Additive manufacturing of polymer-based structures by extrusion technologies


 Extrusion-based additive manufacturing (AM) enables the fabrication of three-dimensional structures with intricate cellular architectures where the material is selectively dispensed through a nozzle or orifice in a layer-by-layer fashion at the macro-, meso-, and micro-scale. Polymers and their composites are one of the most widely used materials and are of great interest in the field of AM due to their vast potential for various applications, especially for the medical, military, aerospace, and automotive industries. Because architected polymer-based structures impart remarkably improved material properties such as low density and high mechanical performance compared to their bulk counterparts, this review focuses particularly on the development of such objects by extrusion-based AM intended for structural applications. This review introduces the extrusion-based AM techniques followed by a discussion on the wide variety of materials used for extrusion printing, various architected structures, and their mechanical properties. Notable advances in newly developed polymer and composite materials and their potential applications are summarized. Finally, perspectives and insights into future research of extrusion-based AM on developing high-performance ultra-light materials using polymers and their composite materials are discussed.


INTRODUCTION
Continuous adaptation and optimization of natural structures under evolutionary pressure and/or environmental conditions have led to the development of unique architectures [1,2], such as bone [3], seashells [4], bird beaks [5], shark teeth [6], and many more [7,8], which are known for their complexity yet exceptional structural and mechanical performance. These architectures have a high load-bearing capacity and offer the advantage of being lightweight, thereby, facilitating their mobility and structural purpose [9]. For example, nacre shells have a unique microstructure consisting of CaCO 3 with organic molecules [10][11][12][13]. Such a microstructure gives rise to enhanced mechanical properties including great strength with low density and thus, nacre is known to be nature's toughest material [13,14]. Because the microstructure provides the ability to dissipate energy upon loading, the shell can protect its soft tissues against parasites and damaging factors such as high underwater pressure [4]. While researchers have pushed for significant advances in the replication of these natural architectures, conventional fabrication processes (i.e. standard extruding, molding and casting) are limited in terms of architectural design [15] and thus their potential is not fully realized [9]. Fortunately, rapid innovation and enhanced mastery of additive manufacturing (AM) techniques, also commonly known as 3D printing, have begun to address these challenges through their high degree of control and flexibility in designing highresolution complex architectures for structural applications. One prevalently used structural material is polymer, which has found manifold applications in the automotive [16], aerospace [17], biomedical [17], and electronics [18] industries. Typical AM techniques used for polymers include binder jetting, sheet lamination, powder bed fusion, direct ink writing (DIW) and fused filament fabrication (FFF) [15]. These techniques can be categorized into two sub-groups: non-extrusion and extrusionbased printing. In comparison to extrusion-based techniques, non-extrusion-based techniques are limited by the lack of material diversity, cost and challenges in the customization of the printing processes. Therefore, extrusion-based printing offers exciting possibilities for advances in materials and their architectures, particularly for structural purposes.
Mechanical properties desired in these structural applications, including tensile strength, flexural strength, impact resistance and toughness, can be acquired through innovative and complex structural designs via 3D printing [19]. The complexity of these structures arises from several geometric characteristics (e.g. hexagonal cells [20], triangular honeycomb [21], octet [22], as well as ink and filament compositions (e.g. carbon fibers [23], platelets [24], and influences the microstructure and stress distribution of the final print. The inclusion of reinforcement materials in the polymer matrix also renders superior mechanical and structural properties to the three-dimensional (3D) printed structures with added functionalities [25]. This review explores the recent progress in extrusion-based AM techniques for polymers and their composites, with a particular focus on structural design and ink/filament composition to improve mechanical performance. Further demonstrated is the ability of AM extrusion-based techniques to fine-tune material properties and enhance their novel structural purposes.

EXTRUSION-BASED AM
Extrusion-based AM is a computer-controlled layer-by-layer deposition of molten and semi-molten polymers, pastes, solutions, and dispersions through a movable nozzle or orifice serving as the extrusion print head [15]. The most common extrusion-based printing techniques are FFF and DIW [15]. These techniques have gained significant attention primarily due to their low cost, rapid manufacturing with limited waste, and printing of complex 3D architectures [26,27].

Fused filament fabrication (FFF)
FFF is a melt extrusion-based method that locally melts solid filaments through a nozzle to fabricate layer-by-layer complex architectures from Computer-Aided Design (CAD) or other modeling softwares [26]. Typically, the automated nozzle patterns the melted filament in the x-y plane (Fig. 1). As each subsequent layer is printed, the build platform moves down incrementally in the z-direction. FFF typically uses thermoplastics as this material can be melt-extruded at or above the glass transition temperature through a heated extruder, and then fused upon contact with the subsequent layer (reversible curing). This property also allows the material to be remolded and recycled [29]. FFF printability requires that the filament have sufficient strength to prevent buckling between the feed pinch roller and the heated liquefier. Additionally, they also need to be flexible enough to enable their spooling and despoiling for printing. Generally, FFF prints display rough surface finishes that can be mended with post-processing such as polishing and sanding. Besides, machine and printing parameters, including the resolution, size, nozzle diameter and printing rate, can affect and determine the final printed structure (Table 1). FFF allows for high resolution at 100-150 mm for the x-y layer with a typical layer thickness of 100-200 mm, indicating the z-resolution. The larger the diameter of the nozzle, the more material can be printed at a time; therefore, the use of a bigger nozzle can decrease the build time, but it compromises the precision and resolution [30]. The minimum feature size generated is $250 mm 9 and typically, FFF operates at the maximum printing speed of 50 mm 3 /s [9,29].

Direct ink writing (DIW)
In contrast to FFF, DIW offers higher versatility as it can be used to print thermosets and thermoplastics as well as multimaterial architectures encompassing polymers, metals, ceramics, and cements [19]. The gel-or liquid-phase ink is driven through a nozzle and then patterned into stacked layers to create a controlled, desired architecture [24] as can be seen in Fig. 2. This process can be customized to occur in a vacuum, under an inert gas such as argon, or under ambient conditions. Solidification occurs once the ink exits the nozzle due to its rheological modifications that allow for rapid dynamic recovery of its yield modulus and shape. Once the structure recovers, the mechanical strength should be high enough so that there is adequate support for the structure and its overhanging components [29]. The structure can also be solidified via an intermediary or external solidification process between printed layers, such as solvent evaporation, gelation or UV curing [27]. Because DIW is compatible with a broad range of ink viscosities, even highly viscous inks can be processed. For example, poly(tetrafluoroethylene) in the viscosity range of 10 6 -10 7 mPaÁs cannot be printable by FFF; however, DIW is successfully able to print this viscous polymeric ink [32]. DIW printability requires that the ink exhibits yield-stress shear thinning behavior; this criterion eliminates the need to apply high internal pressures for the ink to flow. Furthermore, the ink must exhibit appropriate rheological properties, including apparent viscosity, and storage and loss modulus. Suitable ink viscosity for DIW ranges over several orders of magnitude (10 2 -10 6 ) mPaÁs at a shear rate of %0.1 s À1 [33]. Because of the diversity in DIW inks, the nozzle size can range from several micrometers to centimeters. As for further machine and printing characteristics, DIW prints quasi-3D structures with a high resolution of 100-1200 mm for the x-y layer and 100-400 mm for the z-resolution, or layer thickness [29] ( Table 1). The typical minimum feature size that can be printed is $500 mm 9 . DIW operates at a slightly faster maximum production rate of 150 mm 3 s À1 than that of FFF [9].

POLYMERIC MATERIALS FOR EXTRUSION-BASED AM
Owing to intrinsic lightweight, low cost, high modulus-toweight ratio and corrosion-resistant properties as well as potential electrical and mechanical characteristics, polymers have been widely and successfully used for both FFF and DIW [34]. Among the various thermoplastics used for FFF, the most recognized include acrylonitrile-butadiene-styrene copolymers (ABS), polylactide (PLA), polycarbonate (PC) and polyamides (PA) with tensile strengths in the range 20-60 MPa. Thermoplastics can be categorized as amorphous or semi-crystalline; ABS and PC are considered amorphous, while PLA and PA are semi-crystalline. These thermoplastics are most commonly used for their low cost, biocompatibility, reliability and potential scalability. More specifically, PLA is an optimal choice due to its low coefficient of thermal expansion and low printing temperature. Also, PLA has higher tensile strength and better surface finishing compared to ABS [29,35]. For DIW, in addition to the listed thermoplastics, increased diversity of polymeric materials is  printable, such as thermosets like epoxy (e.g. bisphenol-F epoxy $2.0 GPa) that are typically stronger than thermoplastics due to their highly cross-linked structure [23]. The ability for DIW to process more diverse materials than FFF is due to fact that its printability is primarily dependent on the rheological properties of the ink rather than the material itself. While low-cost thermoplastics have their advantages, they are known to have performance deficiencies (e.g. PLA has a poor heat resistance and low ability to deflect heat) that can have an adverse effect on the final printed structure and its efficiency. Therefore, polymers with stronger mechanical properties are desired for FFF and DIW. To meet this need, high-performance thermoplastics, such as polyetheretherketone (PEEK) and polytetrafluoroethylene, as well as high-performance thermosets, such as bismaleimide (BMI), can be used; however, they generally come with the major disadvantage of higher cost than typical commercial polymers, making it difficult to scale-up production to meet industrial demands. FFF and DIW allow for the printing and transformation of low-cost and/or lowperforming thermoplastic and thermoset materials into composite polymers through the use of multiple nozzles or a priori infusion of reinforcements into the polymer matrix. These composite materials offer tunable mechanical properties comparable and/or superior to that of high-performing thermoplastics. One major consideration for composites is their need for exceptional interfacial adhesion and undisturbed polymer entanglement to avoid unwanted porosity, which can undermine the mechanical properties of the material. However, in the case of polymer structures, it is interesting to note that the porosity can have a positive effect on their mechanical performance when cells are intentionally incorporated into the final printed structural geometry [36]. Therefore, simply by printing complex architectures, a remarkable mechanical response can be achieved which would otherwise be inaccessible for simple block structures made from the same material. This highlights the importance of considering both the geometry and ink/filament composition for AM printing.

FFF of polymeric structures
FFF is the most widely used method among all the AM techniques for industrially fabricating thermoplastic polymerbased components with low cost, material versatility, and minimal waste [15]. By incorporating complex, nature-inspired architectures in FFF fabrication, the mechanical performance of commercially produced polymers can be drastically improved (e.g. load-bearing capacity, specific strength, impact resistance).
Schwarzite structures have positive and negative curvature topologies with periodic minimal surfaces, tunable porous size and shape, and possess fascinating mechanical and electrical properties [37]. Sajadi et al. [36] developed schwarzite structures using FFF 3D printing technology and investigated the mechanical behavior of two types of schwarzites (primitive and gyroid) based on atomistic models and finite element simulations ( Fig. 3a and b). They observed, in both experiment and simulation, a unique layered deformation mechanism that emerges in these architectures during compression loading. As the load was applied (pressing from top), initial structural voids/holes started closing from the topmost layer and progressed into subsequent layers. This allowed the structure to densify at higher loads beyond the plastic regime. These structures could find potential use in a wide variety of areas such as automotive, civil, biomedical, and space applications.
Tiwary et al. [4] developed seashell-inspired pure polymeric architectures to study how its complex shape withstands extraordinary water pressures at the bottom of the sea while protecting its soft body (Fig. 3c). Mechanical testing determined whether the structural shape is the dominant factor that contributes to the superior mechanical response of seashells. It was shown that structural complexity aided in stress distribution and concentration at favorable locations within the structure and thus, enhanced the safety of the living species inside the seashell. The 3D printed biomimetic polymer structures showed better mechanical load-carrying capacity compared to natural seashell structures.
First theorized by Ray Baughman [40] in 1993, tubulanes are porous 3D structures of cross-linked carbon nanotubes projected to have incredible mechanical strength and Young's moduli (YM) comparable to that of the diamond. However, the production of these materials has been challenging becuase of the difficulties in manufacturing long chains of carbon nanotubes. Sajadi et al. [38] used the FFF technique to 3D print polymeric versions of tubulanes that exhibited similar behavior to their carbon nanotube antecedents ( Fig. 3d and e). They investigated the mechanical (compressive stress and ballistic impact) behavior of these scaleindependent lightweight structures. For testing the impactresistant properties, bullets traveling at 5.8 km s À1 were fired into the polymeric tubulanes blocks as well as solid blocks made of the same material. Upon comparison, the tubulanes structures could stop the force of the projectiles, while their solid counterparts were crushed by the same ballistic force. The tubulanes demonstrated lamellar deformation attributed to its porous design and flexible matrix across which stress can be distributed and dissipated, resulting in a stronger architecture.
Another complex design with engineered superior mechanical performance is a lightweight sandwich structure. These structures are composed of a thick core made of mechanical metamaterials and two thin solid face-sheets with high flexural stiffness at the top and bottom surfaces exhibit high stiffnessto-weight ratio and a high energy absorption capability. Yazdani Sarvestani et al. [22] studied the structural responses, failure mechanisms and multi-hit and energy absorption capabilities of FFF architected sandwich structures with different core topologies (e.g. cubic, octet and Isomax). They demonstrated that the core topology and geometric parameters of the meta-sandwich structures were critical to their failure mechanism and energy absorption capability. For example, Isomax, octet, and cubic meta-sandwich structures had higher energy absorption than the auxetic core for low impact energy, while, for higher impact energy, octet meta-sandwich structures demonstrated better performance.
Sajadi et al. [39] implemented the 'Boxception' concept commonly used in packaging to design a lightweight, high load-bearing and impact-resistant structures from simple elastic polymers (Fig. 3f). Boxception is an architecture where a box is embedded within another box (hollowed by orthogonal right cylinders) and this configuration is repeated from very small to large size boxes interconnected at their edges. Such a design allows the outer boxes to act as effective shielding members to the inner boxes from impact forces and damage. They also demonstrated that by adjusting the number of repetitive cubes, arranging their other and modifying the material, the mechanical properties can be tailored. The complex topology facilitated the dispersion of the applied force into many smaller components, thus lowering the accumulation of local stresses in certain regions of the structure and minimizing large-scale damage.
Bates et al. [41] created single-density, thermoplastic polyurethane honeycomb cellular structures capable of withstanding repeated compressions to full densification without degradation. The stress-strain profile of the printed hexagonal structures had three deformation regions: linear elasticity, long and flat plateau, and densification. At small compressive strains, the behavior is linear as the cell walls of the structure experienced simple bending. As deformation progressed, the cell walls began to buckle which produced the plateau region, and finally, the opposing cell walls came into contact, leading to densification. At densification, the stiffness of the structure increased steeply. The characteristic long, flat plateau region is particularly desired for energy-absorbing structures and as such, peak energy-absorbing efficiencies of 0.36 were realized.
Basgul et al. [42], created a 3D printed lumbar fusion cage using PEEK. Several attempts were made previously to print the cage structures via FFF out of low-temperature polymers, such as PC and ABS, which inherently offer limited mechanical properties. However, PEEK is a biocompatible polymer with excellent mechanical properties and has been used for intervertebral lumbar cages since the 1990s. Although FFF has been shown to be feasible for PEEK [43], previous research has not focused on specific clinical applications with structural load-bearing demands. The FFF cages exhibited a compressive strength of 8 kN which satisfies the load expected for cages under in vivo loading. Furthermore, the compressive, shear, and torsional strength were above the defined axial compressive shear and rotation limits for such applications. In addition to the mechanical evaluation of the printed structures, the authors investigated the optimum printing speed for PEEK cages. By simply changing the printing speed, they were able to modulate the porosity in the printed construct (from 2 to 4% for slower speeds to 20% for fastest speed).

FFF of polymeric composite structures
The mechanical properties of polymeric structures can be modified by the addition of reinforcements. In particular, carbon- fiber reinforcements have gained significant interest due to their high strength, stiffness and recyclability [26,44]. The improved mechanical performance adds to their versatility in structural applications such as automotive, aerospace and surgical components [26]. Since standard composite materials production (i.e. casting, molding, and autoclaves) are expensive, time-consuming and design limited, FFF is of particular interest for producing these composites [26].
Ning et al. [45] reported a comprehensive study on the mechanical properties of carbon fiber-reinforced ABS (CF/ABS) using varying carbon fiber concentrations (0, 3, 5, 7.5, 10 and 15 wt.%). Generally, FFF printing of composites limits the homogeneous distribution of reinforcing materials and results in incomplete removal of the voids [26]. Scientists addressed this issue by extruding CF/ABS filaments, then cutting and reextruding the filaments. This not only increased the homogeneity of the carbon fiber in the polymer matrix but also resulted in higher bulk density. An increase in carbon fiber content was tied to the enhancement of tensile strength and modulus; however, a peak in these properties was observed for 5-7.5 wt.% carbon fibers. CF/ABS composite also exhibited superior flexural properties in comparison to pure ABS. However, toughness, yield strength and ductility were observed to exhibit a negative trend with respect to increasing carbon fiber content. This trend was attributed to the increase in porosity and fiber pullouts ( Fig. 4a and b) at higher carbon fiber reinforcement content. Nevertheless, FFF printed composites offer a promising alternative to standard polymers.
Typically, FFF printing of composites requires a predetermination of the resin-fiber combination ratio or the utilization of two printer heads. However, this produces composites filled with short fibers or particles that are mechanically inferior to composites reinforced with continuous fibers. Matsuzaki et al. [46] printed continuous carbon fiber-reinforced thermoplastics (CFRTPs) and jute fiber-reinforced thermoplastic (JFRTP) green composites via optimization of fiber alignment (Fig. 4c). Contrasting trends were observed for the stress-strain relationship for both composite types: CFRTPs exhibited a linear stressstrain curve before fracture, while JFRTP showed a nonlinear behavior. Both samples were found to have voids and fiber pullout as a result of the lack of adhesion between the fibers and thermoplastic resin. The continuous fiber composites fabricated via FFF [26] in this study were reported to have superior strength and YM, approximately twice that of composites developed by traditional manufacturing techniques (Fig. 4d).
Carbon fiber as a reinforcement material was further investigated in the study conducted by Dickson et al. [48], where the relative performance of FFF-printed continuous carbon, glass and Kevlar fiber-reinforced nylon composites was evaluated. Based on both tension and flexural tests, it was observed that CFRTP composites, printed in a concentric pattern (spiralshaped framework), demonstrated the largest mechanical strength per fiber volume, followed by glass and then Kevlar fiber-reinforced composites. CFRTP samples exhibited 6.3 times higher tensile strength than that of bare polymer counterparts. However, the increasing volume fraction of these reinforcements led to greater porosity that resulted in weaker bonding between the fiber and nylon layers and thus, mechanically weaker structures. Dickson et al. also confirmed the speculation made by Matsuzaki et al. [46] that the maximum effective fiber content for composites is between 40% and 50%. This study highlighted the challenges faced by FFF composite printing in controlling the porosity of the structure at high fiber volumes.
As previously mentioned, composites run the risk of failure by mechanisms associated with high porosity due to air gaps between the polymer matrix and the reinforcing material. On account of these inherent defects, there have been limited functional materials and commercial applications for FFF. To reduce the formation of pores, researchers have begun to shift their focus toward the incorporation of nanoparticles and nanotubes.
Gardner et al. [44] demonstrates how the integration of FFF with continuous carbon nanotube (CNT) yarn-reinforced polyether imide (Ultem V R ) polymeric composite led to the fabrication of net-shaped, multifunctional load-bearing components. The addition of CNTs to polymers not only increases the mechanical strength of the structure but includes additional functionalities like enhanced electrical and thermal characteristics. A continuous compaction printing method allowed for improved adhesion between the CNT-filament layers and the final printed structures, which was conductive in all directions. This study introduces the potential of using FFF printed CNT yarnreinforced polymer structures for architectures (Fig. 4e) with specific mechanical and electrical functionalities.
Gnanasekaran et al. [49] fabricated CNT and graphene (G)based polybutylene terephthalate (PBT) composites using FFF and compared the mechanical properties and conductivity. While surface modifications of CNT and graphene increased its dispersibility within the polymer matrix, the conductivity decreased. The stiffness of PBT/CNT nanocomposites significantly exceeded the stiffness of PBT/G nanocomposites as the storage modulus for the CNT-based composite was 28% higher than its counterpart.
Biocompatible nanomaterials have also been investigated for bio-based structural applications. One such example is WS 2 nanotubes (WS 2 -NTs), a nontoxic semiconductor that can be dispersed easily in polymers as well as organic solvents. Shalom et al. [47] were the first to demonstrate the mechanical performance of biocompatible WS 2 -NTs in polylactic acid (PLA) and its dispersion through melt-extruded filaments ( Fig. 4f and   g). As PLA is a commonly implemented biocompatible and degradable material, the WS 2 -PLA nanocomposite was found to be biocompatible and biodegradable [50,51]. The addition of WS 2 nanotubes increased the elastic modulus by $20%, yield strength by $23% and the strain-at-failure by $35%. Dispersion of the nanotubes significantly increased from the pre-printed state of the filaments to the final fuse-deposited structure on the order of 10 3 . While a higher degree of crystallinity was found in solvent-cast PLA and PLA/WS 2 -NT, negligible effects on the crystallinity of the FFF architectures were observed.
Another biocompatible nanomaterial is montmorillonite which has been used in catalytic processes [52], as a rheology modifier [53], and found to intrinsically induce antitumor activity [54]. Natural fillers, when reinforced in a polymer matrix, are known to improve the mechanical performance of the polymers [55]. Weng et al. [56] found an increase in tensile strength, flexural strength, flexural modulus, storage modulus and elastic modulus with an increase of organically modified montmorillonite (OMMT) to acrylonitrile butadiene styrene (ABS) for the tested manufacturing techniques, FFF and injection molding. FFF printed structures had a 43% increase in the tensile strength with the addition of OMMT to the polymer matrix and resulted in a maximum elastic modulus of 3.6 GPa. However, maximum tensile strength was found to occur in the injection-molded nanoclay composites. The discrepancy between the strength of the injection molded and FFF architectures was attributed to air gaps between the two composite materials during FFF, while the high pressure of injection molding increased the density of the composite matrix and thus reduced void occurrences. While shrinkage has been often perceived as a major dilemma for 3D printed structures, linear thermal expansion ratio of postdeposition FFF architectures is another crucial factor that contributes to defects such as warping and changes in the dimensional accuracy of the final geometry. By increasing OMMT (5 wt.%), they decreased the composite's linear expansion ratio as the addition of well-distributed nanoclay particles restricts polymer chain movement.

DIW of polymeric structures
DIW prints a wide variety of materials with significantly different process mechnisms. There are three main aspects of DIW that alter the mechanical performance of a print: the machine properties, the ink composition, and the final 3D geometry. Clausen et al. [57] focused on the improvement of the mechanical properties of a silicone-based elastomer through the creation of novel 3D geometries (Fig. 5a). Most naturally occurring materials have a Poisson's ratio greater than or equal to zero. Negative Poisson's ratio (auxetic) materials have been artificially created for applications such as body armor and other shock-absorbing products (e.g. knee and elbow pads or running shoes). However, auxetic materials are currently limited to a small range of strains, thereby reducing their scalability. Using DIW and a nonlinear numerical model, Clausen et al. created a lattice of parameterized super-ellipses with constant width. Remarkably, the extreme shape of the auxetic super-ellipses allowed for a continuous print path typically not found in positive Poisson's ratio designs. These polydimethylsiloxane (PDMS)-based super-ellipses had a Poisson's ratio programmable between À0.8 and 0.8 and displayed a near-constant ratio with up to 20% deformation (Fig. 5 b-d).
One aspect of polymers conducive to DIW is the ability to manipulate the phase and rheology of the material through the addition of catalysts, inhibitors, and rheology modifiers. When Maguire et al. | 7 considering 3D printed thermoset polymers, the time to cure is just as crucial to the success of the print as the phase and viscosity of the ink. By increasing or decreasing curing time, structures like free-form prints or omnidirectional vasculature, respectively, become feasible. Robertson et al. [58] discovered novel synergies between ink composition and the subsequent curing process (Fig. 5e and f). The combination of this modified ink and controlled frontal polymerization in the printed fiber allowed for a reduction in the energy requirements for large manufactured components by a factor of 10. This advantage was also applied to other non-AM processes such as vacuumassisted resin transfer molding [61,62] (VARTM) for highdensity carbon fiber-infused composites. The modified ink was composed of dicyclopentadiene (DCPD) with thermally activated catalyst ruthenium, second-generation Grubbs' catalyst (GC2) and alkyl phosphite inhibitor. These additives extended the liquid processing window up to 30 hours and allowed the ink to slowly transform into a gel ideal for printing due to its shear-thinning effect and shape retention capacity (over the span of 18 hours). The curing process was initiated soon after the gel was extruded from the nozzle by a heated wire or ring, creating a curing front that results in the immediate formation of polydicyclopentadiene (pDCPD). Furthermore, this work demonstrated that simultaneous free-form prints are also feasible.
Wu et al. [63] also utilized ink adaptation to adjust the curing requirements of their final printed structure and increase its mechanical properties. They used BMI, a high-performance resin, and modified it for DIW using a two-part curing process: UV exposure and heat treatment. One major challenge with high-performance resins is post-heat treatment shrinkage. The shrinkage limits the potential applications of resins like BMI as the initial dimensions of the print are not maintained. Wu et al. demonstrated that, for most small complex geometries, a low shrinkage rate of 2-3% could be obtained for their ink when rheologically modified with 33.3 wt% N-vinyl-2-pyrrolidone (NVP). For more porous radial geometries, however, a slightly higher shrinkage rate was observed. Wu et al. also improved the printability of the high-performance resin. The use of NVP improved the dissolvability of the resin and its reactivity during photoinitiated polymerization. With increasing NVP concentration, Wu et al. found an increase in the tensile strength up to 97.2 MPa and a peak thermal stability of the print at 33.3 wt.% NVP. At higher concentrations, tensile properties decreased. This trend was noticeable for other properties including viscoelasticity, hardness and the glass transition temperature at slightly higher concentrations of NVP. Finally, it was determined that ink solutions with maximized mechanical performance were comparable to current industrial high-performance resins, and the two- step curing process of UV and heat treatment successfully produced proper adhesion between layers with no defects.
Taking a cue from nature, rapid improvements in mechanical structures and properties have propelled materials science forward by generations. As further inspiration is drawn from biological and natural phenomena, research on materials with nonhomogeneous or graded properties becomes feasible to model and understand. For instance, Qin et al. [59] modeled spider webs through the geometry of orbital webs to understand how material distribution and topological designs affected the mechanical properties (Fig. 5g and h). Webs of PDMS were printed with DIW, then spiral threads were coated in aqueous glue. The web strength was correlated to the load distribution on the radial and spiral threads. Concentration of the failure load could be altered by varying the thickness of the fibers. This phenomenon was observed in natural orbital spider webs in which the outer spiral silk threads are up to 30% thicker in diameter than silk threads near the center of the web. Qin et al. believed that the distribution of the radial and spiral threads homogenized stiffness across the web. While rupture occurred at the loading point for all samples, the force-displacement altered depending on the number of spiral threads. In nature, smaller orbital webs are used to catch smaller prey and support the weight of the spider. This smaller web design provides maximum strength for point loading. On the other hand, large orbital webs in nature are built to last longer and under prolonged distributed loads such as high-speed winds or rain. The longer lifespan of large webs is possible due to the significantly smaller diameter of the spiral threads compared to the diameter of the radial threads. Qin et al. showed that this diameter ratio between the spiral and radial threads yields higher strength. It also caused localized failures within the web that allow for easy repair. These biomimetic geometries have great potential in applications, such as biomedical scaffolding or composite reinforcement, in which stiffness is critical for operation.
A great deal of material innovation is inspired by the marvel of nature. Regarding 4D printing, botanical shape morphing systems are of particular interest. An example of these systems is the nastic motion response of plants to an environmental stimulus. This motion is possible due to local swelling of stiff cellulose fibrils. Gladman et al. [60] created programmable fabricated architectures inspired by nastic motion that alters its shape upon immersion in water (Fig. 5 i-n). This hydrogel composite was composed of stiff cellulose fibrils embedded in a soft acrylamide matrix. Since natural shape morphing systems are reversible, it was found that replacing N, N-dimethylacrylamide with N-isopropylacrylamide created a reversible system. Similar to Compton et al., shear-induced alignment of cellulose fibrils was deliberately applied during the printing process. A printed floral form with a bilayer lattice of 90 /0 configuration closed upon swelling, while À45 /45 print orientation resulted in a twisted structure. Inter-filament spacing promoted rapid absorption of water, transforming the prints in a matter of minutes. The next challenge in creating these shape-morphing systems was producing a numerical model to translate complex 3D surfaces into two-layer print paths that achieve the desired 3D shape. Gladman et al. successfully developed a mechanical model to determine the architecture of the following layers.

DIW of polymeric composites
Polymer-fiber composites are currently mass-manufactured by VARTM [62], compression molding [64,65] or fiber winding [66]. By adapting DIW for complex fiber-reinforced architecture, the cost of production can be greatly reduced. Lewicki et al. [23] constructed a DIW composite ink containing bisphenol-F epoxy resin modified with colloidal silica and discrete carbon fibers with a high aspect ratio ( Fig. 6a and b). The addition of colloidal silica induced a non-Newtonian fluid effect, increasing its viscoelasticity. It was demonstrated that the higher viscosity silicamodified resin increased drag forces between the fiber and the resin and allowed the fiber phase to be effectively carried and aligned as a contiguous component of the fluid. With this alignment method, Lewicki et al. achieved 8 vol% CF loading, although they hypothesized an increase in the CF loading limit may be feasible with varying nozzle geometries.
Compton et al. [24] also created an ink that induced drag forces between the resin and the CF and silica carbide (SiC) whisker fillers. Their epoxy-based composite enabled the alignment of the CF within the ink by DIW of a balsa wood-inspired cellular geometry (Fig. 6 c-e). Commercial epoxy 'Epon 826' resin was rheologically modified so that reinforcement fillers could be added. A significant factor in 3D printing resins is the shelf life of the ink as this limits the time that the ink may be used for printing. The imidazole-based ionic liquid extended the pot life of the ink to 30 days in ambient conditions and little aggregation or sedimentation was observed. Furthermore, the base ink had a viscosity of four orders of magnitude higher than pure resin at low shear rates ($0.1 s À1 ). During printing, Compton et al. found that the viscosity increased to $20 PaÁs, thereby, increasing the shape retention of the printed structure. The shear forces during extrusion aligned the fibers to such a degree that fibers longer than the inner diameter of the nozzle did not congest the nozzle during printing. Typically, high-loaded CF composites experience defects such as bubbling or debonding of the filament. By 3D printing the filament within the polymer composite, Compton et al. reported no substantial evidence of such defects. The final product was an aligned fiber epoxy cellular composite with a modulus nine times the magnitude of pure epoxy resin cast samples and thus was able to demonstrate that carbon fiber reinforced ink can dramatically increase the storage modulus, shear yield stress, and transverse strength of the original resin.
Another approach to align reinforcement fillers is through a modified DIW process known as 3D magnetic printing. In this manufacturing technique, 3D printed layers of polymercontaining magnetized platelets [64] and/or microparticles [69] are exposed layer by layer to a magnetic field to orient all particles in a layer. Particles can be selectively oriented into volumetric pixels (voxels) within each layer by exposure to the magnetic field through a photocurable mask. This process is then repeated until all particles have an assigned orientation within the polymer matrix before an additional layer is printed. Kokkinis et al. [64] produced two inks created from a base-ink of light-sensitive liquid resin modified with fumed silica. The researchers, in this work, created a multi-nozzle modified system that layered a shaping ink and textured ink to introduce shape-changing properties in their printed architectures and enabled the magnetic alignment of anisotropic particles ( Fig. 6f  and g). The creation of a gradient concentration of dopants was also possible with the static mixing of different ink formulas before extrusion. They used the interactions between capillary and elastic forces to prevent distortion by assessing the Laplace capillary pressure across the surface of a layered edge against the yield stress of the ink to create an edge deformation to an equilibrated radius and curvature. To demonstrate high specificity and control, soft cuboids were printed with programmed concavity to reduce its size. The shape was maintained via mechanical interlocking that occurred by printing bilayer walls of cross-linking polymers with different swelling behaviors. A second cuboid was printed with a mixture of soft and hard materials with an alignment of the particles in the soft polymer ink. This unique architecture exemplified the ability to control swelling and mechanical properties independently from shape-changing effects. Kokkonis et al. speculated that inks could reach up to 27% platelet concentration with the addition of mechanical vibrations to the fabrication process.
Another exciting avenue of biomimetic research is the imitation of mechanical gradients in multi-material interactions present in nature. One such example is the interaction of mussel on a hard rock which connects by the mussel byssus, a soft foot-like muscle. Kokkinis et al. [65] modeled and prototyped these multi-material mechanical gradients in two-and threedimensions to study its effect on interfacial failure, drawing inspiration from bone-tendon interactions and human intervertebral discs (Fig. 6h). They used photocurable methylacrylate/ acrylate monomers and created two fully miscible base resins containing $50% multifunctional polyurethane oligomers and colloidal silica rheology modifiers. While resin A exhibited high stiffness, its counterpart resin B was a soft polymer solution that exhibited lower stiffness. Similar to Wu et al. [63], printed resin structures were UV cured between each layer. The elastic modulus of the resins ranged from 319 MPa for resin A to 0.12 MPa for resin B. The strength of the materials was tuned between 19 MPa and 1.2 MPa while the stretchability was found to have a more comprehensive range (67-753%), than that of commercial resins [70]. Digital image correlation strain maps were also created for four 2D modeled samples consisting of a polymer matrix with a 'glass island' in the center of the material. These four samples were ascending gradient (increasing YM away from the glass island), descending gradient (decreasing modulus towards the glass island), nongraded (no change in elastic modulus throughout material), and soft layer (sudden decrease in elastic modulus outside of the glass island). Kokkonis et al. found that strain energy density effectively described the failure of the models and, while the non-graded and descending gradient models failed at lower strain values, both the ascending and soft layer models failed at a higher global strain. Interestingly, the soft layer and ascending models moved the peak strain away from the interface into the surrounding polymer matrix. The soft layer material, on the other hand, experienced extensive deformation around the glass island but failed within the polymer phase. By controlling the mechanical gradient of a material, Kokkonis et al. demonstrated that the failure location and strength of a material can be customized. Note that, all of the extrusion-printed materials discussed above have been summarized in tabular form in Table 2.

APPLICATIONS
FFF and DIW manufacturing have typically been used for prototyping and rapid manufacturing in industry. The ability of these AM techniques to produce complex architectures easily promotes rapid iterative testing and can reduce the net cost of the design-to-manufacturing process [71]. For FFF, the main advantage is its ability to use recycled plastic waste to create show an initial node rotation failure event (f), followed by damage propagation from that site in the form of elastic wall buckling and tensile fracture (g) [24]. The scale bars are 10 mm. (h-i) SEM images of the failure site show an imperfection in the cell wall, which may have led to the initial node rotation [24]. Copyright 2014, John Wiley and Sons. (j-k) 3D magnetic printing: Design (j) and 3D printed (k) magnetically aligned helicoidal platelet staircase within multi-material architecture [67].
Copyright 2015, Springer Nature. (l) Demonstration of mechanical gradient effect on failure mechanisms using a human intervertebral disc inspired architecture [68]. filaments for extrusion through customized FFF printers, such as recyclebots [72], which create their filaments before extrusion by shredding recycled thermoplastics. Figure 7 shows the hierarchical relationship between the use of polymers for AM techniques and industrial applications. Additionally, new initiatives have been pursued to utilize polymer 3D printing for applications ranging from automotive to biomedical [15,29].
One such application is the improvement of jigs and fixtures, used for positioning and assembling parts during manufacturing. These components are traditionally machined using CNC and require high tolerances to ensure the accuracy of their use. The level of customization required for jigs and fixtures, however, results in long production times at a high cost.
Incorporating FFF 3D printing as an alternative method not only allows for improved precision of the printed components with low material waste but also shortens print and assembly time of consecutively printed iterations. Volkswagen Autoeuropa has currently implemented FFF printed jigs and fixtures that are used on the car assembly line, with a yearly output of 100,000 cars. By transitioning to 3D printing, Volkswagen Autoeuropa saved over 90% in tool manufacturing expenses and increased its productivity [73].
FFF also functioned as a competitive solution for the prototyping company, Peak Additive, which produced a high specific tolerance flash drive cap created from acrylonitrile styrene acrylate. This extrusion-based AM technique was Table 2: Polymers used for extrusion-based AM processes in recent years [24, 36, 38, 39, 41, 44-49, 56-60, 63, 67]  advantageous due to its low-to mid-sized production runs with no additional cost requirements from tooling. Moreover, the build size of the industrial FFF printer also allowed for a large number of the caps to be printed in a single run, further lowering the overall cost [73]. For DIW, there has been ground-breaking research currently conducted on a multitude of materials; however, the adoption of these materials for industry-level applications is still at its infancy. This is due to the fundamental and implicit differences in the manufacturing processes between FFF and DIW; DIW does not melt extrude its material(s) to fuse printed layers together, rather it extrudes non-Newtonian liquids/or gels that can be printed at room temperature. DIW could be beneficial for biomedical and medical research as living cells, and other biological materials [74], as structures can be safely printed under ambient conditions. This manufacturing process is also ideal for biological applications since properties, such as porosity and composite distribution, can be more finely controlled than its extrusion-based counterpart. DIW living cell printers such as EnvisionsTec's 3D bioplotter [75] are currently commercially available, however, given the novelty of the tech, these printers are expensive. Other current industrial applications include biocompatible scaffolding for tissue regeneration and complex biocompatible implants that can be customized to the patient [76]. It is important to note that these medical applications are currently being implemented on an as-needed basis within research institutions and hospitals. While commercially available and affordable, DIW for polymers has yet to reach the market, however DIW of metals is beginning to emerge. Desktop Metal, based in Burlington, MA, is one of the first companies to industrialize this extrusion-based process and has begun to release DIW metal printers for industrial use. [77]. With continued research and development, DIW is expected to expand its impact and implementation in various industries.

CONCLUSIONS AND FUTURE DIRECTIONS
Extrusion-based AM technology offers tremendous opportunities for the development of complex architected polymeric structures and increased production efficiency. The rapid and ongoing engineering of novel materials, such as resins, filaments, inks, and complex or nature-inspired architectures, has propelled these techniques to become frontiers of current manufacturing procedures. As such, several researchers have demonstrated the significant impact of geometry on the mechanical performance of a design and its judicious utilization for applications in medicine, aerospace, and electronics.
Advantageously, FFF and DIW offer great freedom in their manufacturing parameters. This flexibility in determining the printing parameters for and intricate details of the final structure enables the fabrication of complex polymeric architectures and their composites. thereby, enhancing. While thermoplastic filaments can be easily printed via FFF, the printing of high-performance, flexible, biocompatible, and continuous fiber-reinforced composites for engineering applications are now possible with further improvements in process controls and material modifications. Furthermore, DIW has gained increasing attention because of its versatility and its ability to print soft materials and composites for various applications. Even with their progress and popularity, both FFF and DIW are yet to be fully industrialized and commercialized. In the future, the scalability of these extrusion-based AM processes can be achieved with further development; several future considerations for research are outlined below.
• By exploring more natural complex architectures, improved performance and functionality can be discovered through the use of rapid extrusion-based techniques.
• Generating 3D printed macroscale models from atomic models can be used to investigate unusual molecular structures that re-  • Artificial intelligence, in particular machine learning, has proven successful in predicting the relationships between molecular or microstructure and material properties, and the same can be accomplished for architected structures and their advantages. ML methods such as Bayesian modeling and support vector machines can be employed to relate the composite structure information (e.g. reinforcement distribution, alignment, adhesion with the matrix phase) to the target properties (e.g. controlled porosity, load-bearing capacity, impact resistance).
The recent progress in polymer materials for AM is expected to spur further research interests and developments in this field. Given the rapid pace of technological advancements, it is only a matter of time before AM becomes a feasible, complementary technology to traditional manufacturing techniques.