm wide between the cortex and cuticle cells. The CMC is a continuous region that extends throughout the fiber. It contains relatively loosely crosslinked proteins and waxy lipids.

The cell membrane complex connects the cortical cells and plays an important role in the integrity of wool fibres and hair structure.

Due to its slight cross-linking, CMC is more vulnerable to chemical attack than other fibre sections if the fabric is manufactured under extremely high temperatures or strongly alkaline conditions

4. Matrix

The matrix is made up of proteins high in sulfur, making wool absorbent because the sulfur atoms attract water molecules.

5. Helical Coil

The matrix surrounds the coil. It is a tightly coiled structure that naturally forms in wool fibers. It also makes the wool fibers more durable and resistant to breakage. The coils help distribute stress and tension across the fibers, making them less likely to break.

Did You Know?

There are more than 170 distinct proteins in wool. Proteins with distinct structures are found in particular areas of the fibre, and these proteins are not evenly distributed throughout. This heterogeneous composition is responsible for the different physical and chemical properties of different regions of the wool. The proteins in wool are made up of amino acids.

Basic structure of an amino acid. Image source: technologynetworks.com

2. Chemical Structure

Proteins are composed of 20 amino acids, all of which have the same basic structure, differing only in the R-group, or side chain.
Amino acids join together to form long polymer chains. Since an amide group connects each structural unit, these compounds are referred to as polyamides.

Peptide bond formation in the reaction between two amino acids. Image source: researchgate.net

The amide repeat unit (-NHCHRCO-) is called a peptide group when the polymer chain is a protein.

Individual polypeptide chains in wool are joined to form proteins through various covalent bonds, called cross-links, and non-covalent physical interactions.
The most important cross-links are sulfur-containing disulfide bonds formed during the growth of the fibres through the keratinization process. This makes keratin fibres insoluble in water and more resistant to physical and chemical attacks than other types of proteins.
The isopeptide bond is a different type of cross-link that forms between amino acids having basic or acidic groups.

An isopeptide bond between lysine and aspartate. Image source: wikimedia.org

In addition to chemical cross-linking, other types of interactions also help stabilize the fibers in wet and dry conditions. These interactions arise from interactions between the side groups of the amino acids that make up the wool proteins.

Ionic interactions between acidic (carboxyl) and basic (amino) side groups are the most significant non-covalent interactions.
Carboxyl and amine groups are important in wool because they give it pH-regulating properties. These characteristics include its capacity to absorb and desorb alkalis and acids. The ionic groups also control the fiber’s dyeing behaviour by interacting with the negatively charged dye molecules.

Sources

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