Spider Silk: Structure, Engineering, and Applications - Soft Fibrillar Materials: Fabrication and Applications - page 195

6.3.3
Tougher Silk than Natural Spider Silk

Recently it was interestingly found that metals can be infiltrated into inner
protein structures of silk to significantly improve its toughness [92]. This metal
incorporation was achieved through multiple pulsed vapor-phase infiltration with
equipment conventionally used for atomic layer deposition (ALD). Metals like
zinc (Zn), titanium (Ti), or aluminum (Al), were infiltrated with water from
corresponding ALD precursors, into spider dragline silks.
An ALD process conventionally leads to thin deposited layers of metal oxide on
a fiber [92]. Therefore, the question arises whether the increased strength is due
to the deposition of these layers. The authors claimed that, since the increase of
the maximum tensile stress and strain was independent of the contribution of the
Al 2 O 3 or TiO 2 coating, the contribution of the outer metal oxide layer coating on
the fiber to the improvement of its mechanical properties is of minor importance.
On the contrary, the increased toughness of the silk fibers appears to be caused
by an infiltration of inorganic impurities such as Al or Ti, which presumably
react with proteins after a preconditioning by the water penetrating into the fiber
[93, 94], a process probably similar to the hardness- and stiffness-increasing effects
on Nereis jaws produced by a small amount of artificial Zn incorporation into the
jaws [95]. The transmission electron microscopy (TEM) images of SS/TiO 2 /500
show that, along the TiO 2 shell, a region of 100 nm in depth shows a high

image contrast. Considering the relative weight ratio of carbon (C), oxygen (O),
and Ti, a large amount of Ti was infiltrated into this shell region. In the center
part of the silks (folded region), energy-dispersive X-ray (EDX) analysis detected
weak but clear Ti signals (1.42-2.83% by relative weight ratio). In addition, indirect
but similar evidence for the infiltrated Al ions into silk, which interact with the
silk proteins, was observed by magic angle spinning NMR measurements of
SS/Al 2 O 3 /300.
It was suggested that the strong reactivity to chemical bonds [93] and deep
penetrating capability of metal-containing ALD precursors (such as TMA, TIP,
and DEZ) into soft materials such as polymers [94] and the metal ions' prefer-
ential binding features to the proteins [96, 97] could lead to stable metal-protein
compounds by chelating ions [98, 99] such as Al 3+ [100], Ti 4+ [101], or Zn 2+
[102] to form metal-protein complexes. During long exposure to water vapor
(5-40 s), the inner hydrogen bonds of the silk protein are likely to be broken
in some regions upon water vapor attack at 70 ◦ C. Subsequently after long-term
exposure to the metal precursor vapor, Al 3+ ,Ti 4+ ,orZn 2+ is likely to infiltrate
the protein and bind to the broken bonding sites, resulting in the formation of
metal-coordinated or even covalent bonds. As a minor additional effect, methane
(TMA/H 2 O) or isopropanol (TIP/H 2 O) as reaction byproducts may have additional
effects to weaken or break the remaining hydrogen bonds. Consequently, the
recoverable hydrogen bonds may be changed to permanent covalently bonded or
coordinated Al-, Ti-, or Zn-protein complexes. Therefore, unlike native silks, which
are highly sensitive to environmental conditions such as humidity and temperature

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