## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Metal-Based Nanocomposites
#### 3.1.2 Gold Nanoparticle (AuNP)/Nanorod (AuNR) Nanocomposites
This suggests their potential utility in applications such as optical thermal history sensors. AuNRs are widely employed as photothermal agents due to their biocompatibility and tunable SPR in the NIR region, which is achieved by adjusting their aspect ratio [94; 95]. Utilizing this specific longitudinal SPR wavelength along with a polarizer enables polarization-selective photothermal heating in composites containing aligned AuNRs [96]. Consequently, [PERSON] et al. incorporated polyethylene glycol (PEG)-modified AuNRs into a poly(L-lactide-co-\(\varepsilon\)-caprolactone) (PLCL) matrix to introduce NIR responsiveness [80]. In the PLCL-AuNR/PEG composites, the T\({}_{\text{g}}\) is critical for the transition between crystalline and amorphous phases, affecting the SME. The integration of caprolactone units into poly(L-lactide) (PLLA) enhances toughness and lowers the T\({}_{\text{g}}\) of PLCL, which is further reduced by the addition of AuNR/PEG, thus facilitating SME activation at tissue-friendly temperatures [97]. However, achieving direct heating within a living organism is often challenging, which leads to the use of AuNR/PEG as a device to convert energy  
Figure 7: **(a)** SEM images using backscattered electrons and 2D SAXS detector images displaying the arrangement of AuNPs in the samples at various stages: before stretching, during stretching, and after shape recovery. Reproduced with permission from ref. [93]. Copyright 2021 American Chemical Society. **(b)** Graphs showing the temperature changes in response to varying NIR-light intensities and different AuNR concentrations, accompanied by sequential photos depicting the stages of the shape-recovery process for the PLCL-AuNR/PEG-2 nanocomposite. Reproduced with permission from ref. [80]. Copyright 2023 American Chemical Society. from remote stimuli into thermal energy [80]. Under NIR light exposure, the heating rate and equilibrium temperature of the PLCL-AuNR/PEG composites increase significantly with higher light intensity and AuNR/PEG content. This results in enhanced thermal conductivity and rapid heat dissipation, crucial for the shape-recovery process [98]. As illustrated in Figure 7b, the PLCL-AuNR/PEG-2 composite (containing 2 wt.% AuNRs/PEG) demonstrated the ability to transform from a rectangular to an \"S\" shape under NIR light, exhibiting spatiotemporal control. In addition to NIR-induced heating, direct heating leads to excellent shape-memory performance, characterized by a decrease in the shape fixity ratio (R\({}_{\text{f}}\)) and an increase in the shape-recovery ratio (R\({}_{\text{r}}\)) with higher AuNR/PEG content. This indicates the role of AuNR/PEG as a physical cross-linking point that enhances stress storage for shape recovery [99]. Utilizing digital light processing (DLP) 3D printed technology (Figure 8a), [PERSON] and colleagues developed NIR-responsive biodegradable polymer composites based on _d,l_-lactide (LA) and \(\varepsilon\)-caprolactone (CL) by incorporating AuNRs [100]. These composites achieved a temperature of 55 \({}^{\circ}\)C within 20 s under laser exposure, demonstrating rapid and stable NIR responsiveness. The shape-memory properties of the printed cuboid shapes were assessed by evaluating the shape fixity (R\({}_{\text{f}}\)) and shape-recovery (R\({}_{\text{r}}\)) ratios through five cycles of deformation at 80 \({}^{\circ}\)C. These properties exhibited high consistency, with R\({}_{\text{f}}\) and R\({}_{\text{r}}\) values ranging from 97.6% to 100% and 83.1% to 100%, respectively, during one-minute exposure to an 808-nm laser (Figure 8a). In another study, [PERSON] et al. prepared light-induced shape recovery of deformed SMP micropillar hexagonal arrays containing AuNRs [101]. The pillars, bent to varying angles by applying force above their T\({}_{\text{g}}\) and subsequently cooled to room temperature to fix the deformed structures, almost entirely recovered to their original straight state within 5 s under exposure to a 0.3 W green laser.