Material of the month – silk

Materials World magazine
,
2 Nov 2016

Khai Trung Le focuses on applications of silk in both warfare and heathcare.

Stop me if you’ve heard this one before. Mongolian warriors reportedly wore silk garments underneath their armour as additional protection against arrows. With its flexibility and strength, arrows fired from distance would struggle to penetrate the silk, adding to the chance of deflection. If the arrow entered the body but did not pierce the silk, it would enable the warrior to draw the arrow out of the wound quickly and cleanly, as well as preventing poison from entering the bloodstream. So said many frustrated history teachers, including mine, looking to entice young girls and boys into their lessons with the mild promise of historic violence.

But the use of silk has had a persistent presence in military history. In July 2014, the Royal Armouries and National Firearms Centre, UK, explored whether the assassination of Archduke Franz Ferdinand would have succeeded had he been wearing a bulletproof silk vest popular at the time. The vest was created and patented by Polish priest and inventor Casimir Zeglen in 1897, who initially tested the material by firing into wooden planks before realising that wood has no resemblance to flesh, reportedly leading to Zeglen testing silk by firing into cadavers, live dogs and eventually himself.

Although the success of these tests led to the bulletproof silk vest being endorsed by the Chicago police department and US military, heavy production costs and its inability to withstand rifle rounds meant the vests were never adopted in service. But with the proliferation of pistols as the early 19th Century assassin’s preferred implement, being used in the assassinations of King Umberto of Italy in 1900 and USA President William McKinley in 1901, it has been argued that heightened distribution of silk vests may have prevented these attempts. Polish inventor Jan Szczepanik, who partnered with Zeglen in 1897 to mass-produce the vests before an acrimonious split, claimed in 1902 that several heads of state had acquired bulletproof silk vests including German Emperor Wilheim II, King of England Edward VII and President of France Émile Loubet.

The Royal Armouries conducted numerous tests with reproductions of Zeglen’s vests in 2014, measuring a variety of silk vest structures and calibres. However, in the case of Archduke Franz Ferdinand, Lisa Traynor, author of The Archduke and the Bullet-Proof Vest: 19th Century Innovation Versus 20th Century Firepower, published by the Royal Armouries, argues that the bullet that killed Ferdinand had lodged in his jugular vein, rendering the question of whether a silk vest would have prevented his death immaterial.

Silk remains an appealing material for use in modern conflict, although its widespread use has been prevented due to difficulties in farming – silkworm farming is slow and typically involves killing the worm, and notoriously territorial and cannibalistic spiders cannot be farmed in quantity. However, following the success from USA-based bioengineering company Kraig Biocraft in genetically altering silkworms to produce a spider silk approximation, as detailed in the paper, Silkworms transformed with chimeric silkwork/spider silk genes spin composite silk fibers with improved mechanical properties, published in PNAS, the US Army has provided an initial US$100,000 grant to develop Kraig Biocraft’s material, named Dragon Silk, as an alternative to Kevlar for use in body armour.

While Kevlar has a strength of 3GPa, compared with Dragon Silk’s 2GPa, Kevlar has a limited elasticity of 3%, against Dragon Silk’s 30–40%. Kraig Biocraft will produce a series of ballistic ‘shoot packs’ of varying thread counts and thickness to assess the appropriate qualities of Dragon Silk for use as protective clothing. Should initial findings meet expectations, the grant will be increased to US$1 million.

Do no harm

But if Kevlar is synonymous with militaristic connotations, silk has been spared this by the sheer variety of uses it has seen. A recent trend has been in categorising its characteristics – tear-resistance, high elasticity, hydrophobicity, antibacterial and biodegradable, surely a near-complete checklist of desirable qualities – for suitability in healthcare. Sutures and bandages have been heavily sought–after uses, hoping to make the most of silk’s high flexibility and strength. Silk has been used as a suture in ocular, neural and cardiovascular surgery throughout the last 100 years, although its poor microbe-resistance characteristics remain a major drawback. 

It is also an obstacle for bandages, and an Ohio State University, USA, research team hopes to overcome it with a portable adhesive dressing made from silk and silver that drives a small continuous electrical current, intended to fight against infection and stimulate healing. Initial tests in 2016 suggest that the electrical currents interrupt bacteria by either weakening molecular structure of the bacteria’s biofilms, making them more susceptible to antibiotics, or disrupting the production of chemical messages that instruct the bacteria to form biofilms. Dr Vish Subramaniam, Chair of the Ohio State Department of Mechanical and Aerospace Engineering, said, ‘The destruction of the biofilm would enable antibiotics to start killing off bacteria, reduce chronic inflammation and allow the body’s natural immune response to work more effectively.’ Patient trials will begin by the end of 2016.

On a smaller scale, in an attempt to replicate 19th Century Murcia silk for use in regenerative medicine, a Universidad Politécnica de Madrid, Spain, research team has created a variety of hijuela de araña, an unusually strong thread traditonally extracted from silkworms, which they believe to be the strongest ever developed. Hijuela was previously produced using an acid liquid medium that deformed the worms’ silk glands, strengthening the silk. This time, using a similar technique, the Madrid team has been able to deform the silk from the South African Nephila inaurata spider, creating unusually strong and large-diameter fibres, as detailed in the paper, Spider silk gut: Development and characterization of a novel strong spider silk fiber, published in Scientific Reports in 2014. The team believes this could be used in regenerative applications, including scaffolds for damaged tissues and facilitate cell adhesion.

Silk seemingly has a place whether to harm or to heal, and Materials World is hoping that the material can find a home in the latter.