Reversible interactions in self-healing and shape memory hydrogels

Responsive hydrogels have been extensively studied in the past decades because they are able to interact with their biological environment in a pre-programmed manner. Several biomedical applications have already been achieved (or at least approached) by in vivo experiments. As a class, injectable hydrogels gained considerable attention because of their minimally invasive implantation. However, the final shaping of hydrogel implants is not resolved, and their lifetime is limited because of their insufficient mechanical stability. The solution to these challenges can be given by two seemingly independent properties, shape memory and self-healing. Both properties are well-known for conventional polymers but research on the shape memory or self healing of hydrogels is in its infancy. In this study, we introduce the molecular mechanisms behind these two properties with a focus on hydrogels, attempt to provide a general overview on the role of reversible physical and chemical interactions, and discuss the similarities between the background of shape memory and self-healing. There are a number of open questions regarding the uniform characterisation of such hydrogels, and their theoretical description is very incomplete, but the developed systems hold great promise for future applications. In the final part of the paper, we note that the synthesis of hydrogels providing both self-healing and shape memory is a difficult challenge, but some examples do exist. Future research in these fields should focus on a better understanding of structure-property correlations and should uncover additional fields of application for these advanced materials.