## 2 Basis of Ice-Templating  
The use of freezing colloids as a route to template porosity in materials, usually called ice-templating, although known for many years [57; 58; 59], has been the subject of considerable attention over the last ten years [60; 61; 62; 63]. The reasons for the growing interest in utilising ice-templating process in materials engineering stem from its simplicity and versatility and the wide variety of porous materials that this technique can provide. The basic principle of ice-templating is simple: it is a segregation-induced templating of a dispersed (or dissolved) solid phase by a solidifying solvent (Figure 3). The dispersed (or dissolved) phase can be of almost any nature, from ceramic to metal particles or polymer. During the solidification of the suspension, the dispersed (or dissolved) phase is rejected from the growing crystals of the solvent. The solidified solvent is then removed by sublimation, leaving a template porous material where the pores are almost a direct replica of the frozen solvent crystals. In practice, the processing of porous materials by ice-templating can be divided in three main steps (Figure 3). Figure 3: Scheme of the principle of ice-templating. (Adapted from [PERSON] and [PERSON] [52] with permission from the Royal Society of Chemistry). Figure 2: Schematic view of the different processing routes used to obtain nanocellulose-based aerogels and foams. (Adapted from [PERSON] and [PERSON] [52] with permission from the Royal Society of Chemistry); CNF: cellulose nanofibril, CNC: cellulose nanocrystal. Preparation of the suspensions. The solid phase is dispersed (or dissolved) in the liquid medium (solvent). The stability of the suspension should be carefully controlled to avoid any segregation phenomenon taking place in the solidification phase, yielding gradients of porosity in the final materials. Water is the most commonly used solvent because it is easy to use and manipulate and available in large quantities [63]. Other solvents such as camphene [64; 65] and ter-butyl alcohol (TBA) [66; 67] are also frequently used in the ice-templating process. Generally, moderate solid loading is used (10-40 vol.%) to ensure that crystals have enough space to grow and solid particles are redistributed by the crystals. The concentration of solids in the initial suspension should be adjusted to reach the desired porosity in the final materials. * Solidification of the suspensions. During this step, crystals of solvent are formed and grow in the suspension. The solid particles that are dispersed (or solubilised) in the suspension are rejected by the moving solidification front, concentrated, and entrapped between the crystals. This step is crucial and determines the characteristics of the porous structure (e.g., shape, size of the pores) in the final materials. The freezing conditions mainly depend on the choice of the solvent [63]: for instance, low temperatures (\(<\)0 \({}^{\circ}\)C) are required when using water, whereas room temperature is sufficient when using camphene. It is important to notice that the equipment required to fabricate the materials (e.g., the mould) has to accommodate the variations of volume (e.g., \(-3.1\%\) for camphene, 9% for water [63]) that can occur during the solidification of the suspensions. * Sublimation of the solvent. After complete solidification of the suspension, the sample is kept at conditions of low temperature and reduced pressure using a freeze-drying equipment [68]. The sublimation conditions are dictated by the physical properties of the solvent. Under these conditions, the solidified solvent is converted into a gas, leading to a porous structure. The ice-templating process has already been used to fabricate a wide variety of porous materials with a great potential in several engineering fields [63], including construction [69], energy [70], and biomedical [71; 72] sectors. In comparison to other processes, ice-templating routes exhibit several unique features.