The design of materials capable of mechanical responses to physical and chemical stimuli represents one of the most exciting and challenging areas of scientific research because of the huge number of their potential applications. This article is focused on the molecular events occurring in thin films of carboxylated nanocellulose fibers, which are capable of converting water gradients into mechanical movements at the macroscopic scale. The analysis of the mechano‐actuation, and of the conditions to obtain it, shows that the film movement is fast and reproducible, the gradient intensity is transduced into rate of displacement, and the response is observed at vapor pressures as low as 1.2 mm Hg. The actuation mechanism is associated to an efficient and reversible water sorption process by the hydrophilic nanocellulose fibers at the film interface. Conversely, water desorption is slow and follows a kinetic behavior supporting the presence of two binding sites for water molecules. The adsorbed water induces swelling of the surface nanocellulose layers and local structural rearrangement, however transitions between ordered and random coil conformations are not observed. The understanding of the actuation mechanisms of nanocellulose offers exciting opportunities to design macroscopic structures responding to chemical gradients by the assembly of simple molecular components.

Sol-gel transition of carboxylated cellulose nanocrystals has been investigated using rheology, SAXS, NMR and optical spectroscopies to unveil the distinctive roles of ultrasound treatments and addition of various cations. Besides cellulose fiber fragmentation, sonication treatment induces fast gelling of the solution. The gelation is independent of the addition of cations, while the final rheological properties are highly influenced by the type, concentration and sequence of the operations since the cations must be added prior to sonication to produce stiff gels. The gel elastic modulus was found to increase proportionally to the ionic charge rather than the cationic size. In cases where ions were added after sonication, SAXS analysis of the Na+ hydrogel and Ca2+ hydrogel indicated the presence of structurally ordered domains in which water is confined, and 1H-NMR investigation showed the dynamics of water exchange within the hydrogels. Conversely, separated phases containing essentially free water were characteristic of the hydrogels obtained by sonication after Ca2+ addition, confirming that this ion induces irreversible fiber aggregation. The rheological properties of the hydrogels depend on the duration of the ultrasound treatments, enabling the design of programmed materials with tailored energy dissipation response.

Moreover, the implementation of a wearable photonic technology directly in contact with clothes would be light-weight, comfortable, noninvasive, implantable, and inherently low cost. It attracts a strong interest for industry in this futuristic field. Indeed, according to International Data Corporation, the important growth of this technology is forecasted to grow up to 213.6 millions in 2020. [8] Nowadays, examples of commercial real-time applications include textile-based displays, [9] photovoltaics [9] or health monitoring. [9] In spite of this significant progress, wearable devices still require reduced footprint, better coupling techniques, and the integration of more complex optic/electrical functionalities to meet the performances already provided by traditional semiconductors integrated on rigid substrates.Taking into account these current limitations, metal halide perovskites(MHPs) is a promising semiconductor for flexible/wearable optoelectronic devices because of the outstanding capabilities to provide light emission, gain generation, and photodetection functionalities of polycrystalline MHPs films grown at low temperature. Indeed, MHPs demonstrated a broad range of excellent electrical and optical properties, such as long diffusion lengths, [10] high absorption cross-section, [11] high quantum yield of emission at room temperature, [12] or tunable bandgap with the composition. [13] MHPbased devices include highly efficient solar cells, [14] optical active devices, [12,13,15,16] and photodetectors. [17,18] The majority of these publications, however, use a rigid substrate to fabricate the device, being a significantly lower amount of works on MHP flexible devices with a single functionality as solar cells, [19,20] optical switch, [21] or lasing. [22] On the other hand, nanocellulose (NC) [23,24] has been probed as an ideal substrate for wearable optoelectronics. [25] This polymer is obtained from the most common biopolymer on Earth, and it consists of rigid nanocrystals that can be easily assembled into films and gel materials.NC is not only an excellent bendable, deformable and stretchable material, [26] but also exhibits very interesting properties for optoelectronics. Its advantages comprise a very high transparency in the visible, [27] tunable chiral nematic order by the surface chemistry, [28] low roughness, and extremely high gas barrier properties. [29] Nevertheless, despite these promising abilities, integration of optoelectronic devices in cellulose has been elusive, being it polyimide or polydimethylsiloxane Flexible optoelectronics has emerged as an outstanding platform to pave the road toward vanguard technology advancements. As compared to conventional rigid substrates, a flexible technology enables mechanical deformation while maintaining stable performance. The advantages include not only the development to novel applications, but also the implementation of a wearable technology directly in contact with a curved surface. Here the monolithic integration of a perovskite-based optical wave...

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Ca2+ induced external and inverse ionotropic gelation of crystallites of nanocellulose allows the fabrication of small hydrogels with various morphologies using a reaction-diffusion driven process.