Animal fibers are natural materials derived from various animal sources, primarily utilized in textile production. Unlike plant fibers, which are composed mainly of cellulose, animal fibers are entirely protein-based, giving them unique characteristics like elasticity and warmth. This protein foundation is responsible for their ability to wick moisture, maintain shape, and provide insulation. The specific type of protein and its molecular arrangement determine the final physical properties, such as a material’s softness, strength, and durability.
The Chemical Foundation: Keratin and Fibroin
The distinctive qualities of animal fibers originate from two major structural proteins: keratin and fibroin. These two proteins have fundamentally different molecular blueprints, which directly translate into the vastly different textures and applications of the fibers they form. Keratin is the protein found in hair and wool, while fibroin is the core structural protein of silk fibers.
Keratin molecules are characterized by their primary structure folding into an alpha-helix configuration, which is then twisted with another chain to form a coiled-coil dimer. This helical arrangement provides the material with inherent springiness and resilience. A significant component of keratin is the amino acid cysteine, which allows for the formation of strong covalent cross-links called disulfide bonds. These chemical bridges are responsible for the overall mechanical strength and permanent shaping properties of keratin-based fibers.
Fibroin adopts a beta-pleated sheet structure, where polypeptide chains are aligned side-by-side. This structure is stabilized by numerous hydrogen bonds that form between adjacent protein chains. The repeating sequence of small amino acids like glycine and alanine allows these sheets to pack together densely and uniformly, creating crystalline regions within the fiber. This tight packing gives fibroin fibers their notable tensile strength and light weight. The difference between the flexible, coiled alpha-helix of keratin and the rigid beta-structure of fibroin explains the contrast between the elastic resilience of wool and the smooth strength of silk.
Keratin Structures: Wool, Hair, and Specialty Fibers
Keratin fibers, such as those harvested from sheep (wool) or goats (cashmere), are complex structures grown from mammalian hair follicles. The fiber is composed of three main layers: the cuticle, the cortex, and sometimes a central medulla. The outermost cuticle is a layer of overlapping scales that protects the fiber, while the inner cortex makes up about 90% of the fiber’s mass and provides its mechanical properties.
The cortex contains two types of cells, the ortho-cortex and the para-cortex, which expand differentially when they absorb moisture. This uneven swelling causes the fiber to bend, creating a natural wave pattern known as crimp. This crimp is the mechanism by which wool traps vast amounts of air, providing the insulation.
The internal structure features microfibrils, which are bundles of the coiled-coil alpha-keratin proteins, embedded in an amorphous protein matrix. This matrix is highly hygroscopic, meaning it can absorb a large percentage of its weight in water without feeling wet. This moisture absorption, combined with the structural support from the disulfide bonds, contributes to the fiber’s durability, fire-resistance, and anti-static properties. Specialty fibers like cashmere and vicuña are simply finer, softer varieties of this fundamental keratin structure.
Fibroin Structures: Insect and Arachnid Silks
Fibers like silkworm silk and spider silk are composed of fibroin, a protein that is secreted and spun rather than grown from a follicle. The most common textile silk comes from the domesticated silkworm, which secretes fibroin to construct its protective cocoon. This process involves the silk protein passing through a narrow duct, which subjects the fluid protein solution to intense shear forces.
The mechanical stress, coupled with changes in pH within the duct, forces the soluble protein to transition from a random coil or alpha-helix conformation to the highly ordered, insoluble beta-pleated sheet structure. This rapid molecular reorganization results in the extrusion of an extremely fine, solid fiber. Silkworm silk is often coated with a gummy protein called sericin, which is typically removed during processing to reveal the lustrous, smooth fibroin filament.
The resulting fibroin fiber is renowned for its combination of high tensile strength and minimal weight. Spider silk, while structurally similar to silkworm silk, exhibits even greater toughness and extensibility, which is crucial for the function of a web. These fibroin-based structures represent some of the strongest natural polymers known.