researchers have long known of the incredible strength of spider silk,
the robust nature of the tiny filaments cannot alone explain how webs
survive multiple tears and winds that exceed hurricane strength.
a study that combines experimental observations of spider webs with
complex computer simulations shows that web durability depends not only
on silk strength, but on how the overall web design compensates for
damage and the response of individual strands to continuously varying
Reporting in the cover story of the Feb. 2, 2012, issue of Nature,
researchers from the Massachusetts Institute of Technology (MIT) and
the Politecnico di Torino in Italy show how spider web-design localizes
strain and damage, preserving the web as a whole.
research groups have investigated the complex, hierarchical structure
of spider silk and its amazing strength, extensibility and toughness,”
says Markus Buehler, associate professor of civil and environmental
engineering at MIT. “But, while we understand the peculiar behavior of
dragline silk from the ‘nanoscale up’—initially stiff, then softening,
then stiffening again—we have little insight into how the molecular
structure of silk uniquely improves the performance of a web.”
spider webs found in gardens and garages are made from multiple silk
types, but viscid silk and dragline silk are most critical to the
integrity of the web. Viscid silk is stretchy, wet and sticky, and it is
the silk that winds out in increasing spirals from the web center. Its
primary function is to capture prey. Dragline silk is stiff and dry, and
it serves as the threads that radiate out from a web’s center,
providing structural support. Dragline silk is crucial to the mechanical
behavior of the web.
of Buehler’s earlier work showed that dragline silk is composed of a
suite of proteins with a unique molecular structure that lends both
strength and flexibility.
the strength and toughness of silk has been touted before—it is
stronger than steel and tougher than Kevlar by weight—advantages of silk
within a web, beyond such measures, has been unknown,” Buehler adds.
The common spiders represented in the recent study, including orb weavers (Nephila clavipes), garden spiders (Araneus diadematus)
and others, craft familiar, spiraling web patterns atop a scaffolding
of radiating filaments. Building each web takes energy the spider cannot
afford to expend often, so durability is key to the arachnid’s
a series of computer models matched to laboratory experiments with
spider webs, the researchers were able to tease apart what factors play
what role in helping a web endure natural threats that are either
localized, such as a twig falling on a filament, or distributed, such as
our models, we used a molecular dynamics framework in which we scaled
up the molecular behavior of silk threads to the macroscopic world. This
allowed us to investigate different load cases on the web, but more
importantly, it also allowed us to trace and visualize how the web
fractured under extreme loading conditions,” says Anna Tarakanova, who
developed the computer models along with Steven Cranford, both graduate
students in Buehler’s laboratory.
The top illustration shows a detailed view of the molecular structure of spider silk in its natural state, without mechanical load applied, showing the characteristic composite of a semi-amorphous protein phase (thin, wiggly lines) and beta-sheet nanocrystals (thick yellow lines). The bottom illustration shows a detailed view of the molecular structure of silk under extreme stress, showing how the protein chains unwind under stretch and eventually give way to deformation. Credit: M.J. Buehler and G. Bratzel/MIT
computer modeling of the web,” Cranford adds, “we were able to
efficiently create ‘synthetic’ webs, constructed out of virtual silks
that resembled more typical engineering materials such as those that are
linear elastic, like many ceramics, and elastic-plastic materials,
which behave like many metals. With the models, we could make
comparisons between the modeled web’s performance and the performance
seen in the webs made from natural silk. In addition, we could analyze
the web in terms of energy, and details of the local stress and strain,”
which are traits experiments were able to reveal.
study showed that, as one might expect, when any part of a web is
perturbed, the whole web reacts. Such sensitivity is what alerts a
spider to the struggling of a trapped insect. However, the radial and
spiral filaments each play different roles in attenuating motion, and
when stresses are particularly harsh, they are sacrificed so that the
entire web may survive.
concept of selective, localized failure for spider webs is interesting
since it is a distinct departure from the structural principles that
seem to be in play for many biological materials and components,” adds
Dennis Carter, the NSF program director for biomechanics and
mechanobiology who helped support the study.
example, the distributed material components in bone spread stress
broadly, adding strength. There is no ‘wasted’ material, minimizing the
weight of the structure. While all of the bone is being used to resist
force, bone everywhere along the structure tends to be damaged prior to
contrast, a spider’s web is organized to sacrifice local areas so that
failure will not prevent the remaining web from functioning, even if in a
diminished capacity, says Carter. “This is a clever strategy when the
alternative is having to make an entire, new web,” he adds. “As Buehler suggests, engineers can learn from nature and adapt the design strategies that are most appropriate for specific applications.”
when a radial filament in a web is snagged, the web deforms more than
when a relatively compliant spiral filament is caught. However, when
either type fails—under great stress—it is the only filament to fail.
unique nature of the spider-silk proteins enhances that effect. When a
filament is pulled, the silk’s unique molecular structure—a combination
of amorphous proteins and ordered, nanoscale crystals—unfurls as stress
increases, leading to a stretching effect that has four distinct phases:
an initial, linear tugging; a drawn out stretching as the proteins
unfold; a stiffening phase that absorbs the greatest amount of force;
and then a final, stick-slip phase before the silk breaks.
to the researchers’ findings, the failure of silk threads occurs at
points where the filament is disturbed by that external force, but after
failure, the web returns to stability—even in simulations using broad
forces, like hurricane-force winds.
structures are typically designed to withstand large loads with limited
damage, but extreme loads are more difficult to account for,” says
Cranford. “The spider has uniquely solved this problem by allowing a
sacrificial member to fail under high load. One of the first questions a
structural engineer must ask is ‘What is the design load?’ For a spider
web, however, it doesn’t matter if the load is just strong enough to
cause failure, or one hundred times higher—the net effect is the same.
Allowing a sacrificial member to fail removes the unpredictability of
‘extreme’ loads from the design equation.”