The beak of the Humboldt squid Dosidicus gigas represents one of the hardest and stiffest wholly organic materials known. As it is deeply embedded within the soft buccal envelope, the manner in which impact forces are transmitted between beak and envelope is a matter of considerable scientific interest. Here, we show that the hydrated beak exhibits a large stiffness gradient, spanning two orders of magnitude from the tip to the base. This gradient is correlated with a chemical gradient involving mixtures of chitin, water, and His-rich proteins that contain 3,4-dihydroxyphenyl-L-alanine (dopa) and undergo extensive stabilization by histidyl-dopa cross-link formation. These findings may serve as a foundation for identifying design principles for attaching mechanically mismatched materials in engineering and biological applications.Living organisms are functional assemblages of different interconnected tissues. Not infrequently, tissues with highly disparate mechanical properties (e.g., bone and cartilage, shell and adductor muscle, nail and skin) are joined together (1). In practice, the joining of dissimilar materials can lead to high interfacial stresses and contact damage (2,3). In apparent contradiction to this, the contacts between mechanically mismatched biomolecular tissues are remarkably robust. Mechanical-property gradients are increasingly invoked as the principal reason for their mechanical performance. The dentino-enamel junction (4), arthropod exoskeleton (5), polychaete jaws, and mussel byssal threads (6) all exhibit such gradients. Optical properties in squid eyes have also been correlated to a protein-density gradient (7). Although much is known about the mechanical and biochemical properties of the separate tissues, surprisingly little has been done to explain how mixtures of macromolecules are adapted for incremental mechanical effects at interfaces.The beak of the Humboldt squid Dosidicus gigas is an example of a system with grossly mismatched tissues. It is composed of slightly offset apposing upper and lower parts that make no hard pivotal contact with one another and are set into a muscular buccal mass that controls
Formulating effective coatings for use in nano- and biotechnology poses considerable technical challenges. If they are to provide abrasion resistance, coatings must be hard and adhere well to the underlying substrate. High hardness, however, comes at the expense of extensibility. This property trade-off makes the design of coatings for even moderately compliant substrates problematic, because substrate deformation easily exceeds the strain limit of the coating. Although the highest strain capacity of synthetic fibre coatings is less than 10%, deformable coatings are ubiquitous in biological systems. With an eye to heeding the lessons of nature, the cuticular coatings of byssal threads from two species of marine mussels, Mytilus galloprovincialis and Perna canaliculus, have been investigated. Consistent with their function to protect collagenous fibres in the byssal-thread core, these coatings show hardness and stiffness comparable to those of engineering plastics and yet are surprisingly extensible; the tensile failure strain of P. canaliculus cuticle is about 30% and that of M. galloprovincialis is a remarkable 70%. The difference in extensibility is attributable to the presence of deformable microphase-separated granules within the cuticle of M. galloprovincialis. The results have important implications in the design of bio-inspired extensible coatings.
The cuticle of mussel byssal threads is a robust natural coating that combines high extensibility with high stiffness and hardness. In this study, fluorescence microscopy and elemental analysis were exploited to show that the 3,4-dihydroxyphenyl-L-alanine (dopa) residues of mussel foot protein-1 colocalize with Fe and Ca distributions in the cuticle of Mytilus galloprovincialis mussel byssal threads. Chelated removal of Fe and Ca from the cuticle of intact threads resulted in a 50% reduction in cuticle hardness, and thin sections subjected to the same treatment showed a disruption of cuticle integrity. Dopa-metal complexes may provide significant interactions for the integrity of composite cuticles deformed under tension.
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