A mussel-inspired adhesive based on a polyvinylpyrrolidone (PVP) backbone shows a much higher bonding strength under underwater/seawater conditions than under dry conditions. We reasoned that besides catechol moieties, the structure and properties of the backbone also play an important role in the realization of strong underwater bonding.The strong underwater adhesion ability of mussels has attracted tremendous research interest to create biomimetic glues that can perform well under humid conditions or more extremely, underwater conditions. 1 Although the real mechanism for mussel byssal adhesion is still not clear, it is generally accepted that one of the most important moieties in the glue protein secreted by mussels is the dihydroxyphenylalanine (DOPA) residue, which undergoes both covalent cross-linking and metal ion complexation and accounts for the strong underwater adhesion. 2 Inspired by these facts, various polymers bearing catechol sidechains have been designed and synthesized to mimic the strong underwater adhesion properties of mussels. 1a-e,g,3 Good to excellent bonding strength has been achieved when these biomimetic glues were obtained under dry conditions. However, the underwater bonding strength of these biomimetic adhesives is much weaker. For example, poly(3,4-dihydroxylstyrene-co-styrene) exhibits a recordhigh bonding strength up to 11.0 MPa on dry surfaces, 3a while a terpolymer adhesive based on a similar backbone only exhibited an underwater bonding strength around 0.3 MPa. 1e The musselinspired adhesive based on a polyoxetane backbone reported previously by our group showed high bonding strength under dry conditions, 4 however, it failed to bind under the same underwater conditions. Therefore, to realize strong underwater bonding is still challenging to date. It seems that the catechol moiety is not the only factor that governs the underwater bonding performance. Here some questions rise: what kind of role does the protein backbone in the mussel byssal protein play in underwater bonding? What kind of inspiration could we get from the byssal protein backbone in the design of non-protein based mussel-inspired adhesives? Actually, even without catechol moieties, many proteins show some degree of glutinosity. For example, soybean protein has been used as a bio-mass based adhesive. 5 We postulate that the amide bonds together with other functionalities in the proteins might provide an initial driving force for mussel byssal proteins to form strong interaction with the rocky surfaces. During our continuous efforts to prepare adhesives that mimic mussel proteins, 1h,4 we realized that PVP might be a good candidate as the backbone for biomimetic glues, given that PVP itself is glutinous and shows certain similarity in structure to peptides (both containing amide bonds). Herein we would like to report a strong mussel-inspired adhesive that is based on a PVP backbone, the bonding strength of which under underwater conditions is higher than that under dry conditions, which was not reported before. T...
a b s t r a c tMussel-inspired catechol-containing polymers have drawn great attention due to their outstanding adhesive properties. Catechol-containing polyethylene glycol (cPEG) is a well-studied catechol-containing polymer used for tissue repair. Nevertheless, catechols can only be attached to the chain ends of polyethylene glycols thus the bonding strength of the resulting polymers is limited. Aiming at solving the problem, a series of clickable polyoxetane copolymers with grafted catechol moieties were synthesized in an efficient manner. Upon addition of FeCl 3 as the cross-linker, strong bonding strength of the adhesive was achieved. Polymer containing 15.5 molar percent of catechol showed the strongest bonding strength up to 5.59 MPa on sanded stainless steel. It was found that the triazole groups also contributed to the overall adhesive performance. This polyoxetane-based adhesive also displayed strong bonding ability to a variety of other substrates including porcine skin.
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