## 5 Conclusions and Outlook  
Many forming processes using the principle of ice-templating have been developed to obtain porous materials from nanocellulose dispersions. These processing routes commonly involve two main steps: (i) the preparation of the nanocellulose suspension and (ii) the solidification and removal of the suspending fluid by lyophilisation. Depending on the chosen process, nanocellulose-based materials with varied shapes and structures can be manufactured such as semi-structural foam panels, microbeads, filter membranes, and scaffolds. The fabrication methods are also versatile enough to tune easily the process-induced microstructures. A large number of factors related both to the processing conditions (e.g., temperature gradient, cooling rate) and the nature of colloids (particles, size, surface charge density, conductivity, addition of salt or polymer, etc.) can be used to achieve complex and customisable architectures suited for target application requirements. Ice-templated nanocellulose-based materials can exhibit a broad diversity of physical (thermal, acoustic, permeation, etc.) and mechanical properties owing to their tunable microstructure. These materials can exhibit medium to high porosity \(\varepsilon\) (80% \(\leq\varepsilon\leq\) 99%), varied pore morphology (tubular, cellular, and lamellar), and a pore size distribution defined at several length scales (from the nanoscale to the millimetre scale). In addition, the processes using the principle of ice-templating are almost material agnostic, i.e., almost any type of materials can be used. As such, multifunctional porous nanocellulose-based materials can also be designed by combining CNCs or CNFs with polymers [137; 147; 155] or other nanoscale particles such as carbon nanotubes [156; 157], graphene [158], and silver nanoparticles [159]. Ice-templated nanocellulose-based materials constitutes promising biophysical solutions for a wide diversity of engineering fields including construction, transportation, energy, and biomedical sectors. However, the existing manufacturing processes still suffer from important limitations related both to the inherent characteristics of nanocellulose suspensions and the ice-templating technique. Nanocellulose are colloidal fibrous systems that exhibit a complex rheology between that of soft solids and liquids. These suspensions are thus difficult to use in forming processes, particularly in concentrated regimes. Extensive research should be carried out in order to better characterise and understand the rheology of nanocellulose suspensions in concentrated and hyper-concentrated regimes. Then, the existing processes are multi-step processes with long production cycles. In particular, the freeze-drying or lyophilisation phase is time-consuming and research efforts are still required to make this step more efficient. For instance, additives or solvents with low enthalpy of sublimation could be used to increase the sublimation kinetics. In addition, the control of the temperature gradient during the processing of the materials still need to be improved. Most of ice-templated materials exhibit core-shell architectures, microstructure gradients and anisotropic pore morphologies. Theyalso exhibit heterogeneities and numerous structural defects such as dangling masses and torn cell walls. These defects significantly reduce the physical and mechanical properties of ice-templated materials [56]. Research efforts must be done to better characterise and understand the phenomena involved in the formation of foam cells [160; 161; 162] as well as those related to the shrinkage of materials during solidification and freeze-drying of nanocellulose suspensions. Conceptualization, S.G., F.M., L.O. and P.J.J.D. ; writing--original draft preparation, S.G., F.M., L.O. and P.J.J.D. ; writing--review and editing, S.G., F.M., L.O. and P.J.J.D. ; supervision, F.M., L.O. and P.J.J.D. ; project administration, F.M., L.O. and P.J.J.D. ; funding acquisition, F.M., L.O. and P.J.J.D.