Spider Silk: Structure, Engineering, and Applications - Soft Fibrillar Materials: Fabrication and Applications - page 188

Consequently, faster-reeling fibers have a higher proportion of intra-molecular
β

-sheets [30].
The property of strain hardening of spider draglines plays a key role in the
function of supporting a suspended spider. In the case of a spider escaping from a
predator by abseiling [47], the soft spider dragline can effectively buffer the impact
force with its gradually hardening property. The stress-strain profile of the spider
dragline (Figure 6.4) shows that the J -shaped strain-hardening curve accounts for
a very large extension, enabling quick absorption of tremendous energy at low
applied stress, so draglines can facilitate safe and gentle falls.

6.2.3
Environmental Effects on the Mechanical Properties of Spider Silk

Environmental conditions such as ambient humidity [48], temperature [49], and
UV radiation [50] all affect the mechanical properties of native silk. In particular, the
two unusual properties of spider dragline silk, supercontraction and temperature
dependence of mechanical properties, are briefly reviewed in this section.

6.2.3.1 Supercontraction of Spider Draglines
Significant shrinkage of a dragline fiber occurs when it absorbs water. This
process is known as supercontraction . In nature, this characteristic property allows
reorientation of hydrogen bonds between the spider silk protein molecules during
the uptake of water [51-53], thereby plasticizing the thread and changing its
mechanical properties [35, 51]. By this process, ''worn-out'' silk threads within a
spider's net are renewed in the morning dew, and the web regains its rigidity [53, 54].
Remarkably, supercontraction of spider silk takes place at ambient temperatures,
whereas induction of the same process in man-made fibers generally requires
elevated temperatures or harsh solvent conditions [35].
As revealed by solid-state NMR, dragline fibers undergo a collapse of amino acid
chains when contacting with water. The inter-chain hydrogen bonds are broken
by hydration, leading to local phase transitions to a rubbery state in the process of
supercontraction (Figure 6.6). As a result, supercontracted silk exhibit rubber-like
mechanical responses [55].

6.2.3.2 Tough Silk at Low Temperature
The work by Yang et al . [49] reported the unusual mechanical performance of
spider dragline silk at low temperatures. Remarkably, increasing temperature up
to 150 ◦ C produced surprisingly little changes in the tensile behavior of spider
draglines, while cooling down to 60 ◦ C from room temperature doubled the

strength and elongation. This means that at low temperatures the spider silk can
outperform the toughest (i.e., most energy absorbent) synthetic polymer fibers. The
authors suggested that the underlying mechanism is similar to that for enhanced
strength and toughness at a higher strain rate, since for viscoelastic processes low
temperatures are equivalent to high strain rates. This temperature dependence of
mechanical properties of spider silk (as opposed to that of other polymers) is yet

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