The general model of wool fibre structure is represented by figure 8 which shows the overall hierarchy of structural elements. Under the microscope, the wool fiber looks like a long cylinder with scales on it. The fiber is very curly and springy. The fibre is surrounded by cuticle cells which overlap in one direction. The cuticle cell consists of four layers including the epicuticle, the A-layer and the B-layer of the exocuticle and the endocuticle. The cortical cells of spindle form aligned with the fibre axis and with their fringed ends interdigitating with each other, are surrounded by the cuticle. Cell membrane complex (CMC) comprising internal lipids and proteins, separates the cuticle and cortical cells. This CMC is responsible for strong intercellular bonding via proteins called desmosomes. Macrofibrils oriented in the direction of the fibre axis and embedded into the inter-macrofibrillar matrix which contains cytoplasmic residues and nuclear remnants, are confined into the cortical cells. The macrofibrils consist of hundreds of microfibrils embedded in a matrix of interfilament material (KAP). The microfibrils contain pair of twisted molecular chains. The two kinds of cortex cells can be distinguished due to different intensity of staining. The orthocortex cells appear lighter and the paracortex cells appear darker upon staining with silver nitrate in ammonia solution.ImageUpload an image file, pick one from your media library, or add one with a URL.UploadMedia LibraryInsert from URL
The cuticle cell is a rectangular sheet, slightly bent, with a width of about 20mm, a length of 30mm and a thickness of 0.5–0.8mm. The cuticle contributes about 6 – 16% weight to the fibre. The outer cuticle cell is thicker than the cuticle cells lying below it. The cuticle cells have some overlap, with the transition from one cuticle cell to the next being either planar or stepwise. The surface of the cuticle cells contains a covalently bound fatty acid.
This is the outer most layer and is the thin water repellent membrane. It is the only non-protein part of the fiber and it protects the fiber like a covering of wax. However epicuticle composed of many microscopic pores and water vapors passes through these microscopic pores in the sheath. Therefore, it repels water but is permeable to water vapor. The epicuticle is highly resistant to attack from alkalis, oxidizing agents and proteolytic enzymes. It is about 2.5 nm thick and amounts to approximately 0.1% of the weight of the fibre. It has been considered to consist of lipids, proteins, and/or carbohydrates. Due to its chemical inertness it is called a resistant membrane.
The Scale cell layer
Beneath the epicuticle there is a layer of flat, scale-like cells. This layer consists of horny, irregular scales called epithelial scales which cover the fiber. The number of scales varies greatly depending on the fineness of the fiber. In Marino wool the scales could be up to 790/cm and in coarse wool up to 276/cm.
Cortex is enclosed within the cuticle. It is the main central portion of wool fiber. About 90% of the fiber consists of Cortex with ortho cortex being (60–90%) and paracortex cells being (40–10%). The orthocortex is responsible for the crimp in wool fibre. The crimp makes wool feel springy and provides insulation by trapping air.
The cortical cell
It is built up from long, spindle shaped cells which provide the strength and elasticity of the wool fiber. These cortical cells are held together by a strong binding material. The cortical cells are themselves built up from fibrous components called fibrils, which are in turns constructed from protofibrils. These may be seen through the electron microscope. The cortical cells are surrounded and held together by a cell membrane complex (CMC), acting similarly to mortar holding bricks together in a wall. The cell membrane complex contains proteins and waxy lipids and runs through the whole fibre. The molecules in this region have fairly weak intermolecular bonds, which can break down when exposed to continued abrasion and strong chemicals. The cell membrane complex allows easy uptake of dye molecules.
Each cortical cell is composed of 5–20 macrofibrils at the widest point with a diameter of 100–300 nm embedded into the inter-macrofibrillar matrix material comprising OF cytoplasmatic and nuclear remnants of the keratinocytes. These are made up of bundles of even finer filaments called microfibrils, which are surrounded by a matrix region. The matrix consists of high Sulphur proteins. This makes wool absorbent because Sulphur atoms attract water molecules. Wool can absorb up to 30% of its weight in water and can also absorb and retain large amounts of dye. This region is also responsible for wool’s fire-resistance and anti-static properties.
The macrofibrils are composed of bundles of 500–800 microfibrils (KIF), each of them being enveloped by KAPs. There are five acidic (Type I KIF) and five basic (Type II KIF), and more than a hundred KAPs, some of which are heavily crosslinked. The microfibrils in the matrix are rather like the steel rods embedded in reinforced concrete to give strength and flexibility. The microfibrils contain pairs of twisted molecular chains.
Twisted molecular chain and helical coil
Within the twisted molecular chains are protein chains that are coiled in a helical shape much like a spring. This structure is stiffened by hydrogen bonds and disulphide bonds within the protein chain. They link each coil of the helix, helping to prevent it stretching. The helical coil is the smallest part of the fiber and gives wool its flexibility, elasticity and resilience, which helps wool fabric keep its shape and remain wrinkle-free in use.