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Mussel adhesive proteins have received increased attention as potential biomedical and environmentally friendly underwater adhesives thanks to their fascinating properties, including strong and flexible adhesion, adhesion to various material substrates, water displacement, that they are harmless to human body, and controlled biodegradability. Several mussel adhesive proteins have been identified and characterized from mussels, and profound biochemical knowledge for mussel adhesions has been accumulated. In addition, a lot of effort has been put into realizing the promise of these bioadhesive materials from marine mussels. Here, progress in the diverse developmental approaches, with particular emphasis on functional production of mussel adhesive proteins, are reviewed.

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Complex coacervates are a dense liquid phase of oppositely charged polyions formed by the associative separation of a mixture of polyions. Coacervates have been widely employed in many fields including the pharmaceutical, cosmetic, and food industries due to their intriguing interfacial and bulk material properties. More recently, attempts to develop an effective underwater adhesive have been made using complex coacervates that are based on recombinant mussel adhesive proteins (MAPs) due to the water immiscibility of complex coacervates and the adhesiveness of MAPs. MAP-based complex coacervates contribute to our understanding of the physical nature of complex coacervates and they provide a promising alternative to conventional invasive surgical repairs. Here, this review provides an overview of recombinant MAP-based complex coacervations, with an emphasis on their characterization and the uses of such materials for applications in the fields of biomedicine and tissue engineering.

Mussel adhesive proteins (MAPs) have received increased attention as potential biomedical and environmental friendly adhesives. However, practical application of MAPs has been severely limited by uneconomical extraction and unsuccessful genetic production. Developing new adhesives requires access to large quantities of material and demonstrations of bulk mechanical properties. Previously, the authors designed fp-151, a fusion protein comprised of six MAP type 1 (fp-1) decapeptide repeats at each MAP type 5 (fp-5) terminus and successfully expressed it in Escherichia coli. This recombinant hybrid protein exhibited high-level expression, a simple purification and high biocompatibility as well as strong adhesive ability on a micro-scale. In the present work, investigations on the bulk adhesive properties of semi-purified ( approximately 90% purity) fusion fp-151 were performed in air. The unmodified recombinant fp-151, as expressed, contains tyrosine residues and showed significant shear-adhesive forces ( approximately 0.33 MPa). Adhesion strength increased ( approximately 0.45 MPa) after enzymatic oxidation of tyrosine residues to l-3,4-dihydroxyphenylalanine (DOPA) groups. Addition of cross-linkers such as iron(III), manganese(III) and periodate (IO(4)(-)) generally enhanced adhesion, although too much addition decreased adhesion. Among the three cross-linking reagents examined, the non-metallic oxidant periodate showed the highest shear-adhesive forces ( approximately 0.86 MPa). In addition, it was found that adhesive strengths could be increased by adding weights to the samples. The highest adhesion strength found was that of DOPA-containing fp-151 cross-linked with periodate and having weights applied to the samples ( approximately 1.06 MPa). Taken together, the first bulk-scale adhesive force measurements are presented for an expressed recombinant hybrid mussel adhesive protein.