Imagine a material that is tougher than Kevlar or steel, yet remarkably flexible. It's something you can easily find in your attic or a lingerie store. It's as instantly recognizable today as it was to our early ancestors, yet we still aren't sure exactly how it's made.
The miracle thread in question is natural silk, the ubiquitous fibers made by spiders and silkworms, which has been used throughout history for items ranging from stockings and parachutes to surgical sutures. Today scientists and engineers are creating a number of useful materials based on silk research. But many researchers believe these applications may just be the start of a whole web of useful new products and devices, if only we had a better understanding of just how these small creatures spin their precious thread. In recent years, researchers have worked to gain a better understanding of what silk is and how it's made, with the goal of being able to consistently replicate and enhance its production synthetically. In the July 30 edition of the journal Science, two Tufts University researchers, Fiorenzo G. Omenetto and David L. Kaplan, review the state of silk research, the challenges that remain, and why synthetic silk production is so appealing.
According to Omenetto and Kaplan, scientists understand that silk is "a relatively simple protein processed from water." Research has established what those proteins are, and they have determined that the properties of silk can vary a great deal depending on factors such as the outside temperature, how fast the silk is spun, and the exact type of silk created.
But no one knows how exactly the spiders and silk worms actually make silk. Scientists have determined they don't secrete the stuff, but instead pull it out of special glands in very specific ways. Spiders, for example, pull it with their legs, while silkworms perform a 'figure eight' dance with their heads to create the silk threads. Despite this knowledge, Omenetto and Kaplan write, "there are still significant knowledge gaps in understanding how to reverse-engineer silk protein fibers."
The spiders and silkworms have also figured out another neat trick that, according to Omenetto and Kaplan, still evades the capabilities of their would-be mechanical copycats. When the scientists try to store silk proteins in the lab, they find they must do so under exacting conditions, or material will quickly begin to crystallize. Nature's silk makers, on the other hand, don't seem to have this problem. They can store the raw silk materials internally at a variety of temperatures for days and even weeks without encountering the crystallization problem, and at this point in time, the authors write, no one is sure how they do it.
One goal of silk research, Omenetto and Kaplan write, is to find a way to genetically engineer other organisms to produce custom-designed silk proteins that could then be used to produce synthetic silk for specific purposes on a large scale. This has led to genetically modified mushrooms, bacteria and even goats that are able to produce silk protein, yet none of the actual silk produced from these modified organisms matches the qualities of the stuff produced by spiders and silk worms. Once these issues are overcome, however, Omenetto and Kaplan believe that someday, plants could be modified to produce silk as a crop, like cotton is harvested today.
So why all of this focus on silk? Omenetto and Kaplan say that figuring out how to replicate and modify silk could lead to new breakthroughs in medicine, among other fields. Although silk is used in sutures today, the authors explain, it has to be coated in wax, which prevents the sutures from being gradually absorbed into the body. Modified silks could be wax free, Omenetto and Kaplan write, and could be used to safely administer drugs within the body or even create "degradable and flexible electronic displays for improved physiological recording" of a person's body. These and other intriguing possibilities await, Omenetto and Kaplan say, if we can just figure out how exactly the spider spins that web.