## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Metal-Based Nanocomposites  
3.1.1. Fe\({}_{3}\)O\({}_{4}\) Magnetic Nanoparticle (MNP) Nanocomposites  
Magnetically activated SMPNCs represent a pioneering category of smart materials at the intersection of advanced engineering and materials science. These nanocomposites merge the inherent properties of SMPs with magnetic responsiveness through the incorporation of magnetic NPs or fillers, notably Fe\({}_{3}\)O\({}_{4}\) NPs (MNPs). The choice of MNPs is predicated on their favorable magnetic characteristics, low toxicity, and biocompatibility [50, 60]. Embedded within SMPs, these magneto-responsive NPs enable efficient, contactless activation when subjected to an external magnetic field [43]. This mechanism facilitates the shape-memory effect via inductive Joule heating, wherein electromagnetic energy from an alternating high-frequency magnetic field is converted into thermal energy [61]. The implications of this technology are vast, encompassing remotely controlled biomedical applications such as magnetically actuated SMP micro-actuators for thrombus removal, as well as innovations in smart textiles, aerospace, and robotics. By integrating magnetic components into SMPs, a new paradigm of responsive, adaptable materials is being forged, heralding an era of precision-engineered solutions responsive to magnetic stimuli. [PERSON] and colleagues pioneered the development of multistimuli-responsive SMPNCs by incorporating bio-based vanillin-furfurfurylamine benzoxazine/epoxy (V-fa/ECO) copolymers with MNPs (0-5 wt.%). [51]. These SMPNCs exhibit enhanced reconfigurability under the influence of both light and magnetic fields. The introduction of 3 wt.% MNPs improved the shape fixity ratio to 90%, an 8% increase over the unfilled polymer, attributed to the NPs serving as cross-linking points [62]. However, a reduction in mechanical properties was observed at 5 wt.% NP content, likely due to NP agglomeration [63]. The MNPs also facilitated rapid heat generation in a magnetic field by aligning their magnetic dipole moments with the alternating magnetic field, resulting in accelerated shape recovery [64]. High concentrations of MNPs were effective in generating sufficient heat to surpass the glass transition temperature (T\({}_{\mathrm{g}}\)) of the copolymer, enhancing its shape-memory capabilities [65]. For instance, V-fa/ECO copolymers with 5% Fe\({}_{3}\)O\({}_{4}\) NPs demonstrated significant shape-memory behavior, capable of moving objects by transitioning from straight to bent shapes under an alternating magnetic field (Figure 4a). Recently, they synthesized magnetic-responsive triple SMPNCs from bio-based vanillin-furfurfurylamine benzoxazine/polyurethane (V-fa/PU) polymers with incorporated MNPs [66]. In another study, [PERSON] et al. reported on 3D printable SMPNCs with enhanced mechanical strength and magnetic responsiveness, achieved by embedding Fe\({}_{3}\)O\({}_{4}\) particles into polylactic acid/thermoplastic polyurethane (PLA/TPU) polymer blends [67]. The PLA/TPU/Fe\({}_{3}\)O\({}_{4}\) composites showed nearly perfect shape fixing (\(\sim\)100%) and a high recovery ratio (96.4%) within 40 s, indicative of the homogeneous distribution of MNPs. As illustrated in Figure 4b, the shape-memory performance was demonstrated by designing various smart structures, such as a honeycomb and bionic flower-like model, which displayed complete recovery under a contactless magnetic field. [PERSON] and colleagues developed a shape-memory polyurethane (SMPU) nanocomposite using Fe\({}_{3}\)O\({}_{4}\) NPs modified with octadecyl isocyanate (OD), demonstrating outstanding thermo-responsive shape-memory properties [68]. The enhanced performance, including significant shape-recovery and fixity ratios, was attributed to the nanocomposite's crystallizable polycaprolactone (PCL) segments. Specifically, a nanocomposite with 10 wt.% OD-modified MNPs achieved rapid shape recovery in 90 s under a mild alternating magnetic field (350 kHz and 12.9 kA/m). In a separate study, [PERSON] et al.