A review of vat photopolymerization technology: Materials, applications, challenges, and future trends of 3D printing

polymers Review A Review of Vat Photopolymerization Technology: Materials, Applications, Challenges, and Future Trends of 3D Printing Marek Pagac * , Jiri Hajnys , Quoc-Phu Ma , Lukas Jancar, Jan Jansa , Petr Stefek and Jakub Mesicek   Citation: Pagac, M.; Hajnys, J.; Ma, Q.-P.; Jancar, L.; Jansa, J.; Stefek, P.; Mesicek, J. A Review of Vat Photopolymerization Technology: Materials, Applications, Challenges, and Future Trends of 3D Printing. Polymers 2021,13, 598. https://doi. org/10.3390/polym13040598 Academic Editor: Paul F. Egan Received: 30 January 2021 Accepted: 13 February 2021 Published: 17 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Center of 3D Printing Protolab, Department of Machining, Assembly and Engineering Technology, Faculty of Mechanical Engineering, VSB-TU Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic; [email protected] (J.H.); [email protected] (Q.-P.M.); [email protected] (L.J.); [email protected] (J.J.); petr[email protected] (P.S.); [email protected] (J.M.) *Correspondence: mar[email protected] Abstract: Additive manufacturing (3D printing) has significantly changed the prototyping process in terms of technology, construction, materials, and their multiphysical properties. Among the most popular 3D printing techniques is vat photopolymerization, in which ultraviolet (UV) light is deployed to form chains between molecules of liquid light-curable resin, crosslink them, and as a result, solidify the resin. In this manuscript, three photopolymerization technologies, namely, stereolithography (SLA), digital light processing (DLP), and continuous digital light processing (CDLP), are reviewed. Additionally, the after-cured mechanical properties of light-curable resin materials are listed, along with a number of case studies showing their applications in practice. The manuscript aims at providing an overview and future trend of the photopolymerization technology to inspire the readers to engage in further research in this field, especially regarding developing new materials and mathematical models for microrods and bionic structures. Keywords: vat photopolymerization; SLA; DLP; CDLP; CLIP 1. Introduction Additive manufacturing (AM) technology was first introduced in the 1980s with the aim of producing rapid and functional prototypes from various materials. Particularly, in May 1980, Dr. Hideo Kodama from the Nagoya Municipal Industrial Research Institute (NMIRI) applied for a patent that describes a method of curing a spatial model of photopolymer material by exposing it to ultraviolet (UV) light. Nevertheless, Kodama’s efforts toward finding funding for further research and development on the issued patent bore no fruit, even with support from the government. The patent was consequently abandoned and never commercialized [ 1 – 4 ]. Kodama is, however, credited as the first inventor of this technology. The origin of 3D printing with commercial potential can be traced back to 1986 when Charles W. Hull filed a patent for a photopolymerization technology called stereolithography (SLA). The very first printed model was an eye-wash cup. Henceforth, SLA has been developed by 3D Systems Corporation [1–3]. New developments are also closely related to the development of 3D printing in medical and regenerative medicine. Dating back to 2004 in [ 5 ], a novel technique for printing 3D cell patterns was introduced. Later on, in [ 6 ], the author describes the process of manufacturing structures, namely, scaffolds, on which, cells can grow after being implemented into the body. A typical example of 3D bioprinting is artificial vessels, for which, dog bone marrow samples were used [7,8]. 2. Vat Photopolymerization AM technologies are classified as shown below in Figure 1. Each technology differs from another in the manner of the application process, curing principle, and the initial state Polymers 2021,13, 598. https://doi.org/10.3390/polym13040598 https://www.mdpi.com/journal/polymers Polymers 2021,13, 598 2 of 20 of the material. Herein, attention is paid to the historically oldest AM technology, namely, vat photopolymerization. Polymers 2021, 13, x FOR PEER REVIEW 2 of 18 2. Vat Photopolymerization AM technologies are classified as shown below in Figure 1. Each technology differs from another in the manner of the application process, curing principle, and the initial state of the material. Herein, attention is paid to the historically oldest AM technology, namely, vat photopolymerization. Figure 1. Division of the additive manufacturing (AM) technologies according to ISO/ASTM 52900: 2015 Additive Manufacturing—General Principles—Terminology. 2.1. 3D Printing Process The 3D printing production process begins with a 3D model that is designed with computer-aided design (CAD) software or 3D scanned from a physical object. The 3D model is subsequently converted to the standard triangulation language (STL) format. It should be noted that since 1987, STL has been the standard and most frequently used format for preparing data for AM production. Specifically, the STL format solely describes the surface of the 3D model with a network of triangles of different sizes, depending on the required resolution. The smaller the size of the triangles, the more accurately the triangular mesh represents the desired surface, yielding a smoother surface of the to-beprinted object. The stage in which preparing the desired mesh model takes place is called preprocessing [9]. The second production stage is called processing. As a 3D printed object is fabricated layer by layer, each consecutive layer has to be supported by either the platform, the preceding layer, or extra support elements. After designing the correct and optimal orientation of the model and the supports, the STL model, including the supports, is sliced into layers with a plane parallel to the platform surface, namely, the xOy plane. Each layer is then built consecutively in the Oz direction. The layer thickness depends on the printer, AM technology, and quality requirements. The sliced model is subsequently sent to the printer. Differing from the traditional subtractive manufacturing process (machining), where the material is taken out of a workpiece, AM offers more efficient material usage. After the printing is finished, the model is removed from the platform and other technical processing procedures are used to refine the printed object in a stage called postprocessing. In the case of photopolymerization, the as-built models are rinsed in a wash solution, most commonly, isopropyl alcohol (IPA), to get rid of the liquid layer of resin. To enhance the mechanical properties, these models are subsequently artificially cured with UV light or naturally exposed to sunlight. Other processes include support removal, grinding, sealing, gluing, polishing, painting, varnishing, coating, sterilization, inspection, and measurement. There are specific post-processing treatments for photopolymer Figure 1. Division of the additive manufacturing (AM) technologies according to ISO/ASTM 52900: 2015 Additive Manufacturing—General Principles—Terminology. 2.1. 3D Printing Process The 3D printing production process begins with a 3D model that is designed with computer-aided design (CAD) software or 3D scanned from a physical object. The 3D model is subsequently converted to the standard triangulation language (STL) format. It should be noted that since 1987, STL has been the standard and most frequently used format for preparing data for AM production. Specifically, the STL format solely describes the surface of the 3D model with a network of triangles of different sizes, depending on the required resolution. The smaller the size of the triangles, the more accurately the triangular mesh represents the desired surface, yielding a smoother surface of the to-beprinted object. The stage in which preparing the desired mesh model takes place is called preprocessing [9]. The second production stage is called processing. As a 3D printed object is fabricated layer by layer, each consecutive layer has to be supported by either the platform, the preceding layer, or extra support elements. After designing the correct and optimal orientation of the model and the supports, the STL model, including the supports, is sliced into layers with a plane parallel to the platform surface, namely, the xOy plane. Each layer is then built consecutively in the Oz direction. The layer thickness depends on the printer, AM technology, and quality requirements. The sliced model is subsequently sent to the printer. Differing from the traditional subtractive manufacturing process (machining), where the material is taken out of a workpiece, AM offers more efficient material usage. After the printing is finished, the model is removed from the platform and other technical processing procedures are used to refine the printed object in a stage called postprocessing. In the case of photopolymerization, the as-built models are rinsed in a wash solution, most commonly, isopropyl alcohol (IPA), to get rid of the liquid layer of resin. To enhance the mechanical properties, these models are subsequently artificially cured with UV light or naturally exposed to sunlight. Other processes include support removal, grinding, sealing, gluing, polishing, painting, varnishing, coating, sterilization, inspection, and measurement. There are specific post-processing treatments for photopolymer parts. It is noteworthy that the rinsing time, temperature, and duration of the curing process play an important role in determining the mechanical properties of the finished parts. A typical printing process with SLA technology is shown below in Figure 2. Polymers 2021,13, 598 3 of 20 Polymers 2021, 13, x FOR PEER REVIEW 3 of 18 parts. It is noteworthy that the rinsing time, temperature, and duration of the curing process play an important role in determining the mechanical properties of the finished parts. A typical printing process with SLA technology is shown below in Figure 2. Figure 2. A typical production process with photopolymerization. CAD: computer-aided design, STL: standard triangulation language, STEP: standard for the exchange of product data. 2.2. Printing Principle of Photopolymerization In the photopolymerization process, a photopolymer, which is a light-curable resin, is stored in a vat and treated with either visible or UV light. The curing light triggers and manifests the polymerization reaction, which then forms chains of polymers or crosslinks them to form a solid resin. As can be seen in Figure 3, three elements of the photopolymer mixture are monomers, oligomers, and photoinitiators. When exposed to curing light, photoinitiators release creative species that work as catalysts for the chain formation among monomers and oligomers. The chain-forming chemical-thermal process is irreversible and prototypes cannot be changed back to liquid form. Utilizing this principle, consecutive layers of resin are fabricated gradually from a sliced STL file [10–14]. Figure 3. Liquid photopolymer (on the left), induced polymerization by light (small circle—monomer, large circle—oligomer, star—photoinitiator). Photopolymerization is classified with regard to the method of curing (Figure 4), which employ lasers (SLA), digital projection (digital light processing (DLP)), and lightemitting diodes (LEDs) and oxygen (continuous digital light processing (CDLP)/continuous liquid interface production (CLIP)) [9,15–19]. Figure 2. A typical production process with photopolymerization. CAD: computer-aided design, STL: standard triangulation language, STEP: standard for the exchange of product data. 2.2. Printing Principle of Photopolymerization In the photopolymerization process, a photopolymer, which is a light-curable resin, is stored in a vat and treated with either visible or UV light. The curing light triggers and manifests the polymerization reaction, which then forms chains of polymers or crosslinks them to form a solid resin. As can be seen in Figure 3, three elements of the photopolymer mixture are monomers, oligomers, and photoinitiators. When exposed to curing light, photoinitiators release creative species that work as catalysts for the chain formation among monomers and oligomers. The chain-forming chemical-thermal process is irreversible and prototypes cannot be changed back to liquid form. Utilizing this principle, consecutive layers of resin are fabricated gradually from a sliced STL file [10–14]. Polymers 2021, 13, x FOR PEER REVIEW 3 of 18 the curing process play an important role in determining the mechanical properties of the finished parts. A typical printing process with SLA technology is shown below in Figure 2. Figure 2. A typical production process with photopolymerization. CAD: computer-aided design, STL: standard triangulation language, STEP: standard for the exchange of product data. 2.2. Printing Principle of Photopolymerization In the photopolymerization process, a photopolymer, which is a light-curable resin, is stored in a vat and treated with either visible or UV light. The curing light triggers and manifests the polymerization reaction, which then forms chains of polymers or crosslinks them to form a solid resin. As can be seen in Figure 3, three elements of the photopolymer mixture are monomers, oligomers, and photoinitiators. When exposed to curing light, photoinitiators release creative species that work as catalysts for the chain formation among monomers and oligomers. The chain-forming chemical-thermal process is irreversible and prototypes cannot be changed back to liquid form. Utilizing this principle, consecutive layers of resin are fabricated gradually from a sliced STL file [10–14]. Figure 3. Liquid photopolymer (on the left), induced polymerization by light (small circle— monomer, large circle—oligomer, star—photoinitiator). Photopolymerization is classified with regard to the method of curing (Figure 4), which employ lasers (SLA), digital projection (digital light processing (DLP)), and lightemitting diodes (LEDs) and oxygen (continuous digital light processing (CDLP)/continuous liquid interface production (CLIP)) [9,15–19]. Figure 3. Liquid photopolymer (on the left), induced polymerization by light (small circle—monomer, large circle—oligomer, star—photoinitiator). Photopolymerization is classified with regard to the method of curing (Figure 4), which employ lasers (SLA), digital projection (digital light processing (DLP)), and light-emitting diodes (LEDs) and oxygen (continuous digital light processing (CDLP)/continuous liquid interface production (CLIP)) [9,15–19]. Polymers 2021,13, 598 4 of 20 Polymers 2021, 13, x FOR PEER REVIEW 4 of 18 Figure 4. Different curing methods for vat photopolymerization. LEDs: light-emitting diodes. 2.3. Classification 2.3.1. Stereolithography A 3D printer with SLA technology uses a platform immersed in a transparent tank that is filled with light-curable resin (henceforth, simply resin). While the platform is immersed in the resin, the laser beam traces the construction area and cures the areas according to the sliced STL template. After a layer is formed, the platform is lowered or raised in the Oz direction, depending on whether the machine uses a top-down or bottom-up process, by a constant amount equaling the height of a layer. The curing process is repeated layer-by-layer until a 3D model is completed. The height of a layer ranges from 12 to 150 µm. In practice, 100 µm is the most chosen layer height. The print speed of standard SLA printers is in the range of 10–20 mm/h. The principle of a 3D printer with SLA technology is shown in Figure 5. The accuracy of SLA production is related to the diameter of the laser beam at the curing point, namely, the spot size. For example, for a Formlabs Form 2 SLA, the spot size is 140 µm [18,19]. Figure 5. Components of a typical SLA machine: 1—printed part, 2—liquid resin, 3—building platform, 4—UV laser source, 5—XY scanning mirror, 6—laser beam, 7—resin tank, 8—window, and 9—layer-by-layer elevation. 2.3.2. Digital Light Processing DLP technology differs from SLA solely in the curing method. Instead of a mirror to reflect a laser source, a digital light projector is employed. In comparison with SLA techFigure 4. Different curing methods for vat photopolymerization. LEDs: light-emitting diodes. 2.3. Classification 2.3.1. Stereolithography A 3D printer with SLA technology uses a platform immersed in a transparent tank that is filled with light-curable resin (henceforth, simply resin). While the platform is immersed in the resin, the laser beam traces the construction area and cures the areas according to the sliced STL template. After a layer is formed, the platform is lowered or raised in the Oz direction, depending on whether the machine uses a top-down or bottom-up process, by a constant amount equaling the height of a layer. The curing process is repeated layer-bylayer until a 3D model is completed. The height of a layer ranges from 12 to 150 µ m. In practice, 100 µ m is the most chosen layer height. The print speed of standard SLA printers is in the range of 10–20 mm/h. The principle of a 3D printer with SLA technology is shown in Figure 5. The accuracy of SLA production is related to the diameter of the laser beam at the curing point, namely, the spot size. For example, for a Formlabs Form 2 SLA, the spot size is 140 µm [18,19]. Polymers 2021, 13, x FOR PEER REVIEW 4 of 18 Figure 4. Different curing methods for vat photopolymerization. LEDs: light-emitting diodes. 2.3. Classification 2.3.1. Stereolithography A 3D printer with SLA technology uses a platform immersed in a transparent tank that is filled with light-curable resin (henceforth, simply resin). While the platform is immersed in the resin, the laser beam traces the construction area and cures the areas according to the sliced STL template. After a layer is formed, the platform is lowered or raised in the Oz direction, depending on whether the machine uses a top-down or bottom-up process, by a constant amount equaling the height of a layer. The curing process is repeated layer-by-layer until a 3D model is completed. The height of a layer ranges from 12 to 150 µm. In practice, 100 µm is the most chosen layer height. The print speed of standard SLA printers is in the range of 10–20 mm/h. The principle of a 3D printer with SLA technology is shown in Figure 5. The accuracy of SLA production is related to the diameter of the laser beam at the curing point, namely, the spot size. For example, for a Formlabs Form 2 SLA, the spot size is 140 µm [18,19]. Figure 5. Components of a typical SLA machine: 1—printed part, 2—liquid resin, 3—building platform, 4—UV laser source, 5—XY scanning mirror, 6—laser beam, 7—resin tank, 8—window, and 9—layer-by-layer elevation. 2.3.2. Digital Light Processing DLP technology differs from SLA solely in the curing method. Instead of a mirror to reflect a laser source, a digital light projector is employed. In comparison with SLA techFigure 5. Components of a typical SLA machine: 1—printed part, 2—liquid resin, 3—building platform, 4—UV laser source, 5—XY scanning mirror, 6—laser beam, 7—resin tank, 8—window, and 9—layer-by-layer elevation. 2.3.2. Digital Light Processing DLP technology differs from SLA solely in the curing method. Instead of a mirror to reflect a laser source, a digital light projector is employed. In comparison with SLA technology, the DLP process is faster as each layer is entirely exposed to the curing light projected from the digital screen. Since DLP technology utilizes a digital light projector, each layer appears pixelized and the accuracy of the printed part depends greatly on the Polymers 2021,13, 598 5 of 20 projector resolution. A typical DLP machine and its components are illustrated below in Figure 6[18]. Polymers 2021, 13, x FOR PEER REVIEW 5 of 18 nology, the DLP process is faster as each layer is entirely exposed to the curing light projected from the digital screen. Since DLP technology utilizes a digital light projector, each layer appears pixelized and the accuracy of the printed part depends greatly on the projector resolution. A typical DLP machine and its components are illustrated below in Figure 6 [18]. Figure 6. Components of a typical DLP machine: 1—printed part, 2—liquid resin, 3—building platform, 4—light source, 5—digital projector, 6—light beam, 7—resin tank, 8—window, and 9— layer-by-layer elevation. 2.3.3. Continuous Digital Light Processing/Continuous Liquid Interface Production CDLP/CLIP technology is an innovation based on DLP technology [16]. In particular, being different from SLA and DLP technology, CDLP/CLIP employs digital projection with LEDs and an oxygen-permeable window instead of a normal glass window. This oxygen-permeable window forms a so-called dead zone as thick as a human hair, which allows the liquid resin to flow between the interface of the printed part and the window. This uncured resin flow remarkably increases the resolution of the printed part, as well as decreasing the risk of printing failure due to the peeling force. Moreover, as opposed to the layer-by-layer method, CDLP/CLIP machines are designed with continuous movement of the build platform, thus, allowing for undisrupted prototype printing at speeds of several hundred millimeters per hour [15,16,18]. A typical CDLP/CLIP machine and its components are illustrated below in Figure 7. Figure 6. Components of a typical DLP machine: 1—printed part, 2—liquid resin, 3—building platform, 4—light source, 5—digital projector, 6—light beam, 7—resin tank, 8—window, and 9— layer-by-layer elevation. 2.3.3. Continuous Digital Light Processing/Continuous Liquid Interface Production CDLP/CLIP technology is an innovation based on DLP technology [ 16 ]. In particular, being different from SLA and DLP technology, CDLP/CLIP employs digital projection with LEDs and an oxygen-permeable window instead of a normal glass window. This oxygen-permeable window forms a so-called dead zone as thick as a human hair, which allows the liquid resin to flow between the interface of the printed part and the window. This uncured resin flow remarkably increases the resolution of the printed part, as well as decreasing the risk of printing failure due to the peeling force. Moreover, as opposed to the layer-by-layer method, CDLP/CLIP machines are designed with continuous movement of the build platform, thus, allowing for undisrupted prototype printing at speeds of several hundred millimeters per hour [ 15 , 16 , 18 ]. A typical CDLP/CLIP machine and its components are illustrated below in Figure 7. Thanks to the continuous printing process, the problem regarding layer connection is eliminated and the visible staircase effect is minimized. Therefore, the fabricated parts have isotropic mechanical properties, although they may appear anisotropic [ 20 , 21 ]. This technology was utilized to 3D print supramolecular hydrogels, as seen in [17,18]. 2.3.4. Two-Photon Lithography (2PL) 2PL is a direct laser-writing (DLW) method that uses a laser beam to create 3D microstructures with a resolution below the diffraction limit. This is possible thanks to the polymerization of a photosensitive polymeric material, namely, a (photo)resist, based on the two-photon absorption (2PA) principle [ 22 ]. This technology has found its place in medical applications thanks to the ability to produce solid polymer objects, whose resolution is several nanometers. Unlike traditional 3D printing, 2PL technology can freely cure solid polymer in the resin vat, eliminating the need for material deposition in a layer-by-layer manner. Subsequently, the printed parts are treated in a suitable solvent to remove the uncured liquid resin. The smallest polymer identity in 3D printing with 2PL is an ellipsoidal 3D point, which is known as a voxel (volume pixel) [ 22 ]. It should be noted that the printing principle described above is applicable to negative photoresists, whose uncured portion dissolves in the solvent during postprocessing, while the cured portion does not. Polymers 2021,13, 598 6 of 20 Polymers 2021, 13, x FOR PEER REVIEW 6 of 18 Figure 7. Components of a typical CDLP/CLIP machine: 1—printed part, 2—liquid resin, 3— building platform, 4—light source, 5—digital projector, 6—light beam, 7—resin tank, 8—oxygenpermeable window, 9—dead zone, and 10—continuous elevation. Thanks to the continuous printing process, the problem regarding layer connection is eliminated and the visible staircase effect is minimized. Therefore, the fabricated parts have isotropic mechanical properties, although they may appear anisotropic [20,21]. This technology was utilized to 3D print supramolecular hydrogels, as seen in [17,18]. 2.3.4. Two-Photon Lithography (2PL) 2PL is a direct laser-writing (DLW) method that uses a laser beam to create 3D microstructures with a resolution below the diffraction limit. This is possible thanks to the polymerization of a photosensitive polymeric material, namely, a (photo)resist, based on the two-photon absorption (2PA) principle [22]. This technology has found its place in medical applications thanks to the ability to produce solid polymer objects, whose resolution is several nanometers. Unlike traditional 3D printing, 2PL technology can freely cure solid polymer in the resin vat, eliminating the need for material deposition in a layerby-layer manner. Subsequently, the printed parts are treated in a suitable solvent to remove the uncured liquid resin. The smallest polymer identity in 3D printing with 2PL is an ellipsoidal 3D point, which is known as a voxel (volume pixel) [22]. It should be noted that the printing principle described above is applicable to negative photoresists, whose uncured portion dissolves in the solvent during postprocessing, while the cured portion does not. The absorption coefficient of two photons in 2PL is defined by the equation: =2ℏ  = , (1) where ℏ is the Dirac constant, ω is the laser frequency, I is the irradiance or power (radiation flux) per focused volume, v is the rate of transition of 2PA to the voxel, N is the density of the photoresist (number of reaction molecules in the voxel), E is the photon energy, σ is the cross-section of the 2PA (cm4·s/photon). This equation was taken from [23]. 2.4. Accuracy 2.4.1. Accuracy Comparison between SLA, DLP, and CDLP/CLIP The curing light source and the light-projecting method play key roles in determining the accuracy of the printed parts. As for SLA, the footprint of the curing laser beam on the xOy plane is a spot and the accuracy depends on the spot diameter, whereas in DLP and Figure 7. Components of a typical CDLP/CLIP machine: 1—printed part, 2—liquid resin, 3— building platform, 4—light source, 5—digital projector, 6—light beam, 7—resin tank, 8—oxygenpermeable window, 9—dead zone, and 10—continuous elevation. The absorption coefficient of two photons in 2PL is defined by the equation: α2= 2}ω I2v2PA = N Eσ2PA, (1) where ¯h is the Dirac constant, ω is the laser frequency, Iis the irradiance or power (radiation flux) per focused volume, vis the rate of transition of 2PA to the voxel, Nis the density of the photoresist (number of reaction molecules in the voxel), Eis the photon energy, σ is the cross-section of the 2PA (cm4·s/photon). This equation was taken from [23]. 2.4. Accuracy 2.4.1. Accuracy Comparison between SLA, DLP, and CDLP/CLIP The curing light source and the light-projecting method play key roles in determining the accuracy of the printed parts. As for SLA, the footprint of the curing laser beam on the xOy plane is a spot and the accuracy depends on the spot diameter, whereas in DLP and CDLP/CLIP, the accuracy is given by the pixel matrix, which is defined by the resolution of the digital projector. In contrast to the flexibility that SLA offers, thanks to the free movement of the laser beam, DLP and CDLP/CLIP are limited to the network of pixels of the digital screen, which is illuminated wholly one at a time for each layer. As a result, the DLP and CDLP/CLIP deliver less accurate, Minecraft-like parts in comparison with SLA, despite its many-fold smaller individual footprint. A comparison of the production accuracies is illustrated in Figure 8below, where a scan track in SLA is approximated with voxels in DLP and CDLP/CLIP. With regard to this fact, it can be stated that SLA technology can deliver higher accuracy than DLP and CDLP/CLIP at a cost of production time. Additionally, as aforementioned, CDLP/CLIP can fabricate parts with better accuracy and mechanical properties in comparison with DLP thanks to the continuous movement of the platform and the existence of the dead zone. Polymers 2021,13, 598 7 of 20 Polymers 2021, 13, x FOR PEER REVIEW 7 of 18 CDLP/CLIP, the accuracy is given by the pixel matrix, which is defined by the resolution of the digital projector. In contrast to the flexibility that SLA offers, thanks to the free movement of the laser beam, DLP and CDLP/CLIP are limited to the network of pixels of the digital screen, which is illuminated wholly one at a time for each layer. As a result, the DLP and CDLP/CLIP deliver less accurate, Minecraft-like parts in comparison with SLA, despite its many-fold smaller individual footprint. A comparison of the production accuracies is illustrated in Figure 8 below, where a scan track in SLA is approximated with voxels in DLP and CDLP/CLIP. With regard to this fact, it can be stated that SLA technology can deliver higher accuracy than DLP and CDLP/CLIP at a cost of production time. Additionally, as aforementioned, CDLP/CLIP can fabricate parts with better accuracy and mechanical properties in comparison with DLP thanks to the continuous movement of the platform and the existence of the dead zone. Figure 8. Accuracy comparison between SLA and DLP/CDLP/CLIP based on their footprint on building platform, namely, the xOy plane [24]. 2.4.2. Accuracy of 2PL In general, there are two factors that influence the accuracy of the parts fabricated with the 2PL method, namely, the geometric error inherited from the scanning stages and the principal errors of the method itself. As 2PL printed parts are voxel-based, their accuracy primarily relies on how the voxels are spatially arranged and their resolution [25]. The 3D movement of the laser beam is controlled by a 3D piezoelectric stage, a 1D piezoelectric stage in combination with a 2D galvanometer scanner, or a 3D linear motor-driven stage with or without a galvanometer scanner. The 3D piezoelectric stage can provide a resolution of several nanometers, scanner-based systems about 100 nm, and 3D linear motor-driven stage about 400 nm [26]. These are considered geometric errors that can offset the printed parts considerably. On the other hand, the principal errors are inherited from the layer-by-layer fabricating process and from exporting the 3D model to triangularbased STL files. 3. Photopolymer Material Classification After curing, the models are dimensionally stable, hard or elastic (elastomers), and can withstand very low and high temperatures. Each resin has different mechanical and chemical properties and specific uses. In general, photopolymer materials are divided into standard, structural, tough and durable, flexible and elastic, castable wax and ceramic, biocompatible, and bioink. Figure 8. Accuracy comparison between SLA and DLP/CDLP/CLIP based on their footprint on building platform, namely, the xOy plane [24]. 2.4.2. Accuracy of 2PL In general, there are two factors that influence the accuracy of the parts fabricated with the 2PL method, namely, the geometric error inherited from the scanning stages and the principal errors of the method itself. As 2PL printed parts are voxel-based, their accuracy primarily relies on how the voxels are spatially arranged and their resolution [ 25 ]. The 3D movement of the laser beam is controlled by a 3D piezoelectric stage, a 1D piezoelectric stage in combination with a 2D galvanometer scanner, or a 3D linear motor-driven stage with or without a galvanometer scanner. The 3D piezoelectric stage can provide a resolution of several nanometers, scanner-based systems about 100 nm, and 3D linear motor-driven stage about 400 nm [ 26 ]. These are considered geometric errors that can offset the printed parts considerably. On the other hand, the principal errors are inherited from the layer-bylayer fabricating process and from exporting the 3D model to triangular-based STL files. 3. Photopolymer Material Classification After curing, the models are dimensionally stable, hard or elastic (elastomers), and can withstand very low and high temperatures. Each resin has different mechanical and chemical properties and specific uses. In general, photopolymer materials are divided into standard, structural, tough and durable, flexible and elastic, castable wax and ceramic, biocompatible, and bioink. 3.1. Standard Resin Standard resin and its applications span the widest among the photopolymer materials. The most commonly used colors are grey, white, black, transparent, turquoise, and blue. Within the standard resin family, there is draft resin, which can cure 3–4 times faster than the conventional one. Thus, the draft resin is an ideal solution for quick prototyping with only one disadvantage being the height of the layer (300 µ m), resulting in the stair-case effect on the surface of the model [ 27 ]. It should be noted that “green” is used to name parts that are in their as-built condition, receiving no additional post-treatments. The mechanical properties of the standard resin are summarized below in Table 1. Polymers 2021,13, 598 8 of 20 Table 1. Mechanical properties of standard resin [28,29]. Properties Standard Standard (Grayscale, Clear, Colors) Draft Resin Green 1Postcured 2Green 3 Postcured at Room Temperature 4 Postcured at 60 ◦C5 Tensile Properties Ultimate Tensile Strength 38 MPa 65 MPa 23 MPa 28 MPa 36 MPa Tensile Modulus 1.6 GPa 2.8 GPa 0.9 GPa 1.3 GPa 1.6 GPa Elongation at Failure 12% 6.2% 17% 10% 7% Flexural Properties Flexural Modulus 1.25 GPa 2.2 GPa 0.6 GPa 0.9 GPa 1.5 GPa Flexural Stress at Break - - - - - Impact Properties Notched Izod 16 J/m 25 J/m 35 J/m 35 J/m 21 J/m Temperature Properties Heat Deflection Temp. @ 264 psi 42.7 ◦C 58.4 ◦C 43.3 ◦C 44.3 ◦C 50.1 ◦C Heat Deflection Temp. @ 66 psi 49.7 ◦C 73.1 ◦C 50.6 ◦C 50.7 ◦C 63.4 ◦C 1 Data was obtained from green parts, printed using a Form 2, 100 µ m, clear settings, without additional treatments. 2 Data was obtained from parts printed using a Form 2, 100 µ m, clear settings, and postcured with 1.25 mW/cm 2 of 405 nm LED light for 60 min at 60 ◦ C. 3 Data was obtained from green parts, printed using a Form 2, 300 µ m, draft resin settings, washed for 5 min in Form Wash, and air-dried without a postcure. 4 Data was obtained from parts printed using a Form 2, 300 µ m, draft resin settings, and postcured with Form Cure at room temperature for 5 min. 5 Data was obtained from parts printed using a Form 2, 300 µ m, draft resin settings, and postcured with Form Cure at 60 ◦C for 5 min. 3.2. Structural Resin A typical example of structural resin for printing universal prototypes is Grey Pro resin, which can deliver printed parts with high accuracy, slight elongation, and low creep. Thus, it is applicable for conceptual modeling, reusable functional prototypes, and precise templates for molding. As for rigid structural resin, it can produce a combination of remarkably high rigidity and accuracy thanks to the glass reinforcement, yielding a glossy surface. In addition, this resin can be utilized to print thin walls and details, such as turbine blades, fans, connections, tooling, electronic covers, and cabinets in the automotive industry. Among the structural resin family, there is a heat-conductive one so-called High Temp resin, which can withstand temperature up to 289 ◦ C under a pressure of 0.45 MPa. The material is suitable for parts under light pressure and high temperatures, such as injection molding prototypes, heat-resistant fittings, hot gas, liquid piping, and electronics covers. The mechanical properties of the three resins are summarized below in Table 2. Polymers 2021,13, 598 9 of 20 Table 2. Mechanical properties of structural resin [30–32]. Properties * Grey Pro Rigid High Temp Green 1Postcured 2Green 3Postcured 4Green 5Post-Cured 6Post-Cured + Thermally Postcured 7 Tensile Properties Ultimate Tensile Strength 35 MPa 61 MPa 40 MPa 75 MPa 20.9 MPa 58.3 MPa 51.1 MPa Young’s Modulus - - - - - - - Tensile Modulus 1.4 GPa 2.6 GPa 2.2 GPa 4.1 GPa 0.75 GPa 2.75 GPa 2.9 GPa Elongation 32.5% 13% 13.3% 5.6% - - - Elongation at Break - - - - 14% 3.3% 2.4% Flexural Properties Flexural Modulus 0.94 GPa 2.2 GPa 1.37 GPa 3.7 GPa 0.69 GPa 2.62 GPa 2.62 GPa Flexural Stress at 5% Strain 39 MPa 86 MPa 49 MPa 121 MPa - - - Flexural Strength at Break - - - - 24.1 MPa 94.5 MPa 93.8 MPa Impact Properties Notched IZOD - 18.7 J/m - 18.8 J/m 32.8 J/m 18.2 J/m 24.2 J/m Temperature Properties Head Deflection Temp. @ 1.8 MPa - 62.4 ◦C - 74 ◦C 43.6 ◦C 99.2 ◦C 101 ◦C Heat Deflection Temp. @ 0.45 MPa - 77.5 ◦C - 88 ◦C 49.3 ◦C 142 ◦C 238 ◦C Thermal Expansion (−30 to 30 ◦C) - 78.5 µm/m/◦C - 53 µm/m/◦C - - - Thermal Expansion (0 to 150 ◦C) - - - - 118.1 µm/m/◦C 79.6 µm/m/◦C 74 µm/m/◦C * Material properties can vary with the part geometry, print orientation, print settings, and temperature. 1 Data was obtained from green parts, printed using a Form 2, 100 µ m, Grey Pro settings, without additional treatments. 2 Data was obtained from parts printed using a Form 2, 100 µ m, Grey Pro settings, and postcured with Form Cure for 120 min at 80 ◦ C. 3 Data was obtained from green parts, printed using Form 2, 100 µ m, rigid settings, without additional treatments. 4 Data was obtained from parts printed using a Form 2, 100 µ m, rigid settings, and postcured with a form cure for 120 min at 80 ◦ C. 5 Data was obtained from green parts, printed using a Form 2, 100 µ m, High Temp settings, washed for 5 min in Form Wash, and air-dried without a post-cure. 6 Data was obtained from parts printed using a Form 2, 100 µ m, High Temp settings, and postcured with Form Cure at 80 ◦ C for 120 min. 7 Data was obtained from parts printed using a Form 2, 100 µ m, High Temp settings, and postcured with Form Cure at 60 ◦ C for 60 min plus an additional thermal cure in a lab oven at 160 ◦C for 90 min. Polymers 2021,13, 598 16 of 20 5. Future Trends Trends in 3D printing are shaped by the skills and experience of users, as well as the development of printers and printing materials. Indeed, photopolymerization spans a wide variety of fields: industry and engineering, smart composites, soft robotics, flexible electronics, superhydrophobic 3D objects, medical and biomedical, prosthetics and orthotics, sports equipment, jewelry, etc. Recent studies have shown the huge potential of photopolymerization in tomographic printing, 3D nanoprinting, and 4D printing. Current trends are moving toward multimaterial printing, which utilizes single or multinozzle systems to fabricate parts with variable strength and elasticity, living cell structures for bioprinting, and 4D printing applications. Referring to AM with such smart materials, we can begin to use the two concepts, namely, smart 3D printing and smart 4D printing. In addition, we are seeing fundamental changes in the robotics field thanks to the development of printing materials. Progress in smart composites, soft robotics, and flexible electronics is promising to deliver improved capabilities, increased safety for human interaction, lower costs, etc. Progressive and innovative materials technologies are changing the way several mature industries operate, such as medical, industrial, automotive, and aerospace robots. Some typical examples for such an evolution are electroactive polymers or artificial muscles, electroadhesion gripping, and electrolaminates [74]. Furthermore, photopolymerization has found its place in fabricating superhydrophobic objects for water-related applications, e.g., module spacers, novel filtrations, desalination membranes, adsorbents, and water remediation. The promising potential, challenges, and prospects of AM and the materials for water resource and treatment-related applications were all discussed in [74]. In the field of biocompatible materials, attention is paid to developing monomer-free resins to meet health and safety standards. Deploying these materials, dentists and orthodontists can create long-lasting and highly performing dental and other body prostheses without fear of toxicity or side effects. Other developments focus on fiber-reinforced and two-photon resins for 3D printing at the nano level [29,33]. Interest in 3D bioprinting organs is growing and aims to alleviate the shortage of donated organs and eliminate illegal organ trading on the black market. Another advantage is the ability to test new drugs on printed tissues (kidneys and liver) without burdening and exposing the patient’s organs to side effects. Taking another step further, 4D printing produces 3D biological parts whose shapes respond following time-dependent functions. Indeed, time is the additional 1D and the shape change is activated with stimuli from the environment, e.g., water, temperature, or light [ 75 ]. Summarizing from [ 8 , 75 – 79 ], the evolution of 3D bioprinting, Figure 9, and its process, Figure 10, are shown. Polymers 2021, 13, x FOR PEER REVIEW 15 of 18 stimuli from the environment, e.g., water, temperature, or light [75]. Summarizing from [8,75–79], the evolution of 3D bioprinting, Figure 9, and its process, Figure 10, are shown. Figure 9. Evolution of 3D bioprinting. Figure 10. Technological process of a 3D-printing production applied in regenerative medicine [80]. 6. Conclusion Photopolymerization in 3D printing has been used for applications in fabricating rapid and functional prototypes, customized products, and serial production. Over more than 30 years of development on photopolymerization has made great progress on improving the production methods (SLA, DLP, CDLP/CLIP), production speed, and quality of 3D printed products for industrial and research applications. Material-wise, efforts will further be paid to high-performance resins, especially biocompatible resins, and bioink. This article serves as a review of vat photopolymerization printing technology and acquaints a wide audience of readers across various disciplines with the possibilities of their use, as well as the advantages and disadvantages of this technology. In this review, we aimed to summarize all the various types of this technology and the large range of materials used, as well as providing specific applications of this technology in practice, especially in the field of biomedicine and customization. We think that this review is a good follow-up to the already published review of 3D printing, and it is a great addition to this article such that together they form a comprehensive overview of these latest trends in AM. The future of photopolymerization lies in two main categories: material development, especially in medical applications, and applying modern designing methods. Indeed, there is huge potential for material development and the mathematical modeling of 4D printing using multiphysical properties, i.e., variable surface hardness, shape memory, Figure 9. Evolution of 3D bioprinting. Polymers 2021,13, 598 17 of 20 Polymers 2021, 13, x FOR PEER REVIEW 15 of 18 stimuli from the environment, e.g., water, temperature, or light [75]. Summarizing from [8,75–79], the evolution of 3D bioprinting, Figure 9, and its process, Figure 10, are shown. Figure 9. Evolution of 3D bioprinting. Figure 10. Technological process of a 3D-printing production applied in regenerative medicine [80]. 6. Conclusion Photopolymerization in 3D printing has been used for applications in fabricating rapid and functional prototypes, customized products, and serial production. Over more than 30 years of development on photopolymerization has made great progress on improving the production methods (SLA, DLP, CDLP/CLIP), production speed, and quality of 3D printed products for industrial and research applications. Material-wise, efforts will further be paid to high-performance resins, especially biocompatible resins, and bioink. This article serves as a review of vat photopolymerization printing technology and acquaints a wide audience of readers across various disciplines with the possibilities of their use, as well as the advantages and disadvantages of this technology. In this review, we aimed to summarize all the various types of this technology and the large range of materials used, as well as providing specific applications of this technology in practice, especially in the field of biomedicine and customization. We think that this review is a good follow-up to the already published review of 3D printing, and it is a great addition to this article such that together they form a comprehensive overview of these latest trends in AM. The future of photopolymerization lies in two main categories: material development, especially in medical applications, and applying modern designing methods. Indeed, there is huge potential for material development and the mathematical modeling of 4D printing using multiphysical properties, i.e., variable surface hardness, shape memory, Figure 10. Technological process of a 3D-printing production applied in regenerative medicine [ 80 ]. 6. Conclusions Photopolymerization in 3D printing has been used for applications in fabricating rapid and functional prototypes, customized products, and serial production. Over more than 30 years of development on photopolymerization has made great progress on improving the production methods (SLA, DLP, CDLP/CLIP), production speed, and quality of 3D printed products for industrial and research applications. Material-wise, efforts will further be paid to high-performance resins, especially biocompatible resins, and bioink. This article serves as a review of vat photopolymerization printing technology and acquaints a wide audience of readers across various disciplines with the possibilities of their use, as well as the advantages and disadvantages of this technology. In this review, we aimed to summarize all the various types of this technology and the large range of materials used, as well as providing specific applications of this technology in practice, especially in the field of biomedicine and customization. We think that this review is a good follow-up to the already published review of 3D printing, and it is a great addition to this article such that together they form a comprehensive overview of these latest trends in AM. The future of photopolymerization lies in two main categories: material development, especially in medical applications, and applying modern designing methods. Indeed, there is huge potential for material development and the mathematical modeling of 4D printing using multiphysical properties, i.e., variable surface hardness, shape memory, etc. As for the construction methods, the future lies in fabricating topologically optimized, bionic, and microrod structures. Applications of 3D printing and, in particular, photopolymerization has spanned across all industries, e.g., in robotics, medicine, dental, engineering, automotive, aerospace, and water resources and related treatments. To conclude, it can be stated that the future of 3D printing is solely limited by the creativity and technical thinking of designers. Author Contributions: Conceptualization: all authors; M.P. and P.S. conceived the ideas and organized the main content; writing—original draft: M.P., J.M., J.J., and Q.-P.M.; writing—review and editing: J.H., L.J., and Q.-P.M. All authors have read and agreed to the published version of the manuscript. Funding: This paper was completed in connection with the project “Innovative and additive manufacturing technology—new technological solutions for 3D printing of metals and composite materials,” registration no. CZ.02.1.01/0.0/0.0/17_049/0008407 financed by the Structural Funds of European Union project. 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