Amyloid-Based Natural Adhesives and Cements - The Functional Fold: Amyloid Structures in Nature - page 139

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in biological adhesives, that is, functional amyloid formed ex vivo .

A detailed mechanical understanding of the varying amyloid
structures that underpin a broad range of natural adhesives has
been developed.

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8.3.1 Nanomechanical Properies of Natural Adhesives:
Amyloid Structures Provide Mechanical Strength
and Toughness

One of the most important evaluation criteria of materials is their
mechanical stability to applied load and ultimately how they fail
or break as the load increases. To evaluate this for amyloid based
adhesives, AFM has been used to gently pull the material apart.
Locations where amyloid structures may occur are highlighted due
to the very repetitive nature of the mechanical response (Fig. 8.4)
and sometimes also from AFM images of the material which can
indicate the presence of fibrillar structures (Fig. 8.2b). Pulling apart
any repetitive structure is clearly indicated in force-extension curves
produced when an AFM tip binds (non-specific adsorption) to the
sample surface, and the substrate and tip are moved apart under an
increasing tensile load, thus unravelling the structure until it either
breaks or detaches from the tip or substrate

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(Fig. 8.4b).
Resultant force-distance curves, show a repetitive mechanical
response corresponding to the unravelling of the repetitive structural
units of amyloid on retraction of the AFM tip. Observed as a series
of well-ordered, equally spaced sawtooth structures,

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they
represent the repetitive breaking of “sacrificial bonds” within an
intermolecular

β

-sheet, and the subsequent extension of the “hidden
length” of the amino acid chain. This mechanism imparts superior

mechanical strength and toughness when compared to systems of
similar overall length without such “sacrificial bonds”. This is due
to the extra energy required to extend the “hidden length” and
rupture the “sacrificial bonds” before the main structural backbone
is broken.

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This same mechanism has been measured previously
in modular proteins such as titin,

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and the “glue” adding fracture

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resistance to bone. It does not have to be associated with adhesion,

but instead influences the cohesive strength of the material. Also, it
is important to note that in complex materials it is not yet feasible
to identify (sub-)components of the material from the mechanical
response alone, as the uniqueness of mechanical “signatures”