Underwater adhesion: a complex (coacervate) affair

Abstract

Underwater adhesion is technically challenging mainly because of the presence of water which drastically weakens the mechanical properties of a glue and prevents a good contact with the surface. This strongly limits the development of injectable adhesives for wet environments, such as the human body: the medical field, especially when regarding soft tissue repair and wound closure, would have much to gain from the development of glues that would replace patient and surgeon-unfriendly techniques such as suturing and stapling. However, this challenge has been solved long time ago by many natural organisms, such as mussels and sandcastle worms. These underwater living animals are able to release a fluid phase underwater. This substance successively hardens due to a change in the environmental conditions (pH, ionic strength), enabling the organisms to attach to different kinds of surfaces. The natural glue is mainly composed of oppositely charged protein domains and stored in the form of complex coacervate, a fluid and water-immiscible phase. Taking nature as a source of inspiration, in this thesis we report the development of an adhesive by mixing aqueous solutions of oppositely charged polymers modified with thermosensitive units. In the first part of the thesis, we show that the material, liquid-like at preparation conditions, turns immediately into a solid when released in a physiological environment, where the adhesive experiences a gradient in both temperature and salinity. The liquid-to-solid transition is ascribed to the collapse of the thermoresponsive chains at the human body temperature and to the strengthening of the electrostatic interactions in a lower ionic strength medium, resulting in higher toughness and strength. When tested underwater, the material shows high adhesive properties, sticking strongly to many different surfaces like glass, teflon or charged surfaces. In addition to that, in the second part of the thesis, we report the optimization of the material, which can be achieved by properly controlling parameters such as the polymer composition, salt concentration, water content and incorporation of nanofillers.