Role of sonication pre-treatment and cation valence in the sol-gel transition of nano-cellulose suspensions

Abstract

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.

In vitro toxicity assessment of hydrogel patches obtained by cation‐induced cross‐linking of rod‐like cellulose nanocrystals

“…Microenvironmental mechanical stress has been demonstrated to induce apoptosis in cancer cells (Cheng, Tse, Jain, & Munn, ) and warnings about physical trauma have been reported on the European Standards EN ISO 10993‐5 for the evaluation of Biomedical devices by direct contact tests in vitro (ISO 10993‐5, ). Enhanced physical stress can be expected for CNC‐HMs used for this study since they are made of a homogeneous and soft material, as indicated by its rheological parameters (storage and elastic moduli) and the shear‐thinning behavior (Maestri, Abrami, et al, ). These characteristics make the CNC hydrogels easily injectable and suitable for in vivo applications such as drug and cell delivery, absorption of exudates, and exchange of fluids.…”

“…The presence of internalized CNCs further supports the quantitative data of Figure , which demonstrate the nontoxicity of CNCs. Sonication and cross‐linking by cations entangle CNCs in soft hydrogels, the elastic modulus of which depends on the ratio charge/radius of the positively charged cross‐linker (Maestri, Abrami, et al, ). Ca 2+ has been extensively used as cross‐linker of polysaccharides and, recently, to produce CNC hydrogels (Dong, Snyder, Williams, & Andzelm, ).…”

“…At the end of this procedure, a transparent suspension of rod‐like cellulose nanocrystals (CNCs) is obtained. The size of CNCs is rather independent of the plant source and is well known to fit in the range 3–5 nm of width and 200–500 nm of lengths (as from AFM and TEM analysis; Maestri, Abrami, et al, ; Saito & Isogai, ). By a conductimetric procedure we estimated that, the ratio between the carboxylic groups and the cellobiose units is in average 1/2.5 (carboxyl group concentration 9.3 mM for a 8 g/L CNC solution; Maestri, Bettotti, & Scarpa, ).…”

“…Conversely, CNCs have been considered ill‐suited as single component gels due to their limited ability to entangle (De France et al, ). Only recently, extensive sonication and ionotropic gelation have been successfully utilized to obtain soft CNC hydrogel structures of different size and shape with long‐term stability and high transparency to visible light (Maestri et al, ; Maestri, Bettotti, & Scarpa, ). In spite of the many advantages offered by nanocellulose hydrogels, toxicity concern is still an open question.…”

Effect of ultrasonication on physicochemical properties of apple based nanocellulose-calcium carbonate composites

“…Also, the addition of surfactants to CNF suspension enhanced the gelation properties even in low concentrations (Quennouz et al 2016). Most recently, Maestri et al (2017) have shown that application of ultrasounds breaks the TEMPO-oxidized cellulose fibrils in the amorphous region producing isolated nanofibrils which suspension forms weak but uniform and transparent gel (Maestri et al 2017). This separation of fibrils avail the interaction of polymeric surface with water molecules effecting the rheological properties.…”