## 3 Shape-Memory Polymer Nanocomposites (SMPNCs)
### Carbon-Based Nanocomposites
#### 3.2.3 Nanostructured Cellulose Nanocomposites  
Incorporating nanofillers into polymer matrices is a prevalent engineering strategy to enhance material properties and introduce tunable functionalities in nanocomposites [184; 185]. Cellulose nanomaterials (CNs), including cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), are particularly valued as fillers or reinforcing agents in SMPs due to their affordability, high availability, low density, exceptional crystallinity, substantial aspect ratio, remarkable tensile strength and modulus, and biocompatibility [186; 187; 188]. The abundant hydroxyl groups on the surface of CN materials facilitate the formation of hydrogen bonds with the soft polymer matrix during polymerization, resulting in improved interfacial adhesion and increased stiffness [189; 190]. Nevertheless, the fabrication of nanoarchitecture through direct physical encapsulation of CN materials within the polymer matrix is challenging. To enhance the nanoscale dispersibility of CNs and overcome processing challenges, considerable attention has been directed towards implementing bottom-up surface modifications techniques such as chemical covalent bonding, surfactant treatment, wet compounding, polymer grafting, and wrapping [191]. Without undergoing any functional modification, CNs in their pristine state typically exhibit only modest improvements in their characteristics [192]. Recently, functionalized cellulose materials have been investigated as cross-linkers to fabricate nanocomposites with enhanced properties [193; 194]. However, robust cross-linking often leads to decreased free volume and fewer active sites for hydrogen bonding, affecting water molecule diffusion within the microstructure and, consequently, the material's water-induced shape-recovery performance [195; 196; 197]. Hydrated cellulose nanopapers (CNPs), produced from CNFs, offer significant potential due to their nanoscale effects, high specific surface area, and hydrophilicity, making them excellent for functional applications [198]. Yet, their direct application in water-induced  
Figure 18: (**a**) Diagrams illustrating the fabrication process of nanocomposites from SBS, PCL, and CNF, as well as the shape-recovery cycle. Reproduced with permission from ref. [174]. Copyright 2019 Elsevier. (**b**) Plots showing the shape-recovery ratio vs. time for two distinct compositions: POE/LA/CB (50/50/20) (left) and POE/LA/CB (50/50/10) (right), under various voltage conditions. Reproduced with permission from ref. [182]. Copyright 2018 Elsevier. shape-memory materials is limited due to susceptibility to swelling and dissolution, caused by water infiltration weakening the CNFs' interfibrillar hydrogen bonding [199; 200; 201]. This vulnerability restricts the application of CNPs in high humidity and water environments. The preparation methods and characteristics of CNFs predominantly influence the structure and properties of CNP [202]. Formic acid (FA) hydrolysis is one of the clean and sustainable pre-treatment methods for CNF production, offering easy recycling due to the low boiling point of FA [203]. During this process, the introduction of ester groups on the surface of CNFs adjusts their polarity and hydrophilicity, allowing the preparation of flexible CNPs with appropriate hydrophilicity and wet strength [204]. A neutral amino-containing polymer, chitosan (CS), has also been explored to enhance wet strength in CNP as a cross-linker [205]. However, the increased brittleness associated with CS cross-linking poses a challenge for practical applications, limiting folding endurance [206]. Moreover, CS, reported as a shape-memory scaffold for drug delivery, exhibited a relatively low response to water and a rapid decrease in wet strength due to excessive swelling, highlighting the trade-off between water responsiveness and material durability [54]. In recent years, water-induced SMP composites have been investigated more deeply because of the mild and environmentally friendly nature of water as a stimulus [207]. [PERSON] et al.