Mussel adhesive protein is rich in 3,4-dihydroxy phenylalanine (DOPA) residues, thought to mediate attachment through hydrogen bonding, or by interacting with metal ions. By contrast, oysters use a largely inorganic material. "The oyster cement appears to be harder than the substances mussels and barnacles use for sticking to rocks," commented Wilker. "The adhesives produced by mussels and barnacles are mostly made of proteins, but oyster adhesive is about 90 percent calcium carbonate," he explained, adding that calcium carbonate alone is clearly not an adequate bonding material. "The key to oyster adhesion may be a unique combination of this hard, inorganic component with the remaining 10 percent of the material that is protein."

The team determined this composition by thermogravimetric analysis, progressively heating materials from the oysters and recording mass lost at temperatures corresponding at which each material category is vaporised. While the inorganic fraction predominates, the organic component of oyster cement is around five times greater than the shell and ten times more than the shell lining, or pseudonacre. This 10 percent of oyster cement does bear some similarity to mussel glue in its composition of proteins and the presence of iron.

As well as having a cross-linked organic matrix, and an elevated protein content, the cement differs from the surrounding shells in its calcium carbonate crystal form. This was detemined by infra-red spectroscopy, with shell and pseudonacre absorbing predominantly at 876 cm-1, a characteristic wavenumber for calcite. By contrast infra-red absorbance showed only around a third of the oyster cement to made up of calcite, and the remainder absorbing at 858 cm-1, which is characteristic for the aragonite crystal form.

Perhaps more importantly for adhesive applications, the infra-red spectra also showed differences between the proteins found in different parts of the oyster. The cement shows the strongest absorbance in the 1150-950 cm^-1 range, which is put down to phosphoesters and may indicate that the adhesive proteins are phosphorylated in providing a matrix for the calcium carbonate. Similar phosphoserine-containing proteins have also been found in the adhesives of mussels, tube worms, and sea cucumbers.

"With a description of the oyster cement in hand, we may gain strategies for developing synthetic materials that mimic the shellfishs ability to set and hold in wet environments," said Wilker, who has designed synthetic bioadhesives for more than 10 years. The hard cement of oysters could provide a basis for attaching ligaments or tendons to bone or for tooth repair.

The need for more biocompatible inorganic adhesives was illustrated earlier this year by Mainz University researchers who exploited two enzymes to produce silicate dental fillings and attach them to teeth.² One enzyme, silicatein, is used by sponges to form their skeletons from silica and the other, tyrosinase, is responsible for producing DOPA residues. The German researchers combined these with a protein sequence from a silkworm enzyme known as silk fibroin.

When bacteria are made to produce the resulting protein, silicatein catalyses the formation of silica material that can serve as dental fillings. The DOPA-rich protein attaches the resulting material to the enamel of teeth or surfaces of other solid materials including metals, plastics and composites. While this approach is undoubtedly ingenious, and although silica is more acid resistant than normal tooth enamel, simpler approaches inspired by oysters could also be attractive in such applications.

However, what the Mainz team does show is how biological inspirations could effectively become commercial products. Could the kind of bacterial biofactories that they describe indicate a new phenomenon in the adhesive industry, using abilitie