Spider Silk: Structure, Function and Uses

wanderer Silk Structure, Function and UsesSpider silk, also known as gos a handler, is a character spun by spiders. Spider silk is a remarkably strong. Its tensile strength is comparable to that of high-grade steel. The term silk normally refers to a wide range of continuous filaments spun by the several species of Lepidoptera and Arthropoda, utilise for building expressions for various purposes including prey capture. Silk filaments spun by spiders and wild wilkworms possess superior properties than other silk producing insects and more than 2500 orb weaving species existing worldwide 1 4. Spiders have six or seven sets of glands, each producing a different fiber. These glands remained undifferentiated, early in the evolution 4 7. The spinnerets, microscopic tubes originating from glands, are classified into major and minor ampullate. The term ampulla is used to cover the distal part of the secretary zone 8. Unlike synthetic polymers, the biopolymers are composed of numerous monomers arranged in a strictly controlled manner 9. many an(prenominal) attempts have been made in the past to harvest and convert spider silk filaments into fabric form 4, 10, 11. Scientists have been hard at work attempting to marshal the cater of spider silk for a range of medical applications-including wound-care applications suture materials muscle, bone, cartilage, tendon, and ligament repair scaffolds. Spider silk possesses mechanical attributes such as very high tensile strength and elasticity, fashioning it one of the toughest fibers known to man. The problem is that spiders dont constitute enough silk to render it marketable for mass human use.SPIDER WEB AND TYPES OF SPIDER SILK Prior to the exploration of the structure and properties of spider silks, construction and design of netts have been the major area of focus. The spider webs can take a variety of forms but the about common fount is the orb web. Different families of spiders like Araneus, Nephila builds orb web and other families of spiders construct tangle and sheet webs 33, 35. Orb-web spiders invest little expertness in searching for prey. It spends most of its time synthesising silk and constructing webs.An orb web has several spokes laid outward from a common origin. However, this varies amongst the various species of spiders 31. The orb webs are often constructed with an orientation to avoid universe damaged due to the air drag caused by prey capture 24. In a three dimensional web, the energy required to stop a moving insect is dissipated mainly by breaking some of the strands. In a two dimensional orb web, it is achieved through stretching the handbuild threads 29. Due to high-energy requirement in protein synthesis, only(prenominal) the damaged parts of the web are reconstructed instead of the whole web. Large portions of the web are repaired through the enzyme digestion and recycling. Based on the vibrations of the strands, the spider locates the prey accurately.The orb-w eaving spiders are able to synthesize as many as seven different causes of silk 13-15 including dragline by drawing liquid crystalline proteins from separate gland-spinneret complex. The perfume-coated dragline helps to find their mates, swing from place to place, store food, eggs and for reproduction. Capture threads produced by the flagella form glands of Nephila Clavipes is highly compliant. both Araneus and Nephila coat their capture threads with an aqueous solution that forms sticky droplets which enhances damping and harvests water from air 25. Its principal function is to absorb and dissipate the kinetic energy of captured flying insects. 3. The chemical composition of the aqueous solution of the adhesive spiral varies among the species qualitatively and quantitatively. The variation is mainly due to forcible environment, diet, web recycling, and onto genetic changes in the web chemistry.SPINNING OF SPIDER SILK Many spiders are active at night and their colorations are usua lly orange, brown, grey and black, to reduce the spiders visibility during day time. Silk secreting systems of spiders and insects are homologous and linked to the crural gland and cuticular secretions 6. Cephalothorax of the spider attached to an unsegmented abdomen, which has spinnerets at the rear end end 33. N.clavipes spider has three pairs of spinnerets namely, anterior lateral, posterior lateral and posterior median. The titanicst major ampullate gland secretes dragline silk protein, exits from the anterior lateral spinneret. Secretions of proximal land and the distal region together form spider silk. Proximal region secretions are luxuriant in tyrosine residues, sulfhydryl linkages and red-hotophilic nature. They form core of the silk while secretions of distal zone form coating of the fiber, which lacks tyrosine and sulfur contents.A mature Nephila produces dragline silk fiber at approximately 1 cm/sec during web construction and can increase up to 10 generation faste r during a quick descent 52. Spider silk spun under water displays greater stiffness and resilience compared to silk spun naturally in air 53. The diameter of the silk can be controlled by the valve set(p) at the end of the duct 47. The spiders have the ability to withstand temperature variation of up to 30oC and humidity variation of up to 70% 20. Spiders can easily substitute the spinning conditions by their moving speed, building the webs in different times in a day. Spinning speed has less(prenominal) influence on the diameter of the filament when compared to the temperature even though its influence on toughness.COMPOSITION OF SILK Variability in silk spun by the spiders exists at different levels such as in inter-specific (between species), intra-specific (within same species) and intra individual levels 54. The factors that affect variations in silk structure and properties include body dimensions, body weight, rate and temperature of reeling and spinning direction 17, 25. Composition of silks produced by herbivorous spiders is rich in Glycine, Alanine and Serine. This type of silk can be predicted to some extent. However, the silk produced by predatory spider, can non be predicted due to the different types of prey 40, 55. Dietary compositions of herbivorous spiders are energy rich and poor in protein content whereas the diet of predatory spiders is more diverse and rich in protein. Competition for limited or fluctuating supplies of aminic acid perhaps has resulted in the evolution of two different kinds of glands to secrete protein glues and silk fibroin. The spider produces the thread on a very strict energy budget using liquid crystalline polymer.STRUCTURE AND PROPERTIES Spider silk has drawn attention from all the sections of engineering due to its superior properties when compared to existing fibrous materials such as the silkworm silk. Spider silk cannot be compared with silkworm silk. This is because spiders are difficult to raise in large n umbers and their silk lacks the lustre of silkworm silk. The chemical compositions of various silks vary with the type of function they are intended to perform. In addition to the fibroin, other classes like glycoprotein , inorganic salts, sulphur containing compounds, amino acids, and ionic forms of amines are also present in the spider silk 69, 70. Presences of these chemicals play crucial roles in identification of species, regulation of water content of the web and protection against microorganisms. Presence of 12- methyltetradecanic acid and 14-methyl hexadecanoic acid in less amounts impart antimicrobial properties to the spider silk. Wax like esters are also present in the surface of the spider silk.Macroscopic Structure of Dragline Silk Dragline spider silk is deluxe yellow in color and has circular cross section with a mean diameter of about 7 m 19, 38, 54. It lacks glue-like protein, similar to that of silkworm silk, as its associated with dragline fiber 56. The dragline spider silk consists of semi crystalline polymeric structures with numerous small crystallites between amorphous regions. The mechanical properties of the dragline silk are highly influenced by the composition of the amino acids, insect size, diet, body temperature and drawing speed 99, 124. The breaking strength of silk increases linearly with increasing spider weight and breaks at stress of about six times the spiders weight 47, 100, 129. The average tensile strength of the dragline of Nephila clavipes is almost three times that of Bombyx mori (1.3 0.5 GPa, respectively). Tensile strength of spider silk reduces, when it is subjected to acidic rain and UV beam of light 133. Spider silks can undergo large tensile and compression deformations. The ability of spider silk to resist transverse compression is lower than that of many textile fibers like Kevlar 29, nylon 5, polyester and wool.REGENERATED SPIDER SILK PROTEIN BY ARTIFICIAL ROUTEForced Silking (Reeling) of Spider SilkReeli ng devices have been developed for forced silking of dragline from the glands of anaesthetized Nephila clavipes 78, 115,140, 141 to reel about 3-5 mg of silk in one session. conjoin of silk genes into two different cell lines have been tried in the past using bovine mammary cells and hamster kidney cells, to produce large volumes of recombinant proteins 49, 135. Successful sequencing of genes of the flagella form silk of tropic spider Nephila clavipes and N. madagascariensis has been achieved lately 39. Recombinant DNA technology for microbial proteins 144, 150, 151 appears to be advantageous compared to that of chemical synthesis due to low cost, rapid preparation and absence of by-products. A team of researchers at the University of Notre Dame (Notre Dame, IN) the University of Wyoming (Laramie), and Kraig Biocraft Laboratories Inc. (Lansing, MI) have succeeded in producing transgenic silkworms. The advantage of these animals is that they can spin artificial spider silk with str ength and flexibile attributes similar to those of native spider silk. Until this breakthrough, only very small quantities of artificial spider silk had been produced in the laboratories. Kraig Biocraft believed these limitations can be overcome through use of recombinant DNA. This biotechnological approach can be used to produce silk fibers with a broad range of physical properties or with predetermined properties optimized for specific biomedical or other applications.From the known sequence of the spider silk protein, genes are constructed and express using E. Coli as the host, which has been successfully used earlier for silkworm silk 160. Genes of spider dragline silk have been inserted into mammary gland cells along with regulatory elements. Insertion of the genes into eggs of hotshot cell goat has been tried. This was purposely done to produce water-soluble silk protein 154, 156, 165. Few milligrams of genetically engineered silk like protein has been successfully produced based on the sequence of spider protein 155.APPLICATIONS though availability of the dragline silk is limited, it is widely used in defence 4, and medical 11 applications. Structural similarity and comparable properties of dragline and Kevlar 127, 180 makes it more attractive for applications where high performance, in terms of physical properties is in demand.Until World War II, spider silk was used as crossed-hairs in optical devices including microscopes, telescope and bomb guiding systems 4. Silk strands of the web have an ability to elongated when an insect is caught, convert the preys momentum i.e. kinetic energy into heat, and dissipate about 70% of the converted energy. The web also gently rebounds so as not to catapult the insect back out. This ability to dissipate energy at very high strain rates makes spider silk suitable for body fit out system and ideal for ballistic protection 131, 140. Though biodegradability is a helpful aspect for sutures, it is as unwanted in high performance applications such as bulletproof vests. A very low glass transition temperature of -50o C to 60o C enables it to absorb sudden shocks at low atmospheric temperature and makes the spider silk suitable for parachute applications. However, super contraction in water is undesirable for use in the fabrication of parachutes 174, 175.Earlier use of spider silk in the form of web, quite than a fiber, includes wound dressing to help blood clot and fishing nets. Spider silk protein can be used to coat the medical implants for better performance. working(a) thread, biomembranes and scaffolds for tissue engineering are the possible areas of application in biomedical and biomaterial fields. Due to low inflammatory potential of silk proteins and antithrombic nature, recombinant spider silk has potential applications in sutures for oculus surgery, artificial tendon and ligaments for knee construction. Spider silk with higher safety co-efficient can be used in structural applicatio ns like elevator ropes, bridges and pillars 100. deductionThe dragline silk offers excellent physical and chemical properties that can withstand adverse and extreme conditions than many of the existing natural and synthetic fibers. Though the chemical synthesis seems to be unfruitful in many aspects, the recombination method of producing the spider silk using biological hosts proves to be a viable option for producing the spider silk in a large scale. In spite of various successful attempts made in the production of dragline silk in the laboratory scales, controlling the molecular conformation and their aggregation during the spinning for achieving properties similar to the native fiber still remains as a challenge to be addressed through future research.