Living on a Thread of Silk

Mankind’s Present, A Spider’s Reality   

In the past century, we have experienced revolutions after revolutions in technology that have changed our lives. From the introduction of inventions such as cars, planes, televisions, lasers and the Internet to name a few, we have been forcefully made addicted to change. We are addicted to innovation and new ways of doing the same things. We cannot, it seem, stand still. The types of changes have also changed. From heavy machinery and hard metals that have dominated our lives it seems mankind has taken a new direction namely, in the exploitation and development of biotechnology and biomaterials.

This Essay will outline some of the basic knowledge we know of the miraculous silk produced by spiders. We will look into the physical properties and eventually look at how it is being used to change our lives, ultimately what the future has in store for spider’s silk. Each of these sections can in themselves create detailed lengthy articles, to keep within the confines of decency the author will limit to few examples in each section and we divide the essay as follows.

I.             The Spider and it’s Silk
II.            The Physical properties
III.           Getting Creative with Silk

I. The Spider and It’s Silk

Whitman most aptly captures the spider’s life in his poem “A Noiseless Patient Spider” [4],

I mark’d, where, on a little promontory, it stood, isolated;
Mark’d how, to explore the vacant, vast surrounding,
It launch’d forth filament, filament, filament, out of itself;
Ever unreeling them–ever tirelessly speeding them.

The Spider may be the source of great fear for some; however, it has equally, if not to a greater measure, has become the source of great scientific investigation and curiosity. The nature, in specific the silk that it weaves, is so extraordinary that one cannot get enough of it. Found in a varying species numbering over 40,000 as reported in 2008, and divided into 109 families, spiders are found everywhere on earth with the exception of Antarctica [5]. The evolutionary history of spiders can be traced as far back as 386 million years with modern spiders cropping up in the Triassic period some 200 million years ago [6].

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The Spiders Body can be divided into three parts. As shown in figure 1, there are four pairs of legs, with a fused head and thorax part known as the cephalothorax and finally the abdomen. Attached to the abdomen will generally be one to four (normally three) spinnerets that emit silk. Each spinneret has many spigots, which control the flow of silk forming material. In some spiders we can find a modified form of spinneret known as cribellium that can have up to 40,000 spigots! Each spigot will generally be connected to the six silk producing glands (in some spider we may find up to seven glands), where each gland produces a different type of silk.

The Spider’s silk is spun at close to ambient temperatures and pressures, where water is used as the solvent. Silks of spiders and insect have very much the same building blocks, namely proteins that have been made up of non – essential amino acids [1]. Web spiders are unique as we find that just one individual spider can have a specialized glands to produce different types of silks[2].

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This evolution of diverse silk glands is a direct result of the pivotal role played by silk in the life of a spider [3]. The spider uses its silk for a number of ecological uses, namely, prey capture [6], prey immobilization (silk can be used as “swathing bands” to wrap up the prey [2]), reproduction (spider eggs that are lain can be covered in a silk cocoon [7]), food source (some spiders can eat their own unused silk or that of other others [8]), nest lining, nest construction, guide lines, drop lines, anchor lines, alarm lines and pheromone trails (these are lines of silk left behind by spiders that can be used to guide potential mates [9]). All these various function determine the various properties of the silk that are produced. The type of silk produced by the spider also depends on many other varying factors.

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It would take into account other than ecological factors – the environmental factors, the time and type of day it is [1] before producing the silk required. A detailed tabulation of these and more information is given in the table 1. The table show the glands that produce the specific type of silk, with the attached function and the silk composition.

Having briefly discussed the anatomy of the spider and its uses for silk. We take a look now at how the silk itself is produced. Specifically describing the generation of silk from its major ampullate gland shown in figure 2. The secretary part of the gland labelled (1) in figure 2 shows the section of the gland that produces droplets of protein known as Spindroin I and II, that ultimately form the main component of the spider’s dragline. The next part of the process is known as the storage sac, that stores the gel-like unspun silk as well as contribute to the secretion of proteins that coat the final dragline silk (figure 2- (2)).  After this in section (3) of the figure 2 we see a funnel that narrows in diameter to that of the tapering duct. It is important to note that the unspun silk has the properties of viscoelastic non-newtonian liquid, exhibiting nematic behaviour. As the diameter of the funnel decreases a greater strain is applied and we thus observe strain-thinning, implying a decrease in viscosity of the silk solution resulting in a more resistance-free flow.  As it we reach to the tapered duct (Figure 2-(4)), the end of which is a spigot, we  observe the immergence of a tapered fibre. The final part of the journey is through what is believed to be a helical pump that controls the thickness of the fiber but also helps in clamping the fibre as it is released [6].

The spider’s silk itself is a form of protein fibre that is made of polypeptides that contain dominantly crystalline poly-alanine and crystalline glycine producing regions. There are also non-crystalline producing regions. The polypeptide folding forms structures believed to be consist of beta-sheets, though this conclusion is debated and suggestions different types of protein folding to occur in the dragline silk [1]. Knowing the kind of structure these protein folds become is vital as it helps us copy the silk for mass production. However, in the author’s knowledge, this lack of understanding hasn’t prevented the scientific community in producing spider’s silk via genetically engineering a silkworm to produce the same silk as the spiders silk.

We stop here and leave the discussion of genetic energy for section III, instead we move on to looking at the physical properties of the spiders silk that is produced.

II. Physical Properties

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In determining physical properties of each silk type would require great undertaking, which we unfortunately cannot commit to. A very extensive discussion of all the silk types and the respective references to research undertaken throughout the past few decades is well presented by Humank et al [11]. Of all the silks types shown in table 1, the most understood and research silks are ones produced in the major ampullate (MA) glands, and the flagelliform (FL) gland, the silk fibres of which are known as viscid silk. Gosline et al [12] do a very interesting comparison of the two silk fibre types specifically to their mechanical properties. We recall, that the stress of a material is essentially defined as Force applied per Cross-section Area of the silk fibre. The strain is the deformation defined as the change in the fibre length over its initial length of the fibre. The ratio of the stress-strain gives the stiffness of the material known as Young’s coefficient. The area under the stress-strain curve ultimately defines the energy required to break the material, which is used as variable to define toughness of a material. This property of toughness comes into new light when compared with other biomaterials and engineering materials. Table 2 shows this very comparison. We find that the stiffness and strength of spider’s silk being exceedingly lower in comparison to engineering materials such as Kevlar (used for bullet-proof armour), carbon fibre and High-tensile steel. However, comparing other biomaterials, the strength and the stiffness, to a lesser degree, are markedly higher and thus important indicators of its relative mechanical properties. This comparison may lead us to conclude that MA Silk is better than most biomaterial and as would be expected not desirable as Kevlar or carbon fibre. However, taking a look at the toughness of the materials one finds that MA Silk (with breakage toughness at 160 MJm-3) is 3-10 times tougher than its engineering counterparts! Accompanied with a 20-30% greater extensibility than engineering materials, MA Silk is truly a remarkable material! The viscid silk (stiffness at 0.003 GPa), on the other hand, can be considered to be more rubber like (Stiffness at 0.001-2GPa). It’s extensibility also being exceptionally higher than MA Silk and very much like rubber-like materials, however we find that viscid silk comes into its own, with its strength being nearly 10 times stronger than that of other natural or synthetic rubbers as well as having a relatively higher toughness.

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Denny in 1976 [13] expounded on the mechanical function of these particular silks, looking at how the forces acting on a web would affect the energy dissipation. What he found was that the MA Silks and Viscid silks approximately had 65% hysteresis. That is to say, approximately 65% of the kinetic energy is absorbed by the web formation as heat, and therefore is not available to catapult the incoming prey or particle through elastic recoil! These properties are ultimately more consistent with a viscoelastic material than any engineering material. These bizarre combinations of properties give the spiders silk unique characteristics that can be utilised for a plethora of new applications.  The nature of the MA Silk used in catching prey suggested that a study of its strain-rate dependence might illuminate further characteristics of the silk fibre. Denny’s initial exploration of this demonstrated that as the strain-rate is increased from 0.0005 s-1 to 0.024 s-1 the performance of the MA silk is as enhanced in all its properties, namely, stiffness, strength, extensibility and toughness. Gosline et al furthered this by exploring a strain-rate ranging from 20-50 s-1. The result was the achievement of MA Silk for Araneus, reaching an astronomical toughness of 500-1000MJm-3 and its strength (reaching 2.0 – 4.0 GPa) matching that of Kevlar!

Exploring other properties such as optical properties of the spiders silk, we find that the fibres to be highly reflective and diffractive.  These properties primarily reveal some of the workings of the spider, however as we shall see in the section III, how some of the optical properties can be exploited.

III. Getting Creative with Silk

The one of the major aims of the biotechnological industry is to emulate this spider’s silk and ultimately mass-produce it. Extracting this however is not an easy task. Just to make a cape as shown in figure 4, more than million orb-weaving spiders had to be used. However, aesthetically pleasing the dress may be, it is hard to justify mass production when the whole process of spinning and creating the garment can take up to three years [14].

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The solution has been provided recently with the use of silk worms to produce spider’s silk [15] and reportedly in the modified goat’s milk that contain spider’s silk proteins [16]. Silkworm is already of course been used to create spectacular products in its own right.

Fiorenzo Omenetto, most emphatically, standing before a TED audience, presented the true potential of silk, that his group has been utilising [17]. Outlining some of the key features of silk produced from silkworm (not spiders silk), from such things as being biologically friendly to being integrated into modern microelectronics. Omenetta and co has achieved examples of this, where, in [19], silk proteins were integrated into organic, light emitting transistors producing novel materials that are able to project stored holograms when a laser beam is incident upon them. A demonstration of this can be seen in [17]. In addition to this the materials produced are lightweight, and programmable. For todays technologically savvy these are words considered to be superlatives. Silk in general shows certain particular traits that make it a truly futuristic material. During the self-assembly process of the silk proteins, Omenetto showed how during this process one could incorporate such products as drugs, that once placed in water would dissolve making them great delivery systems.  The cocoon like feature of silk that stores other materials in its make up and once delivered to its desired environment recovers that material by disintegrating. Further to this a timing feature can be programmed into such materials so that they only dissolve after a certain time [17]. This is not withstanding the obvious traits of bio-degeneracy, and “green” products riding worlds “rubbish dilemma”.

The reader may be wondering as why we have diverted to the silk of silkworms. The major breakthroughs in products that are currently being developed specifically for medicinal purposes would be ideal place for the spiders silk to play its part. Spider’s silk are predicted to be able to achieve the same type of innovations as the silk from the silk worm. However, with its increased tensile strength and toughness, the spider’s silk promises to be much more useful in the making artificial ligaments [18]. They can be used for making screws or replacements or healing supports to parts of the body as well as integrating medicine within them, which is released to that part of the body!

In Conclusion, even though spider’s silk has been used for thousands of years by mankind, with mass production and industrialisation of this novel material in today’s time and the current methods available to us, the types of products possible is a tantalising prospect.

References

[1] Vollrath F, Knight DP. Liquid crystalline spinning of spider silk. Nature. 2001; 410 (6828), 541-8

[2] Foelix R, Biology of Spiders, Oxford press Oxford, 1996

[3] Paolo Domenici, In Biomechanics in Animal Behaviour , BIOS Scientific, 2000

[4] Whitman,  A Noiseless Patient Spider”, A Passage to India [Online], <http://www.bartleby.com/142/208.html&gt; [Accessed 27 January 2012]

[5] Platnick, Norman I. “The World Spider Catalog, version 9.5”. American Museum of Natural History. 2009

[6] [Online] <http://en.wikipedia.org/wiki/Spider#cite_note-WSC-1&gt; [Accessed 27 January 2012]

[7]  Nentwig, W. & Heimer, S. Wolfgang Nentwig. ed. Ecological aspects of spider webs. Springer-Verlag. p. 211. 1987

[8] Miyashita, Tadashi; Yasunori Maezono, Aya Shimazaki “Silk feeding as an alternative foraging tactic in a kleptoparasitic spider under seasonally changing environments”. Journal of Zoology 262 (03): 225–229. 2004.

[9] Holm, Erik, Dippenaar-Schoeman, Ansie; Goggo Guide; LAPA publishers 2010

[10] Saravanan D. Spider silk structure, properties and spinning. Journal of Textile and Apparel, Technol & Manag. 5(1), 2006

[11] Humenik M, Scheibel T, Smith A. Spider silk understanding the structure-function relationship of a natural fiber. Elsevier Inc. 131-85. 1st Edition 2011

[12] Gosline JM, Guerette P a, Ortlepp CS, Savage KN. The mechanical design of spider silks: from fibroin sequence to mechanical function. The Journal of experimental biology. 202 (Pt 23) 3295-303. 1999

[13] Denny M. The physical properties of spider’s silk and their role in the design of orb-webs. J. exp. Biol. 65: 483, 1976

[14] [Online] <http://www.dailymail.co.uk/femail/article-2090608/Reviving-lost-tradition-Cape-silk-million-spiders-unveiled-new-exhibition.html&gt; [Accessed 27th January 2012]

[15] Teule F, Miao Y-G, Sohn B-H, et al. Silkworms transformed with chimeric silkworm/spider silk genes spin composite silk fibers with improved mechanical properties. Proceedings of the National Academy of Sciences. 109 (3), 923-928. 2012

[16] [Online] <http://www.physorg.com/news194539934.html&gt; [Accessed 27th January 2012]

[17] [Online] <http://www.ted.com/talks/fiorenzo_omenetto_silk_the_ancient_material_of_the_future.html&gt; [Accessed 27th January 2012]

[18] Kaplan F. From Silk Cocoon to Medical Miracle. Scientific American Magazine. 303, 76-77. 2010.

[19] R. Capelli, J. J. Ansden, G. Generali, S. Toffanin, V. Benfenati, M. Muccini, D. L. Kaplan, F.G. Omenetto* and R. Zamboni*, “Integration of silk protein in organic and light emitting transistors,” Organic Electronics, 12 (7), 1146, 2011

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2 comments

  1. “The case of those who take helpers beside Allah is like unto the case of the spider, who makes for herself a house; and surely the frailest of all houses is the house of the spider, if they but knew!” (Surah al-`Ankabut – The Spider – Chapter 29 : Verse 42)

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