# Magnetic Self-Healing Composites: Synthesis and Applications
## 1 Introduction  
Magnetic (nano)particles and derived magnetic composites show changes in their properties (e.g., magnetorheology), structures or positions/motion (electromagnetic transduction) in response to the application of an external magnetic field or as a consequence of inherent magnetic properties [1; 2; 3; 4]. This has triggered the attention of many researchers during recent decades for exploitation for a wide range of different applications. The response of the magnetic composite depends on the type and structure of the magnetic filler used, the properties of the matrix material and the interaction between the magnetic filler and the inert matrix. Hard magnetic (nano)particles can be permanently magnetized by an external magnetic field, whereas soft magnetic (nano)particles lose their magnetization upon removal of the external magnetic field. Magnetic polymer (nano)composites are created using a variety of magnetic particle fillers with different magnetic properties and processed using different physical and chemical techniques, leading to different nanocomposite morphologies and different levels of complexity in their responses to applied magnetic fields. Self-healing materials have been drawing much research interest in the past decades. They exhibit the ability to recover their functional properties by healing incurred damage, thus extending their lifetimes and those of the objects and structures made from theseself-healing materials [5]. Two fundamentally different healing mechanisms can be distinguished. Extrinsic self-healing materials rely on a healing agent that is stored in hollow fibres [6], capsules [7] or microvascular networks [8], mimicking the cardiovascular systems seen in nature. When these reservoirs break due to mechanical damage, the healing agent leaches out of the reservoirs, filling the damage volume. Upon solidification, usually following a chemical reaction, the mechanical properties of the material are recovered. Extrinsic self-healing mechanisms are almost exclusively used for thermosets and related composite materials. Alternatively, the healing ability of intrinsic self-healing materials is inherent to the chemical structures of the materials. Intrinsic self-healing polymers have reversible covalent bonds (e.g., reversible cycloaddition [9], metathesis or exchange reactions [10,11]) or noncovalent interactions (such as hydrogen bonds [12] or ionic interactions [13]) built into their polymer network structures. These reversible bonds and interactions are broken preferentially and in a reversible fashion upon mechanical overloading. After the removal of the mechanical load, these bonds can be reformed either autonomously or upon the application of an adequate stimulus, such as heat or light irradiation. Intrinsic healing systems are used for thermosets, elastomers and derived composites. In thermosets, usually, a thermal trigger is necessary to create the chain segmental mobility required for the effective healing of the incurred damage, whereas certain healing chemistries enable the autonomous damage healing in elastomers. In this regard, the formulation of self-healing magnetic composites has the potential to solve major issues in the respective fields of magnetic materials and related applications and in the developing field of self-healing materials. This mini-review focuses on this intriguing class of smart, functional materials that can respond to many types of stimuli. The ability to repair damage and thus extend the service lifetimes of materials, systems and structures is expected to find application in many different fields. As self-healing materials have significantly maturer in the last decades, the focus has shifted towards the recovery of a wider variety of combinations of functional properties that may be lost after damage.