## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Carbon-Based Nanocomposites
#### 3.2.5 Clay Nanocomposites  
Nanoclays are two-dimensional, ideal reinforced nanofillers typically arranged in a stacked configuration, located at or in close proximity to the Earth's surface [244]. The concept of nanoclay originated in the 1990s, with seminal works by [PERSON] et al. and [PERSON] et al., who investigated polyamide-6 filled nanoclay and referred to the composite as a hybrid material rather than a nanocomposite [245; 246]. The incorporation of clay nanofillers into thermoset and thermoplastic polymers subsequently progressed, with a focus on montmorillonite (MMT), bentonite, mica, and hectorite. MMT became particularly favored due to its high aspect ratio and significant swelling properties in polar liquids [247; 248; 249]. Advances in material science have shown that nanoclays substantially enhance the mechanical, thermal, and shape-memory properties of nanocomposites by altering the crystal structure and improving the polymer matrix compatibility [250; 251]. For example, [PERSON] et al. developed starch/clay bio-nanocomposites using a melting process, showing that nanoclay addition significantly enhanced both the elastic modulus and shape-recovery ratio (R\({}_{\mathrm{r}}\)) [252]. [PERSON] et al. enhanced the mechanical properties of PLA and TPU composites by incorporating cost-effective nano-clay particles through melt mixing, facilitating interactions with functional groups [253]. This modification not only improved the mechanical properties but also extended the biomedical applicability of PLA-TPU composites, including tissue engineering scaffolds and stent manufacturing [254]. The addition of 5 wt.% nanoclay resulted in significant enhancements in shape recovery and fixity ratios, increasing from 15.45% to 60.52% and from 95.81% to 99.76%, respectively, attributable to the uniform distribution of clay nanoparticles within the polymer matrix (Figure 25b). Another study demonstrated the fabrication of high-twist yarn using nanoclay-filled PLA/TPU composites to meet the demand for advanced actuators [59]. The yarn was coiled through mandrel annealing, exhibiting significant two-way SMEs and reversible contraction strokes within a low-temperature range [255]. This development of low programming temperature nano-reinforced PLA/TPU blends focuses on the shape-memory behavior of the composite, targeting applications in smart clothing, self-expandable stents, advanced medical devices, biomedical scaffolds, and SMPs for controlling proof marks in the frozen food industry [3; 256; 257]. [PERSON] et al. explored the thermal, structural, and shape-memory properties of PLA/TPU blends, which were fabricated using melt mixing on a twin-screw extruder and subsequent injection molding [256]. Figure 25a illustrated the distribution of nanoclay into the polymer matrix. PEG was incorporated as a plasticizer at a fixed loading of 10 wt.% to assess its influence on the programming stimulation temperature and SMEs, identifying an optimal blend ratio of 60/40 PLA/TPU for these applications. Thermal analysis confirmed the successful mixing and demonstrated a reduction in the glass transition temperature (T\({}_{\mathrm{g}}\)) with the addition of 5 wt.% nanoclay and 10 wt.% PEG, thus achieving the primary objective of the study. The in situ inclusion of various modified nanoclays during nanocomposite synthesis enhances polymer chain intercalation, leading to increased interaction and reduced crystallinity compared to composites with pristine nanoclays. [PERSON] et al. incorporated both unmodified and organically modified nanoclays into polyurethane matrix through an in situ polymerization technique [258]. At physiological temperature (37 \({}^{\circ}\)C), the nanocomposite containing dimethyl dehydrogenated tallow ammonium cation modified nanoclay (HPL-15A) demonstrated superior shape recovery, attributed to the complete melting of the soft segment and the restricted flipping of the hard segment due to extensive interactions.