## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Carbon-Based Nanocomposites
#### 3.2.3 Nanostructured Cellulose Nanocomposites
The 3D printed snowflake model exhibited excellent magneto-responsive shape memory, gradually unfolding and recovering to its permanent shape within 35 s. This confirms the favorable magneto-responsive shape-memory property of the snowflake model. [PERSON] et al. introduced cellulose nanocrystals (CNCs) to polycaprolactone (PCL)-based SMPU through in situ one-pot reactions [215]. The in situ incorporation of CNCs led to covalent bonding between PCL chains and the CNC surface, enhancing dispersion without the need for conventional pre-modification of CNCs (Figure 21a). Incorporating 10 wt.% CNCs significantly improved the mechanical properties, including tensile strength and modulus, without compromising elongation at break (Figure 21b). Additionally, the composite exhibited remarkable shape fixation and recovery qualities during a stretching test conducted on a 20 mL glass bottle, evidenced by its tight grip and rapid return to its original shape upon heating, highlighting its suitability for biological applications (Figure 21c). Figure 20: (**a**) Schematic diagram of the water-responsive mechanism, shape recover process, and lignin (LIG) content-dependent recovery time graph of the CA/LIG/PVA/CNF composite. Reproduced with permission from ref. [209]. Copyright 2022 Elsevier. (**b**) Shape recovery of a 3D-printed flower-like structure of CNFs embedded nanocomposite. Reproduced with permission from ref. [213]. Copyright 2023 Elsevier. [PERSON] et al. developed SMPCs by incorporating modified cellulose nanocrystals (MCNCs) with urethane and phosphorus-containing flame retardant units through an in situ polymerization method [216]. With a 3 wt.% inclusion of MCNCs, the nanocomposite exhibited excellent dispersibility within the polymer matrix and maintained shape fixity (R\({}_{\text{f}}\)) and recovery (R\({}_{\text{r}}\)) ratios above 90% over five cycles. Similarly, [PERSON] et al. developed a shape-memory polyacrylamide/gelatin hydrogel, integrating 3-methacryloxyppropyltrimethoxysilane modified MCNCs that acted as chemical-physical cross-links, enhancing the mechanical properties and swelling resistance [217]. Optimal performance was achieved with 2 wt.% MCNC addition; the hydrogel exhibited superior mechanical strength, excellent thermally induced shape-memory performance with 76.6% fixity and 96.5% recovery, and maintained cytocompatibility, marking its potential in biomaterial applications. Recent advancements in SMPCs have shifted focus from the traditional reliance on a single triggering mechanism towards the exploration of multi-stimulus-responsive systems, significantly enhancing their applicability across various domains [218]. [PERSON] et al. developed multi-responsive SMPNCs by integrating thermoplastic polyurethane (TPU) with carbomer (CB) and utilizing CNCs as a cross-linking agent [139]. This composite benefited  
Figure 21: (**a**) Depiction of a possible interaction between PCL and CNC; (**b**) relationship between CNC concentration and tensile strength at yield and break; and (**c**) images of grippers taken at room temperature, following mechanical stretching, and after being heated to 40 \({}^{\circ}\)C. Reproduced with permission from ref. [215]. Copyright 2022 Elsevier. from the dual-phase structure of polyurethane, which facilitated thermo-responsive shape-memory behavior through its hard and soft segments, alongside a high concentration of -COOH groups (ranging from 56.0% to 68.0%) of CB, aiding in cross-linking via hydrogen and ester bonds [219; 220]. The nanocomposite exhibited shape-memory behavior through molecular switches activated by stimuli such as water, heat, and pH changes, enabling the programming and recovery of temporary shapes (Figure 22). Chemical and microstructural analyses revealed that the optimal loading of CNCs at 5 wt.% significantly influenced the mechanical properties of the nanocomposites, facilitating the formation of quaternary ammonium ionic and hydrogen bonds.