Characterization of self-healing hydrogels for biomedical applications

Self-healing hydrogels have become attractive biomaterials due to their ability to repair their initial structure and properties in response to damage. When designing ideal self-healing hydrogels the understanding of their properties but also the actual healing process is required. Even though there currently are different characterization methods used, the lack of standardization makes comparison of different hydrogels difficult. The challenges in standardization arise, for example, from the use of different healing methods (i.e. healing environments) or different testing equipments used. In order to help the comparison of hydrogels, a group of characterization methods should be chosen and the measuring parameters and results in the literature should be presented more consistently. The characterization should include methods suitable to determine the presence of reversible interactions and their reversibility study, to investigate the self-healability of hydrogels and to determine the healing efficiencies of hydrogels, not forgetting time dependence and dynamics of self-healing. More quantitative, as well as theoretical studies are recommended. In this review different general characterization methods, including different measuring parameters and environments, used for self-healing hydrogels are charted, but also additional methods suitable for injectable/3D-bioprintable and conductive self-healing hydrogels are discussed. Some challenges of each method and future aspects for self-healing hydrogels and their characterization are also given.