Underwater Contact Behavior of Alginate and Catechol-Conjugated Alginate Hydrogel Beads.

Modifying hydrogels with catechol functionality is a promising approach for improving their mechanical and interfacial properties in water, particularly in biological environments. However, the effects of this modification on hydrogels' contact behavior with soft tissues are not well-studied due to the complexity of hydrogels and lack of suitable techniques to probe this behavior. In addition, modification can alter the mechanical properties of hydrogels, resulting in consequences for adhesive strength as well. In this work, we report an investigation of the contact behavior of alginate hydrogels with and without conjugation of catechol functionality, aiming to elucidate the role of catechol modification on wet adhesion of alginates to a model tissue-like material, gelatin. To directly characterize this soft-on-soft contact, which has commonly been a challenge, we developed an indentation-based contact adhesion measurement using alginate hydrogel beads as the testing probe. We found that <3% conjugation of catechol can significantly improve the adhesion of alginate to gelatin by half an order of magnitude, with this adhesion depending heavily on contact time and pH. In contrast, the reduced elastic modulus from modification resulted in lower adhesive strength on rigid substrates. These findings provide valuable insight into the effects of catechol modification of hydrogels, especially in their interaction with tissue-like soft substrates, as well as a simple method for the direct measurement of time- and pH-dependent hydrogel adhesion behavior underwater.