The structure of fibers varies significantly depending on their type and can be broadly classified into two major categories: natural fibers and synthetic/manufactured fibers.
Structure of Natural Fibers
Cotton Fiber
Chemical Composition: The primary constituent is cellulose (molecular formula: C₆H₁₁O₅).
Microstructure:
The cross-section is kidney-shaped and features a central lumen; longitudinally, the fiber exhibits natural convolutions. It is composed of an outer cuticle, a primary wall, and a secondary wall (comprising layers S1, S2, and S3); of these, the S2 layer accounts for over 90% of the fiber's mass and consists of spirally arranged fibrils, imparting a porous nature to the structure.
Bast Fibers
While also primarily composed of cellulose, they possess a multicellular composite structure and contain 10% to 16% gum.
The structure of individual fibers is complex, exhibiting significant variations across different types, and thus lacks a unified descriptive framework.
Wool (Animal Hair Fiber)
Chemical Composition: Keratin (containing over 20 types of amino acids; rich in disulfide bonds)
Structural Organization:
The outer layer consists of *scale cells* (keratinized dead cells);
The interior consists of *cortical cells* (comprising ortho- and para-cortex, exhibiting a fibrillar structure: protofibrils → microfibrils → macrofibrils → cells);
The cells are bonded together by the CMC (Cell Membrane Complex).
