# 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
\begin{table}
\begin{tabular}{c c c c} \hline
**Method** & **Pros** & **Cons** & **References** \\ \hline Direct mixing and dispersion of magnetic (nano)particles in a polymeric matrix & Simple fabrication & Simple 2D-3D geometries & [30; 39; 40; 41; 42; 45; 49] \\ \hline \multirow{3}{*}{Self-assembled magnetic (nano)particles composites} & Good control over the particle & \multirow{3}{*}{Complex fabrication (Incorporation of additional functionalities)} & \multirow{3}{*}{[32; 33; 34; 36; 48; 51; 52; 53]} \\  & aggregation state & & \\ \cline{1-1}  & Incorporation of additional functionalities & procedure & \\ \hline \multirow{3}{*}{Additive manufacturing} & Fast fabrication & \multirow{3}{*}{Complex control and high cost} & \multirow{3}{*}{[33; 45; 46; 49]} \\  & Multi-material printing & & \\ \cline{1-1}  & High resolution & & \\ \hline AC magnetic field & Fast fabrication & AC magnetic fields & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline Spray deposition & Fast fabrication & Limitation in & [38] \\ \cline{1-1}  & Easy performance & building geometries & \\ \hline \end{tabular}
\end{table} 
Table 1: The most utilized fabrication methods for magnetic self-healing composites together with their pros and cons. followed by moulding and drying at 120 \({}^{\circ}\)C for 12 h, and finalized by several hot-pressing steps. The optimum content of the magnetic filler was determined to be 15 wt% with a tensile strain of 400% and a self-healing efficiency of 62.2% at room temperature. Another interesting example was shown by [PERSON] et al. [42]. In this work, Fe\({}_{3}\)O\({}_{4}\) and CoFe\({}_{2}\)O\({}_{4}\) NPs with spherical, cubic and rod shapes were embedded in the ionomeric elastomer PZn-52. The fabrication was performed by mixing the elastomer and the magnetic NPs under sonication followed by moulding and drying in vacuo for 72 h. The best healing was found for elastomers containing multidomain cubic Fe\({}_{3}\)O\({}_{4}\) NPs under an alternating field of 250 kHz (31.5 kAm\({}^{-1}\)). The proposed materials were successfully exploited as magnetic soft actuators. While casting and moulding methods are the most popular choices for the preparation of magnetic self-healing composites, they exhibit strong limitations concerning magnetic (nano)particle aggregation and sedimentation. This is a key issue for the fabrication of most polymer nanocomposites, where magnetic composites find the extra disadvantage of magnetic dipole interactions when the particles are magnetized [55]. Self-healing materials are usually built up from monomeric mixtures with a low molecular weight that exhibit low-viscosity, liquid-like behaviour. Upon polymerization, the reversible polymer network structure and related viscoelastic properties build up. Mixing magnetic (nano)particles in the initial unreacted monomer mixture, under reprocessing upon degleation or in the presence of a solvent, intrinsically leads to strong magnetic (nano)particle interactions and the sedimentation of aggregates or the formation of strong percolated structures in these low-viscosity systems. These are known to take a predominant role in the rheological behaviour and mechanical performance of the composites [56]. This is detrimental to some applications for which finely dispersed magnetic NPs are required, such as MRI or inductive heating. Hence, alternative synthetic methods or composite (re)processing are required to shatter the agglomerate particle's state and keep the dispersion stable until network (re)formation. Fabrication methods that can improve magnetic (nano)particle dispersion could be divided into two families:  
Figure 3: A sketch of the preparation and characterization of the PMMS-COOH-Fe\({}_{3}\)O\({}_{4}\) polymer. (**a**) The chemical structure of the PDMS-COOH polymer. (**b**) the Schematics for the preparation process. (**c**) The stress-strain results for both the initial and healed polymers. (**d**) The self-healing efficiency of the PDMS-COOH at loading weights from 10 to 25 wt%.