Spinning and Applications of Spider Silk

Spider silk is specially produced from the different species (sp.) of spider throughout the globe, specially the most fine and mechanically sound silk produced from nephila sp. The fibriller parallel model of spider silk gives higher mechanical properties with better Young’s modulus and lower stress than normal silk coming from the silk-moth. In this review work we dicuss about the collection, composition, spinning and molecular structure, mechanical properties with its application of the spider silk.


Introduction
Spider silk refers to a wide range o f continuous filaments spun by the several species of lepidoptera & arthropoda. Spiders have six or seven sets of glands spun different fibers. There is d ifferent type of spider species; but the most common species of the spider which p roduced the spider dragline silk is Araneus diadematus [1,2].
There are various types of spider species available in the nature and their habitat, location, life cycle are also totally different. The web produced by them is different in color, strength, composition according to their lifestyle and food [3].
The molecular co mposition in different silk can be as shown in table 1 and table 2. The glysine, alanine is the major amino acids presents in the spider silk. Due to the heavy benzene aromatic ring presents in these two amino acids the spider gives the higher mechanical strength than the normal silk obtained fro m the silk-moth cocoons [3,4]. The other important components are like side chains, polar heads which makes it strongly hydrophobic in nature. Generally the complex proteins presents in the spider silk is called as 'spidroins'. The average diameter of major ampullate dragline silk spidroin ≈ 2.53 + 0.4 µm. the virgin spider silk is infused within by mucopolysaccharide (highly v iscous in nature, figure 3c) on the surface of the silk fibers so it is removed by toluene treatment for further spinning steps [13,14]. The SEM micrograph shows that toluene treated dragline silk gives a higher smooth and lustrous surface than the normal fiber with an abundant gummy part with a totally fu zzy surface (figure 3 a, b).

Spider Orb Web
The spider web where the spider excretes their saliva in the form o f a web by their ampullatory gland is consisting some of special feature over its construction as follows [17]; [a] Several spokes laid outward fro m a co mmon origin.
[b] ragline, minor ampu llate, and v iscid silks form the major port ion of the orb web.
[c] Drag line silk, in the form of mooring threads, framework and radial thread web do minate the web structure [18][19][20].

To Buil d up Their Web They Follow Pattern
The spider generally builds their web in some stages as illustrated in figure 6. In itially the spider used to make a beam bridge between two supports and after then with the time they spin their web with the help of the excretion fro m their ampullatory gland [24].

Collection of s pi der silk
[a] Naturally spun fibers are either retrieved fro m the spider web or fro m the safety line [21].
[b] Forcib ly silk fibers are obtained by pulling the silk fibre fro m the spider at controlled speed.

Spinning of S pi der Silk
[a] Occur at very low temperature.
[b] Used water as a solvent.
S[e] torage of spinning dope in Lu men of MA Gland.
[f] MA spidroins are stored in the lu men of the gland in the presence of sodium and chloride ions, and during fiber spinning these ions are exchanged for potassium and phosphate ions [23][24].

Fi ber for Mati on During Spi nning
[a] Extension of molecules under stress enables them jo in with hydrogen bonds.
[b] Give anti parallel beta conformation in final thread [26].
[c] As silk protein mo lecules aggregate & crystallize, hydrophilic groups are hidden into anterior.
[e] Facilitate loss of water fro m the surface of the extruded thread [25].
[g] The effect of salt on protein aggregation (figure 9).
[h] Low concentrations of salt imp rove the solubility of proteins (known as 'salt ing in'), due to the format ion of ion-rich hydration layers in the vicinity of charged and polar amino acid residues [27].
[i] High concentrations of salt, causing the protein to precipitate (known as 'salting out').
[j] The effect o f shear on protein aggregation: protein aggregation is increased in presence of shear, in both lo w and high concentrations of salt as shown in figure 10 [25].

The Effect of Shear on Protein Aggregation
[a] Aggregates formed after longer periods of exposure to shear were fibrous with dimensions of micro meter to mm scale.
[b] At first we will not able to follow any stress-strain properties at very load, the response of the curve will show when the spider silk is being stretched t a quietly h igher force.
[c] Rat io of stress to strain provides the Young Modulus which is actually derived fro m slope of curve as shown in the figure 11 (a).
[d] Young Modulus is required to measure the fiber stiffness.
[e] Curve suddenly changes slope because of major structural transition in material [28].
Stress Strain curve vary between same type of silk fro m different spiders like the Silk of Euprosthenopus stiffer require more force to break, less elastic, take up less energy than nephila sp. Silk as shown in figure 11 (b) [6].

Spinning Environment Control Material Properties of Fi nal Silk Thread
[a] Variability is due to differences in both the chemical composition of the dope & condition under which it spun.
[b] Silk reeling speed & body temperature affect the diameter of silk thread & which influences force needed to break thread Action of solvents, Supercontraction as depicted in figure 12.
[c]Spider silk shrin k to half its orig inal length & doubles in diameter (when in contact with water) called super contraction [22].
[d]It involves a reversible uptake of solvent by silk.
[f]Increase fiber extensibility but reduced stiffness.

Supercontraction
[a] Consequence of hydrogen bond destruction by water mo lecules, result in mo lecular chain motion & disorientation.
[b] Larger hydrogen bond destroyed larger shrinkage.
[c] Super contracted in the presence of other NaSCN, urea, methanol., also take place in air (RH% of 90) [d] Can be re extended to their original length in same med iu m [29].

Strees-Strain Graph Of Supercontracted Fi ber
The shadowed area illustrates the large variability observed in the tensile properties of naturally spun fibers as shown in the picture 13. We should be within the tolerance spinning limit for the stretching of fiber during applying the unidirectional force inside the spinneret.

Influence of The Controlled Supercontraction Length on the Stress-Strain Curves of Spider Silk
[a] Forcib ly silk fibers are stiffer than naturally spun ones.
[b] There is a monotonous decrease in the stiffness of the fibre as supercontraction increases.
[c] Supercontraction above 25% gives stress-strain curves that are more co mp liant than those of naturally spun fibers [24].

Significance of Controlled Super Contraction
Supercontraction is introduced as a post spinning treatment to tailor final tensile properties of artificial silk fibers as it provides the opportunities to give the higher unidirectional force wh ich forms the micro-do main in parallel co mbination fibriller model. The supercontracted solution of the poly mer gives the heavy viscous nature with the higher intrinsic viscosity (IV) and it makes easy to stretching and drawing in further steps in a spinning line [8].

Problems with Spider Silk Generation
[a] Spiders cannot be farmed like silkworms since they are cannibals and will simp ly eat each other if in close proximity [b] The silk produced is very fine so 400 spiders would be needed to produce only one square yard of cloth.
[c] The silk also hardens when exposed to air wh ich makes it difficult to wo rk with [6][7][8].

Work Done in Genetic Engineering for The Production of The Excellent Quality Spider Silk
Randolph V. Lewis, Professor of Molecular Biology at the University of Wyoming in Laramie Nexia Biotechnologies Inc in, Montreal, Canada had successfully developed a new type of spider species with a higher luster, mechanical stress over the normal silk by help of gene insertion into fungi and soya plants.