Duplicating spider silk has been a long-standing goal of the fiber industry, and NRDEC has been a leader in the effort since at least the 1960s. The reason for the industry’s interest and for NRDEC’s long-running commitment is that research has shown that fiber toughness is integral to the development of better-performing ballistic protective clothing. NRDEC’s Survivability Directorate has contributed to that body of research, proving over the years that to be effective, the materials used in ballistic protective clothing must be high in strength, high in modulus (a mathematical term that in this case refers to the stiffness of a fiber), and high in elongation to the breaking point. In other words, these materials must be very tough. The Army’s current ballistic protective vest is made of 13 layers of Kevlar®, but NRDEC researchers continue to investigate the performance of other weaves of Kevlar® and additional high-performance fibers like ultrahigh molecular weight polyethylene, liquid crystal polymers, and other aramid-based fibers.
Spider silk, perhaps to the surprise of people outside of the industry, is three times tougher than Kevlar®. Its superior ability to elongate allows it to absorb more energy in breaking and, theoretically, slow down a projectile more effectively. If it could be incorporated into the Army’s protective clothing, it could provide a better barrier to fragmentation. But as a team of researchers from NRDEC’s Science and Technology Directorate knows, until the availability of spider silk is increased, that kind of implementation is still far down the road.
It is not practical to harvest silk from live spiders. According to researchers, arachnids are too territorial and cannibalistic to raise in captivity the way that silkworms are. So the goal of scientists is to mimic the spider silk protein, to move production of that protein out of the spider and into a bacterial agent that can reproduce it in quantity.
Researchers from the Science and Technology Directorate are now well on the way to realizing that goal. In a major breakthrough, they were able to clone the gene to make spider silk from the spider N. clavipes, popularly known as the golden orb weaver. They also made synthetic genes to mimic the spider dragline silk. Once those genes were isolated, researchers inserted them into the DNA of E. coli bacteria, which were then induced to replicate them. Small quantities of recombinant spider silks were produced in a fermenter without the spider present. Fibers have been spun from these materials, and several patent applications on the technology are in the works.
The production of recombinant spider silk and the spinning of silk fibers is an important accomplishment, but NRDEC researchers say that major challenges remain before the worth of the fibers as a ballistic protective material can be proven. Chief among these is the need to ascertain how the bacteria can be coaxed into producing larger quantities of silk. The spider silk protein is uncommonly difficult to work with, both for humans and for the normally prolific E. coli, and spiders, it seems, aren’t giving away any of their secrets.
According to NRDEC researchers, the very properties that make the spider silk protein so potentially valuable in ballistic protection are the ones the make it exceedingly difficult to process. The protein is hard to manipulate and hard to maintain in solution, they say, because it has a tendency to precipitate and is not soluble in many substances. Most synthetic polymers, including nylon and rayon, which were developed as synthetic analogs to silk, are much easier to manipulate in solution or by melting. In fact, researchers say, the entire mechanism of natural spider silk production is very sophisticated, far more so than any synthetic fiber-making process.
Spider silk has captured the interest of researchers at a host of corporations and universities, including DuPont, Hoechst-Celanese, Cornell, and Oxford. Recently, NRDEC signed a Cooperative Research and Development Agreement (CRADA) with Agricola as part of a joint effort to develop technology to produce recombinant spider silk fibers in quantity. The agreement will provide the basis for a scale-up leading to potential commercial production. CRADAs are designed to stimulate the use of federally funded technology developments. They were made possible by Congress’s enactment of the Federal Technology Transfer Act of 1986. The act makes the development resources of federal laboratories available to private industry and to state and local governments.
In addition to its potential value in helping the Army develop superior ballistic vests and helmets, spider silk could take on a number of other military applications as well, including a potential role as a building block of parachutes and parachute cords. Commercially, spider silk could have a vast market. It could potentially be used as tire cord or cables and in a variety of biomedical settings, including the formulation of sutures and artificial tendons.