Core structure of aligned chitin fibers within the interlamellar framework extracted from Haliotis rufescens nacre. Part I: implications for growth and mechanical response

By means of consecutive alkaline and proteolytic treatments of the organic framework’s interlamellar layers extracted from the nacre of H. rufescens, we have exposed a core of aligned parallel chitin fibers. Our findings both verify basic elements of the interlamellar layer structural model of Levi-Kalisman et al. (2001) and extend the more detailed model of Bezares et al. (2008, 2010). We observe via SEM imaging of square millimeter sized samples, which include numerous interlamellar layers and micron sized, yet nanocrystalline, CaCO3 tiles whose native orientation within the shell was first documented, that the chitin fibers in all layers are aligned normal to the growth direction of the shell. Similar alignment has been suggested in the literature for two other classes of mollusks, viz. N. rupertus and P. martensii (Weiner and Traub, 1983; Wada, 1958), suggesting that this may be a more general motif. We find that in order to expose the chitin core it is necessary to first remove protein by an alkaline treatment followed by enzymatic digestion with proteinase-K. We also observe what appear to be the ∗Department of Structural Engineering, University of California, San Diego, La Jolla, CA 92093, e-mail: [email protected] †Department of Structural Engineering, University of California, San Diego, La Jolla, CA 92093, e-mail: [email protected] ‡Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093, e-mail: [email protected]