## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Metal-Based Nanocomposites
#### 3.1.2 Gold Nanoparticle (AuNP)/Nanorod (AuNR) Nanocomposites  
Light-activated SMPs offer advantages of remote, spatial, and temporal control, contrasting with their thermally activated counterparts that require direct heating [78]. A conventional method to develop light-actuated SMPs involves incorporating photothermal fillers into thermally responsive SMPs [79]. Gold nanoparticles (AuNPs), gold nanorods (AuNRs), and gold nanospheres (AuNSs) are extensively utilized as nanofillers due to their distinctive photophysical properties and surface plasmon resonance (SPR) capabilities [80]. These nanostructures efficiently convert light, particularly at plasmonic or NIR wavelengths, into heat, facilitating remote and localized heating [81]. The significant photothermal heating capability of these NPs enables their use in SMPs for applications including therapeutic interventions [82], drug delivery [56], as well as polymer curing [83] and processing [84]. However, incorporation of NPs into SMPs introduces challenges such as NP clustering due to uneven dispersion, adversely affecting the material properties, and potential impacts on photothermal efficiency from surface modification [32]. To address these issues, [PERSON] et al. synthesized a cross-linked PCL polymer embedded with AuNPs/reduced graphene oxide (AuNPs/RGO) nanohybrids, achieving improved photothermal conversion and rapid NIR-light-driven shape recovery [85]. Graphene oxide (GO) acts as a carrier for effective nucleation and immobilization of AuNPs through ion-dipole interactions or metallic surface passivation, reducing NP aggregation and ensuring even distribution within the PCL matrix [86; 87]. Comparative studies showed that PCL/AuNP-RGO composites exhibit faster, complete shape recovery in 53 s, outperforming PCL/AuNP (109 s) and PCL/RGO (87 s), attributed to the synergistic photothermal effects and uniform nanohybrid dispersion [88]. Additionally, the AuNP-RGO embedded nanocomposite demonstrated superior shape-recovery stress, effectively lifting a 10 g weight by 1.5 cm under NIR light, exhibiting enhanced heavy-lifting capabilities (Figure 6). [PERSON] and colleagues developed SMPNCs by integrating AuNPs as photothermal converters into acrylate-based composites, enabling adjustable transition temperatures [89]. These SMPNCs undergo shape transformation upon exposure to LED light, which is  
Figure 6: **(a)** Schematic diagram illustrating the synthesis of AuNP-RGO nanohybrids; **(b)** diagrams depicting the photothermal effect (left) and the ability of PCL/AuNP and PCL/AuNP-RGO to lift a 10 g weight under NIR light (right). Reproduced with permission from ref. [85]. Copyright 2019 Elsevier. facilitated by the conversion of light to heat through the AuNPs, reaching the polymer's T\({}_{\text{g}}\). This method results in exceptional shape-memory properties, with shape fixity and recovery ratios exceeding 95%. [PERSON] et al. designed a promising SMP that possesses optical healing capabilities in addition to light-triggered actuation [90]. This was achieved by chemically cross-linking a crystalline polymer with a minor concentration of AuNPs, enabling optically controlled shape memory and rapid healing through localized heating by the surface plasmon resonance of the AuNPs [91]. SMPs and elastomers containing plasmonic AuNPs exhibit inherent properties that vary with the arrangement of the NPs, which are influenced by their orientation or mutual coupling [92]. [PERSON] et al. investigated the plasmonic coupling of AuNPs in TPU SMPs, assessing changes pre- and post-mechanical stretching as well as thermal shape recovery [93]. Stretching the composite, as depicted in Figure 7a, induces a polarization-dependent optical response due to decreased interparticle distances along the stretching direction. This polarization effect disappears upon shape recovery. Additionally, simulations with AuNP dimers provide a method to estimate interparticle spacings, functioning similarly to a plasmon ruler.