To understand what is going on in this post, take 5 minutes to read this one. It’s an introduction to the concept of Biomimicry Biweekly Challenge, hence a mysterious abbreviation BBC in the title. I also wonder how many people ended up on this page by simply looking for the latest news from British Broadcasting Corporation.
The very hungry caterpillar
Once upon a time, an American Dagger Moth was spotted in my apartment, attempting to lodge itself between the hardcover books for shelter. About a month ago I caught a squirrel red-handed stomping all over my herbs. It was frequenting my 6th floor balcony with a mouthful of peanuts, lunching in oregano flowerpot, scattering husks all over the floor, and escaping by vertical wall Mission Impossible style.
Ever since then, I stopped wondering about random animals showing up in my apartment. This gorgeous caterpillar seemed agitated and worried, looking for a place to start pupating among The philosophy of Zen books. The agitation was passed on to me as I began scouring for books and websites to read up on the habits of dagger moths – here is what a came up with:
And here is what happened three hours later. Such an operative action not only impressed me, but also provoked me for the next challenge.
And then there was Kevlar
Janine Benyus, in her remarkable book ‘Biomimicry: Innovation Inspired by Nature’, challenged our perception of synthetic chemistry more than any author has done before. Janine specifically discussed the manufacturing process of Kevlar, our wonder material. [...] While Kevlar requires processing at high temperatures with very caustic and aggressive solvents, the spider is able to create this material at room temperature, with just a little water, and parts of dead flies. In the creation of new materials, clearly we have a long way to go. – Brian Burns, ‘People Want Toast, not Toasters’
Spider silk has been a rival of Kevlar ever since scientists and material engineers were able to fully test and commercialize its mechanical properties. Then there were forced silking, transgenic plants, transgenic goats, metabolically engineered bacteria, and other attempts at maladapting bio-assistance.
Being a complete layman in materials engineering, yet, still gnawing through numerous scientific papers over the past six days, I’ve drawn several conclusions:
- Mass production of structural protein from animal cells or transgenic animals is too expensive and time consuming in terms of fermentation or animal breeding.
- Forced silking is only suitable for dragline silk, it is time consuming and highly expensive, especially since most spiders are cannibals, rendering farming costly.
- Transgenic tobacco plants are able to produce the necessary protein, but there is no suitable technology to spin such protein into fibers.
Silkworms are the idiot cousins of spiders
I deviated from the cocoon of my beloved American Dagger Moth due to the lack of information on its anatomic structure. The interest did bring me to some extremely interesting articles on the cocoon structure of the infamous silkworm Bombyx mori, that has been providing humanity with this fine, strong, lustrous fiber for many centuries. What drew me further into reading was a fundamental difference of manufacturing process from that of spider.
Some material engineers (infatuated with modulus of elasticity and tensile strength) view silkworms as the idiot cousins of spiders, because mechanically, silkworm silk is much weaker and less extensible as compared to dragline silk of spiders. To fully relinquish the flame, depending on spinning conditions, silkworm silk is either strong or elastic, whereas spider silk combines both properties.
One material + One process = Highly versatile results
This is when I got really excited! As someone, who has little knowledge about polymerase chain reaction, or GPGXX motifs, and absolutely no knowledge of how you would go about reinventing spider silk without bio-utilizing natural organisms, my industrial designer’s instincts started buzzing at the ingenuity of silkworms.
Silkworm caterpillars spin cocoons to protect the moth pupae against possible attacks by predators and bacteria during metamorphosis. A silkworm cocoon is constructed of three parts, the cocoon coating, the inner compact layers and the innermost pelade, which have different microstructures and functions, but are composed of the same composite material made of fibroin (fibers) and sericin (scaffold).
Table Source: Zhao, H., Mechanical properties of silkworm cocoon pelades. Author of diagram: Alëna Konyk
It seems, the following properties of each layer could be isolated:
- Cocoon coating is incompact and brittle (meaning, there is a lot of sericin and very little fibroin). Fibroin diameter is larger.
- Middle compact shell resists external attack force and absorbs external impact energy. Fibroin diameter is smaller than in cocoon coating layer.
- Pelade layer has very low porosity that prevents bacteria and ambient water from destroying the pupa. Lower sericin levels and smaller fibroin diameter than in other two layers. Both static and dynamic properties of pelade are superior to the average properties of cocoon.
The entire system functions as one and each layer is optimized to have different protective functions due to the microstructure. One composite material, one type of manufacturing, three different microstructures. Now, here is an ad for the bulletproof vest:
Three materials are being used for this product:
- Aramid fiber (Kevlar) with coarse weave is the topmost material
- The second layer is made of tight-fine weave
- The third is made of non-woven polyethylene.
It seems, instead of employing the first and second woven layers, it would make sense to take advantage of non-woven material with different porosities depending on the layer. The closer to body, the less porosity. Furthermore, if the story of silkworm cocoon has taught me anything, it seems tensile modulus, strength, and yield stress increase with the decrease in the silk diameter. The silk diameter in the pelade is much smaller than the average silk diameter of the complete cocoon, and thus, the properties are much better. And, finally, instead of trying to control the orientation distribution of fibers in the non-woven material, it makes sense to make it random.
This formula could really be applied to any type of product, whether it’s a bullet-proof vest, or a wound dressing, or runner’s pants, or a coating for a knee arthroplasty. Designers often fall under the spell of millions of materials just ready to be bought and applied to the final design. However, quite often, the solution is in one, just very cleverly manipulated material to fit surprisingly versatile functions. The caterpillar depends on correctly manipulated fibroins and sericin to survive, I would trust it to fit form to function impeccably.
Although the mechanical properties of silk (be it from spider or a silkworm) crucially depend on the proteins involved, it is also extremely important to pay attention to the conditions and patterns of spinning that silk. Until the techniques to produce and engineer natural silk proteins are truly biomimetic and emulate the producers rather than exploit them, it makes sense to divert attention to how the silk is spun and learn to emulate the microstructure techniques with the fibers we have available.