Source: CBS NEWS .

"If you look in a spider web, if you just took one single strand of that spider silk, that strand is stronger than steel as well - a lot like Kevlar," said Mr Tucker Norton, a ballistics expert at duPont.

I'm skeptical of this claim. What are the facts?

  • 2
    Not that unreasonable. Depending on the steel too. When comparing material strength, we often look at strength per unit mass. Under that measure, aircraft spruce is similar to steel. – William Grobman Dec 6 '12 at 21:47
  • To answer this question a good measure of strength needs to be defined. The most simple (but perhaps unintuitive) definition of strenght that allows spider silk to be categorized as stronger than steel would be: How quick does it break when you bend it. – Dennis Jaheruddin Dec 6 '12 at 22:38
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    note that most of the time the comparison is either pound for pound or size for size, which puts steel at a bit of a disadvantage – ratchet freak Dec 6 '12 at 23:06

It depends on what spider you are measuring, what sort of silk from that spider, and what properties you want to measure.

The toughness of forcibly silked fibers averages 350 MJ/m3, with some samples reaching 520 MJ/m3. Thus, C. darwini silk is more than twice tougher than any previously described silk, and over 10 times better than Kevlar®.


Caerostris darwini major ampullate silk is, on average, about twice as tough as typical silks spun by other orbweavers (Figs 3–4, Table 1, Table S1). Spider dragline silk is already deservedly renowned for its high toughness of ∼150 MJm∧-3, which outperforms both steel and Kevlar [32]. Yet, C. darwini silk is far higher performing, absorbing about ten times more kinetic energy before fracturing than does Kevlar.

Wikipedia covers many different mechanical properties of spider silk, including strength (same as some steels, less than Kevlar), strength per weight (where it beats steel and Kevlar), ductility and toughness. This may help you understand how silk beats Kevlar in some measures but not others.


Along the same line as Oddthinking's answer...

It depends on the spider and the application.

The major difference between the spider silk and other high-strength materials, such as steel and aramid fibres, is not strength but the degree that the silk fibres extend before they break. This reasonable balance of stiffness, strength and extensibility makes the silk an ideal multipurpose material for advanced biological network construction.[1]

  • Compared to Kevlar

    An N. clavipes dragline silk shows a Young’s modulus, tensile strength, and stress at break of the same order of Kevlar, which is a benchmark of modern polymer fiber technology but absorbs almost one order of magnitude more energy than Kevlar when breaking.[2]

    ...it is fair to say that spider MA silk is amongst the stiffest and strongest polymeric biomaterials known. Note, however, that the stiffness of MA silk is well below that of Kevlar, carbon fibre and high-tensile steel, engineering materials that are commonly employed to transmit and support tensile forces. Note also that the strength of MA silk is somewhat less than that of these engineering materials. At first glance, we might interpret this information as indicating that MA silk is superior to other biomaterials, such as collagen, but not as "good" as Kevlar and carbon fibres. However, this interpretation is based on the assumption that "good" means stiff and strong.[3]

  • Compared to steel

    In fact, their mechanical properties can be considered above those of steel itself. Its absorbed energy at breaking point is almost two orders of magnitude higher, while its tensile strength [stress] is almost six times higher and the stresses at breaking point are equivalent. Additionally, although the Young’s modulus of steel is about three times higher than the spider-silk modulus, this last material has a much lower density. Its ratio of tensile strength to density is perhaps five times better than steel. Therefore, at equal mass, the spider silk behaves much better than steel. In conclusion, spider-silk fibers are nearly as strong as several of the current synthetic fibers and can outperform them in many applications in which total energy absorption is important.[2]

Material comparisons[4]

Material comparison

Properties of materials[5]

  • Stiffness: a measure of how materials respond to loads (the relationship between stress and strain).
  • Strength: a measure of the stress a material can withstand without failure.
  • Extensibility: a measure, breaking length divided by the original length, of the elongation a material can withstand before failure (extension to failure). see Strain.
  • Toughness: a measure of the total amount of work, or energy, required to stretch the material until it breaks (normalized to the volume of material tested).
    • One joule is equal to the energy expended (or work done) in applying a force of one newton through a distance of one metre (1 newton metre or N·m). One megajoule (MJ) is the approximate kinetic energy of a one metric tonne vehicle moving at 160 km/h (100 mph).
  • Stress: a measure of the average force per unit area of a surface within a deformable body on which internal forces act.
  • Strain: a normalized (dimensionless) measure of deformation representing the displacement between particles in the body relative to a reference length.

Other cool properties

  • Balance of energy absorption and return

    Orb-web-weaving spiders appear to use the minimum amount of silk necessary in their webs to catch prey. The web has to stop a rapidly flying insect nearly instantly, so that the prey becomes entangled and trapped. To do this, the web must absorb the energy of the insect without breaking and yet not act as a trampoline and bounce the insect away from the web.[6]

  • Supercontraction

    Another unique feature of major ampullate silks is their supercontraction when exposed to water. Depending on the spider species and other factors, these silks will contract to 50% or less of their original length in water. This silk fiber supercontraction is the only known example of supercontraction in water. This supercontraction can occur repeatedly with virtually identical results. Suggestions are that it provides an advantage to the spider by tightening the web whenever the humidity is very high by contraction of the attachment lines and the framework of the web.[6]


  1. Sirichaisit, J.; Young, R. J.; Vollrath, F. (2000). "Molecular deformation in spider dragline silk subjected to stress". Polymer 41 (3): 1223. doi:10.1016/S0032-3861(99)00293-1
  2. Rodríguez-Cabello et al. (2006). "Genetic Engineering of Protein-Based Polymers: The Example of Elastinlike Polymers". Ordered Polymeric Nanostructures at Surfaces. Advances in Polymer Science. 200. pp. 119. doi:10.1007/12_047.
  3. Gosline, J. M., P. A. Guerette, C. S. Ortlepp, and K. N. Savage (1999) "The mechanical design of spider silks: from fibroin sequence to mechanical function". Journal of Experimental Biology 202, no. 23 : 3295-3303.
  4. Agnarsson I, Kuntner M, Blackledge TA (2010) "Bioprospecting Finds the Toughest Biological Material: Extraordinary Silk from a Giant Riverine Orb Spider". PLoS ONE 5(9): e11234. doi:10.1371/journal.pone.0011234
  5. Biology 427 Biomechanics: "Everyday stress and strain and the stiffness of biological materials I: terms, definitions and other basics".
  6. Lewis, R. V. (2006). "Spider Silk: Ancient Ideas for New Biomaterials". Chemical Reviews 106 (9): 3762–3774. doi:10.1021/cr010194g

Further reading

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