The EDEXCEL AS physics syllabus requires the AS physics students to undertake a visit or case study and write a report on the subject before carrying out a related experiment.
This year they were asked to research and write about the properties of spider silk. You may be surprised but there is a connection with physics.
Our guide for the day was Dr Beth Mortimer.
Dr Mortimer is part of the Oxford Silk group within the Zoology department at Oxford University.
Dr Mortimer’s research is at the interface of physics, engineering and biology, using physical sciences techniques to help answer biological questions. In particular she has focussed on the mechanical and vibrational properties of silks.
A dragline silk’s tensile strength is comparable to that of high-grade alloy steel (450 – 1970 MPa) and about half as strong as aramid filaments, such as Twaron or Kevlar (3000 MPa) which makes it of particular interest to scientists who would like to reproduce such properties in man-made materials. Unfortunately spiders do not produce enough silk to be used commercially and due to the thickness of the fibres they will break at small forces. But if you can scale its properties up you would have a very strong material indeed.
Research is also being done in whether spider silk can be combined with other materials to produce new products.
The Oxford silk group has discovered that spider silk can be cleaned, treated and remoulded to tune its material properties, so it can be made super-strong, say, or highly biodegradable.
The result is a material with many possible applications. Its low density and high elasticity means that it can be used to create a new breed of lightweight protection, such as cycling helmets or car body panels that keep us safe without weighing us down.
And, because the material is biocompatible, it can be used to create hard-wearing replacement joints or lightweight scaffolds to promote the regrowth of nerves that are readily accepted by the body.
It’s also highly sustainable: natural, environmentally friendly, and potentially straightforward to mass-produce.
Dr Mortimer started the day with a lecture which is actually part of the 3rd year Biomechanics option.
The course is aimed at students interested in biometrics that is mimicking biology and producing natural solutions to modern technical problems.
Researching the Biodiversity of silk
Function – What is silk?
Silks are structural proteins spun into fibres for use outside the body and is made by several different types of organisms. The chemicals involved are stored inside the body as a liquid and genes code for the type of silk made outside the body.
The image above left shows four of the most important domesticated silk moths. Top to bottom: Bombyx mori, Hyalophora cecropia, Antheraea pernyi, Samia cynthia. From Meyers Konversations-Lexikon (1885–1892). The image above centre shows Antheraea assamensis, the endemic silk caterpillar species in the state of Assam, India. The image above right shows an adult female golden silk spider, Nephila clavipes (Linnaeus). Photograph by University of Florida.
Silk has evolved 23 times convergently in the insects.
Silkworms produce silk when undergoing larvae to adult metamorphosis.
Fifth instar silkworm larvae
Raspy crickets produce silk to form nest
Honeybee and bumblebee larvae produce silk to strengthen the wax cells they pupate in
Above left shows a European honey bee carrying pollen back to the hive. Above right shows the buff-tailed bumblebee, Bombus terrestris
Bulldog ants spin cocoons to protect themselves during pupation
Bull ant worker foraging in Swifts Creek, Victoria
Weaver ants use silk to connect leaves together to make communal nests
Weaver ant (Oecophylla longinoda) major worker (Tanzania)
An egg is then laid in each cell and after 5–8 days, it hatches, and becomes a larva. After about two weeks it spins a silk cap over the cell’s opening and, during the next two weeks, transforms into an adult, a process called metamorphosis. The adult then eats its way through the silk cap.
The male silverfish lays a spermatophore, a sperm capsule covered in gossamer (a fine type of silk), which the female takes into her body via her ovipositor to fertilise the eggs.
Mayflies attach themselves to surfaces using suckers, claws, and silk
Some types of thrip wrap their eggs in silk to prevent them drying out. Some also construct domiciles using silk and leaves.
Leafhoppers produce silk nest under the leaves of the trees they live in, to protect them against predators
Adult of two-lined gum treehopper (Eurymeloides bicincta, Eurymelinae)
Some beetles use silk in the construction of nests for moulting, brood production and hibernation.
The lacewing lays its eggs on stalks made of silk and the pupa is generally enclosed in some form of cocoon composed of silk and soil or other debris.
Robert Hooke’s drawing of a flea in Micrographia
The flea larva is enclosed in a silken, debris-covered cocoon.
Black fly larvae tend to stay attached to a substrate in fast-flowing waters by producing a silk thread from their mouths
Some midges are ‘filter feeders’ consuming tiny particles of detritus and diatoms that are suspended in the water. They do this by using silk nets secreted from special glands. Some midge larvae use silk to anchor themselves to the bottom of a stream.
Family: Chironomidae (Midge Flies).
Arachnocampa is a genus of five fungus gnat species where their larvae hang sticky silk threads to ensnare prey.
Luminosa larvae with their prey snares
Adult glow-worm fly beside its empty pupa case.
Caterpillar is the common name for the larvae of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary and the larvae of sawflies commonly are called caterpillars as well.
A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in insects that undergo a complete metamorphosis, going through four life stages: embryo, larva, pupa and imago.
A cocoon is a casing spun of silk by many moth caterpillars, and numerous other holometabolous insect (insects that go through several stages before adulthood) larvae as a protective covering for the pupa.
Cocoons may be tough or soft, opaque or translucent, solid or meshlike, of various colours, or composed of multiple layers, depending on the type of insect larva producing it.
All proper caterpillars make a cocoon for protection.
Bombyx mori is the silk worm.
The silkworm is the larva or caterpillar of the domesticated silk moth, Bombyx mori (Latin: “silkworm of the mulberry tree”). It is an economically important insect, being a primary producer of silk.
Image on the left shows a paired male (above) and female (below). Image on the right shows fifth instar silkworm larvae.
Silkworm cocoons are non-woven composites for protective functions during metamorphosis. Engineers are very interested in this and would like to reproduce such materials for use in cars.
Silk has evolved only once in the spiders. All spiders make silk but do it in different ways and use it for different functions.
Eriophora transmarina from Australia
The typical orb-weaver spiders (family Araneidae) are the most common group of builders of spiral wheel-shaped webs often found in gardens, fields and forests. Their common name is taken from the round shape of this typical web, and the taxon was formerly also referred to as the Orbiculariae.
A Victorian funnel-web spider (Hadronyche modesta)
Atracinae, commonly known as Australian funnel-web spiders, is a subfamily of spiders in the funnel-web spider family Hexathelidae.
Phoneutria, commonly known as Brazilian wandering spiders, armed spiders (“armadeiras”, as they are known in Brazilian Portuguese), or banana spiders, are a genus of aggressive and venomous spiders of potential medical significance to humans.
Wolf spiders are members of the family Lycosidae, from the Ancient Greek word meaning wolf. They are robust and agile hunters with excellent eyesight.
The jumping spider family (Salticidae) contains more than 500 described genera and about 5,000 described species, making it the largest family of spiders with about 13% of all species. It has very good eyesight.
Bolas Spiders are unusual orb-weaver spiders that do not spin the typical web. Instead, they hunt by using a sticky ‘capture blob’ of silk on the end of a line, known as a ‘bolas’.
Ummidia sp. The image above right shows the closed burrow of a Cork-lid Trapdoor spider saved in padded container. Probable genus: Stasimopus
Trapdoor spiders (superfamily Ctenizoidea, family Ctenizidae) are medium-sized mygalomorph spiders (the scientific name comes from the orientation of the fangs) that construct burrows with a cork-like trapdoor made of soil, vegetation and silk. They use silk to line the burrow as this extends their sense area.
Dinopis is the genus name for the more commonly named Gladiator Spider; Ogre faced spider, or Net casting spider. It makes a little web between its front legs.
Simon Peers and Nicholas Godley’s “fairly eccentric adventure” to revive the Madagascan spider silk industry. The project involved 70 people, 4 years and 1 million spiders
Tensile testing, also known as tension testing, is a fundamental materials science test in which a sample is subjected to a controlled tension until failure. The results from the test are commonly used to select a material for an application, for quality control, and to predict how a material will react under other types of forces. Properties that are directly measured via a tensile test are ultimate tensile strength, maximum elongation and reduction in area. From these measurements the following properties can also be determined: Young’s modulus, Poisson’s ratio, yield strength, and strain-hardening characteristics.
Silk is a polymer. It consists of covalently bonded units with hydrogen bonds. Covalent bonds acts like little springs.
Adapted from A. Wynne (1997) Textiles: The Motivate Series, Macmillan Education, Oxford, UK
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. Whereas elasticity is usually the result of bond stretching along crystallographic planes in an ordered solid, viscosity is the result of the diffusion of atoms or molecules inside an amorphous material.
The area under a stress-strain graph gives you some idea of the amount of energy stored in the material under tension. The greater the energy stored the tougher the material. Silks are the toughest-known materials as they can absorb a lot of energy before breaking. Toughness is a measure of the energy stored/absorbed in the material before it breaks. Spider dragline has a large value. Spider silk is one of the best materials for absorbing energy, which is very useful if he prey moves about a lot. Most energy dissipation occurs as heat.
Adapted from A. Wynne (1997) Textiles: The Motivate Series, Macmillan Education, Oxford, UK
An illustration of the differences between toughness, stiffness and strength
The relationship between the stress and strain that a particular material displays is known as that particular material’s stress–strain curve. It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading (stress). These curves reveal many of the properties of a material (including data to establish the Modulus of Elasticity, E).
Stress is the force per unit area applied to a material. It can be tensile (stretching) or compressive (squashing).
Strain is the extension (or decrease in length) divided by the original length of the material.
The limit of proportionality is the point where stress is no longer proportional to strain.
The elastic limit or yield point is the point where elastic behaviour stops and plastic behaviour starts. Elastic behaviour has the material returning to its original shape when the force is removed. Plastic behaviour has the material remaining in the new form when the force is removed. It is permanently deformed. At the elastic limit you are breaking the weaker bonds.
The ultimate stress is the maximum force that the material can withstand before it breaks.
The braking stress (fracture point) is the stress as which the material breaks.
Young modulus is stress divided by strain.
Function – Orbwebs catch prey
Mechanical properties are optimised for function. Spider silk takes a lot of strain to break. Crack propagation is poor. Each type of silk has a different stress-strain graph.
(Porter, D and Vollrath, F. (2006) Soft Matter 2 377-385)
*Known as stabilimenta
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).
Ductility is plastic deformation under tensile stress
A frequent mistake made in the mainstream media is to confuse strength and toughness when comparing silk to other materials. Weight for weight, silk is stronger than steel, but not as strong as Kevlar. Silk is, however, tougher than both.
In detail a dragline silk’s tensile strength is comparable to that of high-grade alloy steel (450 – 1970 MPa), and about half as strong as aramid filaments, such as Twaron or Kevlar (3000 MPa).
Consisting of mainly protein, silks are about a sixth of the density of steel (1.31 g/cm^3). As a result, a strand long enough to circle the Earth, would weigh less than 500 grams. (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.)
The energy density of dragline spider silk is 1.2 x E8J/m^3.
Silks are also extremely ductile; with some able to stretch up to five times their relaxed length without breaking.
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”.
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).
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.
The toughest known spider silk is produced by the species Darwin’s bark spider (Caerostris darwini): “The toughness of forcibly silked fibres averages 350 MJ/m^3, with some samples reaching 520 MJ/m^3. Thus, C. darwini silk is more than twice as tough as any previously described silk and over 10 times tougher than Kevlar”.
Structure – Processing
Natural silk processing occurs by:
– Pulling the protein feedstock through the spinning duct
– Causing it to flow
– Kinetic energy converts to chemical/bonding energy
– Water is stripped from the protein (water-amide H-bond)
– Proteins change conformation
– Aggregates (amide-amide H-bond)
– Falls out of solution (dry fibre)
It has evolved to be denatured
Knight, D. and Vollrath, F. (1999) Proc. R. Soc. B. 266, 519, Joanne Flickr)
The image below is a schematic of a generalised gland (major ampullate gland) of a Golden silk orb-weaver. Each differently coloured section highlights a discrete section of the gland.
1. The first section of the gland labelled in the image above 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 fib, hypothesized to assist in preventing crack formation or even self-healing of the fibre.
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 fib.
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 fibre 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 silk fibre emerges. The tube here tapers hyperbolically; therefore the unspun silk is under constant shear stress, which is an important factor in fibre formation. This section of the duct is lined with cells that exchange ions and remove water from the fib. The spigot at the end has lips that clamp around the fibre, controlling fibre diameter and further retaining water.
5. Almost at the end of the tapering duct is a valve, approximate position marked “5” on the diagram above. 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 fibres acting much in the way of a helical pump, regulating the thickness of the fibre, and/ or clamping the fibre 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. This allows the unspun silk to flow through the duct as a liquid but maintain a molecular order.
Nematic comes from a Greek prefix nemato meaning threadlike and is used here because the molecules in the liquid align themselves into a threadlike shape.
Silk Technology transfer
• High Performance
• Weight for weight silk is 5 x stronger than steel and 5 x tougher than Kevlar
• Produced at room temperatures and pressures
• Oil free
• Super-low energy costs
• Body can break it down
Evolution of silk biodiversity:
• Convergence (e.g. spinning proteins, silk processing)
• Divergence (e.g. innovation of major ampullate)
• Limitations (e.g. secondary structures, viscoelasticity, trade-offs)
• Variation/plasticity in properties as adaptive
• Co-evolution between silk sequence, duct morphology & behaviour
• Domestication of Bombyx
Harnessing biodiversity of silk:
• Biomining (e.g. spider silks, webs, cocoons)
• Energy efficient polymers
• Environmentally friendly material alternatives
• New capacity materials/structures (e.g. shape memory, high-performance, multifunctional, biocompatible & strong)
Acknowledgements and Further Reading
• All members of the OSG
• All their collaborators
• EPSRC ERC, AFOSR, Leverhulme Trust
www.oxfordsilkgroup.com direct links to all references
Wikipedia Article on Spider Silk
Vollrath, Current Biology, “Spider Silk: Thousands of Nano-Filaments and Dollops of Sticky Glue”
Vollrath, Porter, Holland, MRS, “The Science of Silks”
Craig, Ann. Rev. Entomol. “Evolution of Arthropod Silks”
Sutherland, Ann. Rev. Entomol. “Insect Silk: One Name Many Materials”
Brunetta, Craig, “Spider Silk”
Tour of the department
(Beware there are spiders in some of the pictures)
In the photo below you can see Shefali, Sambath, Abinayan and Milan. You can See Dr. Mortimer behind the students talking about the spiders.
Above right you can see some of the spiders
Above you can see some of the spiders, along with their webs and a nest of spider eggs.
Above you can see Vinnoja, Dr. Mortimer, Sambath (who didn’t want to get anything spider related on him) and Ahlaq.
Students listening to Dr. Mortimer, who is rather hidden
In the laboratory Dr. Mortimer is explaining to Karthikeyan, Milan, Ahlaq and Seyar about some of the techniques used to investigate silk in the department.
Karthikeyan listening to an explanation of how the tensometer works. The tensometer measures force and extension. Unfortunately the thickness decreases during the process to about 3 microns. This is below the resolution of a light microscope. A scanning electron microscope is used to examine the fibres but unfortunately this destroys the silk.
The end of the laboratory tour
Just for fun
Don’t be fooled just because the students were in the zoology department it doesn’t mean that physics wasn’t involved. Without physics spider silk could not really be investigated properly.
A nice extra to the visit was a tour of Jesus College Oxford with our guides Adam and Ben. Dr Mortimer is a junior research fellow at Jesus.
Jesus College (in full: Jesus College in the University of Oxford of Queen Elizabeth’s Foundation) is one of the colleges of the University of Oxford in England.