## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Carbon-Based Nanocomposites  
#### 3.2.1 Carbon Nanotube (CNT) Nanocomposites  
Carbon nanotubes (CNTs) are extensively studied within the carbon family for their ability to significantly enhance the mechanical, electrical, and thermal properties of SMP composites [138]. Their integration into SMP matrices not only provides superior flexibility compared to traditional fiber reinforcements but also aims to induce an electroactive SME via Joule heating, leveraging their intrinsic thermal responsiveness [139; 140]. However, integrating CNTs within the SMP matrix is challenging due to their lack of uniform dispersibility [141]. To overcome this issue, several strategies have been employed, such as in situ polymerization, melt mixing, sol-gel, solution mixing followed by casting, solution mixing aided by ultrasonic dispersion, and chemical functionalization of CNTs followed by cross-linking [142; 143; 144]. These integration techniques are robust and effective approaches to enhancing the compatibility between CNTs and SMP matrices through physical and chemical interactions [19]. After preparing the polymeric matrix through conventional methods like extrusion or injection molding, it is subsequently programmed to deform and lock into the desired temporary configuration. Electrically conducting SMP composites are developed to prevent localized heating in surrounding areas, enabling remote control and rapid actuation. For this purpose, CNTs are commonly used in non-conducting SMP matrices as conducting fillers [145]. CNTs can be categorized as single-walled (SWCNTs) or multiple-walled carbon nanotubes (MWCNTs) depending on their wall structure [146]. The latter is widely used for fabricating SMP composites due to its low bulk density and high aspect ratio, providing enhanced electrical and thermal conductivities, along with improved mechanical and shape-memory properties, compared to neat polymers [147; 148]. [PERSON] et al. enhanced the conductivity of polyurethane and poly(vinylidene difluoride) (PVDF) composites by incorporating modified MWCNTs through melt blending, achieving notable improvements in mechanical, dynamic mechanical, and electroactive shape-memory properties [149]. The inclusion of 10 wt.% MWCNTs enabled the composite to recover approximately 95% of its original shape under a 40 V voltage. Similarly, [PERSON] et al. developed a rapid-response electroactive ternary SMP composite combining MWCNTs, poly(propylene carbonate) (PPC), and poly(lactic acid) (PLA), achieving a conductivity of 10 S/m at 30 V, resulting in a surface temperature of 80 \({}^{\circ}\)C and 97% shape recovery within 30 s [150]. Employing digital light processing (DLP) 3D-printing technology, [PERSON] and colleagues created a photocurable, electroactive SMP composite by integrating MWCNTs into an amorphous thermosetting PEG diacrylate/poly(hydroxyethyl methacrylate) matrix [13]. This approach significantly improved the composite's electrical conductivity, enabling remote activation through Joule heating. The shape-memory functionality was demonstrated in a smart, 3D-printed butterfly structure, showcasing high shape fixity (R\({}_{\mathrm{f}}\approx\) 100%) and shape recovery (R\({}_{\mathrm{r}}\) > 95%), illustrating the potential for Joule-heated shape-memory activation. [PERSON] and colleagues conducted a study on the development of covalently adaptable networks (CAN) using conducting poly(\(\varepsilon\)-caprolactone) (PCL-CAN), incorporating MWCNTs into a four-arm star-shaped PCL with maleimide and furan end-groups through melt blending [55]. Both conventional tensile testing and dynamic mechanical analysis confirmed the enhancement of the composite's mechanical properties, coupled with outstanding shape-memory characteristics, achieving recovery and fixity ratios of approximately 99%. To impart conductivity to the PCL-CAN network, 3% of MWCNTs were added to the matrix, although 1 wt.% was sufficient to enable current-driven heating [151].