One of the most enduring of all comic book images is the sight of Spiderman flying between skyscrapers suspended by a strand of silk.
Although no manufacturers are currently working on this mode of transport, (even though this would surely be one of the most enjoyable ways of beating rush hour traffic) the fantastic and wide ranging abilities of spider silk portrayed are not too far from the truth.
Spider silk is a protein fiber produced by spiders via a unique gland. The spider silk is an extraordinary biological polymer that is linked to collagen found in human skin and bone; however, it has a much more complex structure.
The interest in spider silk has increased as further research has shown it possesses a unique combination of mechanical strength and elasticity, making it one of the toughest materials known to man.
Although the usefulness of spider silk has been known for a long time, recent breakthroughs and innovations have shed new light on the elastic properties of the silk and may lead to a new generation of 'bio-inspired' materials.
The mechanical properties of spider silk will vary according to the spider species, but most displays some incredible mechanical properties.
The drag-line silk is said to possess excellent mechanical properties with a unique combination of high tensile strength and ductility.
The ductility of some spider silks enables the silk to be stretched up to five times their relaxed length without breaking the strand. Spider silk is also more elastic and more waterproof than silkworm silk.
Quantitatively, it is reported that spider silk is five times stronger and tougher than steel of the same diameter.
It is finer than the human hair, and can maintain its strength below -40°C. Furthermore, the density of the spider silk is less than cotton or nylon.
Materials Science Applications of Spider Silk
Material scientists around the world are keen to use the spider silk for various applications, and to replicate the spider silk using other means to create new ultra-strong fibres. The Arizona State University (ASU) team of chemists have been studying the spider silk’s molecular structure, so that it can be used to produce various new materials ranging from bullet-proof vests to artificial tendons.
Spider silk is still quite a challenge to many scientists as there are not able to exactly pinpoint and harness the properties of this elusive material; however, the mystery of the spider silk is slowly being revealed in parts.
Researchers from the Technische Universitaet Muenchen and the University of Bayreuth have discovered how spiders construct the elastic fibers. With this new information, they hope to construct an artificial spinning apparatus that will imitate the silk glands of a spider.
The researchers believe that with this apparatus they can create new materials that could have countless number of applications in a wide range of fields. Spider silk sutures and drug delivery systems would be a definite possibility.
The artificial silk material could also be used to form durable artificial ligaments for people with injured knees or shoulders.
Further uses for spider silk include clothing and even musical instruments.
Characterizing Spider Silk
Scientists state that spider silk fibers consist of two types of building blocks, namely, soft amorphous and strong crystalline components.
The soft amorphous subunits provide the elasticity of the silk and also aid in the distribution of stress.
For the silk to gain maximum toughness, a specific amount of crystalline subunits are required. It is also dependent on the way the subunits are distributed in the fiber.
Harvesting Silk from a Spider
15 years of research by the Fritz Vollrath's silk group at Oxford resulted in the creation of a technique to reel silk directly from the spider.
The researchers chose the Nephila edulis species of spider for their experiments. The spider was sedated with carbon dioxide gas and pinned down. Then the silk was pulled from the spinnerets. The silk type is mainly ampullate silk, which the spider uses to form the main structure of its web, and minor ampullate silk, which the spider uses to form the main spiral of its web.
The silk thread is pulled, dabbed with some glue, and then a motor is operated to start spinning the thread onto a spool. Using this method, the researchers were able to harvest between 30 and 80 m of silk in every attempt.
The spider is then released back to its web to feed till the next day’s reeling attempt begins.
Silks are basically large proteins formed by repeating sequences of amino acids enclosed by specific side chains that help determine the chemical behaviour of the protein.
The essential factor in creating the properties of the natural spider fiber is in making the correct side chains in synthetic silks.
Artificial manufacturing of the spider silk has been attempted by many scientists. Researchers from at Utah State University working under the guidance of Utah Science Technology and Research initiative (USTAR) professor, Randy Lewis, experimented with transgenic goats, transgenic alfalfa, transgenic silk worms, and E. coli bacteria to produce the spider silk proteins used to create spider silk.
Similarly, Scientist Omenetto from the Tufts University stated that silk can be harvested from transgenic plants just like cotton. Some other researchers have already created transgenic bacteria and fungi that they believe will help increase silk yields.
Sources and Further Reading