Table 1.

lists the stiffnees (Young's modulus, E); breaking strain (extensibility ε_max); and the proposed molecular mechanisms of extensibility for various protein fibers

Material	E (MPa)	εmax (%)	Proposed extensibility model	References
High extensibility, soft fibers
Crosslinked fibrin fibers	1–10	332	α ↔ β transition, domain unfolding, protofibril sliding	[1, 2, 33]
Uncrosslinked fibrin fibers	1–10	226	α ↔ β transition, domain unfolding, protofibril sliding	[1, 2, 33]
Spider silk (Araneus Flag silk)	3	270	Modular composition; large extensible amorphous or β-spiral regions connected by stiff crystallites	[34–39]
Elastin (Bovine ligament)	1	150	Compact, amorphous, hydrophobic domains (random or β-spiral), which are crosslinked together, entropic, rubber-like elasticity	[40–6]
Resilin (Dragonfly tendon)a	1–2	190a	Compact, amorphous, hydrophobic domains, which are crosslinked	[40, 46, 47]a
Cloned resilinb		313b	together, entropic, rubber-like elasticity	Cloned [48]b
Matrix-free Intermediate filament (mammalianc, hagfishd)	6–300	160c–220d	α ↔ β transition and/or fibril sliding	[49–53]
Fibrillin	0.2–100	> 185	Unstacking of pleated domains	[54–58]
Myofibrils (sarcomere) Titin (Connectin)	1	200	Unfolding of PEVK and Immunoglobulin domains in Titin	[59–61]
Mussel byssus (distal)	10–500	109	Block copolymer: collagen-like central domain (stiff) flanked by crystalline βsheets (stiff) and amorophous or β-sprial motifs (elastic)	[62–64]
Fibronectin	-	200–300	Extension of bent and looped molecules and/or domain unfolding	[65–67] [68]
Low extensibility, stiff fibers
Spider silk (Araneus MA silk)	10, 000	27	Modular composition; small, extensible amorphous or β spiral regions, connected by stiff crystallites	[34–39]
Uncrosslinked, self-assembled collagen-I	-	24–68	Sliding of collagen fibrils	[69, 70]
Cross-linked, self-assembled collagen	5,000–7,500	12–16	Highly regular, paracrystalline structure, small, reversible molecular deformations	[69, 70]
Tendon collagen (mammalian tendon)	160–7,500	12	Highly regular, paracrystalline structure, small, reversible molecular deformations	[71–74]
Actin	1,800–2,500	≤ 15	Highly regular, paracrystalline structure	[75–80]
Microtubules	1,000–1,500	≤ 20	Highly regular, paracrystalline structure	[76, 81–84]
Wet, hard α-keratin in high-sufur matrix (hair, wool)	2,000	45	α ↔ β transition of keratin (embedded in crosslinked, high-sulfur matrix)	[50, 85–90]

There are two reasons for the range of values in the Young's modulus of a given fiber: (1) Authors report a range of values. (2) Strain hardening/softening; the fibers become stiffer/softer as the strain increases. The table includes microscopic fibers (diameter: order of micrometer or less) and macroscopic fibers (diameter: order of several micrometers or larger). Microscopic fibers are fibrin fibers, elastin fibers, resilin fibers, intermediate filaments, fibrillin, myofibrils, fibronectin, actin filaments, microtubules; macroscopic fibers are spider silk, mussel byssus, collagen fibers, wool fibers

^aDragonfly tendon

^bCloned resilin

^cMammalian, matrix free intermediate filament

^dHagfish threads, a model fiber for matrix-free intermediate filament