# 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 grafting of organic molecules on particles surface represents a step forward towards the development of intrinsically magnetic self-healing composites presenting a better compatibility between the magnetic (nano)particles and the polymer matrix. Functionalization strategies are divided into ligand addition and ligand exchange. In the former strategy, amphiphilic molecules interact with the part of the surfactant attached to the particle surface, forming a double layer that covers the particle surface. In the latter strategy, the ligands attached to the particle surface are replaced with new functional compounds by exchange reactions. The new compounds have typically one functional group for binding to the particle surface via strong chemical bonds, e.g., epoxy, catechol, amine, phosphate, and another terminal group that enables the dispersion of magnetic (nano)particles in the polymeric matrix. Surface-modified magnetic (nano)particles with organic groups capable of thermorestible cross-linking are versatile building blocks for magnetic self-healing nanocomposites with improved lives. Within this context, exploiting the reversible Diels-Alder (DA) reaction has been regarded as a useful strategy [37; 47; 48]. [PERSON] et al. developed a system based on 20 nm Fe\({}_{3}\)O\({}_{4}\) NPs that were co-functionalized with alkyl phosphonic acids terminated or by a methyl group or a functional group that can be involved in DA reactions as a furan or a maleimide [46]. The number of DA functional groups was systematically changed from 0 to 100% in 25% steps. The furan-grafted Fe\({}_{3}\)O\({}_{4}\) NPs were chemically crosslinked using a bismaleimide to form polymer networks with magnetic crosslinking centres. Scratch healing was demonstrated at 120 \({}^{\circ}\)C for 1 h, as a significant amount of mobility was required to fill and heal the scratch damage volume. Counter-intuitively, the dilution of the DA reactive groups on the NP surface was shown to lead to lower conversions of the reactive groups into DA adducts. This was explained by steric hindrance. Increasing the spacer length of the grafting agents from propyl to decyl led to a 3- to 4-fold increase in the reaction conversion. Another interesting approach was recently shown by [PERSON] et al. where 20 nm Fe\({}_{3}\)O\({}_{4}\) NPs were functionalized with a self-healing matrix composed of the low-cost commercial monomers acrylamide and n-butyl-acrylate using a graft-from approach [32]. The resulting magnetic self-healing nanocomposite reached a Young's modulus of 70 MPa and extensibility of over 500% while displaying self-healing behaviour at room temperature. Magnetic self-healing thermoplastics have been reported with functionalized magnetic (nano)particles of different sizes and compositions. For example, [PERSON] et al. reported the fabrication of magnetic polystyrene nanocomposites by compression moulding at 240 \({}^{\circ}\)C for 20 min of polystyrene powders containing 20 nm core-shell (CoFe\({}_{2}\)O\({}_{4}\)-MnFe\({}_{2}\)O\({}_{4}\)) NPs with particle content ranging from 3.5 to 10 wt% [34]. When the magnetic polymer nanocomposites were placed in an AC magnetic field, they showed high heat generation due to Neel relaxation and hysteresis of the core-shell magnetic (nano)particles in the solid state. The heat generation resulted in the fusing and healing of large-scale cracks. The maximum temperature generated was for the polymer with 10 wt% of particle content. Another interesting manner of boosting the magnetic hyperthermia efficiency is by increasing the Curie temperature of the particles. [PERSON] et al. reported the preparation of magnetic polyethylene vinyl acetate polymer, functionalizing 20 nm Mn\({}_{\gamma}\)Zn\({}_{1-\gamma}\)Fe\({}_{2}\)O\({}_{4}\) NPs with poly(ethylene-co-acrylic acid) to avoid agglomeration during the processing of the composite [39].