Since my last few blog posts have been all about structural colors, today let’s switch gears and talk about one of my long time personal interests – spider silk. This blog post should serve as an introduction to the efforts that have been made and difficulties encountered when trying to create a synthetic version of spider silk.
First of all, let’s make it clear that spider silk is not a single material. Rather, it’s a class of many different materials. A common orb weaving spider can produce 6-8 different kinds of silk, depending on the species and how you define it. Among them, the most broadly studied and the most “magical” one is the dragline (major ampullate) silk – a long time engineering marvel from nature and the “holy grail” for material scientists that they still have no idea how to synthetically reproduce. What makes the dragline silk so unique and a hot field for Biomimicry is that it is the toughest (both in strength and elasticity) material on earth. Period. There’s no “but”…
However, spiders are territorial and prone to cannibalization. Hence, they cannot be farm-reared economically in large enough quantities and density for silk harvesting as shown in the movie “The Amazing Spider-Man”, or like we have done with domestic silkworms for thousands of years.
Second, the spider silk proteins (spidroins) are a family of very large molecules, averaging around 300 kDa in molecular weight. That’s about 3000 amino acids, or around 9 kbp of DNA just for the encoding region (the gene would be even bigger if it included introns and the regulatory regions). Since the dawn of biotechnology, scientists have been trying to reproduce spidroins in other organisms using genetic engineering techniques. However, there’s no way of doing that with the complete spidroin genes – they’re just too large! Fortunately, spidroins are composed of segments of similar repeats (but not exact replicates), so scientists just inserted a portion of the spidroin genes, expressed fragments of the spidroins, and hoped they work. They have done so successfully in many different organisms, including (but not limited to) goat, silkworm, tobacco, and E. coli. However, when fragments of the full spidroins are inserted, these organisms expectedly don’t produce silk possessing the same toughness as natural dragline silks.
Even if we develop means to reproduce the full version dragline spidroin with biotechnology, we still don’t know much about how to process this raw material. Current knowledge in this aspect is very limited. We know roughly that the pH gradient of the environment, ionic strength (salt concentration), shear stress, spinning speed, and tension when spinning are all contributing factors for the mechanical property of the final silk fiber. But exactly what and how they contribute is still a big mystery. And that’s exactly the reason why trying to re-create a synthetic version of spider silk is still one of the more popular focuses in Biomimicry.
Recently, a handful of start-ups claim that they have gotten really close to being able to mass produce and commercialize some kind of synthetic versions of spider silk in economically viable ways. Without a doubt, they all attracted funding from big time investors. Some of them are summarized very well in this article. However, remember that what makes spider silk such a unique and desired material is its toughness. Therefore, the most revolutionary applications/products of spider silk should tap into that potential head-on and fully utilize this property to solve problems that otherwise cannot be addressed with other materials. All other benefits, such as biocompatibility, biodegradability, breathability, weight, (potential) anti-microbial properties, and eco-friendly attributes are just secondary functions, and add-ons. Because for every secondary function of spider silks listed above, we can probably find some other organisms that do it better than spider silk. Since it’s still difficult to have a synthetic version of spider silk that possesses comparable toughness to the natural spider silk, those start-ups are mostly promoting the secondary functions of spider silks in their spider silk products. Hence, for me, the spider silk products from those start-up companies will stay within a niche market with novelty or gimmick factors, rather than in a consumer market with practical, applied functionalities in the foreseeable future.