# Magnetic Self-Healing Composites: Synthesis and Applications
## 3 State-of-the-Art Fabrication Methods of Magnetic Self-Healing Composites
### Direct Mixing and Dispersion of Magnetic (Nano)Particles in a Polymeric Matrix
(**e**) DSC thermograms of PDMS-COOH and PDMS-COOH-Fe\({}_{3}\)O\({}_{4}\) (15 wt%). Adapted with permission from [30]. Copyright 2021, John Wiley and Sons. * Physical methods: enhance the particle deagglomeration by applying high shear forces that can break down the particle agglomerates to nanoscale dispersed particles. This includes well-known top-down manufacturing methods such as twin-screw extrusion [57; 58], injection moulding [59], roll and ball milling [60; 61; 62] and ultrasonic vibration [63; 64]. The exerted high shear forces are capable of reducing the aggregated particles sufficiently and homogeneously dispersing them. However, there is the great inconvenience that dissociative covalent networks or supramolecular networks rely on the narrow thermal region of the gel transition linked to an abrupt viscosity drop. Thus, precise control of the reaction conversion needs to be maintained to ensure a sufficiently high viscous medium to avoid particle re-aggregation and sedimentation upon curing. With reversible polymer networks, this can be achieved upon thermal dissociation at a well-defined temperature close to the gel transition. In addition, a monodisperse composite cannot be achieved under these conditions as shear forces are not high enough to separate the clusters at smaller size scales. * Chemical methods that can deal with the dispersion issue from a composition perspective: Hindering magnetic (nano)particle interactions (mostly magnetic dipole-dipole interactions, hydrogen bonding and van der Waals interactions) by designing proper formulations has been proven to enhance (nano)particle distribution. This can be approached from either the magnetic (nano)particles (surface functionalization, magnetization state, etc.) or the polymer matrix state (polymer (nano)blends [65; 66; 67; 68], the grafting of diverse functional groups [48; 53; 69], etc.). Additionally, the incorporation of stabilizers [70] or performing the magnetic (nano)particle synthesis of the polymer matrix in situ at a suitable crosslinked state [55] can help further this aim. Dynamic covalent chemistries blur the line between physical and chemical methods given their reversible nature and resulting processability upon the application of an external trigger [71]. Supramolecular networks and dissociative covalent chemistries undergo a shift in their equilibrium state caused by an external perturbation, e.g., thermal heating or light irradiation. In addition, high shear rates can also lead to a mechanochemical dissociation of dynamic covalent crosslinks since these bonds have been found to be the weakest for most of the proposed reversibly crosslinked materials [69]. This enables physical processing methods to be adapted and employed as a consequence of the thermally and mechanically induced reversible chemical changes of the self-healing polymer networks. Analogous strategies are well-known for thermoplastics. While traditional thermoplastics benefit from higher melt strength and extensional rheology or fast crystallization kinetics, following their high molecular weight and thermophysical properties, reversible polymer networks rely on reversible chemical (de)polymerization reactions with associated molecular weight decrease and chemical reaction kinetics. The design of more complex and efficient reversible crosslinked polymers or blends, or the use of exchange reactions that do not show a net change in crosslink density, are potential routes towards the rapid and efficient manufacturing of magnetic self-healing composites [72].