Digital Light Processing 3D Printing Research Articles

Development of a DLP 3D printer for orthodontic applications

Recent advances in Additive Manufacturing (AM) technologies have allowed a widespread diffusion of their use in different fields. 3D printing is becoming commonplace for biomedical applications requiring the custom fabrication of prostheses and appliances fitting patient-specific anatomies. In this work, the feasibility of a vat photopolymerization technology, based on Digital Light Processing (DLP), has been investigated for the manufacturing of polymeric orthodontic appliances. A custom DLP 3D printer has been developed by exploiting an off-the-shelf digital projector, with the aim at studying the influence of printing parameters on the surface roughness. The feasibility of using Dental LT Clear resin, a biocompatible photopolymer specifically designed for SLA technology, has been finally verified.

Mechanics of shape distortion of DLP 3D printed structures during UV post-curing.

Digital light processing (DLP) three-dimensional (3D) printing technology has advantages of fast printing speed and high printing precision. It can print objects with small and complex geometrical features and has been widely used in jewelry manufacturing and dentistry. In DLP printing, it is common to use post-treatment with UV light irradiation to improve the final mechanical properties. However, it was found that the UV post-curing process can lead to shape distortion and thus reduction of dimension accuracy. In this paper, we combined photopolymerization reaction kinetics and Euler-Bernoulli beam theory to study UV post-curing induced shape distortion of thin structures prepared by DLP 3D printing. Experiments were conducted to characterize the evolution of mechanical behavior of printed samples during the post-printing process, which was correlated to printing parameters (printing time of single-layer, height of single-layer and printing UV intensity), post-curing UV light intensity and the thickness of the strip. Moreover, post-curing induced distortion was used for the fabrication of 3D structures.

Formation of a keratin layer with silk fibroin-polyethylene glycol composite hydrogel fabricated by digital light processing 3D printing

Silk fibroin (SF)-incorporated 4-arm polyethylene glycol acrylate (PEG4A) was used as a bioink for digital light processing three-dimensional printing. A visible range of light was adopted for the process to assure the safety of the cells in the bioink. Fibroblast cells were embedded in the printed hydrogels, and the keratinocyte cells were cultured at the surfaces of the hydrogels. The proliferation of cells with the bioink materials, PEG4A and SF-PEG4A, and the toxicity of these materials, were verified by live/dead and alamarBlue assays. The primary human keratinocytes on the SF-PEG4A hydrogel scaffold generated a keratin layer by the tissue culture at the air-liquid interface, promoting maturation and stratification. SF-polyethylene glycol hydrogels showed higher cell proliferation and cell spreading, and the thicker keratin layer was formed with SF-PEG4A hydrogels compared with PEG4A hydrogels.

Mechanical performance of three-dimensional bio- nanocomposite scaffolds designed with digital light processing for biomedical applications

Introduction: The need for biocompatible and bioactive scaffolds to accelerate the regeneration and repair of fractured bones has been considered for bone tissue engineering applications during recent decades. The new methods were developed to produce scaffolds to improve the tissue quality, size of cavities, control the porosity and increase the scaffold compressive strength under different loads. The presence of compounds such as hydroxyapatite (HA), which contains calcium and phosphorus ions, is essential for bone marrow transplantation. The use of carbon nanotubes can be considered to improve mechanical, chemical and biological properties, much like bone collagen. However, the construction of bio-nanocomposite scaffold which includes all of the above properties is very important in accelerating bone repair and also in reducing the problems of using other conventional scaffold methods. Materials and Methods: In this paper, the fabrication of a bio-nanocomposite scaffold with a three-dimensional printing method, digital light processing (DLP) has been investigated. The used compounds of photopolymer resin, carbon nanotubes and hydroxyapatite are solvent by ultrasonic and magnet stirrer, then a scaffold model designed in SOLIDWORKS software applied to the DLP 3D printer. Results: The addition of single wall carbon nanotubes (SWCNT) increases the compressive strength of the samples more than two times, beside it leads to an increase in the porosity of the samples due to the addition of SWCNTs. The scaffold porosity was recorded around 75 and 85%, which is associated with a proper compressive strength around 2 -4 MPa. Addition of 2.5 to 5 wt% SWCNT to HA and photopolymer resin can leads to a better mechanical performance compared to the pure sample. Conclusion: The results obtained from this work showed that the above process was successful methodology in preparation of novel scaffold bio-nanocomposites using DLP technique for using in bone tissue engineering.

3D printing dental composite resins with sustaining antibacterial ability

3D printing brought revolutionary changes to many industries, especially in the field of dentistry. However, dental restoration has a high incidence of secondary caries when patients wearing in the oral cavity. In this paper, we developed an antibacterial stereolithography resins (SLR) for digital light processing 3D printing by adding Ag-carrying halloysite nanotubes (Ag-HNT). The morphology and component of Ag-HNT were characterized by transmission electron microscope (TEM) and energy-dispersive X-ray spectroscopy. The uniform distribution of Ag-HNT in SLR was investigated by TEM. The curing curve showed that photo-curable behavior of the composite resins was hardly affected by the addition of Ag-HNT. Three-point bending test indicated that bending strength of the samples increased by 25% after addition of 3% Ag-HNT. A continuous antibacterial ability of cured Ag-HNT/SLR was evidenced by culturing streptococcus mutans in the leaching solution of resins. L929 cells were cultured in the leaching solution for 48 h, and the results of CCK-8 assay indicated that the composite resins had good cytocompatibility.

Three-Dimensional Printed Photoluminescent Polymeric Waveguides.

In this work, we propose an innovative strategy for obtaining functional objects employing a light-activated three-dimensional (3D) printing process without affecting the materials' printability. In particular, a dye is a necessary ingredient in a formulation for a digital light processing 3D printing method to obtain precise and complex structures. Here, we use a photoluminescent dye specifically synthesized for this purpose that enables the production of 3D printed waveguides and splitters able to guide the luminescence. Moreover, copolymerizing the dye with the polymeric network during the printing process, we are able to maintain the solvatochromic properties of the dye toward different solvents in the printed structures, enabling the development of solvents' polarity sensors.

Reversible shape change structures by grayscale pattern 4D printing

Structures and devices with reversible shape change (RSC) are highly desirable in many applications such as mechanical actuators, soft robotics, and artificial muscles. In this paper, we propose to use 3D grayscale printing method to create reversible self-folding structures. The grayscale pattern was used to control the light intensity distribution of a UV projector in a digital light processing 3D printer such that the same photo irradiation time leads to different curing degrees and thus different crosslinking densities at different locations in the polymer during 3D printing. After leaching the uncured oligomers inside the loosely crosslinked network, bending deformation could be induced due to the volume shrinkage. The bending deformation was reversed if the bent structure absorbed acetone and swelled. Using this method, we designed and created RSC structures such as reversible pattern transformation and self-expanding/shrinking structures, auxetic metamaterial, structures mimicking the blossom of a flower. The grayscale 4D printing method provides us a simple and efficient way to create active structures and has great potential in the application of smart structures, composite materials, soft robotics and endovascular stent.

The fabrication of SiBCN ceramic components from preceramic polymers by digital light processing (DLP) 3D printing technology

We present a novel method to fabricate SiBCN ceramic components with complex shapes from preceramic polymers by using digital light processing (DLP) 3D printing technology in this research work. The photocurable precursor for 3D printing was prepared by blending high ceramic yield polyborosilazane with photosensitive acrylate monomers. The material formulation and printing parameters were optimized to fabricate complicated SiBCN ceramic components with high precision. The printed SiBCN ceramic materials were pyrolyzed at different temperatures, and retained their fine features after pyrolysis. Their microstructures were characterized by FTIR, XRD and TEM respectively. Furthermore, the thermal stability and mechanical properties of the SiBCN ceramic samples were investigated and discussed in detail. The 3D printed SiBCN ceramic material exhibited excellent thermal stability and resistance to high temperature oxidation up to 1500 °C.

Wear Resistance of 3D Printing Resin Material Opposing Zirconia and Metal Antagonists.

3D printing offers many advantages in dental prosthesis manufacturing. This study evaluated the wear resistance of 3D printing resin material compared with milling and conventional resin materials. Sixty substrate specimens were prepared with three types of resin materials: 3D printed resin, milled resin, and self-cured resin. The 3D printed specimens were printed at a build angle of 0° and 100 μm layer thickness by digital light processing 3D printing. Two kinds of abraders were made of zirconia and CoCr alloy. The specimens were loaded at 5 kg for 30,000 chewing cycles with vertical and horizontal movements under thermocycling condition. The 3D printed resin did not show significant difference in the maximal depth loss or the volume loss of wear compared to the milled and the self-cured resins. No significant difference was revealed depending on the abraders in the maximal depth loss or the volume loss of wear. In SEM views, the 3D printed resin showed cracks and separation of inter-layer bonds when opposing the metal abrader. The results suggest that the 3D printing using resin materials provides adequate wear resistance for dental use.

Precisely printable and biocompatible silk fibroin bioink for digital light processing 3D printing

Although three-dimensional (3D) bioprinting technology has gained much attention in the field of tissue engineering, there are still several significant engineering challenges to overcome, including lack of bioink with biocompatibility and printability. Here, we show a bioink created from silk fibroin (SF) for digital light processing (DLP) 3D bioprinting in tissue engineering applications. The SF-based bioink (Sil-MA) was produced by a methacrylation process using glycidyl methacrylate (GMA) during the fabrication of SF solution. The mechanical and rheological properties of Sil-MA hydrogel proved to be outstanding in experimental testing and can be modulated by varying the Sil-MA contents. This Sil-MA bioink allowed us to build highly complex organ structures, including the heart, vessel, brain, trachea and ear with excellent structural stability and reliable biocompatibility. Sil-MA bioink is well-suited for use in DLP printing process and could be applied to tissue and organ engineering depending on the specific biological requirements.

Multiple metals doped polymer-derived SiOC ceramics for 3D printing

Multiple metals doped polymer-derived SiOC ceramics with octet truss structure were prepared by employing a photosensitive methyl-silsesquioxane as preceramic polymer through sol-gel method and Digital Light Processing 3D printing. The physical and chemical properties of the preceramic polymers and printed octet truss structure SiOC ceramics were investigated. Results show that the organosilicon preceramic polymers have outstanding photocuring properties and could transform into amorphous SiOC ceramics at 800–1200 °C. It is illustrated that the excellent mechanical properties of SiOC ceramics with octet truss structure (after 3D printing and pyrolysis) are attributed to the metal elements pinning in the amorphous matrix on the atomic level. Doping other metal elements such as Fe, Ni, Co, Pt, etc, is thought to bring promising properties for the lattice structure SiOC ceramics and potentially further expand its applications in the future.

A digital light processing 3D printer for fast and high-precision fabrication of soft pneumatic actuators

In this paper, we built up a desktop digital light processing (DLP) 3D printer and fabricated multiple size soft pneumatic actuators integrally with fast speed and high precision. The printing process is based on the projection microstereolithography method. The composition of the printing system and key parameters during the printing process were presented. Evaluation experiments demonstrate that our printer can print objects with as small as 87.5 μm size features. We first printed single pneumatic net-works (pneu-net) actuators integrally and conducted a series of actuation experiments and finite element method analyses to test the actuators’ deformation capability. We further designed a soft pneumatic gripper containing three micro pneu-net actuators with 0.4 mm wide square air channels as well as 0.2 mm thick chamber walls and fabricated the gripper integrally in less than 30 min using our printer. The grasping capability of the gripper was verified through experiments as well. Results presented in this work prove the performance of the DLP printer we build up and show the convenience of fabricating micro soft pneumatic actuators integrally using DLP 3D printing approach with fast speed and high precision.

High-Speed 3D Printing of High-Performance Thermosetting Polymers via Two-Stage Curing.

Design and direct fabrication of high-performance thermosets and composites via 3D printing are highly desirable in engineering applications. Most 3D printed thermosetting polymers to date suffer from poor mechanical properties and low printing speed. Here, a novel ink for high-speed 3D printing of high-performance epoxy thermosets via a two-stage curing approach is presented. The ink containing photocurable resin and thermally curable epoxy resin is used for the digital light processing (DLP) 3D printing. After printing, the part is thermally cured at elevated temperature to yield an interpenetrating polymer network epoxy composite, whose mechanical properties are comparable to engineering epoxy. The printing speed is accelerated by the continuous liquid interface production assisted DLP 3D printing method, achieving a printing speed as high as 216 mm h-1 . It is also demonstrated that 3D printing structural electronics can be achieved by combining the 3D printed epoxy composites with infilled silver ink in the hollow channels. The new 3D printing method via two-stage curing combines the attributes of outstanding printing speed, high resolution, low volume shrinkage, and excellent mechanical properties, and provides a new avenue to fabricate 3D thermosetting composites with excellent mechanical properties and high efficiency toward high-performance and functional applications.

High-strength boehmite-acrylate composites for 3D printing: Reinforced filler-matrix interactions

The development of additive manufacturing has offered great potential as well as challenge to resins with good performance. Here we report a facile strategy to obtain significant enhancement of mechanical properties for resins via incorporation of boehmite nanowires surface modified by a new modifier β-carboxyethyl acrylate (β-CEA), instead of the commonly used coupling agent 3-trimethoxysilylpropyl methacrylate (TMSPM). Fourier Transform Infrared Spectroscopy (FT-IR) and thermogravimetric analysis (TGA) characterizations were conducted to demonstrate the grafting of β-CEA onto the surface of boehmite nanowires. Significant mechanical enhancement was demonstrated by manufacturing and evaluation of product quality of delicate Fullerene model with neat acrylate and composite resins with β-CEA-AlOOH via Digital Light Processing (DLP) 3D printing.

Fabrication and characterisation of ceramics via low-cost DLP 3D printing

A stereolithography-based additive manufacturing technique has been used for the fabrication of advanced ceramics. A customised 3D printer using a Digital Light Processing (DLP) projector as UV source has been built to fabricate green bodies from photosensitive resins loaded with 25–60wt% of alumina, 3- and 8-YSZ. The 3D-printed bodies were then sintered in the 1200–1500°C and exhibited thermal stability. As expected, higher ceramic loadings rendered objects with higher density for a given sintering temperature. The limit of solid loading in the resin is approximately 60% and beyond those contents, the extra ceramic appears as powder loosely adhered to the sintered objects. Photogrammetry was used to evaluate the accuracy of the 3D printing process and highlighted a marked deviation between the CAD model and the resulting object, particularly in the top part of the specimens, possibly due to the use of volatile solvents which cause changes in the photoresins used. Nevertheless, that problem may be overcome by thermostatising the printer vat and/or using solvents with higher boiling point. The results obtained suggest the potential application of low cost DLP 3D printing techniques to process ceramics for a number of applications including ceramic fuel cells, piezoelectrics, dental applications, etc.

Thiol-Ene Clickable Gelatin: A Platform Bioink for Multiple 3D Biofabrication Technologies.

Bioprinting can be defined as the art of combining materials and cells to fabricate designed, hierarchical 3D hybrid constructs. Suitable materials, so called bioinks, have to comply with challenging rheological processing demands and rapidly form a stable hydrogel postprinting in a cytocompatible manner. Gelatin is often adopted for this purpose, usually modified with (meth-)acryloyl functionalities for postfabrication curing by free radical photopolymerization, resulting in a hydrogel that is cross-linked via nondegradable polymer chains of uncontrolled length. The application of allylated gelatin (GelAGE) as a thiol-ene clickable bioink for distinct biofabrication applications is reported. Curing of this system occurs via dimerization and yields a network with flexible properties that offer a wider biofabrication window than (meth-)acryloyl chemistry, and without additional nondegradable components. An in-depth analysis of GelAGE synthesis is conducted, and standard UV-initiation is further compared with a recently described visible-light-initiator system for GelAGE hydrogel formation. It is demonstrated that GelAGE may serve as a platform bioink for several biofabrication technologies by fabricating constructs with high shape fidelity via lithography-based (digital light processing) 3D printing and extrusion-based 3D bioprinting, the latter supporting long-term viability postprinting of encapsulated chondrocytes.

High compressive strength metallic architectures prepared via polyelectrolyte-brush assisted metal deposition on 3D printed lattices

Nature controls material properties by complicated assembly and structural organization of matter which contains amount of porosity. In this paper, polymer lattices, including cylindrical simple cubic (CSC), foursquare simple cubic (FSC) and mixed simple cubic (MSC), were built by using curable resin containing a atomic-transfer radical polymerization (ATRP) initiator with Digital Light Processing (DLP) 3D printing technology followed by polyelectrolyte brushes (PMETAC) grafting via surface-initiated ATRP and thereafter electroless plating (ELP) to form metal (Cu and Ni) coatings. The compressive strengths of 3D printing polymer grids before and after surface metallization were investigated experimentally and numerically to find the dependence of the compressive strength of polymer grids on the structure and the enhancement of the electroless deposited metal films to the strength of structure. It shows that the FSC structure exhibits the maximum compressive strength of 3.76 ± 0.31 MPa and specific strength of 1.89 ± 0.16 MPa/g cm−3, and in addition, coated with ∼100 nm metal film, its compressive strength can be 3–4 folded.

Multimaterials 3D Printing for Free Assembly Manufacturing of Magnetic Driving Soft Actuator

AbstractMagnetic Fe3O4 nanoparticles are employed to develop digital light processing 3D printing resin, which is used to fabricate arbitrary‐shape soft actuators with multimaterials printing in a free assembly manner. Various mechanical properties and printed morphology of curing resin with different Fe3O4 nanoparticles content are explored to demonstrate the feasibility and precision. By using magnetic and nonmagnetic resins as the magnetic and nonmagnetic segments, mulitple magnetic devices and actuators are prepared by digital light processing (DLP) 3D printing technology. Moreover, the interfacial binding strength between magnetic and nonmagnetic segments and deflection with different diameters of printed lattices and content of Fe3O4 nanoparticles are investigated. DLP 3D printing enables the free assembly manufacturing of actuators with complicated architectures to achieve bending, deformation, cargo transportation, and so on. As proof‐of‐concept, a flexible gripper is printed through integrating magnetic responsive and nonresponsive materials into one body. This approach with special material will be promising to extend the capability of 3D printing for applications in controllable delivery with remote magnetic control in biological, medical, and robotic fields.

Digital light processing 3D printing of conductive complex structures

3D printing has gained significant research interest recently for directly manufacturing 3D components and structures for use in a variety of applications. In this paper, a digital light processing (DLP®) based 3D printing technique was explored to manufacture electrically conductive objects of polymer nanocomposites. Here, the ink was made of a mixture of photocurable resin with multi-walled carbon nanotubes (MWCNTs). The concentrations of MWCNT as well as the printing parameters were investigated to yield optimal conductivity and printing quality. We found that 0.3wt% loading of MWCNT in the resin matrix can provide the maximum electrical conductivity of 0.027S/m under the resin viscosity limit that allows high printing quality. With electric conductivity, the printed MWCNT nanocomposites can be used as smart materials and structures with strain sensitivity and shape memory effect. We demonstrate that the printed conductive complex structures as hollow capacitive sensor, electrically activated shape memory composites, stretchable circuits, showing the versatility of DLP® 3D printing for conductive complex structures. In addition, mechanical tests showed that the addition of MWCNT could slightly increase the modulus and ultimate tensile stress while decreasing slightly the ultimate stretch, indicating that the new functionality is not obtained at the price of sacrificing mechanical properties.

Characterization of 3D printing techniques: Toward patient specific quality assurance spine-shaped phantom for stereotactic body radiation therapy.

Development and comparison of spine-shaped phantoms generated by two different 3D-printing technologies, digital light processing (DLP) and Polyjet has been purposed to utilize in patient-specific quality assurance (QA) of stereotactic body radiation treatment. The developed 3D-printed spine QA phantom consisted of an acrylic body phantom and a 3D-printed spine shaped object. DLP and Polyjet 3D printers using a high-density acrylic polymer were employed to produce spine-shaped phantoms based on CT images. Image fusion was performed to evaluate the reproducibility of our phantom, and the Hounsfield units (HUs) were measured based on each CT image. Two different intensity-modulated radiotherapy plans based on both CT phantom image sets from the two printed spine-shaped phantoms with acrylic body phantoms were designed to deliver 16 Gy dose to the planning target volume (PTV) and were compared for target coverage and normal organ-sparing. Image fusion demonstrated good reproducibility of the developed phantom. The HU values of the DLP- and Polyjet-printed spine vertebrae differed by 54.3 on average. The PTV Dmax dose for the DLP-generated phantom was about 1.488 Gy higher than that for the Polyjet-generated phantom. The organs at risk received a lower dose for the 3D printed spine-shaped phantom image using the DLP technique than for the phantom image using the Polyjet technique. Despite using the same material for printing the spine-shaped phantom, these phantoms generated by different 3D printing techniques, DLP and Polyjet, showed different HU values and these differently appearing HU values according to the printing technique could be an extra consideration for developing the 3D printed spine-shaped phantom depending on the patient’s age and the density of the spinal bone. Therefore, the 3D printing technique and materials should be carefully chosen by taking into account the condition of the patient in order to accurately produce 3D printed patient-specific QA phantom.