# Magnetic Self-Healing Composites: Synthesis and Applications
## 3 State-of-the-Art Fabrication Methods of Magnetic Self-Healing Composites
### Engineering of the Particle Surface towards the Self-Assembly of Magnetic Self-Heling Composites
The resulting magnetic (nano)particles were added to the polymer by solution casting with final contents from 10 to 16 wt%. The magnetic self-healing was enhanced by tuning the Curie temperature by adjusting the Mn:Zn ratios, which ranged from 0.9 Mn:0.1 Zn to 0.7 Mn:0.3 Zn. The magnetic nanocomposite containing Mn\({}_{0.8}\)Zn\({}_{0.2}\)Fe\({}_{2}\)O\({}_{4}\) reached superior recovery to that of other compositions, as higher temperatures were reached when it was placed in an AC magnetic field, showing better self-healing efficiency than other compositions. The direct polymerization of ligands onto the particle surface is limited by the need for compatibility between the coating material and the reaction conditions, e.g., solubility, pH and temperature. To overcome this issue, the coating and polymerization processes can be split into two or more steps. Magnetic hydrogels with chitosan- Fe\({}_{3}\)O\({}_{4}\) NPs as crosslinking points and telechelic difunctional poly(ethylene glycol) (DF-PEG) as the polymeric matrix were built using a dynamic covalent Schiff-base linkage between the NH\({}_{2}\) groups on chitosan and the benzaldehyde groups on the PEG chain ends [47]. The as-prepared magnetic hydrogel showed self-healing at room temperature without additional stimulus and excellent biocompatibility with HeLa cells. Another interesting example was shown by [PERSON] et al., where Fe\({}_{3}\)O\({}_{4}\) NPs were functionalized with catechol groups inspired by the additive chemistry in mussel bypassal threads (see Figure 4) [36]. Fe\({}_{3}\)O\({}_{4}\) NPs functionalized with polyethylene glycol molecules that ended with a carboxylic acid were mixed with 4-arm catechol-terminated polyethylene glycol. As a result of the interaction between the catechol moieties and the Fe(III), magnetic gels could be obtainedat particle concentrations of 1 vol % and above. The grafting or conjugation of different ligands on the particle surface can contribute to the integration of several functionalities into the final material. Recently, [PERSON] et al. showed an ultrasoft self-healing magnetic hydrogel with conductive and magnetic features in a PVA polymer using MnFe\({}_{2}\)O\({}_{4}\) NPs and nano-fibrilated cellulose functionalized with polyaniline as the substrate [48]. The maximum saturation magnetization and conductivity of the materials were 5.22 emu\(\cdot\)g\({}^{-1}\) and 8.15 \(\times\) 10\({}^{-3}\) S\(\cdot\)cm\({}^{-1}\), respectively. Additionally, the PVA hydrogen showed good self-healing property, as it could be completely healed after the pieces of the hydrogel were put together for several minutes at room temperature. The synthesis of magnetic self-healing polymers using SPIONs was reported in three steps [53]. Magnetic (nano)particles were first activated with the agent KH-570 to enable greater interface compatibility. The resulting activated particles were then functionalized with NVP and DVP monomers and, finally, reacted with the agent TETA and a fatty acid dimer. The superparamagnetic polymer reached 74.3% healing efficiency in 1 h at room temperature and 77% when it was incubated under an alternating magnetic field.