Composition and Structure of Oyster Adhesive Reveals Heterogeneous Materials Properties in a Biological Composite

Abstract

Oyster reefs help maintain coastal ecosystems by filtering water, holding silt in place, and absorbing storm surge energy. We are just beginning to understand the chemical and structural nature of the adhesive used by these animals for building such reef communities. The adhesive has a high calcium carbonate content relative to other bioadhesives, but also appreciable levels of organics, presumably for bonding. In studies presented here we used X-ray absorption near edge structure (XANES) spectroscopy, X-ray photoemission electron microscopy (X-PEEM), scanning electron microscopy (SEM), and micro-hardness methods to understand the composition, as well as the mechanical properties, of this biological material. Oyster adhesive appears to be a heterogeneous mixture of calcium carbonate and silica inclusions arranged randomly within a matrix that lacks any observable structure. Micro-indentation shows inclusions are significantly harder than their surroundings. This hard plus soft strategy has been noted in other biological materials, although not in any adhesives. These compositional and structural insights help us propose a mechanism by which the animals generate their adhesive. Such an intriguing structure, along with resulting mechanical implications, may help explain how oyster reefs can thrive despite being subjected to demanding forces created by predators and the environment around them.

Silk-Like Protein with Persistent Radicals Identified in Oyster Adhesive by Solid-State NMR

“…The two most common crystal forms of calcium carbonate, calcite and aragonite, give distinct 13 C NMR peaks near 168.8 and 170.9 ppm, respectively, with a 2.3 ppm shift difference, while the less common carbonate form, vaterite, exhibits two 13 C NMR resonances with a small splitting of 1.1 ppm. All spectra in Figure showed a calcite peak, as expected for oyster shell. , The spectrum of oyster cement stood out, being the only layer exhibiting two sharp carbonate peaks. The second peak was assigned to aragonite.…”

Changes in Cementation of Reef Building Oysters Transitioning from Larvae to Adults

“…With the animal being older, the adhesive was more mineralized. Further growth into adulthood was consistent with prior findings of an unstructured adhesive layer bonding shell to substrate. − …”

Mineral-Enhanced Polyacrylic Acid Hydrogel as an Oyster-Inspired Organic–Inorganic Hybrid Adhesive

“…Water is present in the form of structural water in ACC. Here, it is noted that both the high content of CaCO 3 in the hybrid hydrogel (∼45 wt %) and its fluidity for adhesion are reminiscent of the oyster organic–inorganic hybrid adhesive, − which also differ this adhesive material from other reported composite adhesives with a majority of organic matrix and inorganic particles only as fillers. ,− …”