## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Carbon-Based Nanocomposites
#### 3.2.3 Nanostructured Cellulose Nanocomposites
addressed the inherent limitations of water-induced SMPCs through the integration of CS-modified CNP, which is derived from CNFs treated with FA hydrolysis to introduce surface ester groups [54]. This modification endowed the CNP with rapid responsiveness to water (<3 s) and humidity (<10 min) without compromising wet strength (Figure 19b). Notably, the composite exhibited a shape-recovery ratio (R\({}_{\text{r}}\)) exceeding 90% across 10 cycles, indicating its durable shape-memory capability in aqueous environments. The observed behavior was attributed to the water-mediated disruption of hydrogen bonds and the reversible activity of CS chains, which facilitated the transition between its temporary folded state and its original configuration [208]. The response benefited from the combined hydration effects of CS and the water-resisting properties of CNFs with ester groups, which together prevented overhydration and enabled a swift shape recovery [204]. The CS-modified CNP further demonstrated significant folding endurance (exceeding 1190 cycles, Figure 19c), exceptional wet tensile strength (65 MPa), transparency, and superior barrier properties, underscoring its potential for diverse applications where water-induced actuation is desirable. Utilizing CNFs, [PERSON] and coworkers developed water-responsive SMPNCs via the evaporation-induced self-assembly (EISA) method, incorporating polyvinyl alcohol (PVA) and lignin as raw materials, and citric acid (CA) as cross-linker [209]. By adjusting the CNF to PVA ratio and the amount of added lignin, the cross-linked network structure was modified, affecting the microstructure, water responsiveness, and mechanical properties of the membrane. However, an increase in CNF concentration resulted in enhanced hydrogen bonding among CNF chains, potentially raising crystallinity and surface density, thereby increasing the hydrophobicity of the composite [210; 211]. The nanocomposite membrane achieved nearly 100% shape recovery in 15 s upon water immersion, with its response time optimized to 3 s at a 20% lignin content, demonstrating exceptional water responsiveness (Figure 20a). In another study, utilizing in situ polymerization technique, [PERSON] and coworkers explored hybrid polymer-polymer nanocomposites using a three-component system (polylactide/polybutylene adipate terephthalate/CNFs) [212]. Unlike conventional polymer blends, the novel formulation exhibited a triple SME characterized by significantly superior strain recovery and fixity ratios, faster recovery rates, and enhanced mechanical properties. [PERSON] et al. engineered 4D-printed thermally responsive SMPNCs by incorporating CNFs into polycaprolactone (PCL)/poly(butyleneadipate-co-terephthalate) matrix through wet melt blending method [213]. The resulting composite exhibited excellent shape fixation and recovery with good shape stability at the transition temperature. However, the incorporation of CNFs did not alter the shape fixity (R\({}_{\text{f}}\)) and recovery (R\({}_{\text{r}}\)) ratios but expedited the recovery process. Utilizing the nanocomposite filament, a flower model was printed and thermally programmed into a temporary shape, showcasing remarkable shape recovery and thus evidencing the achievement of 4D printing capabilities. (Figure 20b). Figure 19: (**a**) Schematic diagram illustrating the synergistic hydration process of the composite; (**b**) shape-memory responses to water for various tape-like CNP samples; and (**c**) double folding times of various composites. (WCNP and DCNP were produced from the homogenization of CNFs in water and dimethylacetamide, respectively). Reproduced with permission from ref. [54]. Copyright 2019 Elsevier. To achieve magnetic-responsive 3D-printed structures, [PERSON] and coworkers added Fe\({}_{3}\)O\({}_{4}\) and CNFs to poly(hydroxybutyrate)/poly(\(\varepsilon\)-caprolactone) (PHB/PCL) blends as functional particles and reinforcing agents [214].