A strong underwater adhesive that totally cured in water

Underwater adhesives have drawn enormous interests for biomedical and engineering applications. In this work, a strong underwater adhesive is developed by addition reaction between bis(3-aminopropyl) terminated polydimethylsiloxane (PDMS) and hexamethylene diisocyanate (HDI). The PDMS-based adhesive was totally cured in aqueous solutions and exhibited strong adhesion with various substrates. The highest underwater adhesion strength of the adhesive on polyethylene terephthalate (PET) substrate reached 2.8 MPa, which is comparable with those commercial glues that cured in air. Such strong underwater adhesion is mainly caused by the combination of chemical crosslinks, hydrogen bonds and other physical interactions. Further study reveals that increasing the curing temperature can decrease the mechanical strength of the adhesive, which leads to a decrease in adhesion strength. Moreover, increasing the salt concentration in aqueous solution would screen the electrostatic charges at the interface and impair the underwater adhesion. Besides, the tensile adhesion tests suggested that the underwater adhesion strength was enhanced by 3 times as the curing time extended from 3 h to 6 h. The increased adhesion is mainly caused by the enhanced mechanical strength of the polymeric adhesive as well as the increased interactions at the adhesive-substrate interface. In addition, the strong adhesion force of the adhesive is also demonstrated from the microscale by employing atomic-force-microscope force measurement. This work provides both practical and fundamental insights into developing underwater adhesives with strong adhesion.