Low Solids Emulsion Gels Based on Nanocellulose for 3D-Printing

Abstract

Multiphase (emulsion) gels with internal phase fractions between 0.1 and 0.5 were formulated at low loadings of cellulose nanofibrils (CNF), alginate and polylactide (PLA). Their properties (rheology and morphology) fitted those of inks used for direct ink writing (DIW). The effect of formulation and composition variables were elucidated after printing cubic scaffolds and other solid designs. The distinctive microstructures that were developed allowed high printing fidelity and displayed limited shrinkage after room temperature and freeze drying (0 and 5% shrinkage in the out-of-plane and in-plane directions upon freeze drying, respectively). The CNF added in the continuous phase was shown to be critical to achieve rheology control, as an effective interfacial stabilizer and to ensure the printability of the ink toward high structural reliability.We found that the extent of shape retention of the dried scaffolds resulted from the tightly locked internal structure. The PLA that was initially added in the non-polar or organic phase (0 to 2 12%), was randomly embedded in the entire scaffold, providing a strong resistance to shrinkage during the slow water evaporation at ambient temperature. No surface collapse or lateral deformation of the dried scaffolds occurred, indicating that the incorporation of PLA limited drying-induced shape failure. It also reduced compression strain by providing better CNF skeletal support, improving the mechanical strength.Upon re-wetting, the combination of the hydrophilicity imparted by CNF and alginate together with the highly porous structure of the 3D material and the internal micro-channels contributed to high water absorption via capillary and other phenomena (swelling % between ~400 and 900 %). However, no shape changes occurred compared to the initial 3D-printed shape. The swelling of the scaffolds correlated inversely with the PLA content in the precursor emulsion gel, providing a means to regulate the interaction with water given its low surface energy.Overall, the results demonstrate that by compatibilization of the CNF-based hydrophilic and the PLA-based hydrophobic components, it is possible to achieve shape control and retention upon 3D printing, opening the possibility of adopting low-solids inks for DIW into dry objects. The dryable CNF-based 3D structural materials absorb water while being able to support load (high elastic modulus) and maintain the shape upon hydration.

Shape fidelity and structure of 3D printed high consistency nanocellulose

“…In previous research the deformation and shape analysis regarding AM cellulose constructs has been assessed with manual measurement tools 21 or qualitative visual inspection 31 . Li et al .…”

“…Arguably, most DIW research with native cellulose materials has been performed with inks consisting of nanostructured cellulose commonly known as nanocellulose. Nanocellulose forms thick shear-thinning gels already at low consistencies which facilitates the printing of high water content freestanding constructs 20 , 21 . One of the most notable applications of AM nanocellulose is biofabrication.…”

Two‐Phase Emulgels for Direct Ink Writing of Skin‐Bearing Architectures

“…The choice of Pickering emulsion stems from the fact that colloids can adsorb irreversibly at the oil/water interface, generating an interfacial barrier that limits droplet coalescence and endows emulsions with superior stability compared to those produced with surfactants . We have previously shown that a mixture of anionic cellulose nanofibrils (CNF) and cationic chitin nanofibers (ChNF) is highly efficient in stabilizing oil‐in‐water emulsions (e.g., PLA/CHCl 3 ‐in‐water Pickering emulsions) . In such systems, cooperative adsorption and network formation at the oil/water interfaces are the main reasons for achieving high stability.…”

“…To tackle the current challenges, we propose multicomponent materials consisting of immiscible but metastable phases given the possibility of spontaneous phase‐separation . For example, we have recently reported on multiphase emulsions to generate complex structures via 3D printing using DIW . While our initial aim was to gain control on the properties of the emulsion ink by changing the formulation (e.g., emulsion morphology), the results hinted to the possibility of achieving hierarchical architectures .…”

Shear melting and recovery of crosslinkable cellulose nanocrystal–polymer gels

“…Methods used for processing CNC composites include solvent-casting, coagulation and spinning, as well as melt extrusion [42][43][44]. More recently, bulk CNC-based structures have been produced by direct-ink writing [45,46]. Consequently, the rheological properties of these composites, and especially the interplay of yielding, flow and recovery are strongly connected to our ability to manufacture CNC-based materials with the desired microstructure and properties.…”