High water-content thermoresponsive hydrogels via electrostatic macrocrosslinking of cellulose nanofibrils

Nanocellulose: Recent Fundamental Advances and Emerging Biological and Biomimicking Applications

In the effort toward sustainable advanced functional materials, nanocelluloses have attracted extensive recent attention. Nanocelluloses range from rod-like highly crystalline cellulose nanocrystals to longer and more entangled cellulose nanofibers, earlier denoted also as microfibrillated celluloses and bacterial cellulose. In recent years, they have spurred research toward a wide range of applications, ranging from nanocomposites, viscosity modifiers, films, barrier layers, fibers, structural color, gels, aerogels and foams, and energy applications, until filtering membranes, to name a few. Still, nanocelluloses continue to show surprisingly high challenges to master their interactions and tailorability to allow well-controlled assemblies for functional materials. Rather than trying to review the already extensive nanocellulose literature at large, here selected aspects of the recent progress are the focus. Water interactions, which are central for processing for the functional properties, are discussed first. Then advanced hybrid gels toward (multi)stimuli responses, shape-memory materials, self-healing, adhesion and gluing, biological scaffolding, and forensic applications are discussed. Finally, composite fibers are discussed, as well as nanocellulose as a strategy for improvement of photosynthesis-based chemicals production. In summary, selected perspectives toward new directions for sustainable high-tech functional materials science based on nanocelluloses are described.

Impact of wormlike micelles on nano and macroscopic structure of TEMPO-oxidized cellulose nanofibril hydrogels

In this work, we investigated the effect of adding surfactant mixtures on the rheological properties of TEMPO-oxidized cellulose nanofibril (OCNF) saline dispersions. Three surfactant mixtures were studied: cocamidopropyl betaine (CAPB)/sodium dodecyl sulfate (SDS), which forms wormlike micelles (WLMs); cocamidopropylamine oxide (CAPOx)/SDS, which forms long rods; and CAPB/sodium lauroyl sarcosinate (SLS), which forms spherical micelles. The presence of micelles in these surfactant mixtures, independent of their morphology, leads to an increase of tan δ, making the gels less solid-like, therefore acting as a plasticizer. WLMs were able to suppress strain stiffening normally observed in OCNF gels at large strains. OCNF/WLM gels have lower G' values than OCNF gels while the other micellar morphologies have a reduced impact on G'. The presence of unconnected micelles leads to increased dissipative deformation in OCNF gels without affecting the connectivity of the fibrils, while the presence of entangled micelles interferes with the OCNF network.

Cationic surfactants as a non-covalent linker for oxidised cellulose nanofibrils and starch-based hydrogels

Rheological properties of hydrogels composed of TEMPO-oxidised cellulose nanofibrils (OCNF)-starch in the presence of cationic surfactants were investigated. The cationic surfactants dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) were used to trigger gelation of OCNF at around 5 mM surfactant. As OCNF and DTAB/CTAB are oppositely charged, an electrostatic attraction is suggested to explain the gelation mechanism. OCNF (1 wt%) and soluble starch (0.5 and 1 wt%) were blended to prepare hydrogels, where the addition of starch to the OCNF resulted in a higher storage modulus. Starch polymers were suggested to form networks with cellulose nanofibrils. The stiffness and viscosity of OCNF-Starch hydrogels were enhanced further by the addition of cationic surfactants (5 mM of DTAB/CTAB). ζ -potential and amylose-iodine complex analyses were also conducted to confirm surface charge and interaction of OCNF-starch-surfactant in order to provide an in-depth understanding of the surfactant-induced gel networks.

Alcohol induced gelation of TEMPO-oxidized cellulose nanofibril dispersions

Solvent-induced physical hydrogels of TEMPO-oxidized cellulose nanofibrils (OCNFs) were obtained from aqueous/alcoholic dispersions of fibrils in lower alcohols, namely, methanol, ethanol, 1-propanol and 2-propanol. The sol-gel transition occurs above a critical alcohol concentration of ca. 30 wt% for all alcohols tested. The rheological properties of the hydrogels depend on the nature of the alcohol: for ethanol, 1-propanol and 2-propanol the magnitude of the shear storage modulus follows the alcohol hydrophilicity, whilst methanol produces the weakest gels in the group. Above a second critical concentration, ca. 60 wt% alcohol, phase separation is observed as the gels undergo syneresis. Analysis of small-angle X-ray scattering data shows that the OCNFs may be modelled as rigid rods. In the presence of lower alcohols, attractive interactions between nanofibrils are present and, above the alcohol concentration leading to gelation, an increase of the OCNF cross-section is observed, suggesting alcohol induced aggregation of nanofibrils.

Tough, Stimuli-Responsive, and Biocompatible Hydrogels with Very High Water Content

Some marine creatures like jellyfish have gel-like bodies consisting mostly of water (above 95 wt%). Yet, their gel-like bodies still show quite good mechanical properties and can respond to external stimuli. Artificial hydrogels with very high water content are generally extremely weak, and hence their practical applications are strongly limited. Inspired by jellyfish, tough and biocompatible poly(vinyl alcohol)/sodium polyacrylate (PVA/PAANa) hydrogels with very high equilibrium water content (98.23-99.58 wt%) are developed. The equilibrium swollen PVA/PAANa hydrogels show good mechanical properties, with elastic modulus, tensile strength, and elongation up to 0.046 MPa, 0.14 MPa, and 206%, respectively, very close to those of jellyfish mesoglea. Moreover, the PVA/PAANa hydrogels can respond to external multi-stimuli distinctly, such as metal cations, pH, and salts. Very impressively, the PVA/PAANa hydrogel can easily distinguish tap water from deionized water, and its detection limit of metal cations can be as low as 10^-4  mol L^-1 . Cell cytotoxicity tests and in vivo biocompatibility tests prove that the PVA/PAANa hydrogels have excellent biocompatibility. The tough, stimuli-responsive, and biocompatible hydrogels with very high water content may find a variety of practical applications in load-bearing biomaterials, detection, sensors, and agricultural fields.

Photodegradable hydrogels for external manipulation of cellular microenvironments with real-time monitoring

A designed hydrogel whose stiffness could not only be controlled but also monitored in situ by fluorescence.