Commonly, cellulose hydrogels have fixed-flexible sheet forms. They are also water-saturated (moist), soft, smooth, transparent, and colorless substances ^[1][2][58,63]; however, the color and transparency may be different depending on the initial cellulosic material (Figure 13a). For example, hydrogels obtained from cotton microcrystalline cellulose (MCC) are white-colored, hydrogels from flax cellulose perform sandy-beige color, and both are completely opaque, while cellulose hydrogels fabricated from waste paper are colorless or yellow, grey, light-brown and were either transparent or semi-clouded ^[3][4][34,64].

Hydrogels formed from bacterial and deciduous cellulose are transparent and colorless, although the latter may acquire a yellowish tone or become white and semi-opaque depending on the conditions of the dissolution ^[1][5][6][58,62,65]. Along with the other physical properties of hydrogels, transparency is desirable for a biomedical application, for example, for a wound dressing, since it allows one to control the wound condition without removing the dressing. As a result, the wound will not be traumatized, and the risk of infection will decrease ^[7][66].

Cellulose-based metallogels borrow the color of the introduced metal (Figure 13b,c); for instance, Ag(0) containing hydrogels have the color from light yellow to dark brown ^{[1][5][6][8][9][10]}[58,59,61,62,65,67], Au(0) composite gels may have purple, pink, brick or red coloration (Figure 13b) ^{[10][11][12][13]}[67,68,69,70], Pt(0)-containing metallogels are colored in black ^[10][67], while calcium based composites are white ^[14][17], reflecting all the spectrum of the visible light. The metallogels with organic-metal complexes of transition metals reveal brown or purple-brown color for Zn(II), Pd(II), and Cr(II)-Cu(II); Co(II) containing composites are almost black, and the ones with Mg(II) are colored in blue (Figure 13c) ^[15][71].

In the series of publications ^[1][11][16][58,68,72], the composites of cellulose hydrogels with Ag(0) and Au(0) nanoparticles fabricated via the diffusion-reduction method  (Figure 1) were studied. It was reported that the intensity of their coloration was in accordance with the aspect ratio between cellulose and metal ions. Generally, the higher the concentration of metal ions in the gel, the more intense the color of the obtained composite becomes. Moreover, the aspect ratio between metal ions and the chemical reducer plays a role in the reduction process, contributing to the resulting metal content and the color of the hydrogels. For instance, the aspect ratio between citrate ion and Ag⁺ was reported to be (1–10):1 mol, respectively ^[16][72].

On the contrary, when the nanoparticles of Au(0) were synthesized in the hydrogel without a chemical reducer due to the reducing ability of the end aldehyde groups of cellulose chains, it was reported that the more Au-ions were introduced in the solution, the slighter the coloration of the metallogel became after reaching the saturation threshold ^[11][68]. The authors explained this fact by the limited number of the end-reducing groups in cellulose capable of interacting with the growing number of Au ions. As a result, not all of Au⁺ were involved in the redox process.

The swelled cellulose hydro- and metallogels are stable while storing in water or humid environment; unlike synthetic hydrogels, they spontaneously lose the moisture when stored in air ^[4][64]. Therefore, the swelled hydrogels are sometimes undergone by freeze-drying for some applications. Although for the majority of the hydrogels the reswelling ability was reported ^[17][18][19][73,74,75], the other cellulose hydrogels crucially collapse while drying, and form slightly accessible structures ^[4][20][64,76] due to the collapse of the H-bonds formed. This is a challenge for the future improvements.

Due to the ability to exhibit strong light absorption, efficient energy transfer, and excellent charge transport makes cellulose-metal complexes play a crucial role in various fields, particularly in the realm of optoelectronics technologies, making them highly valuable for a range of applications such as photovoltaic devices, light-emitting diodes (LEDs), displays, but also sensitive and selective biosensors for different analytes, including biomarkers, pathogens, and pollutants ^[21][22][77,78].

The cellulose hydrogels are super-swollen systems and retain large amounts of water, dozens of times their own dry weight ^[23][24][25][42,79,80]. The swelling of hydrogels is influenced by the available space within the network structure capable of accommodating, absorbing, and maintaining water molecules due to their hydrophilic nature ^[26][81]. The swelling behavior of hydrogels in water or biological fluids is a crucial parameter determining their properties and applications ^[23][42]; for example, the hydro- and metallogels can serve as wound or burn injuries dressings, effectively hydrating dry wound beds and promoting the softening and loosening of slough and necrotic wound debris. The hydrogels exchange moisture with dry wounds, thereby facilitating autolytic debridement and maintaining a moist wound environment that is thermally insulated. They have been shown to promote granulation and epithelialization and reduce the temperature of a wound bed by up to 5 °C ^[2][63]. Metallogels preserve these favorable properties and can be utilized as wound dressings too. Moreover, when specific metals (Ag, Au, Zn) are used, they also confer antibacterial activity to these materials ^[27][28][82,83].

Table 1 presents the properties of non-derivatized cellulose hydrogels obtained by dissolving in organic solvents or in the alkali/urea/water systems. A detailed analysis of the cellulose-based hydrogel preparation using these solvents is provided in the first part of the review ^[29][57]. Swelling in water is usually calculated according to Formula (1) and is given in the articles as a percentage or in grams of water per gram of dry hydrogel (g/g).

where EWC is for the equilibrium water content, m_h is the mass of the swollen hydrogel at the equilibrium, and m_d is the mass of the dried specimen to a constant weight in the vacuum ^[4][64].

Even though the formula is quite simple, there are several discrepancies in its use. Firstly, drying of hydrogels in the vacuum is not mandatory; in some papers, the method of drying is not specified (e.g., in ^[31][84]); as a result, they might be dried in air or by oven heating, which obviously affects the result. Secondly, in some studies, the weight of the initial swollen hydrogel was measured, then it was dried to calculate the swelling ability ^[4][31][64,84], while in the other cases, the hydrogel was dried, then it reswelled in water until equilibrium, and after that, it was weighted. Initial swelling after formation and reswelling after drying may not be the same. Finally, in several studies, the authors changed the Formula (1) as the following:

where EWC is for the equilibrium water content, W_h is the weight of the hydrated sample, and W_d is the dry weight of the sample ^[18][74].

The studies ^[18][32][74,85] attribute the mass of water not to a dry hydrogel but to a swollen one (Formula (2)), so their results cannot be compared to those obtained by Formula (1). Thus, to compare the equilibrium water content (swelling ability, swelling properties, water content, swelling ratio, etc.) of the hydrogels in different studies, the protocol of this measurement should be carefully checked.

Table 1 does not cover all the results obtained in recent years, but it is provided for the purpose of understanding the potential for swelling, as it presents different solvents and different starting cellulosic materials (cellulose, lignocellulose, wastes). It should be noted that Table 1 lists not only hydrogels but also hydrogel films ^[18][33][74,86], for which the swelling properties are lower.

The study by Tovar-Carrillo and co-authors allows one to compare hydrogels obtained in different solvent systems within the same raw material. Thus, the result revealed that of the three solvents, namely NaOH aqueous, NaOH/urea, and N,N-dimethylacetamide (DMAc)/LiCl, the latter one provided the maximum swelling ability (0.31 g/g) of the hydrogel, while for the alkali-based methods, the swelling abilities of the hydrogels were lower (0.11 and 0.14 g/g, respectively) ^[18][74]. These data are methodologically comparable with the swelling ability of the hydrogels obtained from okara fibers (0.88–0.94 g/g depending on the epichlorohydrin (ECH) content which was a cross-linker in the synthesis) using LiOH/urea/water dissolving system ^[32][85]. However, in this study, the authors added ECH as a chemical cross-linker, while in ^[18][74], all the hydrogel films were physical hydrogels.

Among the swelling abilities calculated by Formula (1), the highest result (66.75 g/g) was reported for a composite hydrogel produced from waste paper and acrylamide in NaOH/urea aqueous system with the cross-linker ECH ^[34][87]. An identical dissolving system was applied for waste paper, but the swelling abilities of the hydrogels without acrylamide in the composition of the hydrogel obviously fell drastically to 13.4–27.7 g/g compared with the previous example ^[35][88].

Acrylates are well-known agents for the improvement of the swelling properties of cellulose hydrogels in order to produce super-adsorbents ^[36][89]. For instance, the swelling ability of the hydrogel prepared from maleylated cotton stalk cellulose and acrylic acid was reported to be 1125 g/g ^[37][90]. Another superabsorbent hydrogel obtained from flax yarn waste and poly(acrylic acid-co-acrylamide) revealed a swelling ability of 875 g/g ^[38][91]. The conclusion is that acrylates improve the swelling properties of cellulose-based hydrogels significantly.

The swelling behavior of hydrogels is governed by several factors, including the polymer composition, degree of polymerization, degree of crosslinking, cellulose concentration in the initial solution, cellulose origin (Table 1), and external stimuli. The hydrogels from the solutions of DMAc/LiCl had comparatively high swelling abilities (11.53–28 g/g) depending on the type of initial material. The hydrogels from hardwood lignocellulose had the highest swelling ability, while the sugarcane bagasse hydrogels had the lowest ^[4][31][64,84]. However, the swelling abilities of the hydrogels obtained from agricultural wastes in dimethyl sulfoxide (DMSO)/LiCl, namely thanaka tree heartwood, sugarcane bagasse, and rice straw celluloses, were relatively similar (1.66–1.89 g/g) ^[33][86]. The lower values can be explained by the shape of the hydrogels and hydrogel films prepared in the study, in which swelling abilities were much lower for hydrogels resulting from the (DMSO)/LiCl dissolving system than for those resulting from the DMAc/LiCl.

Several studies ^[17][35][39][73,88,92] compare the swelling abilities of the hydrogels with different concentrations of lignocellulosic solutions. In general, the higher the concentration of cellulose, the bigger the swelling ability of the hydrogel; however, after a certain value of 5% ^[17][35][73,88], it decreases. It is connected to the porous structure of the hydrogel, which allows water to enter the hydrogel, leading to its swelling. As fewer pores are available in the hydrogel samples of a lower percentage of cellulose, such hydrogels demonstrated lower swelling properties. When the cellulose content increases, its chains are easily self-entangled with intra and intermolecular hydrogen bonds, and less space is available for pore swelling ^[17][73].

Moreover, high concentrations of cellulose cause the formation of a higher number of physical entanglements, which act as crosslinking points, and eventually reduce the hydrogel swelling ability ^[40][93]. Not only cellulose but lignin content as well influenced the swelling ability of hydrogels. Thus, the water content of the hydrogel films increased from 11.53 to 15.25 g/g with a decrease in lignin content from 1.62 to 0.68% (Table 1) ^[31][84]. Interestingly, the opposite result was reported by Liu and co-workers ^[41][94], when with the decrease of lignin from 23.24% to 14.24%, the swelling ability also fell from 22.87 to 13.96 g/g with the extremum at 7.71 g/g corresponding to 18.91% of lignin. The non-linear trend (Table 1) was explained by changes in the three-dimensional structure of the polymer caused by delignification, pore sizes, and distribution. Liu et al. revealed the dependence of the degree of hydrogel swelling on temperature. Once the temperature rises, the degree of swelling decreases, which is associated with the compression of the three-dimensional structure from heating.

ThWe researchers aassume that the swelling ability of cellulose-based metallogels depends on the swelling ability of cellulose hydrogels. Song and collaborators studied the swelling ability of composite polyvinyl alcohol/bacterial cellulose (PVA/BC) hydrogels resulting from the dissolution in a NaOH/urea/H₂O system, with the same one with Ag particles. It was demonstrated that metal particles increased the swelling ability of the hydrogel from 13 to 16 g/g due to pore enlargement ^[19][75]. The opposite result was reported when hybrid composite hydrogels were synthesized by immobilization of a 1,10-phenanthrocyanines Zn(II) complex into the cellulose hydrogels ^[15][71]. The swelling ability of the composite hydrogels with Zn(II) organic complexes decreased compared to the hydrogels, namely from 28.00 g/g to 14.80 g/g and from 25.00 g/g to 12.40 g/g for the hydrogels obtained from deciduous and flax cellulose, respectively. The authors explained this distinction using the differences in the chemical and supermolecular structure of the original and composite samples. Another research group reported that during the formation of the bacterial cellulose metallogel with Ni(0) and Cu(0), the swelling-induced adsorption process could control the metal size and dispersion simultaneously ^[42][95]. Thus, the swelling ability of the hydrogel determines not only the applications of the product but may also be an important condition of metallogel production.

External stimuli factors, such as pH, temperature, light, electrical fields, and magnetic fields, or the presence of certain ions, can also impact hydrogel swelling. Some hydrogels exhibit stimuli-responsive behavior, where changes in external conditions trigger volume changes. For example, temperature-responsive hydrogels may undergo swelling or deswelling upon exposure to temperature variations, while pH-responsive hydrogels can swell or shrink depending on the acidity or alkalinity of the surrounding environment ^[43][96].

To conclude this part, the swelling ability of the hydrogels obtained from the solutions of organic and inorganic solvents vary from 1.66 to 66.75 g/g, depending on the following factors: additives (e.g., acrylamide or metal particles), the dissolving system, the cross-linkers (chemical or physical hydrogels), the concentration of lignin and cellulose in the solution, pore size and distribution, temperature and the origin of the initial material.

Cellulose has a 3D-network structure ^[44][97]. Especially after freeze-drying, the cellulose hydrogels reveal a developed system of pores ^[4][64], which is why they are used as porous skeletons to imitate the roles of extracellular matrices and to engineer different tissue types ^[45][98]. The swelling ability of the hydrogels depends on the type and size of the pores. The rate of swelling can be controlled by designing a system of interconnected macropores to obtain a faster reaction and vice versa ^[46][99]. The hydrogel with large pores could not retain a large amount of water, and water absorption was low ^[47][100]. The size of the pores determines the capability to act as a drug carriage in drug-delivery systems ^[46][99], as well as the absorption ability of the hydrogels when they are used as adsorbents ^[48][101].

The proportion of hydrogel volume occupied by the pores (porosity) is calculated according to the following equation:

where m₀ is the weight of the dried hydrogel, g, P is a square of the surface of the hydrogel, cm², L is the thickness of the hydrogel, cm, and ρ is the density of cellulose (ρ = 1.561 g∙cm⁻³) ^[15][71].

The porosity of plant-derived cellulose hydrogels was in the range from 86.8% (cotton) to 98.9% (flax), according to ^[4][64] (Table 1). The hydrogels obtained from waste paper demonstrated the same value of 98.1% ^[34][87]. The average size of the pores increased with the rise of the lignin content in the native lignocelluloses reaching a maximum value of 20.281 nm (Table 1) ^[41][94].

Introduction of a metal phase into the hydrogel decreases the average pore size; for instance, after the formation of the metallogel with Zn(II) organic complex, the porosity of the composite hydrogels fell to 95.8% and 94.8% compared with those of the flax and deciduous hydrogels listed in Table 1, row 3 ^[15][71]. It was also mentioned in the same study that the surface of the hydrogels was loose, and the inner structure of the hydrogels was composed of randomly distributed pores that formed a cellular honeycomb-like structure. As a result, the metallogels retained great amounts of water ^[15][71].

Mechanical characteristics, namely mechanical strength and flexibility, determine the consumer properties of the hydrogels ^[49][102]. For instance, if a hydrogel is applied as a wound dressing or as a wearable sensor, it should conform to the movements of the human body. The tensile strength and elongation of the selected cellulose hydrogels are presented in Table 1. The hydrogel films produced from the solution of bamboo fibers in DMAc/LiCl revealed a tensile strength as high as 66 N/mm² with a 33% elongation, while the hydrogels that resulted from NaOH solutions had lower characteristics of 21–27 N/mm² and elongation of 8–13% ^[18][74]. The tensile strength of the hydrogel films containing trace amounts of lignin decreased from 0.77 to 0.43 N/mm² depending on the duration of NaOH treatment and bleaching, and the relative elongation decreased from 43.8 to 26.5% with a decrease in the trace amount of lignin. The longer exposure time in the NaOH solution reduced the elasticity of the gel. The higher lignin content strengthened the hydrogel films, whereby they showed higher tensile strength and elongation ^[31][84]. The relatively low elongation of 9–35.71% demonstrated that the hydrogel films obtained from the agricultural wastes, such as thanaka tree heartwood cellulose, sugarcane bagasse cellulose, and rice straw cellulose, regenerated from DMSO/LiCl solutions ^[33][86]. The value for the hydrogel from sugarcane bagasse is two times lower than the same one obtained from the solution of DMAc/LiCl ^[31][84]. The lowest stress-at-break was reported for PVA/BC hydrogels manufactured from a NaOH/urea/H₂O solvent system. However, the same hydrogels had the highest elongation, up to 160%.

Generally, the loading of the hydrogels with metal nanoparticles enhances their mechanical strength. Ovalle-Serrano et al. reported the dependence of stiffness of Ag(0) metallogels fabricated from cellulose nanofibers on the molar ratio between reducing functional groups COO⁻ of cellulose to AgNO₃. High COONa:AgNO₃ molar ratios (1:3) produced stiffer hydrogels at the expense of large cubic Ag nanoparticles clusters formation (1 µm), while lower COONa:AgNO₃ molar ratios (1:1) resulted in softer hydrogels with spherical Ag(0) nanoparticles exhibiting diameters between 15 and 80 nm ^[9][61].

Both tensile strength and elongation of the PVA/BC-Ag metallogel improved compared to the PVA/BC hydrogel due to the crosslinking between PVA and BC by silver nanoparticles ^[19][75]. This fact is in accordance with article ^[28][83], where the authors claimed the improvement of the mechanical properties of the hydrogels due to the addition of Ag(0) nanoparticles. However, the values of stress-at-break and maximum elongation-at-break first increased and then decreased with the rise in the amount of Ag(0) nanoparticles. The authors explained the deterioration of the mechanical properties by agglomeration caused by a high concentration of silver nanoparticles, which affected the formation of a hydrogel network ^[19][75]. The same result was reported for a hydrogel obtained from a cellulose derivative when the mechanical properties of carboxymethyl cellulose hydrogels were reinforced via the incorporation of Fe³⁺. The increase of Fe³⁺ content at low iron ions concentrations formed more tridentate coordinates with the carboxyl groups of the hydrogels ^[50][103]. However, the iron ions at high concentrations transformed tridentate into monodentate or bidentate, which deteriorated the toughness of the hydrogels.

Thus, cellulose hydrogel films revealed the following mechanical properties: elongation up to 45.2% and tensile strength up to 66 N/mm². The rise in strength was facilitated by an increase in the content of lignin and the addition of chemical cross-linkers or metal nanoparticles to a certain concentration. Metallogels have better mechanical characteristics compared to cellulose hydrogels; however, there is a certain limit on the metal content, after which the deterioration of the tensile strength and elongation occurs.

The thermal properties of materials refer to their behavior and characteristics in response to changes in temperature. These properties play a crucial role in various applications, ranging from engineering and construction to electronics and biomaterials science. Among thermal conductivity, thermal insulation, thermal expansion, thermal shock resistance, melting point, and specific heat capacity, thermal stability is essential for selecting appropriate materials for specific applications, predicting material behavior under different temperature conditions, and ensuring overall safety and performance. Thermal stability refers to a material’s ability to resist chemical or physical changes when exposed to high temperatures, decisive in applications involving elevated temperatures or thermal processing, as it ensures the material’s integrity and functionality. The thermal properties of materials can be measured using thermogravimetric analysis (TGA).

The thermal behavior of cellulose is the following. Firstly, the slight weight loss at 150 °C is attributed to the vaporization of water trapped in the cellulose materials, then the major weight loss between 250 °C and 400 °C is for the breaking off of glycosidic bonds within the cellulose chain, and finally, the residue at about 600 °C is due to the formation of a high amount of char ^[51][104]. The thermostability of the hydrogels is getting better with the cellulose content increasing with the highest decomposition temperature values from 342 °C to 352 °C (Table 1) ^[39][92]. It is related to the increase in cellulosic OH groups that contribute to hydrogen bonding. The number of hydrogen bonds between the molecules rises, and this enhances the intermolecular forces so that the thermostability of the cellulose hydrogels is improved. The listed results are obtained for the hydrogels regenerated from NMMO solutions ^[39][92]. Lower thermal stability demonstrated the hydrogels obtained via dissolution in a NaOH/urea aqueous system. The heat resistance was up to 250 °C; after that, the weight rapidly decreased due to the dehydration and depolymerization of the hydrogel. The derivative thermogravimetric (DTG) curve revealed a broad peak between 300 °C and 400 °C ^[17][73]. The additive of acrylamide increased the thermostability of the cellulose hydrogels obtained from waste paper in the same solvent system up to 407.72 °C ^[34][87]. The thermal stability of cellulose films regenerated from the solution of inorganic molten salt hydrates was from 231 °C to 307 °C, depending on the salt concentration ^[52][105].

TGA analyses demonstrated that Ag(0), as well as Zn(0) nanoparticles, increased the thermal stability of the metallogel compared to the unmodified hydrogel ^[9][53][61,106]. In some cases, it was reported that thermal stability was firmly preserved for the obtained composites Ag(0)-cellulose ^[54][107]. There were characteristic differences in the influences of metal species embedded into cellulose aerogels on their thermal destruction in air and in nitrogen ^[10][67]. The presence of silver nanoparticles (5 wt.%) caused an increase in the remaining char at 600 °C according to the level of silver deposition. In nitrogen, the Au(0)-cellulose specimen gave a significantly higher char yield at 600 °C than Pt(0)- and Ag(0)-cellulose. The authors explained this fact by a specific interference of gold on cellulose decomposition, which might be attributed to the catalytic effect of gold altering some stage of cellulose decomposition. By contrast, under air, the acceleration of cellulose combustion by metal particles occurred. With Ag(0) and Au(0), the secondary stage of cellulose decomposition (burning of char) between 350 and 600 °C shifted to lower temperatures. The weight loss of the Pt(0)-cellulose specimen took place in nearly one step at 200–250 °C. This is due to the strong catalytic effect of platinum nanoparticles on the oxidation of organic materials ^[10][67].

Thus, the decomposition of cellulose hydrogels is in the wide range depending on the cellulose content and the additives. Metal nanoparticles generally increased the thermal stability of the metallogel compared to the initial hydrogel. However, some metals, such as Au or Pt, can be involved in the catalytic processes of cellulose oxidation, especially in air which impacts the thermal stability of the metallogel.