# Magnetic Self-Healing Composites: Synthesis and Applications
## 4 Applications
### Actuators  
The field of soft actuators is a fast-growing sector arising from the great advances in soft robotics and materials science. Inspired by nature, stimuli-responsive materials have substantially expanded their applicability as soft actuators by promoting their adaptation to the surrounding environmental conditions. Stimuli-responsive actuators are capable of responding to a diversity of external triggers such as heat, light, pH and electric or magnetic fields, leading to modifications of the system performance [81]. This includes the formulation of magnetic soft actuators whose locomotion or properties can be controlled under the application of a magnetic field. Fields such as magnetoreology [82], magnetostriction [83] and untethered robotics [84] have found ample opportunities in robotics due to their fast and reversible responses, versatility and applicability. Magnetic fields are of exceptional interest for this purpose as a consequence of their deep and efficient penetration throughout a broad range of materials, coupled with the quick reaction of magneto-responsive materials to dynamic changes in the magnetic field [85]. Separately, self-healing soft robotics is a new field that emerges as a solution to the inherent vulnerability of soft solids to harsh damage. This extends the performance lifetime of the soft robot part and improves the circularity and reliability as compared with conventional robotic systems due to the ability to heal micro- or macroscopic damages when exposed to a harsh actuation environment. In a recent review exploring the usefulness of self-healing materials for soft robotic applications, magnetic self-healing composites were proposed as promising candidates, using the magnetic field responsiveness for both actuation and promotion of the self-healing mechanism [14]. Thus, the combination of the aforementioned fields can pave the path to the formulation and application of novel materials that can exploit the advantages of both types of materials in a single system. One of the most attractive characteristics of magnetic actuators is their remote controllability when submitted to magnetic fields. Supramolecular reversible networks have attracted great attention for this purpose because of their great stretchability, allowing controlled shape morphing even under the submission of mild magnetic forces. [PERSON] et al. combined dynamic linear backbones (disulfide bonds) and physicochemical interactions (hydrogen and coordination bonds) to synthesize hierarchically structured dynamic networks. During the synthesis, magnetic microparticles are incorporated to also provide a magnetic response to the system. The final composite exhibits a fast recovery, being ableto sustain a 300% stretch after catastrophic damage when healed during 5 s at room temperature [86]. The proposed material is employed to synthesize a variety of different soft actuators, allowing the complex motions such as cargo transport (Figure 6a), multiterrain locomotion (Figure 6b) or climbing plants tendrils (Figure 6c). Composites resulting from reversible networks crosslinked at the particle surface are attractive candidates, suitable for exploitation for soft robotics. [PERSON] et al. prepared a cost-effective magnetic self-healing nanocomposite for which the dynamic reversibility takes place at the particle surface via hydrogen bonding between amide functional groups. The synthesized composites exhibited an enhanced Young's modulus of 70 MPa and extensibility of 500%, sufficient to be efficiently manipulated using magnetic fields, and a self-healing efficiency close to 50% after 5 h under ambient conditions [32]. Similarly, [PERSON] et al. developed a dual-crosslinked hydrogel combining particle-polymer hydrogen bonds and dynamic borate crosslinks. The soft composite exhibits pH responsiveness via borate ion cleavage and efficient magnetic actuation [87].