Research progress of carbon materials in the ﬁ eld of three - dimensional printing polymer nanocomposites

: Three - dimensional ( 3D ) printing technology is an additive manufacturing technology designed to rapidly process and manufacture complex geometrical compo - nents based on computer model design. Based on a 3D data model, materials are accumulated layer by layer through computer control, and the 3D model is ﬁ nally turned into a stereoscopic object. Compared with tradi - tional manufacturing methods, 3D printing technology has the advantages of saving man - hours, easy operation, no need for molds, and strong controllability of compo - nent geometry. With the development of this technology, according to the core materials and equipment and other elements of the printing molding technology, several types of 3D printing technologies such as fused deposition mod - eling, selective laser sintering, stereolithography, and sol - vent cast - 3D printing have gradually formed. This review focuses on the principles and characteristics of several of the most representative 3D printing molding processes. And based on carbon nanomaterial ( carbon ﬁ bers, gra - phene, and carbon nanotubes ) reinforced polymer compo - site materials, the research progress of di ﬀ erent 3D printing molding processes in recent years is reviewed. At the same time, the commercial application of 3D printing molding process in this ﬁ eld is analyzed and prospected.


Introduction
Three-dimensional (3D) printing technology is an important additive manufacturing technology for rapid manufacturing of complex components.This technology uses computer software to assist design, and then uses material-forming equipment to gradually stack up one layer at a time to create an entity consistent with the shape and structure of the imported model.It is very attractive and is widely regarded as a technological revolution in manufacturing [1][2][3].Three-dimensional printing technology is widely used in biological tissue engineering [4], aerospace [5], energy storage [6], electronics and devices [7], In recent years, nanomaterials have been widely used to prepare polymer nanocomposites [12].Among them, carbon material reinforcements mainly include carbon fiber (CF), graphene, and carbon nanotubes (CNTs), etc., and their introduction has given polymer nanocomposites excellent performance and various functions, such as high mechanical strength, electrical conductivity, thermal conductivity, magnetism, and electrical sensing [13].
This review uses nano-carbon material-reinforced polymer composites as a medium to review and analyze the research progress of different types of 3D printing processes in the preparation of composite materials, and analyze and prospect the commercial development prospects of 3D printing technology in this field.
2 Three-dimensional printing technology

FDM
The 3D printing technology based on fused deposition was first invented by Scott Crump in 1989 and applied for a patented technology with FDM as the core.As shown in Figure 1a, FDM is a thermally assisted printing technology.It mainly passes the filamentous material of thermoplastic polymer through a heated nozzle and is melted and extruded with a certain pressure.At the same time, the nozzle is controlled by software to move according to a certain trajectory, and the extruded material accumulates layer by layer, finally form a 3D printed product [14].Due to the low cost and easy operation of FDM technology, it has been widely concerned and loved by people.At present, FDM is widely used in many fields such as medicine, art, industrial design, aerospace, and automobile [15,16].
Compared with other printing technologies, the surface of FDM-printed products is rougher, with a general resolution of 100-800 μm, so FDM-printed products need further post-processing.At the same time, after the material is fused deposited, the compatibility between the layers is poor, and it is easy to produce pores, which leads to the formation of obvious anisotropy in the material, which limits the application of the material in specific fields [17].For the selection of raw materials, traditional FDM is mainly based on heat-assisted extrusion molding, so the raw materials are limited to thermoplastic polymers, and the use of low thermoplastic Figure 1: (a) Schematic of the working principle of FDM.Reproduced with permission from ref. [14], Copyright 2016 Elsevier.(b) Schematic of the molecular design of the phages for use as a nanoink for 3D micro-extrusion bioprinting.Reproduced with permission from ref. [21], Copyright 2016 Elsevier.(c) Illustrations of the SLS system.Reproduced with permission from ref. [23], Copyright 2018 Elsevier.(d) Schematic of 3D SLA printing.Reproduced with permission from ref. [24], Copyright 2017 Elsevier.(e) Schematic of the solvent-cast printing process and side view scanning electron microscopic (SEM) image of a circular spiral.Reproduced with permission from ref. [34], Copyright 2014 American Chemical Society.materials often brings the trouble of nozzle clogging.At the same time, because the materials need to be at a higher temperature under the conditions of melt extrusion, if different materials are mixed and extruded at the same time, the material with lower melting point will often be partially degraded first, which will lead to the reduction of the performance of the final product [18].In response to the above problems of FDM, nanomaterials, such as nano titanium dioxide and cellulose nanocrystals, have been used as fillers for FDM printing materials, and printed together with polymers.The results show that the addition of nanomaterials can improve the fluidity of the printing material in the molten state, enhance the combination between the printing layers, thereby improving the printing resolution and the strength of the printing material [19,20].The emerging FDM technology, 3D micro extrusion, is a technology that uses physical or chemical methods to redistribute and layer extruded materials.The emergence of this technology has greatly broadened the range of raw materials available for FDM.Three-dimensional microextrusion technology can not only print thermoplastic polymers, but also suitable for thermosetting polymers, silicon materials, polymer emulsions, plastisols, hydrogels, multifunctional polymers, etc., and even active biological materials or cells (Figure 1b) can achieve the desired product effect through 3D micro-extrusion technology [21,22].

SLS
SLS is an important branch of additive manufacturing technology and a relatively mature 3D printing technology.As shown in Figure 1c, it can selectively sinter metal, ceramic, or polymer powder by using a carbon dioxide laser beam, where the powder material is evenly spread on the powder bed through a powder leveling roll.In this way, the movement of the powder bed in the Z-axis direction makes it possible to manufacture parts [23].Unlike traditional processing methods, SLS technology is shear-free and free-flowing during the printing process.In addition, another feature of using SLS to prepare polymer composites is that only polymers in powder form can be used, and the materials, fillers, and polymers need to be fully mixed before they become consumables for 3D printing.The materials used in SLS technology are mainly metals, ceramics, and polymer materials.Among them, polymer materials have excellent molding conditions and high molding accuracy.More importantly, they do not have the "spheroidization" effect that is difficult to overcome during metal sintering.Therefore, it has become the most widely used and most successful selective laser sintered printing material.The most widely used thermoplastic polymers and their composite materials are polycarbonate, polystyrene, nylon, polypropylene, polyether ether ketone, and so on.
Because the SLS technology is not formed by applying pressure, it is only welded by the fluidity of the polymer powder after melting.And this kind of weld does not make the powders fully fused, so there will be a certain degree of defects and voids in the polymer SLS products, which causes the mechanical properties of the molded products prepared by SLS technology to be far inferior to the traditional molded product.However, adding a certain amount of inorganic nanomaterials to the SLS printing polymer powder can not only improve the mechanical properties of the molded product to a large extent, but also realize the functionalization of the molded product, such as high conductivity and construction dielectric network.These advantages make the polymer-based nanocomposites prepared by the SLS molding process have huge advantages and potentials in energy storage, biomedical fields, the preparation of conductive devices, flexible circuits, sensors, and wearable devices.However, the "precipitation" phenomenon caused by the density difference between the polymer and the nanofiller is an urgent problem that needs to be solved at the moment.

SLA
The basic structure and printing principle of the SLA technology printer are as shown in Figure 1d.During the printing process, the laser beam reaches the photopolymer through specular reflection, and the photopolymer is polymerized and attached to the printing platform.As the Z axis gradually rises, the photopolymer is polymerized and deposited layer by layer on the printing platform to finally form a product with a 3D structure [24].The printing resolution of SLA mainly depends on the degree of curing of each layer of photopolymer, and the curing degree of the photopolymer is mainly determined by the exposure conditions of the photoinitiator and the bulk polymer in the raw materials.At the same time, additives such as nanomaterials, dyes, and laser absorbers will also affect the polymerization efficiency of raw materials during the printing process.To further improve the resolution of SLA printing materials, two-photon polymerization 3D printing technology, also known as micro-nano light curing 3D printing technology, has been developed rapidly.This technology greatly increases the possibility of preparing materials with a printing resolution below 100 nm, such as printing medical micro-stents.With the characteristics of rapid prototyping and high printing resolution, SLA printing technology has been widely used in aerospace, automation, architecture, art, and medicine [25], especially in the medical field, SLA printed products have applications in surgical models or functional bodies [26,27], tissue engineering [28], carriers for carrying special cells [29], and other fields.
Although SLA printing technology has been widely used, its raw materials are very limited.SLA printing technology is mainly based on photopolymerization, so the raw materials are mainly photosensitive-based liquid materials, such as acrylic-based photopolymers and epoxy resins.In the SLA printing process, the viscosity of the raw material has a greater impact on the quality of the printed product.If the viscosity is not strictly controlled, the quality of the printed material will be significantly reduced [30].At the same time, these photosensitive resin molded products also have problems such as size shrinkage and incomplete curing during printing, which further leads to the degradation of the performance of printing materials [31].Therefore, some inactive additives or nano materials are often used to reduce the viscosity of the printing resin while increasing the curing efficiency of the resin, so as to obtain higher performance printing products [32,33].

Solvent cast-3D printing
As shown in Figure 1e, solvent cast-3D printing technology is an additive manufacturing method based on polymer solvent solidification molding.It is controlled by a robot on a mobile platform to deposit polymer solvent on the platform according to a program model, and solidify the deposited sample by evaporating the solvent [34].This solution-based printing technology is a kind of 3D printing technology with the most development potential.It is simple and easy to operate, low price, high precision, easy to prepare complex structure network, material selection flexibility, and easy to adjust, no need to consider changes in the melting point of composite materials due to the presence of fillers [35].In addition, the precise matching of solvent evaporation rates and printing parameters enables the solvent cast-3D printing technology to fabricate stacked lattice structures with overhangs, and it is possible to manufacture helical-shaped parts without any external support.
Instead of using heat to drive the flow of the polymer, solvent cast-3D printing technology induces the flow of the polymer by dissolving it in a volatile solvent to create an "ink."The solvent evaporates from the ink during extrusion, leaving behind shaped polymer filaments.The technique can be performed at room temperature and can be used with polymers whose processing temperatures are not suitable for melt-based 3D printing techniques.For example, polyaniline has been shown to thermally degrade before reaching its melting temperature.Poly(sulfone) melts are too viscous to be printed using commercially available 3D printing technologies.However, both polymers can be achieved using solvent cast-3D printing technology.Another advantage is that solvent cast-3D printing has been shown to produce stents with smaller feature sizes compared to stents printed using melt-based extrusion.And the polymer scaffolds fabricated using solvent cast-3D printing did not adversely affect fibroblasts and human mesenchymal stem cells.This suggests that solvent cast-3D printing is a suitable technique for the fabrication of biomedical scaffolds.
Solvent cast-3D printing technology is currently only in the laboratory stage, and has not been widely used in industrial production and market promotion, but its application in experimental research is still very effective, especially when preparing composite materials with a high content of nano-filled particles, it has more advantages.
3 Three-dimensional printing of polymer-based composite materials modified by CF CF is a new type of fiber material with a carbon content of more than 95%, high strength and high modulus [93].It is composed of graphite microcrystals and other inorganic fibers stacked along the fiber axis, and processed by carbonization and graphitization processes to obtain microcrystalline graphite aggregates [36][37][38].CF has excellent mechanical properties, it is a good conductor of electricity and heat, meanwhile has excellent corrosion resistance [39,40].And it has been widely used in sports goods, automobile manufacturing, national defense, medical equipment, aerospace, and other fields [41,42].At present, CF/ polymer composite materials have been widely used in various additive manufacturing processes, and it is gradually realizing commercialization.Compared with pure polymer materials, the improvement in mechanical and thermal properties is very significant.As shown in Figure 2a, Jansson and Pejryd [43] studied the reinforcing effect of CF on nylon 12 (PA12) under the SLS process.The results show that, compared with pure PA12 sintered parts, the tensile strength of CF/PA12 sintered parts in the X and Y directions are increased by 28 and 12%, respectively, and the Young's modulus in the X and Y directions is increased by 73 and 53%, respectively.Flodberg et al. [44] analyzed the effect of CF on the porosity and mechanical properties of composites under SLS process, and found that the addition of CF has a certain effect on reducing the porosity of the composite material while improving the mechanical properties of the material.The study found that the average pore diameter of CF/PA12 composite sintered parts was one order of magnitude smaller than that of pure PA12.The author concluded that the smaller pore size of the CF/PA12 sintered part may be due to the composite powder for the absorption of laser energy and internal thermal conductivity are greatly improved.Yan et al. [45] performed oxidation pretreatment on CF, make the distribution of CF more uniform in the matrix.When the CF content is 50%, the flexural strength and flexural modulus of the composite material are increased by 114 and 243.4%, respectively, compared with pure PA12 (Figure 2b).At the same time, the sintering test found that the initial melting temperature of the CF/PA12 composite powder has dropped, thereby reducing the preheating temperature required for SLS, which means lower energy consumption and less thermal degradation during the sintering process.
Liao et al. [46] used the FDM process to prepare CF/PA12 composites and studied their mechanical and thermal properties.It is found that adding 10 wt% CF to the PA12 matrix can significantly improve the mechanical properties of the material without sacrificing impact performance.The tensile strength increased from 46.4 to 93.8 MPa; the flexural strength increased from 35.6 to 124.9 MPa; the tensile modulus increased to 3.66 times the original; and the flexural modulus increased to 4.46 times the original.In addition, as shown in Figure 2c, because CF has established a heat conduction channel in the material, the thermal conductivity of the composite material is increased by 277.8% compared with pure PA12, from 0.221 to 0.835 W/(m K) (Figure 2d).Tian et al. [47,48] used FDM technology to study the molding properties of CF/polylactic (PLA) composites.Through the systematic study of the process parameters and the mechanical properties of the composite material, it was found that when the CF content reached 27 wt%, the bending strength of the composite material reached 335 MPa, which verified the feasibility of the CF/PLA composite material under this process, and laid a foundation for its application in the aerospace field.At the same time, the mechanical properties of CF/acrylonitrile-butadiene-styrene (ABS) copolymer composites containing 10 wt% CF were also studied.The results show that the tensile strength and flexural strength of the CF/ABS composite formed parts are increased to 127 and 147 MPa, respectively.Ning et al. [49] studied the mechanical properties of CF/ABS composites under the FDM process.The results show that, compared with pure ABS formed parts, adding CF can increase the tensile strength and Young's modulus of the formed parts, but it will reduce its toughness, yield strength, and ductility.Among them, the tensile strength of the sample with a CF content of 5 wt% is 22.5% higher than that of the pure ABS formed part, and the Young's modulus of the sample with a CF content of 7.5 wt% is 30.5% higher than that of the pure ABS formed part.As shown in Figure 2e, Tekinalp et al. [50] studied the processing performance, microstructure, and mechanical properties of CF reinforced ABS composite materials as raw materials for 3D printing.The results show that compared to pure ABS, the composite material has increased in tensile strength and tensile modulus to 115 and 700%, respectively.

Three-dimensional printing of polymer-based composite materials modified by graphene
Graphene is a two-dimensional nanosheet carbon material composed of sp 2 hybridized carbon atoms in a hexagonal shape, which is closely packed in a honeycomb shape, with a thickness of only 0.334 nm.Its carbon six-membered ring structure can be considered as the basic building block of all sp 2 carbon materials.Each carbon atom in graphene is connected to and interacts with three adjacent carbon atoms through σ bond.The stronger bond energy endows graphene with excellent mechanical properties.Relevant studies have shown that the strength of single-layer graphene prepared by mechanical exfoliation method is 130 GPa.In addition, since graphene is a zero-bandgap semiconductor, there are four valence electrons in the outer layer of carbon atoms, three of which form σ bonds with the three most adjacent carbon atoms, the remaining one acts as a π electron to form a π bond with the π electron of other carbon atoms, and the electron can move freely within the π bond.As graphene is all composed of carbon atoms, the entire graphene layer forms a π bond region, and electrons can move freely in the entire graphene layer, so graphene has excellent electrical conductivity.Because the lattice structure of graphene is very stable, the bond length of carbon-carbon bond is only 1.42 Å, which makes graphene have good thermal conductivity.Single-layer graphene also has a very high visible light transmittance due to its atomic-scale thickness.
As is well-known, graphene has poor dispersibility in common solvents and is incompatible with organic polymers [92].Adding high content of graphene to polymer composites tends to agglomerate due to its own van der Waals force [91].Fortunately, graphene oxide (GO) is rich in oxygen-containing functional groups [51].These functional groups can be combined with polymer composite materials through non-covalent bonds to improve the compatibility of the two, making GO and polymer composite the materials are evenly mixed.At present, there are three main methods for preparing graphene/polymer composites [52]: solution mixing, melt mixing, and in-situ polymerization.
SLA technology is usually applied to the method of solution mixing.Lin et al. [53] first ultrasonically dispersed a single layer of GO in acetone, then added polymer resin to mix to form a suspension, and finally evaporated the acetone to prepare a composite resin material containing 0.2 wt% GO by SLA 3D printing.It was found that compared with pure polymer resin materials, the tensile strength of the resin material with GO was increased by 62.2%, and the elongation was increased by 12.8%.Xue and Zou [54] mixed GO with propylene glycol monomethyl ether acetate and cyclopentanone under ultrasound to prepare a uniform slurry, then added SU-8 photoresist and ultrasonically stirred, and then dried and evaporated part of the solvent to obtain different GO content SU-8 photoresist.The study found that the addition of GO changed the internal crosslinking of the photoresist, and the interface between the two was strongly bonded, which enhanced the thermal stability of SU-8.Compared with pure SU-8, the tensile strength and Young's modulus of GO/SU-8 with a GO content of 0.8 wt% increased by approximately 81 and 28%, respectively (Figure 3a).Chiappone et al. [55] used water as the solvent for GO dispersion, after ultrasonic treatment, mixed Polyethylene glycol diacrylate (PEGDA) with GO suspension, studied the polymerization kinetics induced by visible light through optical rheology, and adjusted the layer thickness and layer exposure time of 3D printing to obtain the best 3D printing parameters.Manapat et al. [33] formed GO/resin composites by SLA printing, and annealed the GO/resin composites at different temperatures.The tensile test shows that when the annealing temperature is 100°C, the composite material with 1 wt% GO has the highest tensile strength, which is 673.6% higher than that of the pure resin material, which is attributed to the metastable structure of GO (Figure 3b).The bottleneck of this technology is the high cost, and the residual photoinitiator and uncured photosensitive resin may be toxic.In addition, it is necessary to prevent the graphene from settling out of the photosensitive resin during the printing process, causing uneven distribution of the graphene in the workpiece.
The raw materials required by FDM 3D printing technology are standard wires or filaments.Therefore, graphene and thermoplastic materials are generally heated to a molten state first, and the two are uniformly mixed under the action of shearing force, and then the composite material after melting and mixing is made into wire or wire by an extruder for FDM 3D printing.ABS and PLA are the most commonly used polymers for FDM [56].Wei et al. [57] prepared reduced graphene oxide (RGO)/ABS and RGO/PLA composites by mixing the polymer with GO through solution mixing and adding hydrazine hydrate to reduce the composite materials, which were used for fused deposition molding after drawing.Among them, the maximum amount of GO can reach 5.6 wt%, and the conductivity can reach 1.05 × 10 −3 S/m.The addition of graphene increases the glass transition temperature (T g ) of the polymer, so the printing temperature needs to be appropriately increased compared to pure resin.As shown in Figure 3c, Chen et al. [58] used thermoplastic polyurethane (TPU) and PLA and GO through solution mixing to prepare a composite for fused deposition molding.The blending of TPU and PLA makes the composite materials have both toughness and rigidity.The addition of GO not only improves the mechanical and thermal properties, but also has good antibacterial and biocompatibility.The composite material can be used in biology scaffold and tissue engineering after fused deposition molding.Zhu et al. [59] used 6 wt% graphene nanosheets (GNPs) and PA12 to be melt-mixed together for fused deposition molding, and found that GNPs will be oriented during the extrusion process from the nozzle.The thermal conductivity and elastic modulus of the 3D printed parts along the orientation direction are increased by 51.4 and 7%, respectively, compared to the compression molded parts (Figure 3d).Kholkhoev et al. [60] mixed multi-layer graphene and N-methyl-2-pyrrolidone ultrasonically, then added PLA and stirred vigorously.The composite material was obtained through precipitation, filtration, and vacuum drying, and then prepared at high temperature using a benchtop extruder Fiber for FDM 3D printing.Singh et al.
[56] used the GNP/ABS composite material prepared by two methods of mechanical mixing and chemical mixing to be 3D printed into a 3D entity through FDM technology.After testing, it is found that the thermal conductivity and electrical conductivity of the raw materials prepared by the chemical mixing method are better than the mechanical mixing method after 3D printing into a solid.The disadvantage of this method is that the printing accuracy is not high enough; when the graphene is added in a large amount, it is easy to block the nozzle; holes are easy to form in the process of preparing the composite material wire, which affects the printing effect; the parts are easy to warp when the thermal stress is uneven; the resulting article has anisotropy and low interlayer strength.
Gaikwad et al. [61] first melted and mixed GNP and nylon 11 (PA11) with a twin-screw extruder, and then pulverized at low temperature to form a powder for SLS.The amount of graphene added was 1-7 wt%.The addition of graphene improves the Young's modulus, flexural modulus, and thermal stability of PA11, and makes PA11 conductive.Compared with other molding methods, the composite material obtained by the SLS method has better conductivity.In addition, graphene can enhance thermal conductivity, making the laser melting and sintering process easier.

Three-dimensional printing of polymer-based composite materials modified by CNT
CNTs can be viewed as hollow cylinders rolled up by graphene sheets.They can be constructed from a single hollow cylinder, known as single-walled CNTs, or from a set of graphene concentric cylinders, known as multiwalled carbon nanotubes (MWCNTs).There are three main methods for synthesizing CNTs: arc discharge method, laser ablation method, and chemical vapor deposition method.CNTs have excellent thermal and electrical conductivity [62,63].They are long and thin carbon pillars with a high aspect ratio.CNTs have the ability to induce the formation of high-order interphase polymer layers [64].The mechanical strengthening is promoted through the interface stress transfer between the nanotube and the polymer [65,66,94].Therefore, with the maturity of CNTs preparation methods [62,67], the use of 3D printing technology to prepare CNTs composite materials has gradually become a research hotspot.
The printing process of FDM technology is through the accumulation of layers of filaments extruded by the nozzle to finally form a 3D entity.The process of melting and solidification makes the strength of the prepared composite material component more superior, and at the same time, the uniform staggered formation between the layers the conductive network effectively expands the electron migration path, so that the conductivity of the composite material component is significantly improved.Thoams' research group [68] prepared 3D printed components by dispersing MWCNTs of different concentrations into an ABS polymer matrix.The results showed that the high permeability threshold of CNTs did not cause a significant reduction in the printability of the composite material.Around 0.75 wt% of CNTs can significantly improve the conductivity of the printing component, and the increase is most significant when the addition amount is 1 wt%.Schmitz's research group [69] mixed CNTs and carbon black as fillers into ABS, and fabricated composite components with electromagnetic shielding properties through 3D printing.As shown in Figure 4a, the electromagnetic shielding performance of the optimized composite material component can reach 16 dB, which basically meets the standard of effective electromagnetic attenuation.In addition, the author also comparatively studied the influence of different molding directions on the electromagnetic shielding performance.The vertical concentric molding direction can reduce the electrical conductivity of the component through the micropores and cavities without affecting the substantial changes in the mechanical properties of the printing component, thereby achieving the purpose of electromagnetic shielding (Figure 4a).With the wide application of 3D printing based on FDM technology, researchers have become more and more aware of the limitations of this technology.Printing materials are difficult to prepare, inflexible, and inconvenient for material modification, and other factors limit the development of this technology.Therefore, while researchers have modified the materials, Postiglione et al. [70] modified the FDM printer into a machine based on liquid deposition, and called it liquid deposition modeling.Concretely, a syringe is used to replace the standard print head of the original FDM type printer and the corresponding pressure-driven modification is performed on it.After the modification, the liquid will be directly used as the printing material.In the study of conductive polyester nanocomposites, PLA and MWCNTs are dissolved in methylene chloride as printing inks.As shown in Figure 4b, the results show that when the concentration of MWCNTs is 5-10 wt%, the conductivity of the printed sample MWCNTs/ PLA nanocomposite material can reach 10-100 S/m, which is greatly improved compared with FDM technology (Figure 4c).SLS molding technology, as a non-shearing and freeflowing pressureless processing technology, provides a unique method to construct the conductive isolation network of CNTs in the polymer matrix.And it has a very low permeation threshold and is an important method for preparing highly conductive materials.Li's research group [71] used self-made CNTs to wrap TPU powder to prepare flexible TPU conductors.The author used ultrasound to disperse CNTs in ethanol.Then the TPU powder is added to the CNT suspension, and mechanically stirred before filtering and drying to obtain the CNT-wrapped TPU powder used as the raw material for SLS molding.The conductivity of TPU/CNT composites treated by SLS has a low threshold of 0.2%.At the same time, the conductivity of CNT composite materials has been significantly improved.When the concentration of CNTs does not exceed 1 wt%, SLS has a lower permeation threshold and higher conductivity than traditional molding methods.In addition, the TPU/CNT composite components prepared by the SLS molding method can maintain good flexibility and durability, and even after repeated bending tests thousands of times, the resistance can maintain a nearly constant value (Figure 4d and e).Through this method, flexible conductive TPU/CNT composites with complex structures and shapes can be easily obtained.Yuan's research group [23] successfully dispersed MWCNTs uniformly in deionized water through ultrasonic induction and surface modification with sodium bile salt, and obtained functionalized CNTs.Then the polyamide-12 and polyurethane powder are dispersed in deionized water, and the surface of the polymer powder is softened and activated by heating.Finally, the heated functionalized CNTs aqueous suspension is added to the polymer emulsion suspension, and after mixing, cooling, filtering, and drying, functionalized CNT/polyamide-12 and functionalized CNT/polyurethane powder are prepared for raw materials of SLS technology molding.In addition, the isolation microstructure induced during processing is more conducive to the electrical conductivity of the composite material.The low permeability threshold behavior of 0.5% can increase the electrical conductivity of the prepared functionalized CNTs/polyamide-12 and functionalized CNTs/polyurethane by seven orders of magnitude.Although the simultaneous production of the inevitable pores will affect the thermal conductivity, such highly conductive composite parts (functionalized CNTs/polyamide-12) and porous hybrid composite parts (functionalized CNTs/polyurethane) have very great application potential in the fields of aerospace, electronic devices, and automobiles.
For SLA, Eng et al. [72] studied the improvement of mechanical properties of 3D printing components by CNTs on photosensitive resin developed from visible light cured acrylate and epoxy resin-based oligomers as raw materials.In this study, by combining an improved CNTs dispersion process and a thermal curing post-treatment method at the same time, the mechanical properties of the 3D components were significantly improved.The tensile strength and Young's modulus were approximately 48 and 885 MPa, respectively.Although the elongation is still lower than the standard for injection molded parts, this enhancement is already very rare for SLA processes that are not good at mechanical properties, and for thermoset materials, they are often more brittle than thermoplastic materials.Sandoval's research group [73] dispersed MWCNTs into epoxy resin by shearing, ultrasonic, and mechanical stirring, successively and used SLA technology to prepare complex 3D components, and evaluated the components through mechanical performance tests.The results show that only need a small amount of MWCNTs can have a significant impact on the physical properties of the polymer resin.Compared with the control group without MWCNTs, when the concentration of MWCNTs is only 0.05 wt%, the ultimate tensile stress and fracture stress of the nanocomposite are increased by 17 and 37%, respectively.Due to the advantages of SLA based 3D printing technology, such as high forming accuracy, fast curing, high forming quality, and low requirements for nozzle, more and more research is being conducted on the preparation of intelligent and functional materials with this technology.Mu et al. [74] used digital light processing type printers to prepare and study conductive polymer nanocomposites.The printing ink is a mixture of acrylic-based light-curing resin and MWCNTs.N,N-Dimethylformamide (DMF) and polyethylene glycol octylphenyl ether are used as the dispersion solvent for MWCNTs.The results show that when the conductive penetration threshold is only 0.1%, and when the conductivity and printability are considered at the same time, the layer thickness in the optimal printing parameters can be controlled to 19.05 μm, and the curing time is only 40 s.In addition, based on its electrical conductivity, the prepared MWCNT nanocomposite can be used as a smart material with strain sensitivity and shape memory effect.And mechanical performance experiments show that the addition of MWCNTs can slightly increase the modulus and ultimate tensile stress, while accompanied by a small decrease in ultimate elongation at break, which indicates that the new functions of the prepared MWCNT nanocomposite are not obtained at the expense of mechanical properties.
As shown in Figure 4f, Chizari et al. [75] used solvent cast-3D printing technology and used a mixed solution of CNTs and PLA as printing ink to prepare a stent structure for liquid sensors.The printing ink is prepared by dissolving PLA in dichloromethane to obtain a dichloromethane solution of PLA, and then uniformly dispersing MWCNTs into the dichloromethane solution of PLA by a ball milling method, and the resulting CNT/PLA dichloromethane solution is at room temperature dry for 24 h, and finally dissolve the dried CNT/dichloromethane composite material in a certain amount of dichloromethane.Test studies have shown that when the concentration of CNTs is 2 wt%, the conductivity can reach 45 S/m, which is sufficient for the study of the sensitivity of liquid sensors.However, in the actual application of low-voltage state, composite materials usually have high conductivity, which requires the content of CNTs in composite materials to be relatively high.Compared with several other 3D printing technologies, Solvent cast-3D printing technology is easier to obtain high conductivity.Composite material filled with particle content.The author uses ball milling to increase the content of CNTs in PLA/CNTs to 40 wt%.Considering printability, mechanical properties, and conductivity at the same time, 30 wt% CNTs concentration is the best effect (Figure 4g).The conductivity can reach 2,350 S/m, which is much higher than the 100 S/m reported by Postiglione et al., and can be applied to the application of different kinds of electronic components.
6 Commercial application of polymer-based composites modified by carbon materials 6.1 Application in the field of energy storage Based on the excellent electrical conductivity and low thermal expansion coefficient of graphene, the researchers combined graphene with anode or cathode active materials to form 3D lithium-ion batteries or make GNP and sulfur copolymers into battery cathodes through 3D printing, and by using a supercapacitor composed of a thermally responsive ink mixed with GNP and Cu powder, to study the potential application of the high conductivity and low resistance of the composite material with GNP in lithium-ion batteries and supercapacitors.Shen et al. [76] mixed sublimated sulfur and GO solutions to prepare concentrated ink, and then added 1, 3-diisopropenylbenzene to it, and the sulfur copolymer-graphene structure (3DP-pSG) with periodic microlattices was formed by FDM-based 3D printing technology.The study found that the structure has a high reversible capacity of 812.8 mAh g and good cycle performance.Rocha et al. [77] referred to the water-based thermal response formula to mix Cu powder, chemically modified graphene and Pluronic F127 (thermally responsive ink).The electrode was solidified by freeze-drying and the GO was reduced by heat treatment, and an RGO/Cu 3D printed electrode with an interlocking interface was obtained.The test found that the Nyquist pattern generated by the RGO/Cu 3D printed electrode has the same shape as the ideal supercapacitor, indicating that the contact between the RGO electrode and the copper electrode is good.When the content of graphene in polymer composites is high, graphene can form a more compact microstructure and a well-connected conductive network.For example, Huang et al. [78] prepared graphene (200 mg/mL) inks that can be used for printing, and prepared a self-supporting graphene scaffold through FDM-based 3D printing technology.Through compression experiments, it was found that as the compressive strain increases, the resistance change of the 3D printing material with 50 wt% graphene content is slower than that of the composite material with 25 or 37 wt% graphene content.Vernardou et al. [79] used commercially available PLA-based conductive graphene as a raw material, and fabricated graphene pyramids layer by layer through a dual-extrusion FDM 3D printer.A graphene pyramid with a height of 5 mm can be used as an active material, and its current does not change significantly after 1,000 cycles; after charge and discharge analysis, the graphene pyramid's initial discharge specific capacity is 265 mAh/g, after 1,000 cycles the electrical performance after scanning is equivalent to that of the actual lithium battery (Figure 5a).
As shown in Figure 5b, Qi et al. [80] proposed a polyamide/barium carbonate/CNT ternary nanocomposite suitable for piezoelectric devices.In this study, due to the shear-free and free-flowing characteristics of SLS technology, a three-dimensionally isolated permeable network was constructed in polyamide/barium carbonate/CNTs.The results show that the polyamide/barium carbonate nanocomposite powder coated with CNTs has higher laser absorption characteristics and a wider sintering window than the traditional polyamide/barium carbonate composite powder.Its structure is to randomly distribute CNTs in the powder particles, so that the composite powder has better sinterability.In the polyamide/ barium carbonate/CNT module, a 3D isolated conduction path is effectively constructed, and its permeation threshold is only 0.3%.The 3D isolated percolation network greatly enhances the dielectric constant of the polyamide/ barium carbonate/CNT ternary nanocomposite, covering the entire frequency range, because it is formed in the polyamide/barium carbonate/CNT module a large number of microcapacitors.More importantly, the value of the piezoelectric strain coefficient d33 of polyamide/barium carbonate/CNTs is increased to 2.1 Pc/N (Figure 5c).Therefore, the use of SLS technology to construct a 3D isolation percolation network in piezoelectric materials can effectively improve the performance of piezoelectric polymers, which will be an effective means to improve the performance of energy harvesting and energy storage devices.
Research progress of carbon materials in 3D printing technology  1203 6.2 Application in the field of biomedicine Many researchers use 3D printing technology to make biocompatible polymer materials and graphene into hydrogels, 3D scaffolds, catheters, etc., and through in vitro experiments to simulate its impact on bone tissue, nerve tissue, and cell proliferation, it was found that low-content graphene is non-toxic to biological cells [81].Similarly, GO has good biocompatibility, can enhance the stiffness of biomaterials, and promote cell proliferation, providing a good prospect for tissue engineering and biomedical research [66].
The development of 3D printing technology for manufacturing hydrogel structures has made mass production of engineered cartilage tissue possible.Cheng et al. [82] connected the bio 3D printing micro-injection system to the biopolymer reservoir and used chondrocytes to inoculate GO/chitosan hydrogel.The study found that compared with pure hydrogel, 3D printed GO/hydrogel tissue will have thicker new cartilage four weeks after transplantation into cartilage tissue.Wang et al. [83] used NaOH solution to process 3D printed poly(ε-caprolactone) (PCL) scaffolds with low-concentration graphene added, and found that the scaffolds treated with NaOH had higher biocompatibility with cells.Misra et al. [84] used a homogeneous mixing method to prepare the GNP/PCL complex, and used FDM-based 3D printing technology to form a GNP/PCL composite 3D scaffold.After co-cultivating the scaffold and cells for a certain period of time, it was found that the scaffold was harmless to the growing human umbilical vein endothelial cell population.As shown in Figure 5d, they also placed the 3D printed GNP/PCL stent in the coronary arteries of pigs to observe the compatibility of the two, which provides the feasibility of using 3D printed GNP/PCL stents for personalized intervention of coronary heart disease.Shuai et al. [85] incorporated GO as an additive into PVA, and the two were combined through strong hydrogen bonds, and then used SLS technology to fabricate GO/PVA scaffolds.The scaffold has an open, uniform, and interconnected porous structure, and irregularly shaped pores with a diameter of less than 150 μm are randomly distributed on the surface of the pillars.When the amount of GO was 2.5 wt%, the compressive strength, Young's modulus and tensile strength of the stent increased by 60, 152, and 69%, respectively.With the increase of the amount of GO, GO agglomerates, and stacks, resulting in the deterioration of the mechanical properties of the stent.After culturing MG-63 cells on the surface of the scaffold with 2.5 wt% GO for a period of time, it was found that the cells were tightly anchored on the surface of the scaffold and almost completely covered the entire surface, while the cell coverage of the scaffold without graphene was only about 30%.Sayyar et al. [86] incorporated chemically converted graphene into chitinated chitosan (ChiMA) hydrogels, and obtained a cell-compatible matrix with enhanced mechanical properties and cell adhesion characteristics, it is then processed into a stent through FDM-based printing technology.Culturing fiber cells on chitosan and graphene/ chitosan composite film respectively found that compared with pure chitosan film, fiber cells achieved good adhesion and proliferation on the graphene/chitosan composite film, indicating that the addition of graphene changed the adhesion of chitosan.Sayyar et al. [87] mixed a photoinitiator, ethylene carbonate, polytrimethylene carbonate (PTMC), and graphene in a DMF solution, and then used the FDM method to 3D print the PTMC/graphene scaffold under nitrogen pressure.As shown in Figure 5e, cells were cultured on 3D printed PTMC/graphene-3 wt% scaffolds and PTMC scaffolds, and there was no significant difference in the DNA content of the cells on the two scaffolds, indicating that the addition of graphene had no significant effect on the number of cells.Therefore, PTMC/graphene composites with low graphene content have important application prospects in the field of biomedical materials, especially in the development of new conductive scaffolds for tissue engineering.
Graphene nanotechnology has great potential for peripheral nerve repair in clinical applications.In the 3D printed nerve conduit, the graphene nanoparticles are distributed very uniformly, ensuring the ideal electrical conductivity for peripheral nerve regeneration.As shown in Figure 5f, Qian et al. [88] mixed single-layer graphene or multilayer graphene with PCL, and fabricated multilayer graphene/PCL nerve conduits using FDM-based 3D printing technology.Studies have shown that 3D printed nerve conduits help functional sciatic nerve recovery and axon regeneration; 18 weeks after surgery, it was observed that the nerve conduits were softer than when implanted, and with the slow degradation of PCL substrates, graphene did not Causes toxic effects on the target.In addition, Bustillos et al. [89] fabricated PLA and graphene-reinforced PLA/graphene composites through 3D FDM printing, and found that compared to pure polylactic acid materials, the creep and wear resistance of PLA/graphene composites have been significantly improved.The wear resistance has increased by 14%, the creep displacement has been reduced by about 20.5%, and the nano-hardness has increased by 18%.This shows that it is feasible to use 3D printed PLA/ graphene as an orthopedic scaffold.
Lee et al. [90] applied polymer composites containing CNTs in the field of neurotherapy by using SLA-based 3D printing technology.Polyethylene glycol diacrylate and amine-functionalized MWCNTs were used as the raw materials of SLA-based 3D printing technology, are used to prepare nerve scaffolds with controllable voids and complex structures, and the enhancement of electrical properties and nano-characteristics of the scaffold surface has been studied.The main research results show that compared with the scaffold without MWCNTs, the scaffold containing MWCNTs can promote the proliferation of neural stem cells and the differentiation of early neurons (Figure 5g).In addition, the addition of MWCNT makes the nerve scaffolds have conductivity.Experiments have confirmed that by cooperating with corresponding electrical stimulation, it will have a synergistic effect on promoting nerve regeneration in the application field of nerve regeneration therapy (Figure 5h).
In this review, first, the principles and characteristics of several representative 3D printing molding processes, such as FDM, SLS, SLA, and solvent cast-3D printing, are introduced.Subsequently, the application of 3D printing technology in the field of CF, graphene, and CNT-doped polymer composite molding was reviewed.And the application progress of 3D nano-carbon material-polymer composite materials in the field of commercialization.

Figure 2 :
Figure 2: (a) Illustration of the rake spreading a powder layer in the build chamber.Reproduced with permission from ref. [43], Copyright 2016 Elsevier.(b) Variations of the flexural strength and flexural modulus of the CF/PA SLS parts with the carbon fiber content.Reproduced with permission from ref. [45], Copyright 2011 Elsevier.(c) Schematic of heat-channel in the matrix.(d) The thermal conductivity in the matrix of type A (the blue line) and type B (the red line).Reproduced with permission from ref. [46], Copyright 2018 Elsevier.(e) Schematic of 3D-printed CF-reinforced composite by FDM.Reproduced with permission from ref. [50], Copyright 2014 Elsevier.

Figure 3 :
Figure 3: (a) Tensile strength and Young's modulus curves of nanocomposites containing different concentrations of GO (0.4, 0.8, and 1.6 wt%).Reproduced with permission from ref. [54], Copyright 2018 Elsevier.(b) Tensile strength as a function of GO loading and temperature.Reproduced with permission from ref. [33], Copyright 2017 American Chemical Society.(c) TPU/PLA/GO nanocomposites filament preparation and FDM printing process.Reproduced with permission from ref. [58], Copyright 2017 American Chemical Society.(d) Schematic of 3D printed laser-flash diffusivity analysis (LFA) specimens (type A: printing direction perpendicular to through-plane direction, type B: printing direction parallel to through-plane direction); and thermal conductivity of PA12 and PA12/GNPs as measured on type A, type B, and compression molded specimens.Reproduced with permission from ref. [59], Copyright 2017 Wiley-VCH.

Figure 4 :
Figure 4: (a) Total electromagnetic interference shielding effectiveness of ABS carbon-based composites in three different layer-by-layer growing directions: perpendicular (upper graph), horizontal concentric (middle graph), and horizontal alternate (bottom graph).Reproduced with permission from ref. [69], Copyright 2018 Elsevier.(b) Volume electrical conductivity as a function of MWCNT concentration.The inset shows the log-log plot of the electrical conductivity versus the volume fraction of MWCNTs.(c) SEM images of 3D printed MWCNT-based nanocomposite woven structure and used as conductive element in a simple electrical circuit.Reproduced with permission from ref. [70], Copyright 2015 Elsevier.(d) The electrical resistance variation with the bending cycles.The inset shows the bending process.(e)The electrical resistance variation for the composites at a maximum strain of 10% with the stretching cycles.Reproduced with permission from ref.[71],Copyright 2017 Wiley-VCH.(f) Schematic of CNT/PLA composites fabricated via ball mill mixing method, and schematics of 3D printing method used for fabrication of CNT/PLA scaffold structures, and SEM image of a scaffold printed in two layers using the 3D printing method.(g) Liquid sensitivity of CNT/PLA scaffolds (circle) and electrical conductivity (square) of bulk CNT/PLA composites as a function of CNTs concentration.Reproduced with permission from ref.[75],Copyright 2016 Wiley-VCH.

Figure 5 :
Figure 5: (a) Chronopotentiometric curves for 3D printed graphene pyramids with 5.0 mm height under specific current of 40 mA/g, potential ranging −0.5 to −1.0 V and scan numbers 1 and 1,000.Specific discharge capacity of the same sample as a function with scan numbers as inset.Reproduced with permission from ref. [79], Copyright 2017 Springer.(b) Schematic of the SLS process.(c) Piezoelectric strain coefficient d33 of the BT60, PBCNCs, and PBCNCs-3D.Reproduced with permission from ref. [80], Copyright 2018 Elsevier.(d) Computed tomographic scanning of pig heart with deployed 3D-printed PCL-GR stent.Reproduced with permission from ref. [84], Copyright 2017 Wiley-VCH.(e) Actin cytoskeleton (phalloidin) staining of mesenchymal stem cells (MSCs) on 3D printed PTMC-CCG 0.5% and PTMC scaffolds after 3 days of culture in maintenance medium.Scale bars represent 200 μm.Reproduced with permission from ref. [87], Copyright 2016 American Chemical Society.(f) Schematic of graphene nerve conduit fabrication with layer-by-layer casting (LBLC) method.Reproduced with permission from ref. [88], Copyright 2018 Nature Publishing Group.(g) Enhanced neural stem cell proliferation in 3D printed scaffolds with or without MWCNTs after 7 days of culture.(h) Fluorescent micrographs of green fluorescent protein (GFP)-transduced neural stem cells (NSCs) with different electrical stimulation parameters after 4 day of culture.Scale bar = 100 μm.Reproduced with permission from ref. [90], Copyright 2018 IOP Publishing Group.