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

Abstract

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.

Catechol-functionalized hydrogels: biomimetic design, adhesion mechanism, and biomedical applications

“…In hydrogel systems, electrostatic forces can be responsible for the structure of the gel itself, such as ionic cross-linking of calcium alginate gels. 50,51 For adhesion, electrostatic interactions can be used to adhere two hydrogels with oppositely charged polymer networks, 52,53 and can also be used in the general adhesion of hydrogel systems. 54 Electrostatic and van der Waals forces are inversely proportional to the permittivity (or dielectric constant) of the medium (in the case of electrostatic forces, ∝ 1/ε) or its squared value (for van der Waals forces, ∝ 1/ε 2 ).…”

“…This can be particularly useful for hydrogels, where similar refractive indices can make viewing the contact area difficult, particularly underwater. 51 Additionally, both sides of the test setup are flat sheets, allowing for easy formation of both the probe and substrate. 86 This is useful if a hydrogel is being used as a probe and is challenging to form into a sphere or hemisphere.…”

Mussel-Inspired Catechol Functionalisation as a Strategy to Enhance Biomaterial Adhesion: A Systematic Review

“…Aiming at understanding the impact of catechol modification on the mechanical properties, wet adhesive strength and contact behaviour with soft tissues, Cholewinski et al [ 35 ] carried out studies with catechol-modified or unmodified alginate hydrogels, using gelatine as a model tissue-like material. Using hydrogel beads as the testing probe (which allows for testing of interactions with rigid and soft substrates), modified alginate gels showed poor adhesion to hard surfaces (namely glass and gold), although improvements were seen across protein-based substrates.…”

A Robust Salty Water Adhesive by Counterion Exchange Induced Coacervate

“…Marine organisms, such as mussels and sandcastle worms, are impressive for their glue power on diverse submerged, corruptive substrates in seawater through the synergy of catecholic amino acid, 3,4‐dihydroxyphenylalanine (Dopa), adhesive protein sequence, and controlled coacervate . Adhesive proteins with opposite charges were mixed in the worm's gland at a reduced pH, which restored the catecholic moiety and largely screened the electrostatic complexation.…”