Biomedicine, bioengineering or Biomimetics and biomaterials Presentation on Spider Silk, Spring 2015
Facts about spiders There are more than 150,000 different species of spider about 35,000 are identified. Elsewhere I read that as of 2008 43,678 spider species had been identified, but dissension over classification of what is a spider Some are small, some large enough to eat birds in Australia Some bite and some are poisonous They have 8 legs
Introduction Spiders have many uses for the silk: webs or other structures to catch prey nests or cocoons to protect their young suspension to hang from food source Ballooning Many small spiders extrude several threads (gossamer) in the air & let themselves be carried by the wind. Also called kiting
Other Ways Spiders Use their Silk Prey immobilisation As guidelines for a trail to lead back to the nest Drop lines or Anchors an emergency line in case of falling or they can drop down on them if alarmed. Some hang from them while feeding Alarm lines rather than traps, an alarm line can be used to rush out and secure a meal of something small e.g. an ant Pheromonal trail for the opposite sex to find a mate
Different Types of Silk required for all these ecological uses These different types of silk are produced in different glands The silk from a particular gland can be linked to its use by a spider A single spider can produce up to 7 different types of silk each with different properties to match their functions Some silks are primitive and some are very complex
Types of Spider Silk Dragline Silk or Ampullate (major) for a web’s outer rim, spokes and lifeline Dragline Silk or Ampullate (minor) for temporary scaffolding during web construction Flagelliform spiral capture silk – used for capturing lines of web Tubuliform – Egg cocoon silk – used to protect the egg sacs
Types of Spider Silk contd. Aciniform – used to wrap & secure freshly captured prey, used in male sperm webs and as stabilimenta (a conspicuous ultraviolet-light reflecting silken structure like a decoration in the orb web if certain spiders.) Aggregate – a silk glue of sticky globules Piriform – used to form bonds between separate threads for attachment points.
Properties of Spider Silk - 1 Mechanical properties – Each spider AND each type of silk has a set of mechanical properties optimized for their biological function. Most silks especially dragline silks have exceptional mechanical properties with a unique combination of high tensile strength and extensibility (ductibility.) This enables a silk fibre to absorb a lot of energy before breaking (toughness.) Weight for weight, silk is stronger than steel, but not as strong as Kevlar. Silk is however tougher than both.
Properties of Spider Silk - 2 Strength – in detail, a dragline’ silk’s tensile strength (TS) is equal to that of a high-grade steel alloy and about ½ as strong as aramid filaments such as Twaron (also called Arenka) and Kevlar (Dupont.) TS is the maximum stress that a material can withstand before failing or breaking and is NOT the same as compressive strength and the ability to withstand loads tending to reduce size.
A note on aramid fibers These are a class of heat-resistant, strong synthetic fibres used in aerospace and military applications. They are man made and high performance characterized by relatively rigid polymer chains. They are a man made organic polymer, produced by spinning a solid fibre from a liquid chemical, it’s a type of nylon, with high strength, low density and a very high specific strength.
Properties of Spider Silk - 3 Toughness – The combination of strength and ductibility gives dragline silks a very high toughness (or work to fracture) which ‘equals that of commercial polyaramid (aromatic nylon) filaments which themselves are bench marks of modern polymer fibre technology.’
An illustration of the Difference between toughness and strength.
Structure of Kevlar – a para-aramid
The chemical structure of spider silk
Weak hydrogen bonds
Properties of Spider Silk - 4 Density – Consisting mainly of protein, silks are about 1/6 th the density of steel. As a result, a strand long enough to circle the earth would weigh less than 500 grams (18ozs.) Spider dragline silk has a tensile strength (TS) of roughly 1.3 GPa (Giga-Pascal). The TS listed for steel might be slightly higher e.g. up to 1.65 GPa, but spider silk is a much less dense material. This means that a given weight of spider silk is 5 times as strong as the same weight of steel
Properties of Spider Silk - 4 Energy density – Silks are also extremely ductile (some can stretch to up to 5 times their relaxed length without breaking.) This is an aspect of silk’s plasticity i.e the extent to which it can be plastically deformed without fracture.
Properties of Silk - 5 Temperature – While unlikely to be relevant in nature, dragline silks can hold their strength below – 40 ° c and up to 220 ° C (428F) Supercontraction – When exposed to water, dragline silks undergo super-contraction, shrinking up to 50% in length and behaving like a weak rubber under tension. (Theory is that this can automatically tension webs built during the night using the morning dew.)
Macroscopic (visible to the eye) composition Silk as well as many other biomaterials have a hierarchical structure (e.g. cellulose, hair.) The primary structure is its amino acid sequence of mainly highly repetitive glycine and adenine blocks. Hence silks are often referred to as a block co-polymer. On a secondary structure level, the short-sided chin alanine is mainly found in crystalline domains (beta sheets) of the nanofibril. Glycine is mostly found in the so- called amorphous matrix consisting of helical and beta twin structures.
Macroscopic composition Contd 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 fibre’s properties. (See next slide)
Other compounds in Spider Silk Pyrrolidine – has hygroscopic properties that keep silk moist and also wards off ant invasion. It occurs in especially high concentrations in glue threads (aggregate.) Potassium hydrogen phosphate – this releases protons in aqueous solution, resulting in a pH of about 4. This makes silk acidic and protects it from fungi and bacteria that would otherwise digest the protein
Other compounds in silk contd. Potassium nitrate – is believed to prevent the protein from denaturing in the acidic milieu
Models of Silk The very first basic model of silk was introduced by Termonia in 1994 and suggested crystallines embedded in an amorphous matrix interllnked with hydrogen bonds. This model has been refined over the years. Semi-crystalline were also found in a fibrillar skin core model that was suggested for spider silk later. These are nanofibrillar structures and Atomic Force Microscopy (AFM) and Transmission electron microscopy beams (TEM) were used to visualize them using neutron scattering.
Structure of spider silk. Inside a typical fibre there are crystalline regions separated by amorphous linkages. The crystals are beta-sheets that have assembled together
The structure of spider silk
Non-protein composition of spider silk Various compounds other than protein are found such as sugars, lipids, ions and pigments that might affect the aggregation behaviour and act as a protection layer in the final fibre.
Biosynthesis The production of silk differs in an important respect from the production of other fibrous biological materials. They are not continuously grown as keratin (hair) or cellulose in the cell walls of plants, or even the fibres formed from the compacted faecal matter of beetles. Silk is spun on demand from the liquid silk precursor, sometimes referred to as unspun silk dope. It occurs by pultrusion by pulling rather than squeezing. It is pulled through silk glands of which there may be duplicates and differing types in the same spider
The Silk Gland This gland is visible and the external part is called a spinneret. Depending on their complexity spiders will have 2 to 8 sets of spinnerets, usually found in pairs. Behind each spinneret visible on the surface of the spider, lies a gland – see schematic diagram which is based on the major ampullate gland from a golden orb weaving spider, as they are thee most studied and presumed to be thee most complex.
Schematic of a generalised gland of a Golden silk orb- weaver. Each differently coloured section highlights a discrete section of the gland
Human Uses of Silk Biomedical – peasants in the Southern Carpathian mountains cut up the tubes made by Atypus spiders to cover wounds. Reportedly they facilitate healing and even connected skin. This is believed due to their antiseptic qualities & the silk is rich in vitamin K which helps blood to clot. Nephila spider silk has been used recently to help mammalian neuronal regeneration.
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