3D (bio)printing of magnetic hydrogels: Formulation and applications in tissue engineering

Hydrogels have been widely explored in tissue engineering due to their versatile and customizable properties in terms of their mechanical, biological, and chemical features. These properties allow them to recreate the physiological structures of the extracellular matrix in a highly hydrated state. Particularly, magnetic hydrogels have shown great promise due to their biocompatibility, mechanical attributes, and possibility to be controlled remotely. Three-dimensional (3D) (bio)printing has emerged as an efficient method to fabricate 3D complex scaffolds from hydrogels with a defined structure and porous microarchitecture, which is crucial for cell proliferation, migration, and differentiation. Therefore, combining magnetic-responsive biomaterials with bioprinting strategies offers numerous advantages for tissue engineering applications. Despite the large number of reviews on magnetic hydrogels available in the literature, they lack a clear focus on the fabrication of hydrogels through a 3D (bio)printing process. Thus, this review highlights not only the main characteristics and fabrication methods of magnetic nanoparticles (MNPs), but also the strategies for their incorporation into hydrogels. Furthermore, we also provide an overview of the current state of the art in injectable magnetic hydrogels, which have the potential to be used as bioinks for 3D (bio)printing, envisaging several applications in the regenerative medicine and biomedical engineering fields.