Nano-Mechanical Investigation of the Byssal Cuticle, a Protective Coating of a Bio-Elastomer
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Nano-Mechanical Investigation of the Byssal Cuticle, a Protective Coating of a BioElastomer Niels Holten-Andersen1, Nelle Slack2, Frank Zok2 and J. Herbert Waite1 1 Biomolecular Science & Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106, USA. 2 Materials Department, University of California at Santa Barbara, Santa Barbara, CA 93106, USA. ABSTRACT The mechanical properties of the mussel byssal thread have been investigated via nanoindentation, with the emphasis on the differences between the cuticle and the fibrous interior. The cuticle hardness was found to be 30-40% higher than that of the underlying fibrous interior. In contrast, the Young’s moduli in the two regions were virtually identical to one another. Elemental analysis via energy dispersive spectroscopy indicated surprisingly high levels of Al and Br in the cuticle considering the low amounts found in seawater. A potential role of Al in byssal thread mechanics is discussed in light of the unique capability of the cuticle to accommodate strains of 70% by the underlying fibrils in the core without delamination. INTRODUCTION The holdfast structure of mussels, the byssus, has evolved to withstand the challenges of high energy wave action in the intertidal zone [1, 2, 3]. Each byssal thread consists of three functional domains: (i) the glue in the plaques which bonds the thread to a variety of hard surfaces, (ii) the fibrous collagenous core which connects with the glue and undergoes tension due to lift and drag forces, and (iii) the cuticle that covers all parts of the byssus. New byssal threads are formed and cured over a short time-frame (less than 5 min) through a molding process in the groove of the foot. This occurs by secretion of stockpiles of prepolymers of all the molecular constituents from the foot tissue lining the groove [4]. Byssus, as well as its attachment to hard surfaces, is durable for years and hence may serve as an appropriate conceptual paradigm for biomimetic engineering of materials that exhibit excellent extensibility and abrasion resistance. In the distal portion of the thread the cuticle exhibits a granular structural appearance (see Figure 1). This is the part of the thread outside the mussel shell and hence it presumably endows the threads with robust protection against “sand blasting”: abrasive action of sand dispersed in seawater. While abrasion resistance suggests hardness and stiffness, the distal cuticle must also accommodate strains of up to 70% by the underlying fibrils in the distal core without delamination. Hence the distal cuticle material is from an engineering standpoint truly unique since it serves as an example of a protective coating of a highly extensible elastomeric material. The major protein of the cuticle is Mussel foot protein-1 (Mfp-1) [5]. The most reactive functionalities in the sequence are the Dopa residues [6-12]. They tend to auto-oxidize at neutral pH resulting in diDOPA or diquinone cross-linking, but, in the presence of metals they exhibit an even greater
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