One of the greatest challenges of science is to provide solutions to improve human life. Biomimicry (imitating nature) is one of the main tracks of our century. In particular, genetic transmutations, genetics in general, have become a hot question of debate. Should nature be modified, and to what extent, to serve man? Can man allow himself anything with genetics? One of the most unusual but convincing examples is the artificial synthesis of silk.
I. Silk, a genius at the service of Man
A. Synthesis of silk by Man
Since ancient times, man has sought to take advantage of the properties of silk. However, since spider thread is thin and produced in small quantities, the recovery of silk is difficult and laborious, especially since spiders do not lend themselves to intensive breeding: forcing them to live in close proximity to each other would lead them to eat each other. Alternative solutions have therefore been put in place.
1. The first transgenesis
It was not until 1990 that biologist Randy Lewis, from the University of Wyoming, thanks to funding from the US Army, identified and cloned the two genes that produced spider silk. He thus began research into the mass production of fibroin, the protein that makes up spider silk threads.
The first experiments in transgenesis, i.e. the introduction of the gene that allows the production of silk into the genetic heritage of another animal, were carried out on butterflies. Researchers introduced the genes that produce spider silk into these insects using a virus.
A system of host bacteria was also developed, which allowed easy manipulation of the genes. By modifying the genes that make silk possible and implanting them in the host bacteria, the scientists were able to reconstitute fibroin. They obtained threads with specific characteristics by reassembling, thanks to a cloning system, the portions of genes.
In the same way, tests were carried out on hamsters, or even on cow udder cells.
But these experiments did not give any conclusive results; the quantities collected were too small and the quality disappointing. The high resistance of spider silk is due to an intensive repetition of the silk genes; however, in the exam questions studied, the amino acid sequence was broken, causing a decrease in fiber resistance.
2. The true success of transgenesis
However, Nexia, a Montréal-based biotechnology company, obtained Lewis’ patents in the late 1990s. In partnership with the U.S. Army Soldier Biological Chemical Command, it introduced the gene coding for the spider silk protein into two goats.
Jeffrey Turner, leader of the project, had noticed great similarities between the spider cell that synthesizes silk and the goat cell that produces milk. The project, called BioSteel, enabled the offspring of both animals, born with the appropriate gene, to produce proteins in their mammary glands similar to those in spider silk in 2002. “It is not exactly spider webs anymore, but it has all the properties of a spider web,” explained Anthoula Lazarus in the journal Science.
The proteins collected from the goats’ milk are then put into the solution. By applying pressure to the tubes containing the solution, they come together to form a fiber, which is pulled together to stabilize the structure.
One liter provides about 15 grams of protein, while the rest of the milk remains consumable. But although Nexia now has its own herd spread over three farms, two in Quebec and one in the United States, in Plattsburg, where the transgenic goats have been introduced along with a thousand other normal animals, the yield is still too low to consider immediate industrial production.
3. International marketing thanks to the silkworm
However, more recently, American researchers have made a considerable breakthrough. In September 2010, the University of Notre Dame, in association with the University of Wyoming and Kraig Biocraft Laboratories, succeeded in adding two spider genes to the genomes of silkworms, so that the latter can produce arachnid silk.
Malcolm Fraser, one of the main players in this project, in collaboration with Randy Lewis among others, is optimistic: “We could even genetically improve the fibers, which would surpass the remarkable properties of spider silk.”
With silkworm secretions already being commercially exploited, this breakthrough represents the first viable and advantageous solution for mass industrial production, and therefore a true revolution in the field.
B. Exploitation of silk
The qualities of silk are diverse, and therefore exploitable in many fields.
1. Silk in the industrial sector
First of all, in the industrial sector, silk would be used for the elaboration of tires, but also for the chassis of Formula 1 cars, which deform during sudden braking, or because of the shocks caused by the ground. The silk would act as a shock absorber to prevent this torsion from becoming dangerous.
In addition, spider silk would considerably increase the performance of tennis rackets by improving the sieves, i.e. the strung part of the rackets, which would then be made of silk.
Thanks to its flexibility and resistance, silk would be an advantageous substitute for nylon and Kevlar in the production of cables, ropes and fishing line. Researchers are even talking about using silk in the aerospace industry. Mr. Turner, president of Nexia, estimates the market at $500 million per year, or about 370 million euros.
Finally, the textile industry would benefit. For example, Gunma University in central Japan has succeeded in covering silk threads with a metallic film using a plasma metalization system. This innovation would protect firefighters even more effectively against the flames.
2. Silk in the military sector
The military, which has financed a large part of the research on silk, is also very interested in it. Helmets and parachutes would benefit from such a fiber, which would significantly improve their performance.
Nexia, the biotech company mentioned above, is working in partnership with the Pentagon in Washington and the Canadian Department of Defense. Although the details of the agreement remain protected, Dr. Turner mentions in particular the realization of bulletproof vests thanks to the spider silk. Genetically modified goats from the Nexia company would offer a silk capable of stopping a 22-caliber bullet, a result that should be improved in the coming months: “We are at 70% of our objective,” explains Jeffrey Turner. The major advantage of silk is its flexibility and lightness, which is beneficial to soldiers handicapped by the weight of current bulletproof vests.
3. Silk in the medical sector
Finally, silk has important medicinal properties. The Greeks were the first to exploit them. The fiber, which has the advantage of being biodegradable, was used as a suture to heal wounds.
Nowadays, medicine does not require large quantities of silk, which is why the use of silk is developing more than elsewhere – Nexia devotes 20% of its research to it. The University of Ghana has revealed that the properties of silk are conducive to organ regeneration and the reconstitution of biological tissues if the latter have a structural model to follow; without this model, the tissues would grow uncontrollably.
This silk would allow the fabrication of surgical nets, but also the realization of arterial grafts. Finally, it could be used as artificial ligaments or even tendons.
In addition, an ongoing experiment tends to prove the compatibility between fibroids and human cells: Schwann cells, essential cells for the repair of certain nerve fibers. Eventually, silk would help heal nerve damage.
A brilliant product unknown to the general public, spider silk is nonetheless an element that could prove beneficial in many fields. However, the synthesis of this fiber is a real challenge that will require, in the long term, a significant financial and human investment.
II. The manufacture of silk
A. The production of silk
The silk of the spider is the result of a complex process, which takes place in three stages: the manufacture of proteins in liquid form, the solidification of this liquid, and the interweaving of the resulting fibrils.
1. In the abdomen of the spider, the manufacture of proteins
The manufacture of the proteins that form the basis of the final silk produced by the spider takes place in the abdomen. Glands called terrigenous glands produce proteins, or chains of amino acids. Erogenous glands are like pouches that make and store silk. Each gland is divided into two areas: one that makes proteins for the silk bodies, and one that makes proteins for the silk shell. There are nine sericogenic glands, each of which allows the formation of a particular type of silk (to support itself, to swaddle prey, to build the cocoon). The spider decides which type of silk it wants to make according to its needs.
At this stage, the silk is liquid. Among the proteins that constitute it, there are two in large quantities: fibroin, a filamentous protein (about 63.5% of silk) and sericin, which acts as a glue (about 22.5%). Silk is also made up of fatty substances and water. Fibroin is actually a large protein made up of smaller proteins: the proteids. Among these, keratin, which gives silk its strength and elasticity.
2. The solidification of silk
Once the liquid silk is produced by the glands, it passes through the spinnerets, located at the end of the abdomen, in the extension of the glands. The spider has three or four spinnerets in general. They are mobile and articulated excrescences, made up of spindles. They allow the regulation of the flow of the glands and the solidification of the silk.
When the silk passes through a spinneret, it solidifies under the action of ions (hydrogen, sodium and potassium) injected into the silk. These ions facilitate the separation of the water contained in the silk from the proteins (see diagram opposite). Potassium, hydrogen and sodium ions are hydrophilic and attract water molecules. This then facilitates the separation of the liquid proteins from the water and leads to the solidification of the silk.
3. The manufacture of silk thread
After the solidification of the proteins by the spinnerets, the thread is directed to the end of the spinnerets. At this point, there are small tubes that make up the spinnerets: the fissures. These spindles are located at the end of the spider’s abdomen. They allow the complete solidification of the silk and the interweaving of the fibrils. When the silk thread passes through a fistula, the water is then extracted in its entirety, by a pumping system, which allows the silk to solidify permanently.
The threads coming out of the spindle are in fact fibrils, whose diameter does not exceed 0.05 µm, against 15 µm to 70 µm of diameter for a normal silk thread. In order to make the thread even more resistant, the fibrils are intertwined at the exit of the fissures. The speed of secretion of the silk fibrils varies according to the nature of the silk, but generally it is of the order of one millimeter per second.
B. Properties of spider silk
Spider silk is known for its amazing properties in terms of elasticity, strength and lightness. Indeed, with an equal diameter, it is as resistant as Kevlar, a material used in the manufacture of bulletproof vests. It can also stretch from 5% to 35% of its initial length, which shows its elasticity. In addition, the energy required to break it is 120,000 J/kg, compared to 30,000 J/kg for Kevlar and only 2,000 J/kg for steel. These properties are explained by the structure of fibroin, the main constituent of silk.
Fibroin is a filamentous protein composed of units arranged in the form of blocks. It is called a block copolymer. Among these blocks, some are hydrophobic (which repel water), composed of primary structural units repeated several times. The hydrophilic blocks (which attract water) are composed of more diverse and more complex amino acids. The hydrophobic blocks are rich in glycine and alanine amino acids (alanine*n or Glycine-alanine*n motifs). The hydrophilic blocks are rich in glycine (Glycine-Glycine-X*n or Glycine-Proline-Glycine-X*n motifs).
Fibroin is a protein made up of smaller proteins or provides, including keratin. Keratin serves as a structural element in many living things. It is notably present in hair, skin and body hair. A protein is a chain of amino acids. These amino acids are linked together by hydrogen bonds. In the primary structure (the amino acid sequence) of fibroin, glycine and alanine form two regions: one rich in alanine and one rich in glycine. When moving to the secondary structure (the folding of the amino acid sequence of the protein), the two regions acquire different structures.
The alanine-rich region folds into folded sheets or beta sheets. By folding, the protein is densified. The beta sheets then polymerize by hydrogen bonds and become beta crystallites. The beta crystallites give the silk its strong and resistant properties.
The glycine-rich zone folds into helix-like spirals, which give the silk its elasticity. They are similar to springs. No beta crystallites are formed, the structure is non-crystalline.
The silk of the spider thus combines strong, solid, elastic and light properties, thanks to the three-dimensional structure of its proteins.
Thus, it is important to ask what the spider uses the silk it produces for, and thus understand how it is useful to it.
III. The uses of silk by the spider
A. Silk, an essential means of locomotion for the spider
The silk thread constitutes, for the spider, a true all-purpose object. First of all, it allows the spider to move through the air, both vertically and laterally. For example, it uses it as a safety line, also called “trailing line”: the arachnid fixes this safety line at the various places where it passes. Thus, in the event of an unexpected fall or a sudden shock, the silk, which is gradually pulled out by the weight of the spider, constitutes a link between the initial support and the spider. The spider will then only have to cling to it with one of its hind legs, and wait to be carried by the wind.
This thread, contrary to what its name might suggest, is not always used for safety: the spider sometimes uses it as a simple means to cross a watercourse or an obstacle.
The silk is also used as the basis for a thread called “virgin’s thread” to disperse young spiders. These spiders are placed on a support located as high as possible, during the spring or summer days. After being pulled down, these young arachnids put both their lightness and that of their thread (which acts as a parachute) to good use; carried by the upward force of the warm air, they travel over distances that can reach 400 kilometers.
B. Silk, a defense and territory delimitation element in the spider
When a spider hunts, it has the ability, after injecting its venom into its prey, to swaddle it in a special gauze-like silk, like a mummy. These prey is not necessarily simple insects: the Nephila clavipes can occasionally catch a small bird. In most cases, it takes it directly to its retreat, to preserve it for later consumption.
The arachnid also uses its precious thread to make the cocoon protecting the eggs. Thus, she lays her eggs directly in the cocoon, and introduces air bubbles between the threads to protect the future young from the cold. She then hides her offspring and dies before the birth of her offspring; indeed, a spider lives on average only one year.
She can also use her genius to build a shelter: some spiders are able to dig a burrow, which they line the walls with silk and close the entrance with a lid. In this way, they are sure not to be disturbed in their daily life.
C. Silk, the basic element of the spider’s web
First of all, it is important to know that not all spiders spin webs. The proportion of spiders known as “weavers” is about 30%. However, if it is not vital, this genius is a considerable advantage when it is put to use. Here we will study spiders with so-called geometrical webs, knowing that most spiders weave shapeless or horn-shaped webs. Thus, for this type of arachnid, the manufacture of this web is daily, and represents a considerable labor: it requires an effort of nearly an hour and a half, and up to thirty meters of silk. It is generally made by the female, the male ceasing to weave at the adult age.
The geometric or orbicular web is considered a real jewel, both technical in its realization, but also resistant although flexible, and almost invisible. Note that this last property is relative, knowing that an old web will be covered with dust, and will therefore become much more visible to potential prey. Moreover, the arachnid must adapt to the quantity of silk and the space it has, but also to the available attachment points.
1. The construction of the frame and the rays
The construction of the web starts with the construction of a silk thread between two fixed points. The need to establish a link between a point A and B is overcome in a very simple way: the spider, a great worker, is content here to wait for the thread, previously secreted by its spinnerets, to be carried by the wind. The lightness of the thread is then essential. Also, it can reach up to five meters long, and allows crossing obstacles such as paths or rivers.
When the thread reaches the point B on which the spider has set its sights, it clings to the smallest available branch or blade of grass. A bridge is then created, then quickly consolidated by a second layer of silk. The spider begins the rapid realization of the frame, by sticking its threads to the supports which are presented to it.
It then becomes necessary to make the rays of the web, which will be attached to the center, for resistance, thanks to a first circle of silk strands. The number of rays varies according to the species, but generally does not exceed fifty. The spider is, at this stage, in the first phase of its work. For this part, it takes advantage of one of the many possibilities offered by its silk, and uses a strong, thick and not very sticky silk.
2. The elaboration of a sticky and solid spiral
The next step is a crucial phase in the development of the fabric. The hub is woven, an area where extremely tight threads are laid out in a spiral from the first circle of silk previously built in the center of the future web. The spider then uses a thread that is not very sticky, because it is in this area that the spider will wait for its prey, once the web is finished.
It then remains to build the framework of the web, i.e. the essential part of the work. The spider then uses its legs as a compass, and builds a logarithmic spiral starting from the hub, as shown below.
Once it reaches the ends of the frame, the spider has produced a very loose, but already strong web, on which it can move without any problem.
However, this web is only a scaffold for the final work. Indeed, the spokes of the spiral are too far apart, too visible, and not sticky enough to pretend to be a real trap.
Starting from the outside of the frame, the arachnid will progressively destroy its first spiral to replace it by another one, much tighter and made of a double thread, less thick and solid than the previous one, but much more sticky.
The construction of the web is now complete. Note that there is an empty space between the hub and the rest of the web; this space simply allows the spider to move from one side of its web to the other.
3. The uses of the web
It is now appropriate to present the uses of the web, the main one being hunting. Its elasticity allows it to stretch and reach a surface up to 40% larger than its surface at rest. In addition, the sericin used to make the web acts as a glue. Under these conditions, the spider is able to catch numerous prey around it, without the slightest worry of seeing its work damaged by the speed of the latter.
The spider positions itself at the hub of its web and, as soon as an insect is trapped, it is alerted by vibrations and pounces on what will be its future meal. It is important to specify that the spider coats its legs with a particular oil, which prevents it from getting caught in its own trap. However, the task is not as easy as it seems, and it is a matter of being quick and efficient in time: it is estimated that about eight out of ten predators are able to get rid of the web. Even so, the web makes the spider, on its own scale, one of nature’s best-armed predators. The web can also be used as a home, or simply as a passage between its different ends.
The spider is a great recycler: it will eat part of its web in order to recover, in the form of proteins, the maximum energy invested in its production.
The spider’s silk is therefore essential for many tasks carried out daily by the spider. It constitutes a real treasure, which Man hopes to take advantage of.
Spider silk is a unique material in terms of elasticity, strength and lightness. The arachnid, over the generations, has known how to take advantage of this fiber through multiple uses, to the point of making it an essential element for its survival.
It is therefore not likely to consider that silk is as useful to man as it is to the spider.
Nevertheless, the research undertaken by companies such as Nexia for several years tends to bring silk to the fore in a considerable way. This research could meet many expectations in the medical, industrial and military fields. It remains to be seen whether mankind is really able to find long-term solutions to reproduce this very special silk in a strictly identical way.
Bacteria: living prokaryotic micro-organisms, i.e. unicellular and without a nucleus.
Frame: all the strands that make up the outline of the fabric.
Fibroin: transparent protein substance of gelatinous consistency, partially constituting silk.
Fiber: elementary formation, generally in the form of bundles. It is vegetable or animal and of filamentous aspect.
Safety thread: silk thread used by the spider to ensure its safety in case of a fall, and occasionally to cross an obstacle.
Virgin’s thread: silk thread used by young spiders in the form of a parachute to ensure long displacements.
Future: cone-shaped opening located at the end of the spinnerets by which the silk fibrils emerge.
Erogenous glands: In the abdomen of the spider, glands of secretion of silk.
Kevlar: resin is used in the form of fiber in certain composite materials (bulletproof vests).
Protein: biological macromolecules composed of one or more chains of amino acids, linked together by peptide bonds.
Secretion: elaboration of substances by an organism.
Geometric or orbicular web: web composed of a frame, spokes and a logarithmic spiral made of extremely sticky silk.
Transgenesis: the introduction of a foreign gene, called a transgene, into the genetic makeup of an organism.