# Magnetic Self-Healing Composites: Synthesis and Applications
## 3 State-of-the-Art Fabrication Methods of Magnetic Self-Healing Composites  
Self-healing magnetic composites can be prepared by incorporating magnetic (nano)particles containing ferromagnetic or ferrimagnetic materials in self-healing polymer matrices such as soft elastomers or shape memory polymers [4]. The composition, content and surface functionalization of incorporated magnetic (nano)particles are key factors that determine the performances of nanocomposites. The utilization of hard magnetic ferrites (nano)particles in robotics has attracted much attention due to their low magnetic permeability under strong magnetic fields, boosting material programmability [30; 31; 32]. The magnetic response of magnetic self-healing nanocomposites usually scales up with the particle content. However, particle aggregation and phase separation are usual bottlenecks limiting particle loading [4]. Finally, the grafting of inorganic and organic molecules on the particle surface can introduce new functionalities to the final material and enhance the particle dispersion within the polymeric matrix [33; 34; 35; 36; 32].  
So far, two main strategies have been adopted to induce the self-healing of nanocomposites: autonomous and nonautonomous damage healing [37]. The nonautonomous approach relies on external interventions, such as an inductive heating effect under an external stimulus, e.g., induced by an alternating magnetic field for magnetic (nano)particles or near-infrared light for light absorbers [38; 39; 40; 41; 42]. The local heat produced from these processes enables various types of self-healing strategies, such as melting of a thermoplastic phase, facilitating chemical bond exchange or activating reversible reactions. This approach has yielded magnetic self-healing composites with good self-healing and mechanical properties, but it shows the limitation of requiring the application of an external stimulus. Autonomous healing does not require any human intervention and relies either on an embedded healing agent that is released autonomously upon mechanical damage to the reservoir it is stored in or on the intrinsic self-healing potential of polymeric matrices involving dynamic covalent bonds or noncovalent interactions that can reform at the application temperature. Dynamic covalent chemistries include Diels-Alder (DA) reactions, disulfide, imine and boron-based bond exchange [43; 44; 35; 45; 46]. Noncovalent chemistries refer to hydrogen bonds, ionic interactions, metal-ligand bonding, electrostatic interactions and \(\pi\)-\(\pi\) stacking [30; 32; 47; 48; 49; 50]. It is, however, worth mentioning that the distinction between these two strategies is not always clear and that many intrinsic self-healing systems need a temperature increase to reach a fully healed state, which for example can be induced by magnetic inductive heating as a combination of these two strategies [45]. Table 1 summarizes the state-of-the-art fabrication methods of magnetic self-healing nanocomposites. The material geometry and composition determine the final functionalities of magnetic self-healing composites. In a general view, the mixing and dispersion of magnetic (nano)particles in a polymeric matrix can be used to fabricate simple 2D and 3D geometries. Additive manufacturing methods allow for the fabrication of complex structures in short times but at a higher production cost.