Aerogel is a nanostructured material that is gaining popularity as a structural alternative for insulation in a variety of uses, ranging from residences and commercial structures to offshore platforms and spacecraft. Aerogel insulator is thought to provide 40 times the shielding effect of fiber glass, allowing it to be used in space-constrained applications. It is a low-density, high dielectric strength, high specific surface areas, low thermal conductivities, and extremely porous foam with interconnected nanostructures ^[1][2][1,2]. Aerogel is composed of approximately 99.8 percent space, giving it a spectral look, and garnering the name of ‘solid smoke’ [3]. It is typically composed of silica and may take numerous shapes. However, organic polymers, inorganic, carbon allotropes, polysaccharides, transition metals, and nanostructures of semiconductors may also synthesize aerogels [4]. Aerogel is created by drying gels at extremely elevated heat.

In the early 1930s, Kistler and Learned invented the first aerogel by supercritical drying a wet gel and extracting the liquid [5]. It was employed as a tobacco filler and thickener, whereas silica aerogel was used as a thermal insulating blanket. Despite the numerous benefits that silica as well as other inorganic compounds can bring in the production of aerogel, conventional aerogel raw resources are still derived from petrochemical sources. On the other hand, the difficult multistage preparation method stymied the development of aerogel. Nonetheless, native aerogel with a single element is typically afflicted by serious issues such as weak mechanical properties, and a lack of functionalities. The name “aerogel” resurfaced in the 1970s, with the rising use of sol–gel synthesis processes and the usage of aerogel to store rocket fuels [6]. Following that, important efforts were made to simplify the synthesis methods, particularly drying to achieve a low-cost and simple synthesis of aerogel. This paved the way for a wide range of aerogel to be used in various fields of application due to their open structure and lightweight ^[5][7][8][5,7,8]. To improve aerogel performance, significant growth in the emergence of future aerogel with varied physicochemical features and functional abilities is required ^[9][10][9,10]. For example, aerogel-based biomaterials are now made from a variety of sources or components that imitate the structure of a biological extracellular matrix. The tissues that surround this structure serve as support cells and are affected biochemically by it. Even though an aerogel network has also hybridized with a wide variety of nanostructures and improved functional properties such as antifungal or antimicrobial performance.

2. Type of Aerogels and Properties

Different varieties of aerogels were produced during the last few decades as the methods for the synthesis and drying of aerogels improved. They can be classified as inorganic aerogels (silica, alumina, and titania), polymer-based, carbon allotropes (nanotubes and graphene), and natural macromolecule-based aerogels (alginate, starch, gelatin, protein, nanocellulose and chitosan) ^[11][12][13,14]. Typically, silica-based aerogels are the most potential candidate materials owing to their distinctive characteristics, such as low thermal conductivity (15–20 W/mK), low density (0.003–0.5 g/cm³), and large surface area ^[13][15]. They are generally fragile, have poor mechanical properties, and require a lengthy processing technique, hence limiting their application range [10]. Many attempts to increase the quality of silica-based aerogels have already been made, including using (i) adaptable silica catalysts in the strand, (ii) enhanced polymer cross-linking, (iii) accelerated ageing processes in different solutions, (iv) adding nanofillers, and (v) polymerizing the precursor in advance of gelation. For example, it has been shown that the combination of silica with methacrylate polymer to improve the polymerization resulted in enhanced mechanical performance and other parameters, including densities, areas, pore diameters, and void content ^[14][16]. Silica aerogels through polymer modification are illustrated in Figure 1. They are classified as silica aerogels reinforced polymer, fabricated via cross-linked via water-oil aqueous solution in high-internal stage emulsion substance. This novel material shows a superior performance property over pure silica aerogels ^[15][17]. In addition, Posada et al. produced ceria-containing silica aerogel via a three-way catalyst approach in incorporation with a new rapid supercritical separation method. They employed a polyether to strengthen the aging process and accelerate the gelation time ^[16][18]. This innovative technique can reduce the time taken to prepare wet gels, including gelation, ageing, and solvent exchange from days to seconds ^[17][19]. In addition, a nanofiller such as graphene nanoplatelets (GnPs) can also be employed to enhance the mechanical behavior of aerogel. This GnPs can speed up the gelatinization of nanostructures and reduce nanopore shrinkage throughout the hydrothermal process ^[18][20]. In addition, many studies are concentrating on improving the performance of silica aerogel by utilizing various approaches in native silica aerogel. A trifunction alorganoalkoxysilane, such as methyltrimethoxysilane was also used to provide agility to silica aerogel. However, the high costs of these precursors make them unsuitable for long-term use. As a result, many researchers adopted the organic-inorganic hybridization method, which entails cross-linking the silica aerogel with organic molecules ^[19][21]. This distinctive aerogel has a high degree of hydrophobicity and thermal insulation, giving it appealing properties such as self-cleaning, infrared stealth, and heat insulation compared with rival commercial items. The cellular structure shown the construction of multidimensional nanomaterials with synergistic action of organic–inorganic components contributed to the excellent multifunction of aerogel ^[20][21][22,23] and a strong interfacial effect is formed between the two components ^[22][24]. In general, other inorganic aerogels, such as alumina and titania, have garnered huge attention due to their unique microstructures. However, the extreme brittleness and manufacturing expense of these aerogels severely limit their industrial advantages. These aerogels may be modified with other materials, such as organic and polymer substances to provide numerous meshwork formation, high porosity, lightweight structure, moduli of elasticity, and low thermal conductivity ^[23][24][25][25,26,27]. Multifunctional inorganic aerogel with high open porosity and enormous surface area is a promising material that might be extended for extensive applications ^[26][27][28,29]. Additionally, the agglomeration of inorganic nanoparticles and nanofibers are recognized as a very viable approach for creating extremely flexible, readily accessible, and versatile composite aerogels ^[22][24]. Furthermore, polymer aerogels have a variety of forms, including polyamide (PI), polyvinylpolydimethylsiloxane (PDMS), and phenolic-based aerogels. All polymer aerogels have closely similar structures and properties ^[28][29][30][30,31,32]. In contrast to silica aerogel which are fragile and hygroscopic, aerogels derived from polymers have a broad variety of uses owing to their excellent mechanical attributes, such as high strength and fatigue resistance. These organic aerogels have thermal conductivity close to silica aerogel, comparable density and can be produced with very little shrinkage during the manufacturing process. Depending on the polymer type and fabrication circumstances, it may range between sheet-like skeletons and colloidal nanoparticles to nano/micro-fibrillar networks. The structural properties of aerogel materials, such as shape, size, and even pore ordering, have a substantial impact on their ultimate mechanical performance ^[31][33]. For example, a PI reinforced graphene oxide/cobalt (PI/rGO/Co) polymer produced by a unique cross-linking process demonstrated great heat stability and low thermal conductivity ^[32][34]. Additionally, multifunctional polyvinylpolydimethylsiloxane (PDMS)-based aerogels were reported to have high hydrophobicity and super-flexibility, thermal superinsulation, effective water, and oil separation, integrate selective absorption, and strain sensing ^[33][35]. In contrast, cellulose-based aerogel offer high porosity, higher surface area, and lightweight ^[34][36]. Aerogels containing organic precursors such as resorcinol formaldehyde, phenol formaldehyde, or melamine formaldehyde, on the other hand, have extremely poor electrical conductivities and dramatically lowered heat transmission throughout the aerogel’s backbone phase. Compared with cellulose-based aerogels, they may also be mechanically more flexible and confined to surface areas of less than 1000 m²/g ^[35][37]. Meanwhile, carbon allotrope aerogel is generally porous materials made up of small interstitial pores (less than 50 nm) and interconnected with homogeneous carbonaceous particles (3 nm–30 nm) ^[36][38]. This aerogel has strong thermal and electrical conductivity. It provide a more brittle structure with higher backbone porosity due to micropore structures at specific areas of approximately 2000 m²/g for certain meso- and macrostructures ^[37][39]. The typical synthesis process of polymer or carbon aerogel is illustrated in Figure 2. Macromolecules or polysaccharides-based aerogels are made from biopolymers derived from renewable raw materials such as cellulose, chitosan, alginate, chitin, and protein. For example, cellulose aerogel is identical to ordinary silica and polymeric aerogel in terms of compressive stress (5.2 kPa–16.67 MPa) and better recyclability [7]. As stated by Gong et al. the spongy morphology of this aerogel was steadily enhanced with the raising of the carboxyl proportion of nanofibrils in the structure. Carboxymethyl element could also effectively increase the total area of aerogel, due to the elimination of horrification ^[38][40]. Moreover, chitosan-based aerogel has much better physicochemical properties of the functional groups than cellulose-based aerogel and can be used in biomedical applications. When it was incorporated with graphene oxide, the adsorption capacity of this material improved ^[39][41]. In contrast, alginate-based aerogel is highly promising for low-flammability performance; however, it exhibits poor mechanical properties ^[40][42]. Interestingly, with the addition of graphene oxide, the catalytic property of this biomass aerogel can be increased by 30 times, resulting in an improvement in its mechanical property. The properties of different types of aerogels are shown in Table 1.