A comprehensive review of graphene-based aerogels for biomedical applications. The impact of synthesis parameters onto material microstructure and porosity

Graphene-based aerogels (GA) have a high potential in the biomedical engineering field due to high mechanical strength, biocompatibility, high porosity, and adsorption capacity. Thanks to this, they can be used as scaffolds in bone tissue engineering, wound healing, drug delivery and nerve tissue engineering. In this review, a current state of knowledge of graphene (Gn) and graphene oxide (GO) aerogels and their composites used in biomedical application is described in detail. A special focus is paid first on the methods of obtaining highly porous materials by visualizing the precursors and describing main methods of Gn and GO aerogel synthesis. The impact of synthesis parameters onto aerogel microstructure and porosity is discussed according to current knowledge. Subsequent sections deal with aerogels intended to address specific therapeutic demands. Here we discuss the recent methods used to improve Gn and GO aerogels biocompatibility. We explore the various types of GA reported to date and how their architecture impacts their ultimate ability to mimic natural tissue environment. On this basis, we summarized the research status of graphene-based aerogels and put forward the challenges and outlook of graphene-based aerogels dedicated to biomedical usage especially by formation of joints with biocompatible metals.


Introduction
Aerogels are highly porous and ulthralight materials in which nearly 95% air is trapped by the replacement of solvent in the mesh of material during drying process. Graphene (Gn) is a nanomaterial made of 1 atom-thick honeycomb carbon which was discovered in 2004 by Andre Geim and Konstantin Novoselov [1]. Since that moment, the Gn family has captured the interest of an increasing number of scientists working in different fields including electronic [2], energy, photonics [3], and biomedical engineering [4].
Gn can lead to numerous biotechnological applications in phototherapy, bio-imaging, drug, and gene delivery, and biosensing [5]. High biocompatibility of Gn and graphene oxide (GO) with mammalian cells makes them also promising material in tissue engineering field (Fig. 1). The application of graphene-based materials in biomedical field is strongly conditioned by the need to handle and control their microstructure in different range as well as physical and chemical properties. Recently, more studies have shown that three-dimensional (3D) Gn structures can provide suitability conditions for mimicking in vivo environments of tissue [6], affording effective cell attachment, proliferation, and differentiation [7]. Among all forms, graphene-based aerogels (GA) have gained much interest due to its high surface area which exhibits exceptional properties like high conductivity [8], biocompatibility [7], high physical and chemical stability [9], high elasticity [10], and low density which make them very absorbent [11]. Thus, it is a promising materials for biomedical application including bone tissue engineering, wound healing, drug delivering and biosensing [12]. Mentioned excellent properties come from unique structure and morphology of GA, which is formed by Gn, GO or reduced graphene oxide (rGO) sheets with gas as dispersed phase [13]. Thanks to that, GA represent one of the world's lightest materials, with an extraordinarily low density and porosity approximately 90% [14]. High surface-to-volume ratio and 3D porous structure provide a homogeneous/isotropic growth of tissues and rapid mass and electron transport kinetics which are required for chemical/ physical stimulation of cells [15]. The direct interaction with biological molecules, cells and tissues requires specific microstructure of GA to mimic extracellular matrix environment. What is more, the mechanical and physical properties of biomedical scaffold should be similar to those of natural tissue. Because of that, it is crucial to control the GA architecture in nano-, micro-, and macro-meter scale. This remains a challenge due to impact of many different parameters (e.g., synthesis method and condition, precursor properties and concentration, drying method) onto final material structure. This requires complex control over synthesis process and repeatability which is difficult to obtain.
The objective of this review was to summarize current state of knowledge in the impact of synthesis techniques onto GA microstructure. Emphasis onto relation between synthesis conditions and drying method onto pore size and distribution is presented. Regarding the extent of the large possibilities of graphene-based structures, we endeavor to give a brief overview of their use in biomedical field. In this context, we will focus on bone tissue engineering, drug delivery, wound healing and biosensing of GA. The possibilities of forming connections with biocompatible metals are discussed in the last chapter. This subject is extremely important in terms of the synthesis of complex materials with a wide spectrum of biomedical applications.

Preparation of graphene-based aerogel. The impact of synthesis method and drying technique onto aerogel microstructure
One of the most ambitious goals in the field of structural and functional GA for biomedical engineering is to achieve exceptional mechanical and physical properties. Obtaining an ideal combination of light weight and high strength which mimic the native tissue. The mechanical properties of GA strongly depend on their microstructure. In turn, their microstructure can be tailored during the synthesis procedure by precisely controlling over synthesis parameters. According to the literature, there are several techniques which lead the fabrication of GA [2]. These techniques are based on chemical and physical methods including the following: (1) hydrothermal reduction [5], (2) chemical reduction [6], (3) sol-gel transition [7], (4) cross-linking [13], and (5) template-directed reduction methods like freeze casting [24]. The advantages and disadvantages of more often used techniques are listed in Table 1. Depending on chosen technique, randomly oriented or tailored microstructure of final aerogel can be obtained [35] (Fig. 2). Up to now, several different tailored microstructures of GA have been proposed, including the following: (1) cellular [35], (2) honeycomb like [37], (3) lamellar [32], (4) radial [25], and (5) isotropic microstructure with open-cells [39] (Fig. 2). However, it remains difficult to translate the unique intrinsic mechanical properties of single graphene-based flakes into a 3D macroscopic aerogel. This is due to the low controllability over structure during fabrication of macroscopic networks, especially when building blocks with a high aspect ratio are used. Although some progresses have been made in this direction, there is still no systematic summary linking the properties of the building block (e.g., flakes size, concentration, dispersion), the synthesis conditions (e.g., pH, temperature, time), and the drying method (supercritical CO 2 drying , freeze drying) with final aerogel microstructure. Thus, in this chapter the most popular synthesis and drying techniques are explained in detail. Moreover, the impact of the specific synthesis parameters and drying methods onto microstructure of GA are described. Thanks to this, we demonstrate the importance of synthesis and drying process control onto final aerogel microstructure and physicochemical properties. Table 2 summarizes the impact of main synthesis parameters onto pore size of GA according to selected, currently presented scientific reports.   [35]; b lamellar [38]; c cellular network [36]; d isotropic microstructure with open-cell [39]; e randomly oriented microstructure with pores of various sizes: in the range of 1-10 μm (red arrows) and in the range of 10-50 μm (white arrows) [28]; f honeycomb like structure [37]; g radial (Adapted with permission from [25]. Copyright 2018 American Chemical Society) Table 2 Methods of obtaining GA and impact on main synthesis parameters onto aerogel pore size Synthesis method Parameter Impact onto GA pore size References Hydrothermal reduction Temperature Increasing temperature is followed by smaller pores size [40] Precursor concentration Increasing concentration reduces the pore size [30] Reducing agent Depends on reducing agent, e.g., using Vitamin C as reducing agent causes higher pore volumes in comparison to aerogel obtained without reducing agent [41] Surfactant Depends on used surfactant, e.g., adding cetyltrimethylammonium bromide (CTAB) reduces the average pore size of GA [42] Time By increasing the time of hydrothermal reduction, the pores size decreases [43] pH As the pH increases, the aerogel is less dense, and the pore size is larger [4]

GO dispersion
The enhancement in π-π interactions between GO sheets in primary solution results in higher pore volume [44] Post-treatment With annealing the finals aerogel, the quantity of pore diameter increase [30] Sol-gel method Precursor concentration Higher graphene concentration leads to smaller pore diameter [45, 46] pH In acidic conditions total pore volume is lower than for basic conditions [47,48] Resorcinol-formaldehyde (RF) sol-gel method Decrease in RF content causes drops of pore diameter [49] Surfactant Depends on used surfactant [50]

Dispersion
The enhancement in π-π interactions between GO sheets in primary solution results in higher pore volume [44,51] Temperature Higher temperature affects higher number of pores created inside the hydrogels and consequently, higher specific surface area of aerogel [48] Freeze

Reduction methods
Reduction methods are conventional and economical methods for the preparation of GA, and thus are widely used [54]. We can distinguish two most popular reduction methods for preparing GA as follows: (1) chemical, which requires reduction agents, and (2) hydrothermal in which specific condition of high temperature and pressure are needed [55]. As precursor for these methods GO is most common used due to its excellent hydrophilicity [56]. This high hydrophilicity is the effect of the oxygen-containing groups such as oxygen, hydroxyl (-OH), carbonyl (-CO-), carboxyl (-COOH), alkoxy, and epoxide, which are presented on the basal planes and edges of GO [57]. During the reduction processes, most oxygen-containing functional groups of GO are eliminated by reducing agent or/and by specific hydrothermal conditions, and the rGO is formed [12]. The chemical reduction is carried out at low temperature; thus, this technique does not require special equipment. However, the final material microstructure and physical properties strongly depend on reduction agent and solvent composition. Various inorganic and organic reducing agents have been explored, including hydrazine [11], sodium ascorbate [58], sodium borohydride [59], hydroxylamine, and strongly alkaline solutions [12]. Hydrazine is known to be the most powerful and efficient reductant in terms of giving rGO with good qualities. However, hydrazine as well as most of mentioned reduction agents is toxic, and thus limited in usage especially for biomedical applications. For this reason, there are numerous attempts to carry out chemical reduction with the use of environmentally friendly reducers, such as plant extracts (e.g., carrot root [60], tea [61], mushroom extracts [6]), microorganism (e.g., bacteria Escherichia coli [7]), sugars (e.g., glucose [8]), proteins (e.g., bovine serum albumin [9]), and amino acids (e.g., cysteine [10]). Unfortunately, rGO synthesized by most of the green reductants has limited application because of the difficulties in making stable dispersions of rGO [62]. Above all, green reducing agent, ascorbic acid (Vitamin C) has gained much interesting due to its non-toxicity and applicability like toxic alternatives [63]. It was reported that ascorbic acid matches the efficiency of hydrazine in the elimination of labile functional groups on graphene sheets. Sui et al. [64] have studied the impact of Vitamin C onto microstructure of carbon nanotube-graphene hybrid aerogels. They obtained hierarchically porous structure of GA with specific surface area and total pore volume lower than that for aerogel obtaining without reducing agent. Other studies have shown that higher concentration of Vitamin C affects lower total pore volume of GA aerogel for both, acidic and basic conditions [48]. Although Vitamin C as reducing agent possesses many advantages, it shows several challenges, like hygroscopic nature and the need to use a stabilizer in the synthesis process [63]. Fan et al. [65] have demonstrated the impact of three different reducing agents, (l-ascorbic acid (LAA), hydrochloric acid (HI) and sodium hydrogen sulfite (NaHSO 3 ), on GA morphology and properties. Each reducing agent is characterized with different reduction effectiveness, which consequently resulted in differences in surface area of aerogel. The more efficient reducing agent (e.g., HI) removes more functional groups (-OH, -COOH, -O-, etc.) during the reduction processes, and forms more bonding and stacks between Gn nanosheets. In consequence, the surface area and density of final product decreases. As a result, more densely packed nanostructure enhances the electron transport, thereby improving the electrical conductivity of aerogel. In brief, finding a suitable reducing agent for GA synthesis by chemical reduction is a key issue, which has a direct impact on microstructure and physicochemical properties of obtained product. The alternative method of synthesis graphene-based hydrogels is the reduction in hydrothermal conditions, and after drying (either freeze or supercritical), forming a GA. This method involves the self-assembly of the GO sheets which are homogeneously dispersed in solution by electrostatic repulsion [67]. As the gelation proceeds, in the hydrothermal environment of high temperature (above 100 °C) and high pressure (above 1 atm), the functional groups on GO are removed and/or transformed. In this technique, reduction agents also can be added to accelerate the reduction process and increase the yield of reduction [69]. Consequently, it results in the production of CO 2 which is partially trapped inside dense multilayer shell and forms macroscopic voids in the gel [70]. The reduction of C-OH and C-O groups causes the break/cut off the rGO sheets into small pieces, which then agglomerate to reduce the surface energy by hydrogen bonding between the remaining oxygen groups, or by van der Waals forces between interlayers (Fig. 3) [66]. Self-assembling of rGO sheets leads in monoliths and 3D graphene-based structures formation. The production process by hydrothermal reduction is green, environmentally friendly, and has a low production cost. The microstructure and physicochemical properties of the resulting products might vary depending on the control parameters used in the reduction process like pH, time, temperature, and used reduction agent [71] (Fig. 3). Mei et al. [67] have shown that higher temperature (180 °C) can make a more thorough reduction (decreases in O/C atomic ratio) of GO to rGO, which is followed by more thoroughly peeled-off the layers (Fig. 3c). However, high temperature can destroy the p-conjugated structures of GO sheets, and thus, can have serious consequence in final aerogel structure. Above 180 °C the 3D structure of GA could be destroyed, while below 80 °C the hydrogel cannot be formed [72]. What is more, with increasing reaction temperature, in range between 80 and 180 °C, graphene-based sheets transform from smooth layers to rough. It is worth noticing that adding reducing agent can change the wide of temperature window. In consequence, the demands for temperature could vary with different reducing agents. Generally, increasing temperature is followed by smaller pore size of GA. Another important parameter which has direct impact onto final aerogel microstructure is pH of solution. Bosch-Navarro et al. [4] demonstrated the impact of pH on final rGO parameters during hydrothermal reaction. According to the results, acidic conditions (pH 3) lead to a higher number of defects in the rGO samples, with smaller sizes of the sheets. On the other hand reduction under more basic conditions promotes a decrease in the number of defects present in the resulting rGO and affect bigger graphitic domains (Fig. 3a). Not only the size of the domain, but also the type and amount of oxygenated surface groups, depends on pH conditions. For basic pH, carboxylic and epoxy groups are intensively removed. Simultaneously, nitrogen is incorporated in the Gn lattice. In consequence, the Gn nanosheets are twisted and bent for basic pH, while they are flatter for acid pH. Finally, at acid pH, the flatter morphology of the nanosheets and the stacking via hydrogen-bonds between the more abundant oxygenated groups at the basal planes finally result in less porous and denser aerogels, than for basic pH value. At higher pH, nanosheets connect through the bent borders leading to more open structures with higher porosity [43] (Fig. 4ab). The type and number of functional groups on surface of GO sheets is strongly related with time of hydrothermal reduction [66]. With the increase in reduction time (up to 8 h), the oxygen content as well as the C-O/C-OH fractional content decreases. In consequence, the layered nature becomes disordered, crumpled, and smaller (Fig. 3c). What is more, deoxygenation results in removing some of the carbons from the graphitic structure, leaving pores and defects. In consequence, deteriorated structure is formed, thus increasing the reduction time tune the structure and properties of rGO, affecting lower oxygen content, smaller interlayer spacing, and crumbled, or wrinkled structure (Fig. 3b). At the beginning of hydrothermal treatment, the pore volume declines sharply, and then it decreases more slowly as the hydrothermal proceeds further [43]. With time increase, first thinner Gn sheets tightly inlay on the large tracts of Gn sheets. As the Gn sheets become smaller, they adhere to each other more tightly, affecting smaller pore size (Fig. 4ac). Apart from controlling hydrothermal reduction synthesis conditions, there is strong correlation between precursor solution concentration, GO dispersion, sample posttreatment annealing, and reducing agent onto microstructure of aerogel. Similar to chemical reduction, there are lots of reducing agents which can be used for hydrothermal reduction. Each off them is characterized by different effectiveness of reduction and, thus, leads to different morphology of aerogel. What is more, reducing agent can also modify the way in which final aerogel microstructure corresponds to other synthesis parameters, like temperature, pH, or reduction time. For example, Serrapede et al. [41] used LAA as reducing agent to fabricate rGO aerogels. The presence of LAA in the slurry, subjected to the hydrothermal process, induces the formation a more open structure with mesopores in range of 2 to more than 20 nm. In comparison, rGO fabricated without adding reducing agent is composed of micropores and narrow mesopores (pore width smaller than 3 nm). In turn, Cheng et al. [30] have shown that posttreatment thermal annealing at 1500 °C affects higher pore diameter in comparison to aerogel without posttreatment. They also have demonstrated the impact of GO concentration on aerogel pore size (Fig. 4ef). The increases of precursor concentration affect the reduction of pore sizes. What is more, GO dispersion in solution plays a key role in final aerogel structure and properties. This was analyzed by Hu et al. [44] who have determined the dependence of the balance between the forces that determine GO colloidal stability on final aerogel properties. For this reason, to increase the π-π interactions between GO sheets they first freeze-dried GO and then annealed it at low temperatures (50 °C to 130 °C) prior to hydrothermal gelation. The enhancement in π-π interactions reduces the area accessible to the solvent and weakens hydrophobic attraction during gelation, which in turn suppresses volume shrinkage of the hydrogel, and, upon freeze drying, leads to ultra-low density elastic rGO aerogels with tunable pore size. In consequence, the pore volume increases in comparison to conventional prepared GO aerogel. Additionally, the formed structure is composed mostly of open pores, while for standard GO aerogel both closed and open pores are presented.
To summarize, controlling the microstructure and surface chemistry of GA by reduction methods is difficult to obtain, due to many parameters that needs considering. One parameter is closely related to the other, so it is important to have a holistic view of the course of the synthesis process. Although reduction methods make it possible to precisely control the microstructure of final aerogel, it is necessary to clarify the reduction mechanisms and structural changes throughout the process, since the structure is strongly related to the final aerogel properties [30].  [43]; d Morphology of the GA prepared by hydrothermal reduction of GO dispersed with different concentrations: e 1 mg/ml −1 , (f) 10 mg/ml −1 [30]. The red arrows show the direction in which the pore size of the GA aerogels increases while the blue arrows show changes in pH, time, and precursor concentration

Sol-gel method
Sol-gel method is the simplest and most-effective technique used for GA synthesis. As precursor, GO is used instead of pristine Gn sheets, due to its low solubility in virtually all solvents. GO contains abundant oxygenated functional groups, such as hydroxyls, epoxides, and carboxylic acids, which make GO sheets dispersible in various solvents. What is more, the functional groups on GO provide sites for crosslinking chemistry and can be eliminated during the gelation process. This leads to the restoration to Gn-like structure, which is termed as rGO, and the production of the hydrogel and, after drying, aerogel. However, due to the high degree of oxidation (e.g., sp3 carbon) present in GO, some reduction steps (chemical or thermal) are required to recover the desired Gn-like properties. During sol-gel procedure the following steps occur: (1) hydrolysis; (2) condensation, and polymerization of monomers into form chains; (3) the growth of these particles; (4) agglomeration of the polymer structures; (5) the creation of networks. During the process, different interactions between GO sheets occurs,e.g., van der Waals interaction, n-n stacking, the formation of hydrogen bonds, and electrostatic interaction [4].
To determine the final aerogel structure, the following synthesis parameters should be considered: the temperature and the pH of the solution, the solution concentration, and the precursor dispersion. Among others, the most important parameter is precursor concentration. Huang et al. [45] have demonstrated the impact of Gn concentration onto pore size in aerogels obtained by sol-gel method ( Fig. 5a-d). With the increasing concentration, the Gn layer structure gradually crosslinks together to form a three-dimensional porous structure, featured with the pore size of about 1-10 μm for concentration 7 mg/ml (Fig. 5a) and 9 mg/ml, respectively (Fig. 5c). Meanwhile, the crosslink ability of the Gn sheets becomes distinct gradually, with the increasing concentration of Gn, so the size and density of pores become smaller. In sol-gel technique, GA can be obtained from pure GO suspensions or via resorcinol-formaldehyde (RF) sol-gel chemistry. In second method, resorcinol and formaldehyde polymerize into a gel through hydrolysis and condensation reactions. Thus, RF plays a significant role in cross-linking the GO sheets. In consequence, the aerogel microstructure strongly depends on RF concentration (Fig. 5e-h). Worsley et al. [49] demonstrate that with decreasing in RF content, the sheets in the aerogel are thinner and the size of the pores between sheets is smaller, particularly in the aerogel without any resorcinol. Another important parameter which determines the final aerogel morphology are pH and temperature of primary solution. Kondratowicz et.al [48] studied the impact of both, pH, and temperature onto final aerogel microstructure. According to the results, GA prepared in acidic media exhibited many visible voids and well-developed, porous surface, in comparison to the sample prepared in basic conditions, which exhibit a smooth surface. What is more, for basic conditions surface area of aerogel Fig. 5 The impact of Gn concentration onto pore size of aerogel obtained by sol-gel method. Morphology of GA with different graphene concentrations (a): 5 mg/ml (GA-5); (b); 7 mg/ml (GA-7); (c); 9 mg/ml (GA-9); (d) quantitively analysis of pore size distribution for various graphene concentration [45]. Morphology of GA aerogel with different RF concentrations: (e) 4 wt% RF; (f) 2 wt% RF; (g) 0 wt% RF; (h) normalized pore size distribution of GA for different RF concentration. Adapted with permission from [49]. The red arrows show the direction in which the pore size of the aerogels increases while the blue arrows show changes in Gn and RF concentration as well as pore size were higher than for acidic conditions. The reason for that is the differences in behavior of GO in primary solution with various pH. It is known, that GO possesses carboxylic groups that are ionized and, therefore, repulse single GO sheets, preventing the aggregation in water. Upon adding an acid, the repulsion is weakened, whereas adding a base resulted in an increase in the repulsion force and more stable GO dispersion. Due to this phenomena, more aggregated Gn sheets are observed on the surface of aerogel prepared in acidic conditions, and on the contrary, more separated sheets compose the surface in basic conditions. When it comes to temperature, higher temperature affects higher number of pores created inside the hydrogels and consequently, higher specific surface area of final aerogel.

Crosslinking agent
Three-dimensional interconnected porous GA is composed of Gn, GO or rGO sheets that are holds together by interlayer interactions, which affecting stress transfer between them [13]. However, weak connections formed between sheets result in low-stress transfer efficiency inside macroscopic structure of aerogel, and thus, require additional interconnections between adjacent layers [16]. In this strategy, oxygenated groups on GO sheets, such as -COOH, -OH, or aldehyde are the main active non-or covalent bonding sites for these additional binders. To provide more links between sheets various binders can be used, including metal ions [17], small organic molecules [21], or even polymers [18]. For biomedical applications, the selection of crosslinker used to obtain GA is relevant to improve mechanical properties and give the necessity of high biocompatibility of material. The natural origin agents, such as chitosan (CS), collagen (COL), chitin, and cellulose, among others, are most preferred for this purpose [19]. For example, GO reinforced CS systems with chemical interfacial bonding have been produced through the amide reaction,= that significantly improved the interfacial interactions and the mechanical properties of the composites [21]. Moreover, the synthesis of supramolecular hydrogels of CS and GO have been reported in literature, with the self-assembly of CS chains with GO nanosheets that works as the two-dimensional crosslinker of the composite material, exploiting noncovalent interactions [20]. The GO/CS aerogels poses high biocompatibility, biodegradability, and mechanical performance, what makes them promising materials for bone tissue scaffolds. When GO is combined with CS, the carboxyl group on GO reacts with amino group on CS, which indicates that GO not only improves the mechanical properties of CS aerogels not only by physical enhancement, but also by electrostatic interaction and van der Wales forces between GO and CS [74]. For example, Tang et al. [76] fabricated the chemical crosslinked GO/CS composites dedicated for tissue engineering. They improve the mechanical properties of material by adding (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride) (EDC) as crosslinker which causes the formation of interfacial chemical bonding (-CN bond) through the amide reaction between GO and CS. The obtained materials, besides remarkable mechanical properties, possess good biocompatibility which is crucial for biomaterials. According to the results obtained by Fang et al. [22], a higher concentration of GO enhanced the mechanical property of CS while declining the plasticity of the materials. Chemically crosslinking by EDC was also use by Liu et al. [23] to performed GO/COL aerogel with unique, folded microstructure dedicated for bone tissue engineering. What is more, the elastic modulus of aerogel was enhanced with the GO concentration increased from 0.05% to 0.2%. Besides high mechanical strength, this material possesses good biocompatibility and enhance osteoinductivity. In turn, Zhang et al. [75] synthesized composite aerogel with excellent mechanical properties by mixing cellulose nanofibers and Gn through atmospheric pressure drying technology. When nanocellulose is added into Gn as a nanofiller, it interferes with hydrogen bond formation between Gn sheets, slows down π-π stacking, and enhances mechanical strength [74].

Template directed reduction methods-freeze casting
Ice-template, also known as freeze casting, is a common, chemical free, and easy way for the preparation of GA. The template-guided approach tailors the morphology of the aerogel through ice crystal nucleation and growth, resulting in well-controlled, biomimetic porous materials [77]. This technique is governed by complex and dynamic liquid-particle and particle-particle interactions. Oxygen-containing groups on GO can form H-bond with water and influence the growth of ice crystals. During the freezing process, the temperature gradient affects arranged grown of ice crystals with GO sheets accumulated between them (Fig. 6a). The morphology of the colloids during freezing is dominated by the ice growth direction and the ice crystal size. By sublimation of ice crystals in freeze dryer the entrapped GO sheets can form a continuous 3D network with high porosity (Fig. 6a). The microstructure of aerogel, especially the pore size and surface area, can be easily modified by precisely controlling nucleation and growth of ice crystals in different freezing temperature (Fig. 6b). Zhu et al. [53] have demonstrated the impact of freezing temperature onto final aerogel microstructure in wide range of temperature (−10 °C to −196 °C) ( Fig. 7a-e). In the whole temperature range, the obtained aerogels were characterized by a similar density, but differed in morphology. At -10 °C rod-like structure of aerogel was obtained with mean pore size of 241.7 μm.  Such large dimensions were the result of low nucleation and growth rates followed by longer crystallization period. In consequence, the larger ice crystals were formed, and the pore size of the derived GA was also large. The highly directional growth of ice columnar crystals at temperature −20 °C and −40 °C affects formation of tube-like structures with lower pore size (65.9 to 24.1 μm,, respectively). Further lowering the process temperature from −70 °C to −196 °C causes decrease in average grain size of the ice crystals due to the high nucleation rate, which restricts the growth of ice crystals. As a result, highly directional structure with uniform and refined morphology of aerogel was formed with small pores (pore size at −196 °C was 5.9 μm). Therefore, as the freezing temperature decreases the temperature inflection point decreases, the crystallization period gets shorter, and the mean pore size is reduced (Fig. 6b). Besides temperature, the other parameters like GO flakes size, freezing speed, and the suspension viscosity have direct impact onto microstructure of aerogels fabricated by freeze casting method. for example, lower speed resulting in aerogel with larger pore size [80], while higher suspension viscosity usually leads to smaller pores formation [81]. Gao et al. [52] have reported the influence of Gn flakes size onto aerogel microstructure, strength, modulus, and fatigue resistance ( Fig. 7f-i). They proved that the pore wall constructed by large GO (~80 µm) has less defects. The reason is the stronger interaction between flakes which made it more resistant to slip between adjacent flakes during deformation. Thus, the aerogel with larger flake size showed both, higher strength, and fatigue resistance. In contrast, the assembly with small size of GO (~1 µm) usually takes more flakes, and thus, more overlap joints of GO flake in pore wall can be observed (Fig. 6f). By controlling the direction of ice crystals' growth during freeze casting, different tailored microstructure of aerogels can be obtained. Many reports show the formation of tailored microstructure (honeycomb, lamellar, or radical) of GA using freeze casting technique [78] (Fig. 8). For this purpose, different temperature gradient is used, including unidirectional, bidirectional, and radial [68]. During the unidirectional freezing, single temperature gradient occurs by placing polymer mold onto cooper finger immersed in liquid nitrogen (Fig. 8a). The obtained temperature gradient leads bidirectional; c radial. Adapted with permission from [85]. Tailored microstructure of GA fabricated by freeze casting; SEM microstructures on the top and schematic representation of synthesis method on the bottom; a honeycomb like structures performed by unidirectional freeze casting [37]; b lamellar structures obtained by bidirectional freeze casting [38]; c radial microstructure obtained by radial freeze casting. Adapted with permission from [25] 133 Page 12 of 39 to directionally growth of ice crystals, sublimated to anisotropic porous structure (Fig. 8b). This results in small-scale (multiple domain) honeycomb like microstructure of aerogel. In bidirectional freezing, ice crystals grow both, vertically and horizontally, under dual-temperature gradients, which are formed by placing polydimethylsiloxane (PDMS) wedge inside polymer mold (Fig. 8b). Thus, large-size single-domain (centimeter-scale) aligned lamellar microstructure is created. Two temperature gradients are also used in radial freeze casting. However, in this technique, cooper mold is used instead of polymer mold (Fig. 8g). Thus, the temperature gradient is forming along Z-axis of the mold and radially to the material resulting in radially aligned porosity (Fig. 8c). To obtain GA with tailored microstructure simple orientation, custom-made devices can be used [93]. Liu et al. [78] have demonstrated that the directional freezing of a CS/GO suspension on a copper plate cooled by liquid nitrogen reassembles the GO nanosheets into a 3D monolith. As a result, honeycomb-like structure of aerogel is formed. In this case, directional freezing of the suspension leads to the formation of μm-sized ice rods within the suspension, which act as a template to form straight macropores. Bidirectional freeze casting was used by Zhou et al. [24] to fabricate lamellar structure of GA. They have placed Gn/polyvinyl alcohol/cellulose mixture into the shaping mold protected by the thermal insulation material. Then, the bottom of the mold was placed on copper base contacted with liquid nitrogen to freeze the mixture. Finally, in the freeze dryer, the ice crystal of directionally growing was sublimated to anisotropic porous structure in the form of nanofiber aerogel. In turn, Liu et al. [27] have constructed lamellar-structured polyamic acid salt/GO aerogels with vertically aligned graphene lamellae using rectangular silicone mold that was placed on a copper bridge. To form a bidirectional temperature gradient, one end of the copper bridge was inserted into liquid nitrogen. The cooper bridge was also used by Mi et al. [26] to fabricate long-range lamellar Gn aerogel. As shaping mold, they used stair-like PDMS cavity. The temperature gradient was achieved by immersing one end of the copper bridge in liquid nitrogen and the other end was in a water-ice mixture. After the mixtures were frozen completely, the generated ice crystal pillars were sublimated in a freeze dryer. What is more, Yang et al. [24] have used bidirectional freezing method to fabricate GO and poly(vinyl alcohol) (GO/PVA) aerogels with lamellar, hierarchical architectures mimicking the plant stem (Thalia dealbata). Achieved biomimetic architecture of lamellar layers with interconnected bridges was created by introducing a PDMS wedge with a slope angle of around 15° between the cooling stage and precursor suspension. It causes the generation of dual temperature gradients both horizontal (ΔTH) and vertical (ΔTV) during freezing. As a result, they obtained biomimetic architecture which conditioned exceptional strength and resilience of aerogel. In turn, radial freeze-drying technique was used to form radial and centrosymmetric structure of GA by Wang et al. [25]. They manufactured a special device which was composed of the copper rod with cylindrical hole and PDMS gasket at the bottom of the hole. The construction was immersed in liquid nitrogen, to cool the upper cylindrical mold and to generate two temperature gradients inside it: one along the Z-axis of the copper rod ("bottom to top") and the other in the radial directions ("outside to inside"). Thus, the ice crystals within the copper mold were radially aligned, resulting in the formation of radial microstructure of aerogel.

The impact of drying method into GA microstructure
Drying method has a direct impact on aerogel morphology and physical properties and thus plays important role in fabrication process. During formation of 3D aerogel structure, the removal of a large amount of water from the highly porous network interconnected by partial overlapping or coalescing flexible Gn sheets occurs. Thus, macropore (> 50 nm), mesopore (2-50 nm), and micropore (< 2 nm) formation in the structure depends on the type of drying process of alcogel or hydrogel to remove the solvents and substitute them with air [79]. Two most commonly used standard drying techniques are supercritical CO 2 (scCO 2 ) drying and freeze-drying (FD) (Fig. 9a,b). In scCO 2 drying, carbon dioxide is used due to its low critical temperature (31 °C), environmentally friendly nature, and non-flammability. During the process, the liquid solvent in the hydrogel is extracted and replaced with supercritical CO 2, which can quickly diffuse through the pores of hydrogel. Due to these gas-liquid properties of scCO 2 , the structure of the dried material remains preserved with higher porosity than for FD aerogel. What is more, high extent lot of larger macropores appear along with a small number of mesopores. FD method based on sublimation process of frozen solvent to the gaseous state [82]. In consequence, as the solvent is discharged, the cellular structure is maintained. The freezing temperature and freezing rate influence the nucleation and growth of ice crystals, and thus, have a direct impact onto aerogel microstructure. Decreases of freezing temperature causes smaller average pore size of aerogel [53]. In the FD process, a high number of mesopores fuse together into larger ones, causes higher specific surface area, and pore volume [24]. Xie et al. [29] have performed comprehensive studies onto the influence of drying technique onto microstructure and mechanical properties of GA aerogel synthesized by sol-gel method. According to the results, the scCO 2 dried sample had a higher compression strength due to the plastic collapse of the mesopore walls. In comparison, the FD GA aerogels exhibited a nonlinear super elastic behavior due to the random out-of-plane buckling of the thicker pore walls. Using FD they obtained honeycomb like, oriented microstructure of aerogel, due to slow nucleation of ice crystals but high grow rate. On the other hand, the microstructure achieved by scCO 2 was composed of randomly distributed pores with wrinkled texture (Fig. 9c-i).

GA applications in biomedical engineering
GAs attract growing interest in biomedical research and applications due to high biocompatibility, high surface-tovolume ratio, 3D porous structure (which provide a homogeneous/isotropic growth of tissues), highly favorable mechanical characteristics, and rapid mass and electron transport kinetics (which are required for chemical/physical stimulation of differentiated cells). The easy tunable microstructure of GA (see chapter 2) enables to mimic the in vivo conditions of natural tissue in micrometer scale. In consequence, GA provide structural support and exert biochemical effects to surrounding cells in tissues. What is more, the macrostructure of GA can be tailored which extends the range of possibilities of their use in biomedical applications. According to the literature, different types of macrostructures can be obtained including 3D scaffolds [83], sponge-like structures [84], fibers [85], microspheres [73], thin films [86] or 3D printed constructs [87] (Fig. 10). Recently, extensive studies are focused onto their usage in bone and neuronal tissue engineering, wound dressing, drug delivering systems, or biosensors. Within this chapter, the current state of knowledge of using GA for mentioned applications is described. Examples of selected GA dedicated for biological application are listed in the Table 3.

Bone tissue engineering
Biomaterials used in the skeletal system of the human body are widely used, however, scientists are still looking for material whit exponential mechanical properties, high porosity, and biocompatibility. GA fit perfectly into this area of application, and thus, is the subject of extensive studies. In literature, the examples of using GA for bone tissue engineering (Fig. 11a) as well as cartridge tissue reconstruction can be found (Fig. 11b-e). Hong et al. [89] have performed cartilage-inspired superelastic ultradurable Fig. 9 Comparison between freeze-drying and supercritical drying methods: a typical phase diagram of solvent in the gel and the pressure-temperature changes for drying the gels to make aerogels. b Schematic illustrations of FD and scCO 2 [81]. c Schematic represen-tation of the formation mechanism of GA using FD and scCO 2 drying. d, e SEM images and f the geometric models of GA by the FD. g, h SEM images and i the geometric models of aerogel by the FD and scCO 2 drying. Adapted with permission from [29] Gn aerogels nanocomposite which exhibit fully reversible structural deformations and high compressive mechanical strength (Fig. 11b-e). Within the joint architectures, small amounts of soft cartilage are essential for connecting stiff but hollow bones and, hence, building an effective loadbearing system. By selectively gluing the intersheet joints with soft materials (PDMS) inside the 3D internetworked GA, they convert fragile 3D graphene networks into superelastic and ultradurable material. Consequently, the GA/ PDMS nanocomposites exhibit reversible structural deformations with rapid response and recovery rates and high compressive mechanical strength. To reconstruct the hard bone tissue there is growing need of biomaterial which can mimic natural osteocytes and osteoblasts environment and create functional alternatives to regenerate bone. An ideal scaffold should have biocompatibility, suitable mechanical properties, high porosity, and gradient pore to induce osteointegration, provide mechanical stability, and integration with the bone structure [90]. Biocompatible 3D aerogel materials exhibit appropriate properties, due to their high porosity (interconnected mesopores) which can mimic the porous, flexible, resistant, and light structure of bone [91] ( Fig. 11a). What is more, open, and interconnected pores allow nutrients and molecules to transport to inner parts of a scaffold to facilitate cell growth, vascularization, as well as waste material removal. GO aerogels are excellent candidate for bone tissue, due to their high mechanical resistance and porosity. According to their surface chemical reactivity, they can facilitate osseointegration and osteogenesis and help provide absorption, conservation, and transport of biological fluids [14]. The rich functional groups found on the surface of GO offer GO-composite scaffold a tunable, unique platform for bone regeneration if combined with e.g., exogenous seed cells or growth factors [23]. The large surface area and functional groups also help GO hybrid aerogels to form bone-like apatite. Several studies have shown the potential of GA in bone tissue engineering [90]. For this reason, different hybrid GO composites was fabricated including COL or CS. COL is one of the most used scaffold materials for bone tissue engineering, due to its excellent biocompatibility and biodegradability. However, the extremely poor mechanical strength and osteoinductivity of COL limit its wider applications in bone regeneration field [93]. Thus, GO/COL hybrid scaffold can combine high biocompatibility of COL with high porosity and good mechanical strength of GO. This combination was performed by Fang et al. [22] to achieved biocompatible GO/COL aerogel which promoted the survival and osteogenic differentiation of bone mesenchymal stem cells and enhanced the osteogenesis. According to the obtained results, GO/COL materials exhibit optimal mechanical properties at 2% collagen concentration, high biocompatibility, improves angiogenic processes, and confers osteoinductive properties. GO/COL hybrid aerogels with highly interconnected microstructure  [88]; c 3D printed constructs [87]; d scaffold (yellow arrows indicate the orientation of the formed pores) [83]; e fiber [85]; f film [86]. Adapted with permission from 87] [127] and porosity up to 90% was also studied by Xu et al. [71]. They performed in vivo test which shows that 0.1% GO/ COL aerogel exhibited high biomineralization rate and cell compatibility. 3D porous scaffolds made of rGO and COL was synthesized by chemical crosslinking and freeze drying by Bahrami et al. [94]. The scaffold showed interconnected porosity which can mimic the natural bone tissue and enhance bone marrow-derived mesenchymal stem cells (BMSCs) adhesion, proliferation, penetration into pores, and cell-cell contact. Another widely used material for bone tissue engineering CS. Liu et al. [78] have fabricated GO/ CS composite with a well-organized structure and interconnected porous channels, which simulate the bone lamellar structure. They investigate, that the incorporation of GO (1-5 wt%) into the CS matrix enhanced the compressive strength of scaffold and improved its capability for protein adsorption. This scaffold could induce cell aligned growth along the longitudinal direction, which confirms, that GA are a good guidance of cell alignment and simulation of bone lamellar structure. The biocompatibility properties of GO/ CS aerogels strongly depend on oxygen content in GO what was described by Francolini et al. [95]. High oxygen content in GO can promote aggregation of GO sheets in the CS matrix and, hence, reduce its reinforcement effect. On the other side, increasing the amount of oxygen can promote the generation of reactive oxygen species and thus reducing the biocompatibility of aerogel. So, finding the optimal amount of oxygen is crucial for this purpose. In turn, Pandele et al. [92] have reported novel chitosan-poly(vinyl alcohol) and graphene oxide (CHT-PVA/GO) biocomposite which poses unique ability to form hydroxyapatite (HA) crystals on the surface and pores, due to the carboxylic groups attached on the GO and CHT surface. Thus, this composite can be successfully used in the mineralization process of bone. HA is widely used in bone tissue engineering for its bioactivity, biocompatibility, and osteoconductivity [96]. Nie et al. [97] have proposed nano-hydroxyapatite (nHA) composite with rGO which stimulate mineralization and promote the in vivo defect healing of bone tissue (Fig. 11b). The scaffold was fabricated by simple self-assembly technique with different concentration of nHA (20%, 40%, 80%). According to the results, the 20% nHA/rGO scaffold showed the best cytocompatibility and promote the defect healing in 6 weeks, while the blank rGO scaffold has little effect on diminishing the defect area. When designing scaffolds for bone tissue engineering, high structural complexity is a key issue, which enable control both, macrostructure, and microstructure of material. For this reason, 3D printing seems to be good choice. 3D printing can produce customized scaffolds that are highly desirable for bone tissue engineering. Zhu et al. [98] have used this technique to fabricate gellan gum (GG)/GO . e A photograph of the GA/PDMS nanocomposite, which shows its flexibility and compressibility. Adapted with permission from [89] scaffold for tumor therapy and bone reconstruction (Fig. 12). They introduce curcumin (Cur) into obtained aerogel to improve anticancer properties of material. The obtained GG/GO/Cur scaffold was pH responsive and could effectively kill the tumor cells and suppress the tumor growth in vitro via curcumin release in the acidic environment of tumor cells. Simultaneously, the scaffold supports the attachment and proliferation of osteoblasts. In turn, Nosrati et al. [99] have printed 3D rGO/HA/Gelatin scaffolds with high mechanical properties. The material was fabricated employing two steps: (1) a hydrothermal autoclave with hydrogen gas injection to synthesize 3D Gn/HA powders and (2) a hydrogel 3D-printing method to fabricate the scaffolds. rGO were used in this study to enhance the physical strength of the gelatin scaffolds.

Wound healing
Aerogel-based biomaterials for wound dressings are expected to form active barrier between wound and environment, which apart from the absorption of exudate and toxic substances, allow gas exchange and prevent the invasion of microorganisms [28]. Tissue injures involves the local synchronization of a variety of cell types, growth factors, and is divided into a set of consecutive and overlapping in following stages: (1) hemostasis which aims to stop bleeding, (2) inflammatory focused on the destruction of bacteria, (3) proliferative to fill and cover the wound with new tissue, and (5) maturation focused on restoration of a normal skin appearance and functionality [100]. The unique properties of aerogels like high biocompatibility, open porosity, tunable chemical functionalities, and broad portfolio of formats makes them good candidate for wound dressing application. Aerogels can absorb a large quantity of aqueous fluids and providing an adequate moisture balance and pH in the wound site, thanks to highly porous structures with large surface areas. GA have additionally unique ability to absorb plasma, allowing blood cells to accumulate on the surface, and further promoting blood coagulation on the wound surface. Singh et al. [101] have reported that GO sheets elicited platelets aggregation through activation of Src kinases and release of calcium from intracellular stores. Platelets' aggregation is primary step in blood coagulation process, which is followed by thrombus formation. The combination of hemostatic performance of GO with high absorbent capability of aerogels makes graphene sponge aerogels a superb hemostat.
For wound healing application, the sponge like GA with different crosslinking agents are most common used (Fig. 13). Chemically linking of graphene-based sheets has exhibited prominent advantages including large pore volumes, low density, and ideal structural stability, which make such prepared GA suitable for hemostatic applications. It was firstly reported by Quan et al. [102] who have demonstrated an ethanediamine (EDA) cross-linked graphene   [102]. GA sponge crosslinked with DapA: e schematic represen-tation of the preparation route using respectively EDA and 2,3-DapA as crosslinking agents and the hemostatic mechanism of aerogels; f photograph of the DCGS and cross section of the DCGS with layers stacking structure; g SEM image of the interior porous structure of the DCGS. Adapted with permission from [103]. GA sponge crosslinked with DOPA: h schematic representation of synthesis method; i photograph of the polydopamine crosslinked GO aerogel (DCGO); j SEM image of the interior porous structure; k interaction between red blood cells and the DCGO; l interaction between platelets and the DCGO. The red and yellow arrows show the fibrin and active platelet, respectively. Adapted with permission from [104] sponge (CGS) aerogel which possess ability to absorb blood plasma very fast and allowing the accumulation of blood cells on its surface, which further promoted blood clotting (Fig. 13a-d). These materials have gained considerable interest due to low cost, ultra-light, portable, long shelf life, and nontoxicity. The hemostatic process on the sponges' surfaces mainly depended on the physical absorption process and can be improved by increasing the carboxyl groups at its surface through the crosslinking with 2,3-diaminopropionic acid (DapA), a medicinal amino acid [103] (Fig. 13e-g), or with oxygen containing groups by a mild cross-linking with polydopamine [104] (Fig. 13h-l). As a result, high surface charge affects strong erythrocytes and platelet stimulation parallel to strong mechanical properties of aerogels and high absorbability, resulting in remarkable hemostatic performance. What is more, on surface o CGS formation of fibrin filaments, platelets activation as well as pseudopodia formation can be observed. The stimulation effect on erythrocytes and platelets was also observed for GO aerogel crosslinked via Bletilla striata polysaccharide (Bsp) [84]. Bsp exhibited biocompatibility, biomedical activity, and low toxicity, and because of that is widely used in Chinese medicine. Chen et al. [84] used this polysacharyde to improve the cytotoxicity of GO. They obtained uniform, fluffy, porous cavity structure in the form of sponge which promotes red blood cell aggregation, accelerated fibrin formation, and induced blood coagulation. On the other hand, surface active chemical structure of GO with hydrophilic edges and hydrophobic basal plane causes high hemolysis rate [105], potential thrombosis [101], and cytotoxicity [106] of GO. To overcome this limitation, the GO aerogels hybrid composites are widely used with promising results. What is more, such composites are often loaded with Pais grape extracts (PAs), high in proanthocyanidins, due to its unique physiological properties, including antioxidant, anti-inflammatory, anticarcinogenic, and antimicrobial activities [107] (Fig. 16). Mellado et al. [108] proposed an aerogels-based of GO and poly(vinyl alcohol) (PVA) combine with natural Pais grape extracts of seed and skin rich designed to host hemostatic agents and to absorb water and blood without disintegration (Fig. 14a-d). They showed, that release of the Pais garpe extract significantly shortened the coagulation time, while the negative surface charge of GO contributed to the accumulation of blood cells on the surface of the aerogel. Gelatin-GO composite aerogels reinforced with PAs were proposed by Borges-Vilches et al. [109] (Fig. 14e, f). Gelatin was used to limit the cytotoxic effect on GO onto cells. What is more, the addition of PAs increased the negative surface charges of aerogel and promotes blood absorption with great potential for wound healing processes. As a result, they obtained highly resistant porous composite, capable of absorbing more than 50 times their weight, when in contact with liquid media under physiological conditions. The release of PAs promotes clotting cascade activation followed by the thrombus formation. The same author, in another research, has used PAs to improve hemostatic properties of GO/polyethylene glycol (PEG) aerogel [110] (Fig. 14g-j). The phenolic groups of the PAs act as hydrogen-bond donors, forming strong bonds with PEG. The release of grape seed extract from the GO-PEG/Ex aerogel following mainly Fickian diffusion, although the influence of non-Fickian diffusion mechanisms in this system was observed. Similar as in another research, the incorporation of PAs affects improvement in hemostatic properties of GA by inducing blood clotting.

Drug delivery
GO, due to its high reactivity, easy dispersion in solutions, and the wide possibilities of functionalization, attracts attention as a potential carrier of therapeutic agents [111]. Aerogels can increase the bioavailability of poorly soluble drugs and improve both, the stability and release kinetics of drugs, thanks to its high surface area [112]. The ratio of the reactive surface to the size of GO is almost four times greater than other materials, that compete with it for the position of the most efficient drug delivery systems. The fact, that it is made of a single layer of atoms is also important, because the increases in the number of layers reduces the transport capacity of such carriers [113]. Another important parameter is the stiffness of the material, which is indirectly related to the number of layers. The more layers a nanoparticle has, the more integral it structure is, unfortunately, it also becomes inflexible and may damage its components during penetration into the cell [114]. The large, flat surface structure of GO and enriched oxygen-containing groups provide its biocompatibility and solubility. These properties are suitable for the delivery of drugs in the body. What is more, GO contains, among others, -COOH and -OH groups, which easily allow the attachment of biomolecules [115]. The loading of molecular compounds into aerogel pores can be achieved by mixing them with the gel precursors during synthesis, diffusion from solvents before or after supercritical drying, or diffusion from supercritical CO 2 during the drying process (Fig. 15a,b).
Within chapter 3.2, the examples of GAs used as active component carries are mentioned. In this case, Pais grape extract are used as hemostatic agents to promote blood coagulation for GA dedicated for wound healing application. Besides, chemophotothermal therapy for cancer treatment has shown promise thanks to a GO system with a high uptake rate in tumors. Researchers investigated the use of GO for the delivery of anti-cancer and anti-inflammatory drugs. Ayazi et al. [116] have synthesis graphene aerogel nanoparticles (GA/NPs) for in-situ loading and pH sensitive releasing of anticancer drugs (Fig. 15c-e). They prepared the GA NPs through reduction/aggregation of GO sheets in an environment of L-ascorbic acid at a low temperature (40 °C). Electrostatic interaction of ionized drugs with GO through H-bonding was the dominant mechanism of drug loading. The in-situ loaded GA NPs showed no remarkable drug release during the harsh conditions of the environment (severe sonication). It makes them promise in upcoming nanomedicine required nanotechnology-based therapeutic processes in tumors having acidic media. Another pH responsive drug carries aerogel was proposed by Wang et al. [31]. They have reported hybrid aerogel of chitosan, carboxymethyl cellulose, and graphene oxide (CS/CMC/ Ca 2+/ GO), which were synthesized using calcium ion as the crosslinker (Fig. 15f, g). This aerogel was used to encapsulate 5-fluorouracil (5-FU)-an effective chemotherapeutic agent in the treatment of cancers. They reported existing of The macro-morphology of the sponge-like GO-PVA aerogel. c SEM image of GO-PVA aerogel shows heterogeneous porous structure with many openings, measuring from tens to hundreds of micrometers. d Photographs of the corresponding aqueous solutions of hemoglobin after 60 s shows lower absorbance values of the aero-gels than control groups (blood, gauze) and thus more blood absorption. Adapted with permission from [108]. e Schematic representation of Gelatin GO aerogel with PAs. f SEM images of blood cell adhesion on gelatin-GO aerogel surface with different PAs content. Adapted with permission from [109]. g GO/PEG aerogel with PAs: h Photograph of GO-PEG aerogel; i SEM image of GO-PEG aerogel and j SEM image of GO-PEG/PAs aerogel. Adapted with permission from [110] π -π stacking and hydrogen bonding between GO nanosheets and 5-FU, which plays a significant role in the sustained release of 5-FU from the CS/CMC/Ca 2+/ GO aerogel. This comparison clearly indicates that the introduction of GO could greatly delay the release of 5-FU and overcome the burst release problem associated with traditional polysaccharides-based drug carriers [31,110]. In turn, Lim et al. [32] loaded lipophilic Coenzyme Q10 pharmaceuticals into GO-doped carbon aerogel by solution-phase impregnation technique (Fig. 15h-j). In this case, GO enhances the Fig. 15 Schematic representation of active compound introduction into aerogel during synthesis process (a) and during supercritical CO 2 drying (b) [115]. c Schematic illustration of the application of GO sheets in fabrication of GA and drug loading, during formation of aerogel in the drug medium (the in situ method (route A)), and after synthesis of aerogel (step by step method (route B)). d SEM images of GA before sonication (on the left) and graphene-based NPs obtained after 2 h sonication (on the right). e SEM images of in-situ doxorubicin hydrochloride (DOX) -loaded GA before sonication (on the left) and DOX-loaded GA NPs obtained after 2 h sonica-tion (on the right). Adapted with permission from [140]. f Schematic illustration showing the process for the preparation of 5-fluorouracil (5-FU) loaded hybrid aerogels. g SEM images of CS/CMC/Ca2 + / GO. Adapted with permission from [31]. h Schematic of crystalline drug (CoQ10) loading on carbon aerogel based on RF polymeric gels. i SEM images of GOA and (j) CoQ10-loaded GOA. Adsorbed CoQ10 adopts crystalline phase. (k) Absorption of CoQ10 to GOA over 12 h-UV-vis spectrum. Decrease in intensity of absorption band centered at 273 nm due to adsorption of CoQ10 within the aerogel's pores [32] aerogel's loading capacity and rate of uptake for CoQ10. The adsorbed CoQ10 adopts a crystalline phase that differs from the bulk material, not only offering the potential advantages of crystalline materials for drug g delivery, but also a means to control the microstructure of this important lipophilic pharmaceutical (Fig. 15i, j).

Biosensors
Monolayer graphene sheets are a promising material for use in biosensors, as their properties include high mechanical strength, thermal conductivity, as well as a tunable electronic band gap. Graphene can also be easily functionalized to create a biocompatible surface through both, covalent and non-covalent bonding of small molecules. Thus, GA can be used in electrochemical immunosensors in which, antigen-antibody complexes are detected on the surface of the electrode useful for the diagnosis of diseases. They can also be used for detecting specific biological molecules or thank to high flexibility, pressure sensing application (Fig. 16). The holey nitrogen-doped graphene aerogel (HNGA) was synthesized and applied to the concurrently electrochemical determination of small biological molecules including ascorbic acid (AA), dopamine (DA) and uric acid (UA) by Feng et al. [119]. Nitrogen doping affect the distribution of electric charge and spin density of carbon atoms, which form an "active domain" and can straightway take part in the catalytic reaction and improve the reactivity and electrocatalytic performance of graphene. What is more, Mariyappan et al. [117] have reported a molybdenum tungsten oxide nanowire intercalated GA (Mo-W-O/GA) nanocomposite-based sensor for the simultaneous detection of DA and tyrosine (Tyr) (Fig. 18a-d). The unique 3D coral reef architecture with large surface area and high catalytic activity of the Mo-W-O/GA nanocomposite favorable to adsorb more DA and Try molecule, thereby, increased the sensitivity. The prepared sensor was used for real sample analysis to calculate the amount of DA and Tyr in human urine and blood  [117]. GA aerogels for pressure sensing application: e schematical illustration of the overall preparation procedure of rGO/BC aerogel; SEM images of (f, g) neat BC aerogel; (h, i) and rGO/BC aerogel [122] serum samples, with high accuracy and good recovery (%). In turn, Ruiyi et al. [118] have reported folic acid and octadecylamine-functionalized graphene aerogel microspheres (FA-GAM-OA) which act as electrochemical sensors for detection of cancer cells circulating in blood. This micrographene aerogel modified sensor shows an exceedingly large specific surface area and great selectivity to cancer cells. To improve the detection sensitiveness of material, different nanoparticles (NPs) can be introduced into GA structures, including gold (Au) and silver (Ag) NPs. Biasotto et al. [119] have reported hybrid aerogel based on rGO decorated with AgNPs exploitable for surface-enhanced Raman scattering (SERS) (Fig. 17a-e). The obtained sponge-like material was suitable for highly sensitive label-free detection of chemical and biological species (rhodamine 6G (R6G), 4-mercaptobenzoic acid (MBA), and microRNAs). In the light of SERS sensing applications, a 3D porous sponge-like nanoarchitecture, such as GO/rGO aerogels, provide both, a high surface area and a homogeneous spatial distribution of adsorbed AgNPs, arranged to maximize the hot spots density. Thus, prepared microfluidic chip by integrating the rGO and AgNPs aerogel enable detection of the target molecule at low concentrations. Sroysee et al. [120] have prepared allergen-detecting biosensor developed from sulfite oxidase enzyme, AgNPs, and 3D GA. AgNPs decorated on 3D GA provide high electrical conductivity or fast charge transfer and serve as an enzyme anchoring site via a stable thiol bonding. The rGO/Ag aerogel was initially functionalized with the folic acid (FA) and cysteine (Cys), namely rGO/ Ag-Cys-FA before immobilized with the sulfite oxidase enzyme (SOx). The resultant rGO/Ag-Cys-FA-SOx material displays remarkable sulfite detection ability, in terms of both, sensitivity and selectivity. In turn, Sun et al. [34] have fabricated chemiluminescence biosensor based on aptamer and oligonucleotide-AuNPs functionalized nanosilica GOA for insulin (INS) detection (Fig. 17f-l). Prepared nanosilica functionalized GOA (SiO2/GOA) showed rich pore distribution, large specific surface area, and good biocompatibility. As a biorecognitions element they used insulin aptamer (IGA3) while as chemiluminescence signal amplification materials they used oligonucleotide functionalized gold nanoparticles (ssDNA-AuNPs) [34]. The insulin binding to  [119]. GAs with AuNPs: (f) the preparation of ssDNA-AuNPs/ IGA3/SiO2/GO; the SEM images of GO (g), SiO2 (h) and SiO 2 /GO (i, j); the TEM images of AuNPs (k) and ssDNA-AuNPs (l). Adapted with permission from [34] the IGA3 is followed by release of ssDNA-AuNPs which catalyze the luminescence of luminol and H 2 O 2 .
Furthermore, GA with high flexibility can be used in human motion detection systems. Mao et al. [121] have fabricated GA in the form of spheres dedicated to usage as flexible sensors for many applications, including tissue elasticity sensing, finger posture sensing, and kinetic physiological signal monitoring such as swallowing and pulse detection. Zhai et al. [83] have proposed carbon nanotubes/ graphene oxide/chitosan aerogel (CNTs/GO/CS) for detecting various human motions. The function of CS was to stabilize the CNTs/GO suspensions and to obtain aerogels with high reliability, whereas the CNTs was responsible for excellent electrical conductivity of aerogel. To improve resilience and mechanical durability of aerogel they performed in situ growth of CNTs/GO/CS aerogel in an open cell polyurethane foam through a unidirectional freeze-drying process. The resulting composite showed an anisotropic structure consisting of a well-oriented porous cavities generated by the formation of parallel ice columns during the freeze-drying process. Wei et al. [122] have proposed flexible, green synthesized bacteria cellulose (BC) and caffeic acid-rGO composite aerogel for pressure sensing application ( Fig. 16e-i). The high conductivity of rGO and a strong hydrogen interaction with the BC skeleton affects high electrical and mechanical properties of composite. The synergetic effects of pressure-generated microcracks and interconnection of conductive skeleton endow the rGO/BC aerogel sensors with high sensitivity in a large-scales of strain.

Other biological applications of GA
The perspectives of usage GA in biomedical applications are promising due to the growing number of scientific research in the field of new aerogels. Although GA dedicated to bone tissue engineering, wound dressing, drug delivering, and biosensing are most common described, there are several reports which shows other possibilities of used them in biomedical field (Figs. 18, 19). For instance, Wang et al. [123] have developed polydopamine (PDA)/rGO aerogel dedicated to skeletal muscle atrophy regeneration. PDA/ rGO aerogel combined with electrical stimulation enhances myoblast differentiation and induces myotube contraction in vitro, which is a key issue in regeneration of injury muscle  [123]. GA for nerve tissue engineering: e The process of preparing the hybrid scaffold PVA/Gelatin + GOA; f SEM image taken from the cross-section of hybrid scaffolds made from GOA and filled with PVA/Gelatin; g SEM images from the cellular structure of 5% GOA; h SEM image of the horizontal (left) and vertical (right) cross-sections of PVA/Gelatin aerogel. Adapted with permission from [126]. 3D printed GOA for tissue reconstruction: i schematic illustration of 3D printing process; j photographs of printed microlattices; k SEM images of the 3D architecture. Adapted with permission from [87]. GA for bilirubin adsorption from blood: l schematic representation of synthesis method; m optical photograph of rGO beads n; SEM morphologies of the rGO beads o and the internal structure of rGO beads [88] ( Fig. 20a-d). In this case, rGO facilitates proper electrical signal propagation through scaffold and affects synchronous contraction of myotubes in PDA/rGO aerogel. Thanks to high absorption ability of rGO to proteins, aerogel enhances the differentiation and myogenic gene expressions of myoblasts and thus, exhibits the efficient treatment for preventing denervation-induced skeletal muscle atrophy in vivo. What is more, excellent electrical conductivity of GA makes them promising candidate in nerve tissue engineering. Although scientists are just discovering the possibilities of using GA in the reconstruction of the nervous system, the advantages are already visible. GOAs pose stiffnesses comparable to those of brain tissues [124] and can enhance the formation of interconnected neural network. Zeinali et al. [125] have prepared rGO scaffold with high porosity, open channels, and flexibility which supports the neural and glial cell adhesion, growth, and differentiation of nerve cells. The same authors, in another research, have fabricated hybrid scaffold composed of hollow GO aerogel filled with porous poly(vinyl alcohol)-gelatin aerogel (PVP/G) for regeneration a central nervous system (Fig. 28e-h) [126]. They obtained tubular-shaped macrostructure with interpenetrating polymer network and oriented microstructure, which provides suitable conditions for neuronal tissue. In this study, GOA surrounding the neural canal prevents the formation of fibroglial tissues and promotes the proliferation and growth of neural stem cells. Moreover, the gelatin scaffold, with a tubular structure in the canal, promotes the growth of the nerve cells. In turn, Mansouri et al. [127] have fabricated composite sponge-like scaffold composed of sodium alginate (Na-ALG) and GO with desirable pore sizes greater than 50 μm needed for neuron cell culture. The mechanical property of fabricated scaffolds matches up with the native tissue, and thus could serve as a suitable matrix to support cellular responses for the three-dimensional culture of neural cell types. The possible usage of GA in tissue engineering is a subject of extensive research. Development of biocompatible porous supports with controllable macrostructure is a promising strategy for variety of tissues. 3D printing offers the potential to establish customized, transplantable   [156] implants, which provide cell support by biomimic the native tissue environment. Li et al. [128] have 3D printed cytocompatible rGO/alginate scaffolds for mimetic tissue constructs. High biocompatibility of designed composite makes aerogel adaptable to support a range of cell-types and engineer a variety of tissues for research and translation. In turn, Jiang et al. [87] have fabricated 3D printed GOA microlattice for wide applications, from energy storage to tissue engineering scaffolds (Fig. 20i-k). Another interesting application of GA was proposed by Li et al. [126]. High absorbent capability of GA makes them promising candidate as hemoperfusion materials. Thus, they have fabricated regular macromesoporous rGO beads for bilirubin adsorption from blood ( Fig. 18l-o). The GO hydrogel beads were produced by dropping the GO/ascorbic acid solution into the coagulation bath (2% CaCl 2 in a mixture of deionized water and ethanol) and freeze drying. The surface of coagulation bath was covered by dimethyl silicone oil to achieve regular uniform rGO beads. They used three different concentrations of GO: 8, 12, and 16 mg/ml. With increasing GO concentrations, the surface of the rGO beads became smooth with lower randomly oriented porosity ranging between one and 10 μm. However, the high GO concentration decreases the specific surface area of the beads and causes decreases in adsorption capacity of rGO beads for bilirubin. They also confirm that rGO beads besides good absorbing properties are characterized by high hemocompatibility.

Challenges and future perspectives
In many cases, the application of GAs in practice will require them to be permanently joined with other materials. Another potential area of practical application of GAs in combination with different metals is synthesis of ultra-light metal-ceramic composites because theoretical predictions suggest the possibility to design the materials even lighter than air [143].
For synthesis of metal-ceramic composites [144] as well as for joining dissimilar materials such as a metal (Me) and a ceramic [145], high-temperature liquid-assisted processes are commonly used and they seem to be the most suitable to obtain permanent Me/GA bonding. Compared to GAs, other materials, especially bulk metals, have dramatically different thermophysical properties and thermomechanical behavior. Therefore, joining metals with GAs without losing their unique set of properties presents a new challenge for scientists and engineers. Moreover, liquid-assisted bonding dissimilar materials requires a good wetting between them [146]. Most literature data on experimentally estimated wetting properties of different Me/ceramic systems were obtained on bulk substrates [147] and they may differ from wetting behavior of liquid metals on highly porous GA substrates. Nevertheless, these data can be used for the preliminary choice of kye joining parameters. Among different metals of practical importance, liquid Ag, Au, Cu, and Sn form high contact angles (θ > > 90°) on all known forms of carbon (graphite, diamond, glassy-carbon, carbon nanotubes) [148]. Non-wetting behavior of these Me/C systems is related with their non-reactive character because in both liquid and solid states, Ag, Au, Cu, and Sn do not form carbides, neither do they dissolve carbon. For bulk non-reactive substrates, the wettability improvement can be achieved by the following two approaches: (1) alloying a metal with reactive additions forming a thin and well-wettable interfacial layer of carbide [149]; (2) deposition of technological coating on a substrate that assures a good wetting either by dissolution mechanism or by the formation of wettable interfacial reaction product [150].
For 2D materials such as graphene or rGO sheets, the situation is unclear because they represent crystalline materials consisting of single-or few-layer atoms, in which the in-plane interatomic interactions are much stronger than those along the stacking direction. This specific feature of 2D materials affects also interatomic interaction with different liquids. Several studies evidenced that graphene is transparent for non-metallic liquids (e.g., for water because on different substrates coated with only few graphene monolayers, both experimental and theoretical (Fig. 19b) values of contact angle do not correspond to those on graphite ( Fig. 1a and 1b, respectively). They are like contact angle values recorded on materials used as substrates for graphene deposition while wetting behavior shifts to the bulk graphite as the number of layers is increased.
This phenomenon, called graphene wetting transparency [151], was also seen with other 2D materials and non-metallic liquids [152]. However, new experimental data suggest no such dependence of contact angle value on the number of graphene layers [153]. It agrees with recent improved computational research indicating graphene translucency, i.e., monolayer graphene is not entirely transparent to wetting and it partially screens the long-range interactions from the underlying substrate [154]. Thus, wetting transparency of graphene due to its transparency to chemical, van der Waals*** and electrostatic interactions with water and many non-metallic liquids remains a much-debated question.
Despite numerous experimental works supported with advanced computer simulations, progress in better understanding of complex topic of interaction between graphene and even a seemingly "simple" liquid like water is hampered by methodological difficulties and experimental shortcomings since the wettability at the molecular level cannot be directly compared with the macroscopically observed wettability. The number of problems increases with temperature and the use of metals as a liquid in contact with graphene. Therefore, information on high-temperature interaction of liquid metals with graphene-coated substrates is limited to only a few experimental works [155]. Moreover, there are no systematic studies on the effects of temperature and atmosphere on structure and properties of graphene itself. This makes difficult understanding its high-temperature behavior in contact with molten metals and alloys whose surface properties are strongly dependent on these parameters. In addition, wettability tests with liquid metals on graphenecoated substrates should be performed using special testing procedures allowing to reduce the effects of native oxide film on metal drops and heating history that mask high-temperature interaction between contacting materials and affect the reliability of contact angle measurements. Unfortunately, such advanced high-temperature wettability testing is available in only a few laboratories in the world.
It is important to note that none of literature data on interaction between liquid metals and graphene-coated substrates gave an unequivocal and direct confirmation of the wetting transparency of graphene. The first published research by Sobczak et al. [154] was performed by a sessile drop method in vacuum with oxide-free liquid Sn, in situ produced directly inside high-temperature chamber by squeezing the drop through a graphite capillary and immediately deposited on the Cu foil, coated by graphene transferring technique. Despite a good wetting noted with a pronounced delay, as compared to uncoated substrates, which would suggest graphene wetting transparency, structural characterization of solidified couples evidenced that graphene layer did not play the role of a barrier for Sn atoms because in the tests performed, liquid Sn transferred through graphene and Cu foil to form the second drops under the Cu substrate and interfacial intermetallic compound layers. These experimental findings are typical for the interaction between uncoated Cu and liquid pure Sn [155]. Thus, they contradict with main expectations as follows: (1) densely packed crystalline structure of graphene should not allow to pass through it even for helium, the smallest atom from the periodic table; (2) under experimental conditions used, interaction between liquid Sn and carbon should not destroy the graphene layer because carbon does not react neither with solid nor with liquid Sn and its solubility in Sn is negligible. Therefore, Sobczak et al. [154] suggested that with oxidefree liquid Sn, a good wetting of graphene-coated Cu is a consequence of more complicated phenomena than that with water, among which the presence of structural discontinuities in graphene layer is the dominant factor. Sobczak et al. [155] suggested that such discontinuities could be formed by different mechanisms taking place at high temperature under UHV in the presence of liquid tin and resulting in damage or reconstruction of the graphene layer. Whatever is the mechanism of the formation of discontinuities in graphene layer, they make possible local direct contacts of a liquid metal with uncoated substrate and immediate interaction between them, complemented with local dissolution of Cu substrate in molten Sn and the formation of intermetallic phases at the Sn/Cu interface. Consequently, these two phenomena mask the graphene wetting transparency that could be observed in the areas free of discontinuities in graphene layer.
Pstrus et al. [156] performed classical sessile drop tests accompanied with contact heating of Sn-Zn and Sn-Zn-Cu alloys on graphene-coated Cu substrates at 250 °C as follows: (1) in presence of flux without argon protective atmosphere and (2) in argon atmosphere without flux. A good wetting observed in flux-assisted tests was explained by the formation of discontinuities in graphene layer due to chemical attract of molten flux (Fig. 20) while non-wetting behavior of the same alloys on graphene-coated substrates, contrary to uncoated Cu substrates, Pstrus et al. justified by the presence of graphene as a barrier layer. It must be noted that in the analysis of factors affecting the interaction of liquid Sn alloys with any solid substrates, the role of native oxide film on the drops formed in classical sessile drop tests should not be omitted. It is true for easily oxidized Zn-containing Sn alloys, the use of which as solder materials in joining metals requires highly reactive fluxes to remove surface oxides and to protect the solder before father oxidation during soldering in air. Therefore, we suggest that dense nanometer-scale native surface oxide layer on the Sn-alloy in the tests performed by Pstrus et al. dominated the effect of monoatomic graphene layer. From this point of view, their results are in a good agreement with findings of Sobczak et al. [155], i.e., good wetting in the Sn/graphene/Cu couples takes place with oxide-free drops on discontinuous graphene layer, independently on mechanisms responsible for the formation of discontinuities (e.g. by thermal or chemical degradation, reconstruction or damage during contact heating of the couple to the test temperature due to the mismatch of coefficients of thermal expansion between Sn alloy and Cu).
Homa et al. examined wetting behavior of liquid Cu [157] and Ag [158] drops produced by non-contact heating of metal samples placed in a capillary above SiC single crystal substrates coated with graphene. For this, the same testing equipment and procedure were used as those applied for the Sn/graphene/Cu couples in [155] while graphene layer was produced by sublimation of Si from the surface of SiC substrate upon its heating at T > 1000 °C under vacuum. Important to note that both Cu and Ag are non-reactive with respect to carbon and carbon solubility in these metals at the liquid state is extremely low, showing < 0.01 at. % C in liquid Cu [157] and about 10 -7 at. % C in liquid Ag [158]. Even though the wettability tests did not delivery clear evidence of wetting transparency effect in both Ag/ Gn/SiC and Cu/Gn/SiC couples, the real-time observation showed their unusual behavior which was regarded as an indirect manifestation of graphene wetting transparency. Moreover, similar to the graphene-free SiC substrate but contrary to the graphite substrate, the Cu drop showed a strong adhesion to graphene-coated SiC in few attempts to debond the drop from the substrates by rising up a graphite capillary. Post-testing structural observations evidenced that, similar to the Sn/graphene/Cu couple [155], the presence of graphene layer on the SiC substrate suppresses but does not completely prevent chemical interaction between liquid Cu and SiC. Moreover, for Cu/graphene/SiC couple, chemical "etching" of graphene layer due to its contact with liquid Cu was well evidenced, thus confirming the possibility to form structural discontinuities in the graphene layer during high-temperature wettability tests. Using the same graphene-coated substrates and the same testing procedure, Homa et al. [158] examined interaction with liquid Ag with the same findings. Their detailed structural characterization of solidified sessile drop samples also confirmed different chemical interaction phenomena occurring at the interface in an intimate contact between liquid Ag and SiC substrate and leading to detachment and movement of graphene fragments to the top of the Ag drop. The observed effect of "washing off" graphene from the SiC surface was allowed by the appearance of discontinuities in the graphene layer at the triple line, basically produced by thermomechanical stress and drop movement, coupled with local dissolution of carbon into liquid Ag. These phenomena are accompanied with reorganization of carbon atoms dissolved in a liquid metal by segregation, nucleation, and growth to form secondary graphene layer at the Ag drop surface (Fig. 21).
Recent sessile drop wettability tests with graphenecoated Cu substrates, performed by Drewienkiewcz et al. [159] using contact heating, also showed a good wetting by liquid Ag caused from direct chemical interaction between liquid Ag and graphene-free Cu in the areas of structural discontinuities in the graphene layer. However similar to finding reported in [21], the presence of even discontinuous graphene layer affects wetting kinetics and interface structure, resulting in higher contact angles, as compared to uncoated Cu substrates.
The investigation of high temperature behavior of liquid Mg in contact with aerogel made of reduced graphene oxide (rGO) was firstly presented by Sobczak et al. [155]. Magnesium, as the lightest of all metals used as the basis for constructional, biocompatible, and biodegradable alloys due to its lightweight, strength and corrosion resistance, is considered as attractive metal matrix for reinforcing with light carbon phase, particularly with carbon nanotubes and porous carbon preforms [187]. However, liquid Mg does not wet all allotropes of carbon because Mg neither dissolves carbon nor forms stable carbides, e.g., the contact angle formed between oxide-free Mg drop and graphite at 700 °C is about θ ~ 150° [159]. In [155], the sessile drop method was adopted for real-time observation of wetting behavior and adhesion between molten Mg and rGO aerogel substrate at 700 °C in flowing argon using non-contact heating and in situ removal of native oxide film from Mg drop directly in high temperature by capillary purification. Despite non-wetting behavior (θ ~ 140°), a few attempts made to disconnect the rGO substrate from Mg drop by moving up the capillary were unsuccessful. Contrary to the same tests with graphite substrates, it was found that liquid Mg exhibits good adhesion to the rGO substrate and forms a permanent bond with it, which does not degrade during the solidification of magnesium and cooling of the Mg/AC couple. The detailed structural studies with the use of SEM, TEM, HRTEM and EDS methods [155] allowed to explain the mechanism of formation of a permanent connection between liquid magnesium and  [158] carbon aerogel, recorded both on the drop surface (Fig. 22) and inside the drop in the vicinity its contact with rGO aerogel sample (Fig. 23). It was related with the presence of residual oxygen in the aerogel that improves physicochemical compatibility between liquid Mg and rGO through the formation of interfacial oxide phase MgO.  Even though for far-reaching practical applications of carbon aerogels, there is a strong need of a deep and comprehensive understanding of the behavior of graphene-based materials at different temperatures in various environments. This will create new possibilities for the use of aerogels in contact with metals used in medicine as well as in other industries.