Silk – What’s Old is New Again
Spider silk has been touted for a couple of decades – alongside graphene and carbon nanotubes – as the “armor material of the future.” But unlike those carbon nanostructures – which were surprising discoveries with little connection to traditional engineering materials – spider silk is similar in composition to silkworm silk, one of man’s earliest armor materials. (Note: That statement is generally true of all spider silks, but in this post, “spider silk” refers specifically to the strong dragline silk of the orb-weaving spider, Caerostris darwini.)
As Bashford Dean, in his 1920 book Helmets and Body Armor in Modern Warfare, noted:
“In the Far East, silk was discovered to be extremely useful in a defensive costume, certainly at the beginning of the Christian era. As early as the year 600, the Chinese developed armor of padded silk and a similar type of military costume shortly appeared at other points in the Orient. Thus in Japan it is known from the seventh century. . .
“Padded costumes of silk, cotton or linen appear to have been used until comparatively modern times in almost every country.
“Curiously enough, soft armor was quite in vogue at the time of the colonization of America. In 1663 Roger North records that ‘an abundance of silken back and breast plates were made and sold that were pretended to be pistol proof in which any man dressed was safe as in a house, for it was impossible that any one could strike at him for laughing, so ridiculous was the figure, as they say of “hogs in armor.”‘”
Dean goes on to describe various silk-based armor systems experimented with in the 19th and early 20th centuries. These include a silk collar developed during WWI intended to counter artillery fragments and shrapnel, an oversized silk tunic, and, most notably, the Zeglin Silken Body Defense—an essentially modern soft armor system complete with a steel “ICW” up-armor plate!
In a long footnote, Dean also recounts how, during WWI, British Government armor specialist William A. Taylor compared the virtues of silk with those of steel:
“The writer finds from a note furnished him by Captain Ley of the Munitions Board in London that certain of the earlier tests on the ballistic virtues of silk were quite remarkable: Bombs were exploded in the ‘fragmentation hut’ at Wembley (1915?); sample pads of silks were used for comparison with plates of helmet steel (Firth) of twice their weight; the silk pads were the better; they kept out 74 degrees of ‘medium shrapnel bullets at 600 foot seconds.’
“Mr. Taylor summarizes his results as follows:
‘The only material that gives materially better results than manganese steel is pure woven silk which, against shrapnel bullets up to a velocity of 900-1,000 foot seconds, has a distinct advantage, weight for weight, over steel. For example, silk weighing 10.8 oz. per sq. ft. is proof against shrapnel at 800 foot seconds, whereas steel to give the same resistance would weigh about 20 oz.
“The relative advantages and disadvantages of silk as compared with steel for body armor may be summarized as follows:
“Silk does not give nearly the same resistance as steel against high velocity or pointed projectiles (e.g. rifle bullets or bayonet thrusts) but on the other hand it does not deform a bullet when perforated. A bullet after passing through steel is deformed and would cause a very serious wound.
“Against low velocity blunt projectiles (e.g. shrapnel shell splinters, bomb fragments) up to a certain velocity silk is superior to steel, weight for weight.
“Silk sits better on the wearer than steel on account of its flexibility.
“For infantry, silk would probably be uncomfortably warm in summer and would require to be made water and vermin proof.
“Silk is more costly and difficulties of supply would be greater than with steel.”
Indeed, the limitations of silk became glaringly apparent in the trenches of WWI, where it was frequently put to the test. Cost and availability were always issues. Most troubling, perhaps, was that silk armor, being wholly organic, “deteriorated rapidly as trench materiel.”
Steel eventually became preferred as an armor material, and soft armor dropped off the map entirely until nylon was discovered and its usefulness became apparent. Still, silk was assuredly the first soft armor material of the modern era.
A Complex Biopolymer
Fast forward about a hundred years, and now people are talking about spider silk as an armor material. Specifically, the focus is on the strong dragline silk from c.darwini. As with silkworm silk, all spider silk is composed of similar amino acid building blocks – although these blocks are arranged differently. All of them are complex; silks are multicomponent polymeric biomaterials and are of much greater structural complexity than the synthetic polymers made by man.
UHMWPE, admittedly an extreme example, is nothing but [CH2]-[CH2]-[CH2]-. . .-[CH2].
Kevlar is a little more complicated, but it still consists of a single repeat unit that’s drawn into long fibers.
Spider silk, however, is something else entirely; its structural complexity makes it simply impossible to produce in a lab using traditional polymer chemistry techniques. It consists of spidroins—the silk proteins responsible for its structure and mechanical properties. The strong dragline silk of C. darwini comprises multiple spidroin types (e.g., MaSp1, MaSp2, MaSp4, MaSp5), and this complexity is increased by processes such as protein folding, post-spinning crystallization, and hybridization with other silk proteins like the low molecular weight non-spidroin protein “spider-silk constituting element” (SpiCE).
To simplify, spider silk largely consists of hard crystalline segments rich in the amino acid alanine, joined by softer, more flexible amorphous segments rich in glycine. The alanine-rich crystallites are very small—typically under 10 nm in each dimension—and are embedded within a glycine-rich amorphous matrix. Total crystallinity varies widely but is often around 20–30%. These semi-crystalline protein chains are drawn and oriented into long fibers and coated in natural, non-protein chemical lubricants and preservatives.
Here’s a visual overview of spider silk’s structure, from the Wikipedia page, drawn by Yue Zhao:
This is not the sort of thing that can be synthesized using traditional chemical methods.
You Can’t Farm Spiders, but You Can Genetically Modify Silkworms
Though spider silk cannot be synthesized in a lab, it may be “farmed” – that is, produced in large quantities through biotechnological means. Orb-weaving spiders—aggressive, territorial, and cannibalistic—cannot be farmed for silk, but the “instructions” for spider silk are encoded in their DNA. If the correct genetic sequences (those coding for spidroins, SpiCE, and related folding and processing signals) are isolated, they can be implanted into other organisms via CRISPR or other genetic modification techniques. This was done with silkworms in 2014; a spider’s dragline silk gene was isolated and implanted into silkworms using a plasmid-based gene transfer technique. The silk produced by these modified silkworms, though neither as strong nor as tough as true spider silk, was much stronger than what they would otherwise have made, and the silkworms produced enough of it to create a few prototype articles of clothing. Gene transfer technologies have improved substantially since 2014, so silkworm-derived “spider silk” might be viable in the near future.
It is reported that a company called Kraig Biocraft Laboratories has spider silk in production via gene-modded silkworms. Their products, known as “Monster Silk” and “Dragon Silk,” are currently in production, and are said to have been evaluated by the US Military in 2018, though the results of any evaluation have not been made public.
What’s particularly interesting is that there’s no reason to suppose that the dragline of c.darwini is the final word in ultra-high-strength silk. With further genetic tinkering, silkworms might be induced to produce silk fibers that are stronger than any natural spider silk, while maintaining or even improving properties such as high toughness and elasticity. It is also possible that post-spinning treatments – like stretching and heat-treatment – could further enhance the properties of spider silk beyond those found in nature.
Spider Silk: Implications for Modern Armor
Just as silkworm silk was once used as a soft armor material, there’s simply no other way to use spider silk: Its properties naturally lend themselves to soft armor applications. Spider silk is weaker than UHMWPE and aramid fibers in terms of sheer tensile strength, but has the advantage in toughness, flexibility, and elasticity. It also exhibits better heat transfer, which – combined with its flexibility – should make it more comfortable to wear. This may enable the development of light, ballistic-resistant articles of clothing that look and feel like regular garments. For example, it was once suggested that spider silk be investigated for use in ballistic-resistant undergarments for groin protection from fragments. The silk collar – a WWI idea for neck protection – could be revisited with spider silk, as could other applications requiring a light, flexible, frag-resistant garment. (For e.g., integration into the arms of combat shirts.)
In all other applications, even the strongest known spider silk has no clear advantage over conventional armor materials such as UHMWPE and aramid fibers, and may even be substantially inferior. To reiterate: Spider silk is notably weaker than UHMWPE fiber. Another issue is that spider silk is not stiff enough to serve as a support layer in ceramic armor systems. A very real concern is that it might be too flexible and elastic for certain applications; although a spider silk-derived armor panel might be strong and tough enough to catch bullets and fragments, its elasticity could cause it to deform (“pencil in”) against the structure or body behind it. This is a serious consideration, though proper design – such as adding a stiffening layer of another material on the body side of a soft armor panel – might mitigate the issue. (Hybridization with a stiffer material, however, would reduce some of spider silk’s inherent benefits.)
It’s also worth noting that many of spider silk’s theoretical benefits – flexibility, air permeability, and toughness – are shared by nonwoven aramid and polyethylene materials, such as aramid felt. As mentioned in our post on ambient armor, aramid felt is known for its high performance and excellent flexibility and comfort, yet its propensity for excessive deformation has limited its use as a soft armor material.
A final concern is that spider silk is a biodegradable protein. Being composed mostly of alanine, glycine, and other amino acids bonded by amide bonds, it is not especially resistant to decomposition or attack by bacteria and fungi. (Humans can eat spider silk – and so can certain bacteria and, of course, moths.) Moreover, historical accounts from the trenches of WWI noted that silk armor degraded quite severely when exposed to the elements.
There may be ways to mitigate decomposition with coatings and treatments, but the same is true for materials like Zylon – which is roughly three times stronger than spider silk – and few are willing to take that risk. Zylon also has only one known failure mode, whereas spider silk can degrade in multiple ways, with microbial attack being particularly likely, unpredictable, and difficult to mitigate.
In armor applications, spider silk is an interesting material with potential niche uses, particularly in integration into light wearable garments. However, it is unlikely to be a game-changer, and issues surrounding degradation and loss of strength may prove insurmountable. If produced commercially in the future, it might find use in luxury scarves and other clothing applications where the potential loss of strength due to environmental exposure is less of a concern.
Spider silk’s susceptibility to biodegradation may even be advantageous in certain medical applications, such as stents, sutures, and drug delivery systems – but that is an entirely different matter.
