Spider silk

Uses

All spiders produce silks, and a single spider can produce up to seven different types of silk for different uses. This is in contrast to insect silks, where an individual usually only produces one type of silk. Spider silks may be used in many different ecological ways, each with properties to match the silk's function (see Properties section). As spiders have evolved, so has their silks' complexity and diverse uses, for example from primitive tube webs 300–400 million years ago to complex orb webs 110 million years ago.

Types

Meeting the specification for all these ecological uses requires different types of silk suited to different broad properties, as either a fiber, a structure of fibers, or a silk-globule. These types include glues and fibers. Some types of fibers are used for structural support, others for constructing protective structures. Some can absorb energy effectively, whereas others transmit vibration efficiently. In a spider, these silk types are produced in different glands; so the silk from a particular gland can be linked to its use by the spider. See the later section for details on the mechanical properties of silk and how the structure of silk can achieve these different properties.

Mechanical properties

Each spider and each type of silk has a set of mechanical properties optimised for their biological function.

Most silks, in particular dragline silk, have exceptional mechanical properties. They exhibit a unique combination of high tensile strength and extensibility (ductility). This enables a silk fibre to absorb a lot of energy before breaking (toughness, the area under a stress-strain curve).

A frequent mistake made in the mainstream media is to confuse strength and toughness, when comparing silk to other materials. As shown below in detail, weight for weight, silk is stronger than steel, but not as strong as Kevlar. Silk is, however, tougher than either.

It is important to note that the variability of mechanical properties of spider silk fibers may be important and it is related to their degree of molecular alignment. Besides, mechanical properties depend strongly on the ambient conditions, i.e. humidity and temperature.

Strength

In detail a dragline silk's tensile strength is comparable to that of high-grade alloy steel (450 - 2000 MPa), and about half as strong as aramid filaments, such as Twaron or Kevlar (3000 MPa).

Density

Consisting of mainly protein, silks are about a sixth of the density of steel (1.3 g/cm³). As a result, a strand long enough to circle the Earth would weigh less than 500 grams (18 oz). (Spider dragline silk has a tensile strength of roughly 1.3 GPa. The tensile strength listed for steel might be slightly higher—e.g. 1.65 GPa, but spider silk is a much less dense material, so that a given weight of spider silk is five times as strong as the same weight of steel.)

Energy density

The energy density of dragline spider silk is roughly 1.2x10⁸J/m³.

Extensibility

Silks are also extremely ductile, with some able to stretch up to five times their relaxed length without breaking.

Toughness

The combination of strength and ductility gives dragline silks a very high toughness (or work to fracture), which "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fibre technology".

Temperature

While unlikely to be relevant in nature, dragline silks can hold their strength below −40 °C (-40 °F) and up to 220 °C (428 °F). As occurs in many materials, spider silk fibers undergo a glass transition. The glass-transition temperature depends on the humidity, as water is a plasticizer for the silk.

Supercontraction

When exposed to water, dragline silks undergo supercontraction, shrinking up to 50% in length and behaving like a weak rubber under tension. Many hypotheses have been suggested as to its use in nature, with the most popular being to automatically tension webs built in the night using the morning dew.

Highest-performance

The toughest known spider silk is produced by the species Darwin's bark spider (Caerostris darwini): "The toughness of forcibly silked fibers averages 350 MJ/m³, with some samples reaching 520 MJ/m³. Thus, C. darwini silk is more than twice as tough as any previously described silk, and over 10 times tougher than Kevlar".

Adhesive properties

Silk fiber is a two-compound pyriform secretion, spun into patterns (called "attachment discs") that are employed to adhere silk threads to various surfaces using a minimum of silk substrate. The pyriform threads polymerize under ambient conditions, become functional immediately, and are usable indefinitely, remaining biodegradable, versatile and compatible with numerous other materials in the environment. The adhesive and durability properties of the attachment disc are controlled by functions within the spinnerets. Some adhesive properties of the silk resemble glue, consisting of microfibrils and lipid enclosures.

Types of silk

Many species of spider have different glands to produce silk with different properties for different purposes, including housing, web construction, defense, capturing and detaining prey, egg protection, and mobility (gossamer for ballooning, or for a strand allowing the spider to drop down as silk is extruded). Different specialized silks have evolved with properties suitable for different uses. For example, Argiope argentata has five different types of silk, each used for a different purpose:

Macroscopic structure down to protein hierarchy

Silks, as well as many other biomaterials, have a hierarchical structure (e.g., cellulose, hair). The primary structure is its amino acid sequence, mainly consisting of highly repetitive glycine and alanine blocks, which is why silks are often referred to as a block co-polymer. On a secondary structure level, the short side chained alanine is mainly found in the crystalline domains (beta sheets) of the nanofibril, glycine is mostly found in the so-called amorphous matrix consisting of helical and beta turn structures. It is the interplay between the hard crystalline segments, and the strained elastic semi-amorphous regions, that gives spider silk its extraordinary properties. Various compounds other than protein are used to enhance the fiber's properties. Pyrrolidine has hygroscopic properties which keeps the silk moist while also warding off ant invasion. It occurs in especially high concentration in glue threads. Potassium hydrogen phosphate releases protons in aqueous solution, resulting in a pH of about 4, making the silk acidic and thus protecting it from fungi and bacteria that would otherwise digest the protein. Potassium nitrate is believed to prevent the protein from denaturing in the acidic milieu.

This first very basic model of silk was introduced by Termonia in 1994 who suggested crystallites embedded in an amorphous matrix interlinked with hydrogen bonds. This model has refined over the years: Semi-crystalline regions were found as well as a fibrillar skin core model suggested for spider silk, later visualized by AFM and TEM. Sizes of the nanofibrillar structure and the crystalline and semi-crystalline regions were revealed by neutron scattering.

It has been possible to relate microstructural information and macroscopic mechanical properties of the fibers. The results show that ordered regions (i) mainly reorient by deformation for low-stretched fibers and (ii) the fraction of ordered regions increase progressively for higher stretching of the fibers.

Non-protein composition

Various compounds other than protein are found in spider silks, such as sugars, lipids, ions, and pigments that might affect the aggregation behaviour and act as a protection layer in the final fiber.

Biosynthesis and fiber spinning

The production of silks, including spider silk, differs in an important respect from the production of most other fibrous biological materials: rather than being continuously grown as keratin in hair, cellulose in the cell walls of plants, or even the fibers formed from the compacted faecal matter of beetles, it is "spun" on demand from liquid silk precursor out of specialized glands.

The spinning process occurs when a fiber is pulled away from the body of a spider, be that by the spider’s legs, by the spider's falling under its own weight, or by any other method including being pulled by humans. The name "spinning" is misleading because no rotation of any component occurs, but rather the name comes from analogy to the textile spinning wheels. Silk production is a pultrusion, similar to extrusion, with the subtlety that the force is induced by pulling at the finished fiber rather than being squeezed out of a reservoir. The unspun silk fiber is pulled through silk glands of which there may be both numerous duplicates and different types of gland on any one spider species.

Silk gland

The gland's visible, or external, part is termed the spinneret. Depending on the complexity of the species, spiders will have two to eight spinnerets, usually in pairs. There exist highly different specialised glands in different spiders, ranging from simply a sac with an opening at one end, to the complex, multiple-section major ampullate glands of the golden silk orb-weavers.

Behind each spinneret visible on the surface of the spider lies a gland, a generalised form of which is shown in the figure to the right, "Schematic of a generalised gland".

Gland characteristics

1. The first section of the gland labelled 1 on Figure 1 is the secretory or tail section of the gland. The walls of this section are lined with cells that secrete proteins Spidroin I and Spidroin II, the main components of this spider’s dragline. These proteins are found in the form of droplets that gradually elongate to form long channels along the length of the final fiber, hypothesized to assist in preventing crack formation or even self-healing of the fiber.
2. The second section is the storage sac. This stores and maintains the gel-like unspun silk dope until it is required by the spider. In addition to storing the unspun silk gel, it secretes proteins that coat the surface of the final fiber.
3. The funnel rapidly reduces the large diameter of the storage sac to the small diameter of the tapering duct.
4. The final length is the tapering duct, the site of most of the fiber formation. This consists of a tapering tube with several tight about turns, a valve almost at the end (mentioned in detail at point No. 5 below) ending in a spigot from which the solid silk fiber emerges. The tube here tapers hyperbolically, therefore the unspun silk is under constant elongational shear stress, which is an important factor in fiber formation. This section of the duct is lined with cells that exchange ions, reduce the dope pH from neutral to acidic, and remove water from the fiber. Collectively, the shear stress and the ion and pH changes induce the liquid silk dope to phase transition and condense into a solid protein fiber with high molecular organization. The spigot at the end has lips that clamp around the fiber, controlling fiber diameter and further retaining water.
5. Almost at the end of the tapering duct is a valve, approximate position marked "5" on figure 1. Though discovered some time ago, the precise purpose of this valve is still under discussion. It is believed to assist in restarting and rejoining broken fibers, acting much in the way of a helical pump, regulating the thickness of the fiber, and/or clamping the fiber as a spider falls upon it. There is some discussion on the similarity of the silk worm’s silk press and the roles each of these valves play in the production of silk in these two organisms.

Throughout the process the unspun silk appears to have a nematic texture, in a similar manner to a liquid crystal, arising in part due to the extremely high protein concentration of silk dope (around 30% in terms of weight per volume). This allows the unspun silk to flow through the duct as a liquid but maintain a molecular order.

As an example of a complex spinning field, the spinneret apparatus of an adult Araneus diadematus (garden cross spider) consists of the glands shown below. Similar multiple gland architecture exists in the black widow spider.

Artificial synthesis

In order to artificially synthesize spider silk into fibers, there are two broad areas that must be covered. These are synthesis of the feedstock (the unspun silk dope in spiders), and synthesis of the spinning conditions (the funnel, valve, tapering duct, and spigot). There have been a number of different approaches discussed below.

Feedstock

As discussed in the Structural section of the article, the molecular structure of unspun silk is both complex and extremely long. Though this endows the silk fibers with their desirable properties, it also makes replication of the fiber somewhat of a challenge. Various organisms have been used as a basis for attempts to replicate some components or all of some or all of the proteins involved. These proteins must then be extracted, purified and then spun before their properties can be tested. The table below shows the results including the true gold standard - actual stress and strain of the fibers as compared to the best spider dragline.

Geometry

As was shown in the biosynthesis section, spider silks with comparatively simple molecular structure need complex ducts to be able to spin an effective fiber. There have been a number of methods used to produce fibers, of which the main types are briefly discussed below.

Syringe and needle

Feedstock is simply forced through a hollow needle using a syringe. This method has been shown to make fibers successfully on multiple occasions.

Although very cheap and easy to assemble, the shape and conditions of the gland are very loosely approximated. Fibers created using this method may need encouragement to change from liquid to solid by removing the water from the fiber with such chemicals as the environmentally undesirable methanol or acetone, and also may require post-stretching of the fiber to attain fibers with desirable properties.

Microfluidics

As the field of microfluidics matures, it is likely that more attempts to spin fibers will be made using microfluidics. These have the advantage of being very controllable and able to test spin very small volumes of unspun fiber but setup and development costs are likely to be high. A patent has been granted in this area for spinning fibers in a method mimicking the process found in nature, and fibers are successfully being continuously spun by a commercial company.

Electrospinning

Electrospinning is a very old technique whereby a fluid is held in a container in a manner such that it is able to flow out through capillary action. A conducting substrate is positioned below, and a large difference in electrical potential is applied between the fluid and the substrate. The fluid is attracted to the substrate, and tiny fibers jump almost instantly from their point of emission, the Taylor cone, to the substrate, drying as they travel. This method has been shown to create nano-scale fibers from both silk dissected from organisms and regenerated silk fibroin.

Other artificial shapes formed from silk

Silk can be formed into other shapes and sizes such as spherical capsules for drug delivery, cell scaffolds and wound healing, textiles, cosmetics, coatings, and many others.

Research milestones

Due to spider silk being a scientific research field with a long and rich history, there can be unfortunate occurrences of researchers independently rediscovering previously published findings. What follows is a table of the discoveries made in each of the constituent areas, acknowledged by the scientific community as being relevant and significant by using the metric of scientific acceptance, citations. Thus, only papers with 50 or more citations are included.

Human uses

Peasants in the southern Carpathian Mountains used to cut up tubes built by Atypus and cover wounds with the inner lining. It reportedly facilitated healing, and even connected with the skin. This is believed to be due to antiseptic properties of spider silk and because the silk is rich in vitamin K, which can be effective in clotting blood.

Some fishermen in the Indo-Pacific ocean use the web of Nephila to catch small fish.

The silk of Nephila clavipes has recently been used to help in mammalian neuronal regeneration.

At one time, it was common to use spider silk as a thread for crosshairs in optical instruments such as telescopes, microscopes, and telescopic rifle sights.

Due to the difficulties in extracting and processing substantial amounts of spider silk, the largest known piece of cloth made of spider silk is an 11-by-4-foot (3.4 by 1.2 m) textile with a golden tint made in Madagascar in 2009. Eighty-two people worked for four years to collect over one million golden orb spiders and extract silk from them.

In 2011, spider silk fibers were used in the field of optics to generate very fine diffraction patterns over N-slit interferometric signals utilized in optical communications. In 2012, spider silk fibers were used to create a set of violin strings.

Spider silk is used to suspend inertial confinement fusion targets during laser ignition, as it remains considerably elastic and has a high energy to break at temperatures as low as 10-20K. In addition, it is made from "light" atomic number elements that won't emit x-rays during irradiation that could preheat the target so that the pressure differential required for fusion is not achieved.

Attempts at producing synthetic spider silk

Replicating the complex conditions required to produce fibers that are comparable to spider silk has proven difficult to accomplish in a laboratory environment. What follows is a miscellaneous list of attempts on this problem. However, in the absence of hard data accepted by the relevant scientific community, it is difficult to judge whether these attempts have been successful or constructive.