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Exploiting spiders’ silk
Exploiting spiders’ silk
by Paula Gould
Scientists and engineers have long envied spiders’ ability to manufacture a material that is simultaneously strong, fine, and tough. This combination of properties makes spider silk an extremely attractive candidate for numerous applications in medicine, defense, and the leisure industry. Research teams worldwide are gradually piecing together the relationship between the silk’s make-up and its unusual – but highly useful – behavior. At the same time, efforts are underway to mass-produce a similarly super-strong fiber that displays both the strength and elasticity of its natural, backyard counterpart.
42
December 2002
As today’s film-goers and readers of the original Spiderman comic strip will know, spider silk makes an ideal tool for a modern day super-hero. The fine line can be produced at will, holds our hero’s weight as he swings between tall buildings, and then clumps together to trap dastardly villains in a tangled, sticky mess. Highly useful if your daily job description includes rescuing distressed damsels from improbable locations and preventing a thoroughly nasty goblin from wreaking havoc on your home city. Fiction aside, the fibrous threads produced by our eightlegged friends really do have extraordinary properties1. Spiders can spin up to seven different types of silk, each one tailor-made to fit a specific function. To date, most researchers have focused their attention on ‘dragline’ silk, used by spiders as a safety line and also as the frame for their webs (Fig. 1). Weight-for-weight stronger than steel, finer than human hair, more resilient than any synthetic fiber, and completely biodegradable, dragline silk is every inch the 21st century super-material. Suggested applications have ranged from surgical sutures and ligament repair to parachute straps and high-performance, flexible body armor. No wonder the field is attracting interest from entrepreneurs and military personnel, as well as university researchers. So what is it that gives spider silk its amazing properties? That is something Helen Hansma and colleagues from the Department of Physics, University of California, Santa Barbara (UCSB) would like to find out. Keen to look closer at the material’s make-up, the UCSB team used atomic force microscopy (AFM) to analyze a protein solution that had
ISSN:1369 7021 © Elsevier Science Ltd 2002
APPLICATIONS FEATURE
been synthesized from spider dragline silk by researchers at the US Army Natick Research and Development Center in Massachusetts2. They observed the solution self-assemble into nanofibers, each with a segmented substructure. Measurements of the nanofiber segments suggest that each contains a stack of slab-like molecules. The stacking pattern effectively produces a series of nanocrystals within an amorphous matrix, matching with previous observations of the silk’s composite structure from nuclear magnetic resonance (NMR) and X-ray diffraction. In short, they found themselves watching how spiders’ liquid crystalline ‘dope’ turns into stretchy strands (Fig. 2). Hansma’s team is using single-molecule force spectroscopy to see precisely what happens when you pull on the silk. Data on exactly how the protein unfolds will be used to improve models for the structure and mechanics of silk molecules and fibers. However, in addition to pulling on the synthetic dragline silk protein, they have also been experimenting with bulk capture web, a silk noted for its extremely high elasticity. These tests required some cooperation from the lab’s smallest recruits, living spiders similar to those inhabiting Hansma’s backyard. For physicists perhaps more used to working with inanimate objects, the project required a new set of skills altogether. “We started catching spiders and keeping them in aquariums, and then specially made spider boxes. We experimented with feeding them fruit flies, raised on things like cantaloupe rinds,” Hansma says. “The graduate students and post-docs all seemed to enjoy sitting and watching the spiders in the aquarium. I called them ‘physicists at work’.”
Fig. 1 Dragline silk is used by spiders as a safety line and also as the frame for their webs. (Courtesy of E. Oroudjev and H. Hansma.)
Carl Michal is another physicist who now finds himself sharing his workspace with several small, eight-legged creatures. He first started working on spider silk as a graduate student at Cornell University under the supervision of Lynn Golinski, using NMR techniques and computer simulations to examine the orientation of crystalline amino acids in dragline silk3. Michal is now continuing this line of enquiry at the University of British Columbia (UBC) in Vancouver, with the
Fig. 2 Nanofiber (~0.5 µm long) formed spontaneously from depositing a solution of soluble recombinant spider dragline silk protein on a freshly cleaved mica surface. Bumps along the fiber reveal that it is composed of segments. (Courtesy of E. Oroudjev and H. Hansma.)
December 2002
43
APPLICATIONS FEATURE
help of his own graduate students. They feed lab-housed spiders a solution containing amino acids in which one of the atoms has been replaced by a specific stable isotope label, e.g. C-13, N-15, or deuterium. Harvested silk is put inside an NMR probe. Team members then work backwards from the resulting images to determine how the labeled amino acids are distributed. “As we stretch or relax the silk, we’re hoping to be able to see in our model the chains stretch out along the direction of the silk and the crystals orient or disorient as we relax it,” Michal explains. “We’ve got a better feel now for how much of the silk is crystalline, what the crystals are doing, and what the chains are actually doing as the material is performing.” Rather than trapping the spiders themselves, the UBC researchers get their spiders specially shipped from Florida. They work with Nephila clavipes, a golden orb-weaving spider whose silk has been studied in detail by numerous groups worldwide (Fig. 3). The availability of so much background information is not the only thing going for their chosen subject, though. Nephila clavipes is a pretty big spider, which makes routine operations that little bit easier (Fig. 4). “For our experiments, we actually need a fair bit of silk, so having a big spider is really advantageous,” Michal says. “They are also a lot easier to handle than some of the smaller spiders.” While content with the investigation’s status as a pure research project, Michal is hopeful that the advanced models his team is generating will be of interest to others planning to market materials based on dragline silk. His methods of obtaining the raw material, however, will be of little
Fig. 3 The large Nephila clavipes golden orb-weaving spider is favored by most spider silk researchers.(Courtesy of C. Michal.)
44
December 2002
relevance to groups with commercialization in mind. On a good day, UBC researchers might harvest 1.5 mg of silk, a small but significant contribution towards the 50 mg needed to run NMR tests. Clearly, this is nowhere near enough to be commercially viable. And farming the spiders just isn’t a workable option owing to their highly territorial and aggressive nature.
Milking it Faced with this apparent obstacle, Jeffrey Turner, CEO of Nexia Biotechnologies, Inc. decided to do away with spiders altogether. If the arachnids were not going to cooperate, why not simply copy their production techniques? His faith in biomimicry paid off and, in January 2002, the company announced the successful production of BioSteel®, a material based on recombinant spider silk proteins, but derived from the milk of transgenic goats4. Even the Spiderman scriptwriters could not have come up with such an unlikely means of production. Yet Turner’s description of the steps leading up to the silk’s production makes the whole process sound entirely logical in retrospect. Having ruled out traditional fermentation techniques, Nexia researchers took some genes from spider dragline silk, attached some regulatory elements to switch the genes ‘on’ and ‘off’, and then inserted them into so-called ‘MAC-T’ cells, the mammary gland cells usually used for making milk. “And it was really the ‘Eureka!’ moment,” Turner says. “The mammary cells took the spider silk genes in all their complexity and made a beautiful, watersoluble, authentic spider silk. That was a really incredible time.” But this was only step one. The team had simply found an efficient means of producing the complex protein in a watersoluble format. Nothing more, nothing less. What they really needed was a way to turn their watery solution into silk fibers, something US Army researchers at Natick could help with. Not only had this group been developing synthetic silk proteins, but they also had considerable expertise in spinning fibers from the protein solutions. Turner’s enthusiasm is infectious as he describes what happened next. “It was unbelievable,” he says. “The protein came zipping out of the syringe tip and when the water was removed, it selfassembled into a beautiful silk. I’m not sure who was more excited, them or us.” Of course, the researchers still needed a viable production line, so they inserted individual spider silk
APPLICATIONS FEATURE
genes into single cell goat eggs. Milk from the resulting transgenic goats duly contained water-soluble silk proteins. Turner is understandably reluctant to say precisely how much silk solution a single goat will generate, though he notes that just one animal can produce ‘literally miles’ of silk. The number of transgenic goats in Nexia’s farms has grown to around 100, with the total herd size topping 1500 (Figs. 5 and 6). This level of production could be sufficient to meet the needs of the medical devices industry, though a herd ten times as big would be needed to satisfy industrial demands. The company is also planning to work with a major textiles manufacturer, who can help spin high quantities of the fiber at a more commercially-viable rate (Fig. 7). Biodegradable fishing line is likely to be the first Nexia product to reach consumers, with a market launch expected within the next two years. The biphasic modulus of dragline silk is the key selling point here, according to Turner. Initially stiff when pulled, spider silk elongates before breaking. Where existing lines might snap under the force of a large
fish biting and then trying to flee the scene, a spider silk line would hopefully respond to any sudden dashes for freedom by giving a little. This stiffness/stretchiness combination has also led Turner to consider touting BioSteel® as a vital ingredient in tendon and ligament repair. Rupturing an anterior cruciate ligament can mean the end of an athlete’s sporting career. But if a biocompatible material could be found that does not become fatigued when flexed regularly, is strong enough to withstand regular impact, and does not snap under pressure… how much would that be worth to a star athlete facing premature retirement? But this is still a good few years away, owing to necessarily complex regulations surrounding such product development. A simpler product destined for the medical market, and one more likely to appear by the end of 2004, is the BioSteel® microsuture. Here, the fineness and knotability of silk will be stressed, in addition to its strength and biodegradability, to persuade ophthalmic and neurological surgeons to switch from nylon-based threads.
Fig. 4 Physicists at the University of British Columbia have developed new hands-on experimental skills in their work with living spiders. (Courtesy of C. Michal.)
45 December 2002
APPLICATIONS FEATURE
Fig. 5 Peter and Webster – Nexia's first transgenic goats. (Credit: Sean O'Neill.)
Fig. 6 Nexia's rather unusual production line is expanding to meet the needs of the medical and industrial markets. (Credit: Sean O'Neill.)
Last, but by no means least, Nexia is working to create better ballistic protection for policemen and women and the armed forces. Turner points to the inflexibility and heavy weight of today’s standard issue bullet-proof vests. Body armor woven from dragline silk would not only be more comfortable, but it would also be more effective at distributing the force from an explosion or gunshot over a wider area, lessening the blow. “When a fly hits a spider’s web, about 70% of the kinetic energy is converted into heat and dissipated. That’s really important,” he says. “In the case of ballistics, the amount of deformation or shock that the soldier or police officer would feel from a bullet would be considerably reduced. It’s this energy management of spider silk that has people really excited.”
Viney’s chief concern is that the promise of these superstrong synthetic materials relies on laboratory tests that do not mirror the situations in which the products will have to perform. In fact, sometimes they do not even get close. As a prime example, he cites controlled experiments routinely used to measure the strength, stiffness, and toughness of silks that are conducted at a constant strain rate for less than an hour. Yet many engineering functions envisage the new materials carrying high loads for long periods of time, making the behavior of spider dragline under a constant load for days, weeks, or perhaps even years a more appropriate study. He points also to the new materials’ expected durability. Many of the anticipated applications will require man-made spider silks to remain stable during changes in moisture and temperature, and possible exposure to ultraviolet radiation. The natural product has no such built-in longevity. “Spiders produce their dragline and webs to do a specific job, which is accomplished in minutes, e.g. the use of dragline to expedite a vertical ascent or descent, or hours, e.g. the use of a web to catch prey,” Viney says. “Webs are repaired or replaced daily. These silks are not designed to last!” His experiments into the behavior of wetted dragline silk have thrown up some perturbing findings. These relate to the silk’s tendency to ‘supercontract’ when wet, halving in length and almost doubling in diameter. Viney and his colleagues at Heriot-Watt measured the supercontraction stresses in 30 mm lengths of Nephila clavipes dragline exposed to
What everyone wants But is there a danger of getting over-excited? Even Nexia are still only working towards prototypes. What if commercially-manufactured spider silk fibers don’t match up to expectations after all? And even if companies do replicate the properties of natural spider silk in a man-made material, what if no one is willing to take the risk on a new product? Chris Viney, head of materials chemistry at Heriot-Watt University, Edinburgh, admits to adopting a more cautious approach than many others in the field. His caution is borne out of over ten years’ research experience in the area.
46
December 2002
APPLICATIONS FEATURE
increasing humidity5. Their findings proved surprising in two ways. First, the stresses generated were transiently higher than expected. Second, the stresses relaxed after just a few minutes, indicating the wet dragline’s inability to maintain tension – or support a load – indefinitely. Not only does the material abandon its significant load-carrying responsibilities when wet, but it is also likely to creep rapidly and change its shape, according to Viney. This has serious implications for the use of man-made spider silks in high tensile strength fibers. “The longer I work in this field, the more I worry that natural silk many not have the ability to maintain its desirable properties for useful times under likely in-service conditions,” he says. “My impression is that current researchers tend to overlook this inconvenient property of silk when they promise great things from this material.” Michal is also interested in what happens to the wetted silk, though he admits that devising a practical experimental setup for this can pose many challenges. A single 50 mg silk sample can contain thousands of individual strands. Each fiber must be stretched uniformly for the results to be meaningful. While his table-top experiments cannot emulate real-life situations, he suggests that the knowledge derived from small-scale investigations will be invaluable to producing a commercially-viable product. “You can take whatever proteins you like and spin them into fibers,” he says. “And usually the chances are good that they’ll not be very good fibers. So if you want to produce a fiber that has the
properties you want, it’s really important to understand why the material you’re trying to copy has the properties it does.” Careful tailoring of properties could well be the key to commercial success. So dragline silk isn’t matching up to the hype, then? Well, maybe that is because it is designed to meet spiders’ needs, not humans’. Rather than struggling to produce an exact copy of dragline silk, perhaps we should take a leaf out of the spider’s book and learn to manufacture appropriate materials tailored to specific human needs. And this shouldn’t be too difficult, according to Frank Ko, director of the Fibrous Materials Research Center at Drexel University in Philadelphia. Ko believes that materials engineers will have no trouble at all in achieving specific functionality by incorporating nanoparticles into a man-made solution of spider silk protein. When this technique was tested in his laboratory at Drexel, and also during a sabbatical at the University of California in Los Angeles, Ko and his colleagues successfully produced nanocomposite fibrils when the polymer solution underwent electrostatic spinning. “Electronic, magnetic, biological, and structural functions of the fibers can be tailored by using different types of particles and the amount of the particles,” Ko says. Back at Nexia, Turner accepts that they still have some way to go in matching their spider silk materials to intended jobs. The fibers spun from goat milk already meet the mechanical requirements for fishing line. However, further work is needed to increase BioSteel®’s strength and reduce its elongation if they are to market clothing woven from the material as superior ballistic protection. “People say: ‘Oh, spider silk. Everybody’s working on it. That's easy to make.’ But it isn’t,” Turner says. “We’re spending about $1 million a month now on developing this product. We’ve got a group of really talented, serious people doing the development, and big partners, and it’s still taking time. That’s an important message.” MT
REFERENCES 1. Vollrath, F., et al., J. Biotechnol. (2000) 74 (2), 67-83 2. Oroudjev, E., et al., Proc. Natl. Acad. Sci. (2002) 99 (Suppl. 2), 6460-6465 3. Simmons, A., et al., Science (1996) 271 (5245), 84-87 4. Lazaris, A., et al., Science (2002) 295, 472-476 Fig. 7 Highly magnified photo (x200) of a BioSteel® fiber. ( Credit: S. Arcidiacono et al., SBCCOM, Natick, Massachusetts.)
5. Bell, F. I., et al., Nature (2002) 416, 37
December 2002
47
by Paula Gould
Scientists and engineers have long envied spiders’ ability to manufacture a material that is simultaneously strong, fine, and tough. This combination of properties makes spider silk an extremely attractive candidate for numerous applications in medicine, defense, and the leisure industry. Research teams worldwide are gradually piecing together the relationship between the silk’s make-up and its unusual – but highly useful – behavior. At the same time, efforts are underway to mass-produce a similarly super-strong fiber that displays both the strength and elasticity of its natural, backyard counterpart.
42
December 2002
As today’s film-goers and readers of the original Spiderman comic strip will know, spider silk makes an ideal tool for a modern day super-hero. The fine line can be produced at will, holds our hero’s weight as he swings between tall buildings, and then clumps together to trap dastardly villains in a tangled, sticky mess. Highly useful if your daily job description includes rescuing distressed damsels from improbable locations and preventing a thoroughly nasty goblin from wreaking havoc on your home city. Fiction aside, the fibrous threads produced by our eightlegged friends really do have extraordinary properties1. Spiders can spin up to seven different types of silk, each one tailor-made to fit a specific function. To date, most researchers have focused their attention on ‘dragline’ silk, used by spiders as a safety line and also as the frame for their webs (Fig. 1). Weight-for-weight stronger than steel, finer than human hair, more resilient than any synthetic fiber, and completely biodegradable, dragline silk is every inch the 21st century super-material. Suggested applications have ranged from surgical sutures and ligament repair to parachute straps and high-performance, flexible body armor. No wonder the field is attracting interest from entrepreneurs and military personnel, as well as university researchers. So what is it that gives spider silk its amazing properties? That is something Helen Hansma and colleagues from the Department of Physics, University of California, Santa Barbara (UCSB) would like to find out. Keen to look closer at the material’s make-up, the UCSB team used atomic force microscopy (AFM) to analyze a protein solution that had
ISSN:1369 7021 © Elsevier Science Ltd 2002
APPLICATIONS FEATURE
been synthesized from spider dragline silk by researchers at the US Army Natick Research and Development Center in Massachusetts2. They observed the solution self-assemble into nanofibers, each with a segmented substructure. Measurements of the nanofiber segments suggest that each contains a stack of slab-like molecules. The stacking pattern effectively produces a series of nanocrystals within an amorphous matrix, matching with previous observations of the silk’s composite structure from nuclear magnetic resonance (NMR) and X-ray diffraction. In short, they found themselves watching how spiders’ liquid crystalline ‘dope’ turns into stretchy strands (Fig. 2). Hansma’s team is using single-molecule force spectroscopy to see precisely what happens when you pull on the silk. Data on exactly how the protein unfolds will be used to improve models for the structure and mechanics of silk molecules and fibers. However, in addition to pulling on the synthetic dragline silk protein, they have also been experimenting with bulk capture web, a silk noted for its extremely high elasticity. These tests required some cooperation from the lab’s smallest recruits, living spiders similar to those inhabiting Hansma’s backyard. For physicists perhaps more used to working with inanimate objects, the project required a new set of skills altogether. “We started catching spiders and keeping them in aquariums, and then specially made spider boxes. We experimented with feeding them fruit flies, raised on things like cantaloupe rinds,” Hansma says. “The graduate students and post-docs all seemed to enjoy sitting and watching the spiders in the aquarium. I called them ‘physicists at work’.”
Fig. 1 Dragline silk is used by spiders as a safety line and also as the frame for their webs. (Courtesy of E. Oroudjev and H. Hansma.)
Carl Michal is another physicist who now finds himself sharing his workspace with several small, eight-legged creatures. He first started working on spider silk as a graduate student at Cornell University under the supervision of Lynn Golinski, using NMR techniques and computer simulations to examine the orientation of crystalline amino acids in dragline silk3. Michal is now continuing this line of enquiry at the University of British Columbia (UBC) in Vancouver, with the
Fig. 2 Nanofiber (~0.5 µm long) formed spontaneously from depositing a solution of soluble recombinant spider dragline silk protein on a freshly cleaved mica surface. Bumps along the fiber reveal that it is composed of segments. (Courtesy of E. Oroudjev and H. Hansma.)
December 2002
43
APPLICATIONS FEATURE
help of his own graduate students. They feed lab-housed spiders a solution containing amino acids in which one of the atoms has been replaced by a specific stable isotope label, e.g. C-13, N-15, or deuterium. Harvested silk is put inside an NMR probe. Team members then work backwards from the resulting images to determine how the labeled amino acids are distributed. “As we stretch or relax the silk, we’re hoping to be able to see in our model the chains stretch out along the direction of the silk and the crystals orient or disorient as we relax it,” Michal explains. “We’ve got a better feel now for how much of the silk is crystalline, what the crystals are doing, and what the chains are actually doing as the material is performing.” Rather than trapping the spiders themselves, the UBC researchers get their spiders specially shipped from Florida. They work with Nephila clavipes, a golden orb-weaving spider whose silk has been studied in detail by numerous groups worldwide (Fig. 3). The availability of so much background information is not the only thing going for their chosen subject, though. Nephila clavipes is a pretty big spider, which makes routine operations that little bit easier (Fig. 4). “For our experiments, we actually need a fair bit of silk, so having a big spider is really advantageous,” Michal says. “They are also a lot easier to handle than some of the smaller spiders.” While content with the investigation’s status as a pure research project, Michal is hopeful that the advanced models his team is generating will be of interest to others planning to market materials based on dragline silk. His methods of obtaining the raw material, however, will be of little
Fig. 3 The large Nephila clavipes golden orb-weaving spider is favored by most spider silk researchers.(Courtesy of C. Michal.)
44
December 2002
relevance to groups with commercialization in mind. On a good day, UBC researchers might harvest 1.5 mg of silk, a small but significant contribution towards the 50 mg needed to run NMR tests. Clearly, this is nowhere near enough to be commercially viable. And farming the spiders just isn’t a workable option owing to their highly territorial and aggressive nature.
Milking it Faced with this apparent obstacle, Jeffrey Turner, CEO of Nexia Biotechnologies, Inc. decided to do away with spiders altogether. If the arachnids were not going to cooperate, why not simply copy their production techniques? His faith in biomimicry paid off and, in January 2002, the company announced the successful production of BioSteel®, a material based on recombinant spider silk proteins, but derived from the milk of transgenic goats4. Even the Spiderman scriptwriters could not have come up with such an unlikely means of production. Yet Turner’s description of the steps leading up to the silk’s production makes the whole process sound entirely logical in retrospect. Having ruled out traditional fermentation techniques, Nexia researchers took some genes from spider dragline silk, attached some regulatory elements to switch the genes ‘on’ and ‘off’, and then inserted them into so-called ‘MAC-T’ cells, the mammary gland cells usually used for making milk. “And it was really the ‘Eureka!’ moment,” Turner says. “The mammary cells took the spider silk genes in all their complexity and made a beautiful, watersoluble, authentic spider silk. That was a really incredible time.” But this was only step one. The team had simply found an efficient means of producing the complex protein in a watersoluble format. Nothing more, nothing less. What they really needed was a way to turn their watery solution into silk fibers, something US Army researchers at Natick could help with. Not only had this group been developing synthetic silk proteins, but they also had considerable expertise in spinning fibers from the protein solutions. Turner’s enthusiasm is infectious as he describes what happened next. “It was unbelievable,” he says. “The protein came zipping out of the syringe tip and when the water was removed, it selfassembled into a beautiful silk. I’m not sure who was more excited, them or us.” Of course, the researchers still needed a viable production line, so they inserted individual spider silk
APPLICATIONS FEATURE
genes into single cell goat eggs. Milk from the resulting transgenic goats duly contained water-soluble silk proteins. Turner is understandably reluctant to say precisely how much silk solution a single goat will generate, though he notes that just one animal can produce ‘literally miles’ of silk. The number of transgenic goats in Nexia’s farms has grown to around 100, with the total herd size topping 1500 (Figs. 5 and 6). This level of production could be sufficient to meet the needs of the medical devices industry, though a herd ten times as big would be needed to satisfy industrial demands. The company is also planning to work with a major textiles manufacturer, who can help spin high quantities of the fiber at a more commercially-viable rate (Fig. 7). Biodegradable fishing line is likely to be the first Nexia product to reach consumers, with a market launch expected within the next two years. The biphasic modulus of dragline silk is the key selling point here, according to Turner. Initially stiff when pulled, spider silk elongates before breaking. Where existing lines might snap under the force of a large
fish biting and then trying to flee the scene, a spider silk line would hopefully respond to any sudden dashes for freedom by giving a little. This stiffness/stretchiness combination has also led Turner to consider touting BioSteel® as a vital ingredient in tendon and ligament repair. Rupturing an anterior cruciate ligament can mean the end of an athlete’s sporting career. But if a biocompatible material could be found that does not become fatigued when flexed regularly, is strong enough to withstand regular impact, and does not snap under pressure… how much would that be worth to a star athlete facing premature retirement? But this is still a good few years away, owing to necessarily complex regulations surrounding such product development. A simpler product destined for the medical market, and one more likely to appear by the end of 2004, is the BioSteel® microsuture. Here, the fineness and knotability of silk will be stressed, in addition to its strength and biodegradability, to persuade ophthalmic and neurological surgeons to switch from nylon-based threads.
Fig. 4 Physicists at the University of British Columbia have developed new hands-on experimental skills in their work with living spiders. (Courtesy of C. Michal.)
45 December 2002
APPLICATIONS FEATURE
Fig. 5 Peter and Webster – Nexia's first transgenic goats. (Credit: Sean O'Neill.)
Fig. 6 Nexia's rather unusual production line is expanding to meet the needs of the medical and industrial markets. (Credit: Sean O'Neill.)
Last, but by no means least, Nexia is working to create better ballistic protection for policemen and women and the armed forces. Turner points to the inflexibility and heavy weight of today’s standard issue bullet-proof vests. Body armor woven from dragline silk would not only be more comfortable, but it would also be more effective at distributing the force from an explosion or gunshot over a wider area, lessening the blow. “When a fly hits a spider’s web, about 70% of the kinetic energy is converted into heat and dissipated. That’s really important,” he says. “In the case of ballistics, the amount of deformation or shock that the soldier or police officer would feel from a bullet would be considerably reduced. It’s this energy management of spider silk that has people really excited.”
Viney’s chief concern is that the promise of these superstrong synthetic materials relies on laboratory tests that do not mirror the situations in which the products will have to perform. In fact, sometimes they do not even get close. As a prime example, he cites controlled experiments routinely used to measure the strength, stiffness, and toughness of silks that are conducted at a constant strain rate for less than an hour. Yet many engineering functions envisage the new materials carrying high loads for long periods of time, making the behavior of spider dragline under a constant load for days, weeks, or perhaps even years a more appropriate study. He points also to the new materials’ expected durability. Many of the anticipated applications will require man-made spider silks to remain stable during changes in moisture and temperature, and possible exposure to ultraviolet radiation. The natural product has no such built-in longevity. “Spiders produce their dragline and webs to do a specific job, which is accomplished in minutes, e.g. the use of dragline to expedite a vertical ascent or descent, or hours, e.g. the use of a web to catch prey,” Viney says. “Webs are repaired or replaced daily. These silks are not designed to last!” His experiments into the behavior of wetted dragline silk have thrown up some perturbing findings. These relate to the silk’s tendency to ‘supercontract’ when wet, halving in length and almost doubling in diameter. Viney and his colleagues at Heriot-Watt measured the supercontraction stresses in 30 mm lengths of Nephila clavipes dragline exposed to
What everyone wants But is there a danger of getting over-excited? Even Nexia are still only working towards prototypes. What if commercially-manufactured spider silk fibers don’t match up to expectations after all? And even if companies do replicate the properties of natural spider silk in a man-made material, what if no one is willing to take the risk on a new product? Chris Viney, head of materials chemistry at Heriot-Watt University, Edinburgh, admits to adopting a more cautious approach than many others in the field. His caution is borne out of over ten years’ research experience in the area.
46
December 2002
APPLICATIONS FEATURE
increasing humidity5. Their findings proved surprising in two ways. First, the stresses generated were transiently higher than expected. Second, the stresses relaxed after just a few minutes, indicating the wet dragline’s inability to maintain tension – or support a load – indefinitely. Not only does the material abandon its significant load-carrying responsibilities when wet, but it is also likely to creep rapidly and change its shape, according to Viney. This has serious implications for the use of man-made spider silks in high tensile strength fibers. “The longer I work in this field, the more I worry that natural silk many not have the ability to maintain its desirable properties for useful times under likely in-service conditions,” he says. “My impression is that current researchers tend to overlook this inconvenient property of silk when they promise great things from this material.” Michal is also interested in what happens to the wetted silk, though he admits that devising a practical experimental setup for this can pose many challenges. A single 50 mg silk sample can contain thousands of individual strands. Each fiber must be stretched uniformly for the results to be meaningful. While his table-top experiments cannot emulate real-life situations, he suggests that the knowledge derived from small-scale investigations will be invaluable to producing a commercially-viable product. “You can take whatever proteins you like and spin them into fibers,” he says. “And usually the chances are good that they’ll not be very good fibers. So if you want to produce a fiber that has the
properties you want, it’s really important to understand why the material you’re trying to copy has the properties it does.” Careful tailoring of properties could well be the key to commercial success. So dragline silk isn’t matching up to the hype, then? Well, maybe that is because it is designed to meet spiders’ needs, not humans’. Rather than struggling to produce an exact copy of dragline silk, perhaps we should take a leaf out of the spider’s book and learn to manufacture appropriate materials tailored to specific human needs. And this shouldn’t be too difficult, according to Frank Ko, director of the Fibrous Materials Research Center at Drexel University in Philadelphia. Ko believes that materials engineers will have no trouble at all in achieving specific functionality by incorporating nanoparticles into a man-made solution of spider silk protein. When this technique was tested in his laboratory at Drexel, and also during a sabbatical at the University of California in Los Angeles, Ko and his colleagues successfully produced nanocomposite fibrils when the polymer solution underwent electrostatic spinning. “Electronic, magnetic, biological, and structural functions of the fibers can be tailored by using different types of particles and the amount of the particles,” Ko says. Back at Nexia, Turner accepts that they still have some way to go in matching their spider silk materials to intended jobs. The fibers spun from goat milk already meet the mechanical requirements for fishing line. However, further work is needed to increase BioSteel®’s strength and reduce its elongation if they are to market clothing woven from the material as superior ballistic protection. “People say: ‘Oh, spider silk. Everybody’s working on it. That's easy to make.’ But it isn’t,” Turner says. “We’re spending about $1 million a month now on developing this product. We’ve got a group of really talented, serious people doing the development, and big partners, and it’s still taking time. That’s an important message.” MT
REFERENCES 1. Vollrath, F., et al., J. Biotechnol. (2000) 74 (2), 67-83 2. Oroudjev, E., et al., Proc. Natl. Acad. Sci. (2002) 99 (Suppl. 2), 6460-6465 3. Simmons, A., et al., Science (1996) 271 (5245), 84-87 4. Lazaris, A., et al., Science (2002) 295, 472-476 Fig. 7 Highly magnified photo (x200) of a BioSteel® fiber. ( Credit: S. Arcidiacono et al., SBCCOM, Natick, Massachusetts.)
5. Bell, F. I., et al., Nature (2002) 416, 37
December 2002
47