In nature, tiny amounts of inorganic impurities, such as metals,
are incorporated in the protein structures of some biomaterials
and lead to unusual mechanical properties of those materials.
A desire to produce these biomimicking new materials has stimulated
materials scientists, and diverse approaches have been attempted.
In contrast, research to improve the mechanical properties of
biomaterials themselves by direct metal incorporation into inner
protein structures has rarely been tried because of the difficulty
of developing a method that can infiltrate metals into biomaterials,
resulting in a metal-incorporated protein matrix. We demonstrated
that metals can be intentionally infiltrated into inner protein
structures of biomaterials through multiple pulsed vapor-phase
infiltration performed with equipment conventionally used for
atomic layer deposition (ALD). We infiltrated zinc (Zn), titanium
(Ti), or aluminum (Al), combined with water from corresponding
ALD precursors, into spider dragline silks and observed greatly
improved toughness of the resulting silks. The presence of the
infiltrated metals such as Al or Ti was verified by energy-dispersive
x-ray (EDX) and nuclear magnetic resonance spectra measured
inside the treated silks. This result of enhanced toughness
of spider silk could potentially serve as a model for a more
general approach to enhance the strength and toughness of other
biomaterials.
