Structure of wool
Basic wool stuff
Really basic
This blog section only includes cuticular cells, cortex cells, and keratin. If you are looking for other information, such as breeds, fire resistance, dyeing and finishing, or other related information, please wait for further updates! You can also learn more from the Woolmark Company or other fascinating research from different scholars.
You can also take a look at:
Wool
The fibre in the undercoat of sheep is hydrophobic and hydrophilic simultaneously.
Structure of wool
Cuticle
Cortex
Keratin
Cuticle
Outer shell layer of wool fibre.
Envelops the cortical by forming a sheath of overlapping scales 4
Each cuticle cell in the wool fibre includes a layered structure in mainly five layers 5
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F-layer
-
Epicuticle
-
Exocuticle A Layer
-
Exocuticle B Layer
-
Endocuticle
The F-layer and epicuticle are usually marked as the same layer, called the ‘proteolipid layer’, ‘resistant membrane’ or the ‘fibre cuticle surface membrane’ (FCUSM ) 6
F-layer consists of a lipid layer that provides a hydrophobic feature to the fibre
F-layer
and
epicuticle
Without this layer, e.g. delipidated wool with felt-resist treatment, water can be rapidly spread across the fibre surface and wet the fibre 6
The epicuticle layer is made of protein and resists oxidation by chlorine 1
In the general wool process, this membrane is usually avoided to be damage, except some chemical finishes are designed to break down this layer 6
The exocuticle layer is firm and highly cross-linked 1
Acts as a protective shell and resists the wool fibre swells in water.
Exocuticle
Overlapping scale structure creates small spaces between the scales, which allow water vapour enters the fibre slowly 7 8
The exocuticle has two layers: the harder A-layer and the B-layer 4
The exocuticle A-layer: is mainly made by cystine residues and is highly crosslinked 6
This large amount of disulphide cross-links: Hydrophobic9
Structure of the cuticle
Endocuticle
The weakest part of the cuticle and is less cross-linked
It has holes which could let the water vapour pass 7
Swells more in the water when compared with other parts of the cuticle
Primary responsibility is to swell the cuticle cells in water and lift the scale edges
Lipid layer
and Intercellular layer& Cell membrane complex
Lipid layer: hydrophobic
Cell membrane complex (CMC)
Found between cortical cells and cuticle cells 10
Cross-linked by isopeptide bonds 10 11
Cortex
Wool fibre's internal cells layer, surrounded by the cuticle cells 1
There are three types of cortical cells in wool fibres 12
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Ortho-cortical
-
Para-cortical
-
Mesocortical
Differed by their crystallinity, sulphur content and rate of water uptake 12
Ortho-cortical
Para-cortical
Cotrical cell
(Source: Woolmark Learning Centre.)
https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-3the-cortical-cells-of-the-wool-fibre/
Ortho-cortical
Higher crystalline with less sulphur and a lower water take-up rate 12
Para-cortical
Less crystalline and contain more sulphur 12
Have a more uniform keratin structure and are more resistant to chemical and mechanical attacks than the ortho-cortex 13
Higher water-update rate and swell at different rates and degrees making the wool fibre bend and create crimps 13
Mesocortical : around 4% of the wool fibre and has less impact on water absorption 12
Macrofibril
Matrix
Microfibril
Long filaments, bundles of microfibrils 14
Consists of high sulfur proteins which attract H2O 14
-
hydrophilic; absorb and retain liquid and solvent
-
Tends not to create static electricity: anti-static 15
High glycine-tyrosine proteins 10
-
Contributes to the mechanical strength and shape of hair 16
Give strength and flexibility 14
Contain pairs of left-hand curled protofibril with right-hand curled molecular chains
Keratin
Cross-links in Keratin groups 14
A significant component of wool fibre 2
A tough fibrous protein
Structure:
Molecular structure of folded polypeptide chains that situate parallelly to each other along the axis of the fibre.
Main cross-links that secure the structure
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Hydrogen bonding (N-H…O=C)
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​Disulphide bonds
-
Salt linkages
A large number of highly polar peptide bonds in keratin increase the number of hydrogen bonds 14
Disulphide bond:
Critical cross-link on keratin to prevent enzymatic digestion.
The high quantities of amino acid cysteine in keratin contain sulphur
-
Disulphide linkages link these cysteines together and comprise cystine molecules 17 18
Salt linkages:
Formed to connect the free amino and acidic carboxyl groups 14
These two cross-links help keratin resist enzymatic degradation.
I have only mentioned the rudimentary knowledge of wool structure, and I highly recommend the free e-learning platform from Woolmark, which offers all-round information.
What are your thoughts?
References
1 Lakshmanan, A. (2022). Physical and chemical properties of wool fibers. In Wool Fiber Reinforced Polymer Composites (pp. 49–71). Elsevier. https://doi.org/10.1016/b978-0-12-824056-4.00011-x
2 Eslahi, N., Dadashian, F., & Nejad, N. H. (2013). An investigation on keratin extraction from wool and feather waste by enzymatic hydrolysis. Preparative Biochemistry and Biotechnology, 43(7), 624–648. https://doi.org/10.1080/10826068.2013.763826
3 Qiu, J., Wilkens, C., Barrett, K., & Meyer, A. S. (2020). Microbial enzymes catalyzing keratin degradation: Classification, structure, function. In Biotechnology Advances (Vol. 44). Elsevier Inc. https://doi.org/10.1016/j.biotechadv.2020.107607
4 Ammayappan, L. (2013). Eco-friendly Surface Modifications of Wool Fiber for its Improved Functionality: An Overview. Asian Journal of Textile, 3, 15–28. https://scialert.net/fulltext/?doi=ajt.2013.15.28
5 Woolmark Learning Centre. https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-2-the-cuticle-cells-of-the-wool-fibre/
6 The Woolmark Cpompany. THE CUTICLE CELLS OF THE WOOL FIBRE. Wookmark Learning Centre. Retrieved November 12, 2022, from https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-2-the-cuticle-cells-of-the-wool-fibre/
7 Scanavez, C., Joekes, I., & Zahn, H. (2004). Extractable substances from human hair: A discussion about the origin of the holes. Colloids and Surfaces B: Biointerfaces, 39(1–2), 39–43. https://doi.org/10.1016/j.colsurfb.2004.08.021
8 Wang, L., & Wang, X. (2009). Effect of structure–property relationships on fatigue failure in natural fibres. Fatigue Failure of Textile Fibres, 95–132. https://doi.org/10.1533/9781845695729.2.95.
9 Das, D., & Das, S. (2022). Wool structure and morphology. In Wool Fiber Reinforced Polymer Composites (pp. 13–32). Elsevier. https://doi.org/10.1016/b978-0-12-824056-4.00013-3
10 Jones, L., & Rogers, G. (2009). 2. Structure and Composition of Wool. Wool Biology and Metrology.
11 Rice, R.H., V.J. Wong, and K.E. Pinkerton. 1994. Ultrastructural visualization of cross-linked protein features in epidermal appendages. Journal of Cell Science.:1985-1992.
12 Woolmark.. THE CORTICAL CELLS OF THE WOOL FIBRE. Woolmark Learning Centre. Retrieved November 12, 2022, from https://www.woolmarklearningcentre.com/program-library/wool-education-program/wool-fibre-science/module-2-the-structure-of-wool-fibres/topic-3the-cortical-cells-of-the-wool-fibre/
13 Jones, D., & Brischke, C. (2017). Nonwood bio-based materials.
14 Mathison (1964), as cited in Brown, R. M. (1994). The microbial degradation of wool in the marine environment.
15 Woolmark. The Properties of wool. https://www.learnaboutwool.com/globalassets/law/resources/factsheets/secondary/gd3270-secondary-fact-sheet_2019_j.pdf
16 Matsunaga, R., Abe, R., Ishii, D., Watanabe, S. I., Kiyoshi, M., Nöcker, B., Tsuchiya, M., & Tsumoto, K. (2013). Bidirectional binding property of high glycine–tyrosine keratin-associated protein contributes to the mechanical strength and shape of hair. Journal of structural biology, 183(3), 484-494.
17 Hearle & Peters (1963), as cited in Brown, R. M. (1994). The microbial degradation of wool in the marine environment.
18 Martin (1990), as cited in Brown, R. M. (1994). The microbial degradation of wool in the marine environment.