The demand for lightweight, high-strength materials is increasing in various engineering sectors. Polymeric composites, including nanofillers, offer outstanding improvements at relatively lower loadings than micro-sized fillers, helping achieve high-level performance improvements across various advanced applications (Quaresimin et al., 2012). Recently, the mechanical properties of engineered structural components, including structural integrity and durability, produced by vat polymerization have been enhanced by incorporating second-phase nanofillers.

Nanoparticles are widely used to enhance the mechanical properties of vat polymerization polymers because of their low cost and the significant enhancements they deliver, of which, SiO₂ nanoparticles are the most widely used. In an instance, (Weng et al., 2016) and colleagues demonstrated that the absorption and scattering of UV light by SiO₂ nanoparticles were weaker than that of other 1D and 2D materials containing silicon elements (i.e., attapulgite and montmorillonite), which improved the tensile strength and elastic modulus of SiO₂-containing nanocomposites. In another instance, Chiappone and coworkers (Chiappone et al., 2016) introduced the precursor tetraethoxysilane to a photosensitive resin and post-treated samples with acidic vapor to generate SiO₂ nanoparticles in situ by the sol-gel reaction of the precursor in the polymer network after DLP printing. The generation of a SiO₂ inorganic phase contributed to significant enhancement of the tensile and compressive strengths, modulus, and surface hardness of the fabricated samples due to the preferential growth of induced nanoparticles on the surface of the samples. Moreover, different surface treatments create diverse nanoparticle interfacial adhesion and dispersion stability, leading to altered mechanical properties of various nanocomposites (Song et al., 2018). Some other 1D nanomaterials, including nano-TiO₂ (Duan et al., 2011), Cu nanopowder (Vidakis et al., 2022), and polyhedral oligomeric silsesquioxane (Li et al., 2019), have been used as reinforcing phases for photosensitive resins. In addition, the specific orientation of reinforcing fillers is believed to supply the creation of composites with design properties (Martin et al., 2015).

Carbon nanotubes (CNTs) are structurally stable, long, thin carbon columns with high aspect ratios. These CNTs often exhibit excellent polymer reinforcement due to their ability to cause the formation of higher-order interphase polymer layers and promote mechanical strength through interfacial stress transfer between nanotubes and polymers (Wong et al., 2003; Spitalsky et al., 2010). Creating CNT composites with photosensitive resins greatly improves the performance of the matrix. Sandoval et al., (2007) demonstrated that the ultimate tensile and fracture stress of SLA epoxy-based resins increased by 17% and 37%, respectively, with the introduction of .05% (w/v) multiwall CNTs (MWCNTs). Introducing MWCNTs would create strong interfacial bonding between the SLA epoxy matrix and buckled MWCNTs. However, the resultant mechanical properties of nanocomposites depended on the arrangement of CNTs in resins (Chavez et al., 2019). In addition, other 1D nanofillers, such as boehmite nanowires (Han et al., 2018), Al₂O₃ nanowires (Han et al., 2017), and cellulose nanocrystals (Kumar et al., 2012), have been incorporated into photosensitive resins for vat polymerization to enhance mechanical properties.

Graphene, a single layer of carbon atoms, consists of a backbone of sp² hybrid bonds filled in a honeycomb lattice. 2p orbitals forming the π state bands delocalize over the carbon layer and result in the highly stiff characteristic graphene (Papageorgiou et al., 2017), which is often applied in photosensitive resins (Li et al., 2018). The combination of enhanced strength and flexibility in SLA manufacturing is realized by introducing graphene oxide (GO), which is related to increasing the crystallinity of nanocomposites (Lin et al., 2015). Nanocomposites prepared by the bottom-up vat polymerization exhibit significant anisotropic mechanical properties because graphene nanosheets tend to be arranged more parallel in resins during 3D printing (Markandan and Lai, 2020). The annealing process after the completion of printing also affects the mechanical properties of nanocomposites (Manapat et al., 2017). In addition, Ti₃C₂ MXene, a novel 2D ultrathin nanomaterial, is an excellent candidate for enhancing the tensile strength of photosensitive resins to obtain reliable elastomers (Li et al., 2022a). The dispersion effect of nanofillers in photosensitive resins contributes directly to the strengthening development of 3D printed objects. A uniformly dispersed nanofiller can effectively absorb and dissipate energy, improving the mechanical properties of nanocomposites in terms of fracturing. Agglomerated nanofillers induce the formation of micro flaws, deteriorating the mechanical properties. Therefore, there is an optimum loading level for incorporated nanofillers to increase the mechanical strength of nanocomposites from vat polymerization.

A growing task in the design and application of advanced devices is heat management. In this context, an expanding demand for high thermal conductivity materials that can dissipate waste heat from the operation processes of devices exists. The preparation of composites by introducing fillers, such as graphene, metal, and inorganic nanomaterials, is a promising strategy for improving the thermal conductivity of photosensitive resins (Lee et al., 2021; Pezzana et al., 2021). Carbon fibers, due to their remarkable thermal conductivity and high aspect ratio, are also widely used to enhance the thermal conductivity of polymers, which is beneficial for forming nanocomposite percolating networks (Hong et al., 2010). In a case, acidified vapor-grown carbon nanofiber (VGCF)-supplemented dual-cure photosensitive resins were prepared by the SLA technology (Li et al., 2022b). The addition of 2% w/w VGCFs increased the thermal conductivity of a composite by 79% compared to control through the transfer of thermal energy by VGCF phonons.

High filler loadings are typically necessary to achieve appropriately enhanced thermal conductivity in polymeric matrices. It has been reported that the 7.4 wt% loading of a polymer with graphene caused nozzle clogging in a fused filament fabrication printer (Wei et al., 2015). For vat polymerization, using liquid resin would avoid the problem of the repeated clogging of nozzles or spraying devices due to the introduced nanofillers. This would create a quantum leap in the thermal conductivity of printed products through the high loading of nanofillers. However, vat polymerization remains a significant challenge due to the processing requirements of photoactivity and viscosity. Yang et al., (2022) prepared a nanocomposite with a high loading (60 wt%) of micro silica, resulting in the improved thermal conductivity of .44 W m⁻¹ K⁻¹, with 196.91% compared with the pure matrix. Nanocomposites assembled with high thermal conductivity due to closely arranged phonon transportation pathways in resin matrices could be applied to fabrication injection and vacuum casting molding to extend the service life. In addition, the fabrication of ultralow thermal conductivity thermoelectric materials can be realized by mixing thermoelectric materials, including Bi_0.5Sb_1.5Te₃ (He et al., 2015) and Ag₂Se (Park et al., 2021) materials, with photosensitive resins.

Incorporating nanoparticles also offers the possibility of improving the thermal stability of vat-polymerized printed fabrications. However, it should be noted that the thermal stability of the composite is influenced by the organic modifier, filler content, and structural features of nanocomposites (Leszczyńska et al., 2007). The introduction of nanoparticles can also effectively enhance the thermal resistance of nanocomposites (Chiu and Wu, 2008). Zhang et al., (2015) and colleagues dispersed SiO₂ nanoparticles, which modified the silane coupling agent, into a photosensitive resin by ultrasonic and a planetary ball mill treatment, followed by 3D printing. The glass-transition temperature of the nanocomposites increased from 67.28°C for the pure resin to 80.18°C for the nanocomposites with the addition of .7 wt% SiO₂. Similarly, the addition of a 1D nanomaterial, microcrystalline cellulose, had a formative influence on the thermal properties of the photosensitive resin, which increased the degradation temperature of the nanocomposite, and the β-relaxation of the photosensitive resin tended to move toward higher temperatures (Han et al., 2017).

3D printing has been used as an alternative approach for manufacturing conductive composites due to the advantages of reduced cost and eco-friendly features, which are of unique interest in various functional applications, such as electrodes (Ahn et al., 2009; Rymansaib et al., 2016), wearable devices (Hao et al., 2021), and sensors (Leigh et al., 2017). This method balances a low-cost, fast, and versatile production process with the excellent performance of the resultant products.

Metal fillers with good conductivity have been used for conductive materials in vat polymerization. Fantino and coworkers (Fantino et al., 2016) introduced silver nitrate to a photosensitive resin. The silver nitrate was reduced to silver nanoparticles via a UV post-curing process after DLP printing to prepare conductive nanocomposites. The direct loading of the filler affected the electrical conductivity of the composites: Increased amounts of silver nitrate increased the silver nanoparticle content in the matrix, thereby promoting the formation of charge movement paths between the nanoparticles, which led to a decline in the electrical resistance of the nanocomposites. However, due to its prohibitive cost and ease of oxidation, the application of metal has been limited.

In comparison, carbon fillers exhibit considerable stability and high conductivity, which makes them easier to obtain and more widely used (Gonzalez et al., 2017; Zhang et al., 2017). Therefore, CNTs are widely used due to their excellent electrical properties arising from the freedom of the movement of unpaired electrons in orbitals perpendicular to the π-π plane within the plane. Mu et al., (2017) and colleagues combined commercial photosensitive resin with MWCNTs to obtain conductive composite structures using a DLP printer. To optimize printability and conductivity, .3 wt% of MWCNTs was added, increasing the nanocomposite conductivity to .027 S m⁻¹. Guo and coworkers (Guo et al., 2020) prepared a DLP-printed conductive nanocomposite with 2 wt% carboxyl CNTs that exhibited an electrical conductivity of .13 S m⁻¹. These shreds of evidence indicated that the composites with high conductivity, high gauge strain sensors, and superior cycling stability could provide stable signal information for human motion monitoring.

The use of graphene is also an effective strategy for increasing the electrical conductivity of photosensitive resins. Since GO has better compatibility with photosensitive resins, GO can be used directly as a filler. It is reduced to graphene during post-processing after printing, and different post-processing processes affect the electrical conductivity of the samples (Chiappone et al., 2017). Nanocomposites post-cured by UV light exhibit a non-linear electrical correspondence due to uneven internal curing, while homogeneous thermally post-cured models show a linear electrical response. In addition, the combination of 1D and 2D nanofillers provides effective conductive pathways in nanocomposites and has been used to produce conductive structures (Han and Cho, 2018).