Lessons from the web

Call it silk envy.

For years, engineers and materials scientists have marvelled at the strength that spider silk displays — topping that of steel. Now a research team has discovered how the silk responds to stress — a response unlike that in any material humans have engineered. The results appear in a recent issue of the journal Nature.

In essence, when pressure is applied, the silk first stiffens. As pressure increases, it stretches, only to stiffen again as the pressure reaches the silk's breaking point. These traits help explain why damage to a spider web from daily use remains local, preserving the integrity of the web as a whole, the researchers say.

It is an explanation with potential benefits for human engineering, says Markus Buehler, an associate professor of engineering at the Massachusetts Institute of Technology in Cambridge, who led the research team. Indeed, the information gleaned could help humans take advantage of unique building blocks for new materials, such as carbon nanotubes.

"These are superb materials" at scales of billionths of a metre, Buehler says. But so far, adapting nanotubes to human-scale projects has proved difficult. "If you want to actually use these [materials] to make a car or a coating or a building, it doesn't work, because we lose all the strength" exhibited by the pint-size samples, Buehler says.

But the new analysis of spider silk could allow scientists to use such building blocks to craft materials that take advantage, on human scales, of properties that once appeared only at nanometre sizes.

In demonstrating how the silk contributes to the hardiness of the network of strands that make up a web, Buehler says, the results eventually could help engineers craft power grids, data networks and even buildings to survive natural or human assaults with minimal damage.

Spiders have had nearly 400 million years of evolutionary history to hone their chemistry and engineering tools, notes Jessica Garb, an ecologist at the University of Massachusetts at Lowell. Developing the ability to excrete a silk thread that responds differently to different levels of stress comes from trial and error, through random genetic mutations, she says.

Spiders with slight genetic mutations that generated a mix of proteins that built the silk best suited for webs capturing the most prey survived to keep the evolutionary wheels turning. As part of this process, spiders have developed an array of silk types. Single spiders are capable of producing several types — from structural silks to sticky, prey-snagging strands, to glues that fasten a web's main structural threads to bush branches or barn beams.

For its study, Buehler's team selected orb-weaver spiders . The webs are roughly circular, with spokes. They are filled in with threads that spiral out from the hub. The spoke-like strands are the strongest. The spirals are weaker and coated with a sticky adhesive to trap insects.

Buehler's team used a detailed computer model to produce virtual silk threads, starting with the amino acids that made up the proteins involved. Then the team used the model to explore the silks' properties and gauge their contributions to a web's robustness. Materials that humans engineer — from rubber to steel — tend to grow stiff until they break — or they grow pliable under increasing stress, Buehler explains. But the modelling revealed that the silk threads forming the spokes had the ability to stiffen, stretch, then stiffen again as stress was added. A spiral thread may break as a flying insect collides with it, but the damage is limited to the span between spokes. If the bug hits a spoke, the silk thread yields before stiffening again and breaking. This reduces the shock of impact on other spokes and allows them to hold the web together. The marvel, Buehler says, is that the silk is made at ambient temperatures inside a spider from simple building blocks whose ability to bond to one another is so weak that engineers would never consider using them for human-scale applications.

"Yet if you look at the system with engineering tools, it's absolutely stunning," he says. "We don't have anything like this. Yet a spider makes this on the fly."