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Introduction
Aerogels have become one of the most exciting materials of the 21st century. The unique processing strategy produces materials with extremely high porosities and low densities, high specific surface areas, high dielectric strengths, and low thermal conductivities. These properties have made aerogels novel and intriguing materials for applications in aerospace, energy generation and storage, biomedical devices and implants, sensors, and coatings. Since the introduction of the silica aerogel by Kistler in the 1930s, aerogels have been made out of a variety of materials including metal oxides, chalcogenides, biopolymers, and resins to name a few. More recently, aerogels have entered the realm of nanotechnology, incorporating a variety of nanomaterials into the aerogel matrix and using such materials to create composite aerogels.

While an aerogel’s network contains pores with diameters on the order of nanometers, incorporating nanomaterials into an aerogel has further enhanced the functional properties of the aerogels. Since this first use of carbon nanomaterials in the production of an aerogel structure, the utilization of a variety of nanomaterials for the development of high-performance aerogel structures has grown exponentially. For example, carbon nanomaterials such as carbon nanotubes, graphene, and carbon nanofibers have been incorporated into aerogels to improve the electrical conductivity and performance for applications such as supercapacitors, sensors, and batteries. In other earlier works, metal chalcogenide nanoparticles were used as quantum dots to create semiconductor aerogels for photovoltaic and sensing applications. In another effort, synergistic composites of nanostructured aerogels and metal oxides were fabricated using atomic layer deposition for catalytic membranes and gas sensors.

In this review, recent trends regarding the fabrication and utilization of nanomaterials in high-performing composite aerogels are discussed. General aerogel fabrication schemes are outlined first, including a review on the fabrication of aerogels via 3D-printing techniques. Next, 1D and 2D nanomaterials commonly used for the synthesis of composite aerogels are discussed, including the nanomaterial’s fabrication schemes. Finally, the application of aerogel composites of nanomaterials in environmental remediation, energy storage, controlled drug delivery, and tissue engineering are discussed.

2. Aerogel Fabrication Strategies
The preparation of aerogels typically involves three distinct steps: the sol-gel transition (gelation), the network perfection (aging), and the gel-aerogel transition (drying).

Once the desired materials are selected for the fabrication of the aerogel, the precursor materials are dispersed in a liquid (i.e., colloidal dispersion) and allowed to gel, thus forming a continuous network of solid particles throughout the liquid.

For some materials, the transition from a colloidal dispersion into a gel happens without the addition of crosslinking materials. For others, crosslinking materials are added to the dispersion to promote the strong interaction of the solid particles in order to form the gel. The gelation time depends heavily on a variety of factors such as the chemical composition of the precursor solution, the concentration of the precursor materials and additives, the processing temperature, and the pH. Many materials may require additional curing after gelation (i.e., network perfection) in order to strengthen the aerogel network. Once the gelation is completed, the gel is dried in such a way as to minimize the surface tension within the pores of the solid network. This is typically accomplished through supercritical fluid extraction using supercritical carbon dioxide (scCO2) or freeze-drying. More recently, the aerogel fabrication scheme has been revolutionized in order to create 3D-printed aerogels. This section briefly describes and compares the processing strategies of supercritical drying, freeze-drying, and 3D printing.

Supercritical Drying
To dry the gel, while preserving the highly porous network of an aerogel, supercritical drying employs the use of the liquid-gas transition that occurs beyond the critical point of a substance (Figure 1a and Figure 2). By using this liquid-gas transition that avoids crossing the liquid-gas phase boundary, the surface tension that would arise within the pores due to the evaporation of a liquid is eliminated, thereby preventing the collapse of the pores. Through heating and pressurization, the liquid solvent reaches its critical point, at which point the liquid and gas phases become indistinguishable. Past this point, the supercritical fluid is converted into the gaseous phase upon an isothermal de-pressurization. This process results in a phase change without crossing the liquid-gas phase boundary. This method is proven to be excellent at preserving the highly porous nature of the solid network without significant shrinkage or cracking. While other fluids have been reported for the creation of supercritically dried aerogels, scCO2 is the most common substance with a relatively mild supercritical point at 31 °C and 7.4 MPa. CO2 is also relatively non-toxic, non-flammable, inert, and cost-effective when compared to other fluids, such as methanol or ethanol. While being a highly effective method for producing aerogels, supercritical drying takes several days, requires specialized equipment, and presents significant safety hazards due to its high-pressure operation.

Freeze-Drying
Freeze-drying, also known as freeze-casting or ice-templating, offers an alternative to the high temperature and high-pressure requirements of supercritical drying. Additionally, freeze-drying offers more control of the solid structure development by controlling the ice crystal growth during freezing (Figure 1b). In this method, a colloidal dispersion of the aerogel precursors is frozen, with the liquid component freezing into different morphologies depending on a variety of factors such as the precursor concentration, type of liquid, temperature of freezing, and freezing container. As this liquid freezes, the solid precursor molecules are forced into the spaces between the growing crystals. Once completely frozen, the frozen liquid is sublimed into a gas through lyophilization, which removes much of the capillary forces, as was observed in supercritical drying (Figure 2). Though typically classified as a “cryogel”, aerogels produced through freeze-drying often experience some shrinkage and cracking while also producing a non-homogenous aerogel framework. This often leads to freeze-drying being used for the creation of aerogel powders or as a framework for composite aerogels, as will be discussed in Section 3 and Section 4 of this review.

2.3. 3D Printing
The three-dimensional printing (3DP) of aerogels is revolutionizing the field by enabling a fast and accurate fabrication of complex 3D porous structures, thereby introducing new functionalities, lower costs, and higher reliability in aerogel manufacturing. 3DP, in general, is a type of additive manufacturing technique that builds 3D objects through a layer-by-layer growth process. This technique makes it possible to fabricate highly customizable and complex structures for many industrial sectors in significantly reduced times while using a variety of materials such as polymers, ceramics, and metals. The 3DP of aerogels is considered a hybrid fabrication technique to produce extremely lightweight 3D structures, employing new depositional strategies for the creation of the 3D gel constructs while utilizing the common drying methods of supercritical drying and freeze-drying, as discussed previously.

3DP of aerogel techniques are categorized depending on the sol-gel transitions during the printing process. These categories include: direct ink writing (DIW), where a gel is formed prior to printing ; stereolithography (SLA), where the sol-gel transition occurs during printing ; and inkjet printing (IJP), where the sol-gel transition occurs after printing (Figure 3). SLA is a technique that prints 3D structures using photocurable materials through a process called photopolymerization, in which polymer layers solidify upon exposure to specific laser wavelengths (Figure 3a). IJP is a non-contact, droplet-based material deposition process, with the potential to be modified to deposit photocurable materials to achieve patterning with high resolution (Figure 3b). DIW is an extrusion-based printing technique that involves the deposition of continuous ink filaments, in a layer-by-layer fashion, to realize the 3D constructs (Figure 3c).

Similar to the traditional aerogel synthesis techniques, the 3D printing of aerogels comes with its own limitations. For instance, SLA requires the use of only photocurable materials, whereas DIW and IJP offer a lot more versatility when it comes to material selection. SLA often requires expensive accessories, masks, and tools, whereas DIW and IJP do not. However, DIW and IJP both suffer from nozzle blockage caused by nanofiller clusters or high viscosity materials, which is not observed in SLA techniques. SLA provides the advantage of rapid gelation, which is desirable as gelation is known to be time consuming, with some materials requiring significant curing (i.e., aging) in order to produce a structurally sound gel. DIW methods do not offer such rapid gelation; however, it is the most common 3DP technique for aerogels. IJP is a unique technique known for its novelty, simplicity, and ability to fabricate highly modified 3D aerogels as demonstrated in studies discussed later in the review.

As an example of 3DP, silica aerogels were produced through SLA where gels were 3D printed by illuminating photosensitive precursor solutions with a green laser beam (λ = 532 nm). Gelation of the precursor solutions containing an alkoxy-silane, a monomer, and a visible-light photoinitiator (Eosin Y) were triggered, not by the heat from the laser, but by the heat produced from polymerization. The internal heat evolution from the polymerization of the monomer (hexadiodiacrylate) overcame the activation energy for the condensation reaction of the alkoxy-silane (teteraorthosilicate), consequently forming a silica network. The gel was then supercritically dried, leaving behind a polymer crosslinked silica aerogel. Although the drying methods were conventional, the laser induced rapid gelation process added the benefit of ease in fabrication by cutting down the time required for gelation. A similar sol modification followed by SLA has realized hybrid constructs with crosslinked hexagonal honeycomb structures and aerogel cores. These modifications of the sol prior to gelation is a common theme in the fabrication of 3D printed aerogels. In some cases, the use of nanomaterials can mitigate issues that occur when a material is not directly printable using 3D printing technologies. In other cases, the incorporation of nanomaterials into 3D printed aerogels can create aerogels with optimized properties and added functionalities. The following sections will provide details regarding the use of nanomaterials in the 3D printing of aerogel macrostructures.