# Magnetic Self-Healing Composites: Synthesis and Applications
## 3 State-of-the-Art Fabrication Methods of Magnetic Self-Healing Composites
### Additive Manufacturing  
Since its invention in the 1980s, additive manufacturing, popularly named 3D printing, has attracted much attention owing to its revolutionary ability to rapidly manufacture hierarchically complex geometries in a simple and customizable manner [73; 74; 75]. This fabrication method is used to construct an object layer by layer. By controlling the printing direction, structures with complex geometries and magnetization distributions can be fabricated for the desired application. Extrusion-based 3D printing is one of the most recent and attractive methods for manufacturing self-healing hydrogels, as optimal mechanical properties along with their self-healing ability facilitate gel stacking and interfacial bonding during the fabrication of 3D geometries. [PERSON] et al. showed the versatility of the extrusion-based printing method in the fabrication of a magnetic self-healing hydrogel formed from the polysaccharides glycol chitosan and oxidized sodium hyaluron in the presence of  
Figure 4: (**a**) The 4 cPEG and mPEG-COOH structures of the polymers. (**b**) On the top is shown the preparation procedure for the Fe\({}_{3}\)O\({}_{4}\) particles-cross-linked hydrogel. At the bottom are shown representative pictures at each state. From left to right: Fe\({}_{3}\)O\({}_{4}\) dry powder, stabilized Fe\({}_{3}\)O\({}_{4}\) NPs in aqueous dispersion before gel assembly, the self-standing solid hydrogel obtained after assembly with 4 cPEG, and magnetic attraction of the resulting gel. Adapted with permission from [36]. Copyright 2016, American Chemical Society.  
SPIONs without the addition of any chemical crosslinkers [44]. The printing was performed with a 25-gauge needle as a nozzle, and the ferrogel printing speed varied from 3 to 7 mm/s. The prepared hydrogel showed nonsignificant toxicity with ATDCS cells and a maximum self-healing when the SPION content was \(\geq\)5 wt%. [PERSON] et al. demonstrated the high performance of the 3D printing method in fabricating the complex structures (see Figure 5) of a magnetic self-healing hydrogel composed of a double network of chitosan-polyolefin matrix and bondable with 50 nm Fe\({}_{3}\)O\({}_{4}\) NPs [47]. The magnetic hydrogel showed high strength (\(>\)2 MPa), self-healing behaviour after thermal treatment, good magneto-thermal and MRI properties and a good cytocompatibility on the L929 cell line after up to five days of incubation.  
The fused filament fabrication (FFF) of the thermoplastic elastomers embedded within a silicone rubber matrix has also been reported for polycaprolactone and thermoplastic polyurethane using carbonyl iron particles with sizes ranging from 5 to 100 nm with a filler content from 40 to 80 wt% [31]. The prepared polymers showed excellent mechanical property recovery after five healing cycles as a consequence of the intermolecular diffusion provided by PCL when heated above the melting temperature. Additive manufacturing is shown as a promising processing approach to building magnetic self-healing structures. To date, there have only been a few attempts at the additive manufacturing of magnetic self-healing composites, focused primarily on filament printing techniques. Other common additive manufacturing techniques such as stereolithography (SLA), direct ink writing (DIW) and selective laser sintering (SLS) remain unexplored.