Underwater and wet adhesion strategies for hydrogels in biomedical applications

Hydrogel is a class of biomaterials that is widely adopted in tissue engineering due to its versatility and biocompatibility in mimicking natural tissue properties. For these hydrogels, the ability to adhere in wet and underwater conditions would be beneficial since physiological environment is in general wet or even underwater. As a result, various chemical, physical, and structural principles are exploited to modify the adhesiveness of existing hydrogels. These include catechol adhesion, hydrophobic modification, or microneedles, and more. Yet, to yield optimal performances, the adhesion strategies have to be designed to match the properties of the tissues to be applied on, and at the same time fulfil the requirements of the biomedical application. To give a systematical overview in determining the appropriate adhesion strategies, we first summarize major factors in facilitating wet and underwater adhesion for hydrogels, then comment on the difference in wet adhesive hydrogels among common clinical scenarios in literature. After that, we discuss the current trends and possible improvements in commercialized hydrogels, and finally comment on how adhesiveness could enhance hydrogel applications as other aspect of hydrogels made its progress. Through this review, we hope to provide a holistic view on the role of adhesion strategies in different biomedical applications, and hence to help identify actionable clinical scenarios that underwater and wet adhesive hydrogels could improve upon.