Resin Vat Research Articles

Design considerations for digital light processing bioprinters.

With the rapid development and popularization of additive manufacturing, different technologies, including, but not limited to, extrusion-, droplet-, and vat-photopolymerization-based fabrication techniques, have emerged that have allowed tremendous progress in three-dimensional (3D) printing in the past decades. Bioprinting, typically using living cells and/or biomaterials conformed by different printing modalities, has produced functional tissues. As a subclass of vat-photopolymerization bioprinting, digital light processing (DLP) uses digitally controlled photomasks to selectively solidify liquid photocurable bioinks to construct complex physical objects in a layer-by-layer manner. DLP bioprinting presents unique advantages, including short printing times, relatively low manufacturing costs, and decently high resolutions, allowing users to achieve significant progress in the bioprinting of tissue-like complex structures. Nevertheless, the need to accommodate different materials while bioprinting and improve the printing performance has driven the rapid progress in DLP bioprinters, which requires multiple pieces of knowledge ranging from optics, electronics, software, and materials beyond the biological aspects. This raises the need for a comprehensive review to recapitulate the most important considerations in the design and assembly of DLP bioprinters. This review begins with analyzing unique considerations and specific examples in the hardware, including the resin vat, optical system, and electronics. In the software, the workflow is analyzed, including the parameters to be considered for the control of the bioprinter and the voxelizing/slicing algorithm. In addition, we briefly discuss the material requirements for DLP bioprinting. Then, we provide a section with best practices and maintenance of a do-it-yourself DLP bioprinter. Finally, we highlight the future outlooks of the DLP technology and their critical role in directing the future of bioprinting. The state-of-the-art progress in DLP bioprinter in this review will provide a set of knowledge for innovative DLP bioprinter designs.

Investigating Microstructural and Mechanical Behavior of DLP-Printed Nickel Microparticle Composites

The study investigates the fabrication and analysis of nickel microparticle-reinforced composites fabricated using the digital light processing (DLP) technique. A slurry is prepared by incorporating Ni-micro particles into a resin vat; it is thoroughly mixed to achieve homogeneity. Turbidity fluctuations are observed, initially peaking at 50% within the first two minutes of mixing and then stabilizing at 30% after 15–60 min. FTIR spectroscopy with varying Ni wt.% is performed to study the alterations in the composite material’s molecular structure and bonding environment. Spectrophotometric analysis revealed distinctive transmittance signatures at specific wavelengths, particularly within the visible light spectrum, with a notable peak at 532 nm. The effects of printing orientation in the X, Y, and Z axes were also studied. Mechanical properties were computed using tensile strength, surface roughness, and hardness. The results indicate substantial enhancements in the tensile properties, with notable increases of 75.5% in the ultimate tensile strength and 160% in the maximum strain. Minimal alterations in surface roughness and hardness suggest favorable printability. Microscopic examination revealed characteristic fracture patterns in the particulate composite at different values for the wt.% of nickel. The findings demonstrate the potential of DLP-fabricated Ni-reinforced composites for applications demanding enhanced mechanical performance while maintaining favorable printability, paving the way for further exploration in this domain.

Multi-color dual wavelength vat photopolymerization 3D printing via spatially controlled acidity

Dual wavelength vat photopolymerization (DW-VP) has emerged as a powerful approach to create multimaterial objects. However, only a limited range of properties have been showcased. In this work, we report the 3D printing (3DP) of multi-color objects from a single resin vat using DW-VP. This was accomplished by concurrently curing resin with visible light and modulating local resin color with 365-nm ultraviolet (UV) light. The key advance was to use a photoacid generator (PAG) in combination with pH responsive dyes in the 3DP resins. The specific color is dictated by the extent of reaction, or local acidity in our case, and controlled by the light dosage and pattern of UV light applied. Multi-color object formation was implemented in two-step processes involving first 3DP to set the object structure, followed by UV exposure, as well as single processes that leveraged DW-VP to create a broad range of vibrant colors and patterns.

Dual‐Wavelength Vat Photopolymerization 3D Printing with Hybrid Acrylate‐Epoxy Resins: Influence of Resin Composition on Microstructure and Mechanical Properties

Dual‐wavelength vat photopolymerization 3D printing represents a convenient technology for the fabrication of objects with heterogeneous and locally controlled mechanical properties. By using two λ‐orthogonal cross‐linking reactions, it is possible to produce soft and stiff photopolymers with a single resin vat by switching the light source. Herein, hybrid acrylate‐epoxy resins are selectively cured by using either visible or UV light. At 405 nm, a free radical curing of the acrylate monomers is induced while irradiation with 365 nm triggers an additional cationic ring opening reaction of the epoxy monomer yielding interpenetrating photopolymer networks. In a comprehensive approach, the influence of the resin composition and the applied wavelength on cure kinetics, film morphology, (thermo)mechanical properties, and printability are studied. Fully separated as well as homogenous network morphologies are obtained depending on the ratio between acrylate and epoxy monomers, cure rate and applied light source (405 vs 365 nm). In general, glass transition temperature, stiffness, and tensile strength of the photopolymers increase with rising epoxy content. In contrast, a higher epoxy concentration in combination with a higher amount of the cationic photoinitiator compromises on the system's orthogonality, giving rise to the important role of the resin composition in dual‐wavelength vat photopolymerization 3D printing.

Methods for modeling and real-time visualization of CLIP and iCLIP-based 3D printing

Resin 3D printing is experiencing greater adoption in real-world fabrication settings for both prototyping and manufacturing. To optimize the speed of these fabrication methods, few techniques for predicting and visualizing such processes, and its variants, in real-time have been proposed and rigorously evaluated. In this work we outline and validate two complementary methods to monitoring resin 3D printing, specifically continuous liquid interface production (CLIP), in real-time for optimal print parameter selection. Namely, we use: (1) a load cell mounted on the build platform capable of measuring tensile forces during printing, and (2) our novel approach to optical coherence tomography scanning aligned with the printer resin vat. After describing our technical implementation of each of these strategies below, we assess the benefits of each in turn, and evaluate their relative limitations.

Low-Cost, Compact Quadrupole Mass Filters with Unity Mass Resolution via Ceramic Resin Vat Photopolymerization.

This study reports novel, compact, and additively manufactured quadrupole mass filters (QMFs) with adequate filtering performance for practical mass spectrometry applications. The QMFs are monolithically fabricated via vat photopolymerization of glass-ceramic resin using 57µm×57µm×100µm voxels, and selective electroless plating of nickel-boron. Experimental characterization of QMF prototypes at 1.74MHz using FC-43 yields 131Da peaks with 0.50Da full width at half maximum (260 resolution), surpassing the resolution of reported miniaturized counterparts under similar conditions, and being on par with commercial, non-miniaturized, heavier devices. The sensitivity of the 3D-printed devices is estimated at 0.13mATorr-1 (comparable to that of optimized, commercial counterparts), while the devices attained up to 250Da of mass range (limited by the driving electronics). The work is of interest to low-cost, capable mass spectrometry, 3D-printed instruments, and in-space manufacturing of complex instrumentation.

Upconversion 3D printing enables single-immersion multi-material stereolithography

Photopolymer 3D printing techniques have a common limitation: the layers have to be deposited in a sequential order, which constrains multi-material fabrication to techniques where the vats have to be exchanged many times, in addition to repeating the cleaning step for each vat change. In this work, we present a technique that allows multi-material stereolithography by overcoming the layer-by-layer limitation, enabling one single immersion per resin. This is enabled by selective crosslinking of voxels at any depth and position inside the stereolithography (SLA) resin vat. The concept uses invisibility windows and upconversion to achieve selective volumetric crosslinking. This enables printing inside and through previously 3D printed parts. A photopolymer resin is developed, in which a visible light photoinitiator (fluorinated diphenyl titanocene) with a broad absorption and an optical absorber (naphthalimide class dye, maximum absorption at 445 nm) are carefully matched with the upconversion emissions of lanthanide-doped phosphors (both micron-sized phosphors and customized core/shell/shell nanoparticles). The lateral printing resolution of the formulation was found to be 134 ± 13 µm, which was improved to 103 ± 11 µm through the addition of light-absorbing dye. The vertical resolution was defined by the layer thickness achieved by lowering the build platform, which was set to 100 µm. The NIR light used for the excitation of upconversion phosphors enabled enhanced penetration depths of up to 5.8 cm, which allowed SLA printing of interwoven multi-color samples, multi-material rigid/flexible (acrylate/elastomer) samples and dielectric/metallic plated samples. This opens up a myriad of new possibilities, such as 3D printing objects inside cavities in a different material, restoration of broken objects and artefacts, 3D circuitry, insitu bioprinting, etc.

Wavelength Selective Multi-Material 3D Printing of Soft Active Devices Using Orthogonal Photoreactions.

Orthogonal photoreactions provide a unique way to locally and independently control (thermo)mechanical properties and functionality of polymer networks simply by choice of the wavelength. Herein, a library of acrylate functional coumarin monomers is synthesized, which are cured by sequence-dependent wavelength orthogonality. In the presence of a long wavelength absorbing photoinitiator, the monomers undergo rapid curing by visible light induced radical chain growth polymerization. Subsequent irradiation with light in the UV-A region selectively initiates the [2+2] photocycloaddition of the coumarin chromophores, which is confirmed by FTIR and UV-vis experiments. Through a well-targeted design, acrylate-based and thiol-acrylate resin formulations are prepared, whose fast curing rate, low viscosity, and prolonged storage stability enable the one-step fabrication of multi-material structures by digital light processing (DLP) 3D printing. By using a dual-wavelength printer, which operates at two different wavelengths (405 and 365nm), objects comprising soft (ε =22%, σ =7.5MPa) and stiff (ε=2%, σ =8.3MPa) domains are printed with a single resin vat. Along with tensile properties, the wavelength selective change in the network structure features a local control of the glass transition temperature (ΔTg =17°C) in the 3D-printed objects. Soft active devices are fabricated by dual-wavelength DLP 3D printing, with distinct domains having a higher Tg and the local programming of multi shapes is demonstrated.

Spatially controlling the mechanical properties of 3D printed objects by dual-wavelength vat photopolymerization

To date, the 3D printing of polymers with heterogeneous and locally controlled material properties is still a challenging area in additive manufacturing. In terms of vat photopolymerization 3D printing, the fabrication of multi-material objects typically relies on an automatic material exchange of different resin vats. However, along with the high complexity of the printing equipment, this technique suffers from a low build speed and often yields 3D printed objects with week interlayer adhesion across the various material interfaces. Herein, we use chemo-selective wavelengths to fabricate objects with multi-material properties by dual-wavelength vat photopolymerization 3D printing employing a single vat. The photopolymers’ stiffness and flexibility are conveniently controlled by two photoreactions working at two different wavelengths. In particular, a dual photocurable resin is applied containing multi-functional acrylates, which are cured by a radical induced chain growth reaction at 405 nm, and bi-functional epoxy monomers, which additionally undergo cationic curing upon UV exposure (365 nm). FT-IR experiments confirm the wavelength selective network formation whilst dynamic mechanical analysis and tensile tests give evidence of the distinctive difference of the related mechanical properties. By being able to produce soft (ε = 24%, σ = 1.0 MPa) and stiff (ε = 4%, σ = 39.1 MPa) networks with a single resin vat, we demonstrate the efficient fabrication of 3D structures with locally controlled mechanical properties using a dual-wavelength 3D printer operating at 405 and 365 nm. In contrast to previous work in this field, we were able to significantly expand the range of mechanical properties by appropriate selection of the acrylic components and to drastically accelerate the build speed by changing the cationic photoinitiator and using a customized printer with high intensity LED sources.

Dual-wavelength volumetric stereolithography of multilevel microfluidic devices.

Microfluidic devices are typically fabricated in an expensive, multistep process (e.g., photolithography, etching, and bonding). Additive manufacturing (AM) has emerged as a revolutionary technology for simple and inexpensive fabrication of monolithic structures-enabling microfluidic designs that are challenging, if not impossible, to make with existing fabrication techniques. Here, we introduce volumetric stereolithography (vSLA), an AM method in which polymerization is constrained to specific heights within a resin vat, allowing layer-by-layer fabrication without a moving platform. vSLA uses an existing dual-wavelength chemistry that polymerizes under blue light (λ = 458 nm) and inhibits polymerization under UV light (λ = 365 nm). We apply vSLA to fabricate microfluidic channels with different spatial and vertical geometries in less than 10 min. Channel heights ranged from 400 μm to 1 mm and could be controlled with an optical dose, which is a function of blue and UV light intensities and exposure time. Oxygen in the resin was found to significantly increase the amount of dose required for curing (i.e., polymerization to a gelled state), and we recommend that an inert vSLA system is used for rapid and reproducible microfluidic fabrication. Furthermore, we recommend polymerizing far beyond the gel point to form more rigid structures that are less susceptible to damage during post-processing, which can be done by simultaneously increasing the blue and UV light absorbance of the resin with light intensities. We believe that vSLA can simplify the fabrication of complex multilevel microfluidic devices, extending microfluidic innovation and availability to a broader community.

Reducing lateral stair-stepping defects in liquid crystal display-based vat photopolymerization by defocusing the image pattern

Vat photopolymerization-based additive manufacturing (AM) technology has emerging success in efficiently fabricating complex and high-resolution objects. However, the lateral and vertical stair-stepping defects limit vat photopolymerization in manufacturing smooth surfaces, such as optical components. Vertical stair-stepping was mitigated by thinner layer thickness or layer-less fabrication. It is still challenging to reduce the lateral stair-stepping defects caused by the pixelated projection pattern, especially for Liquid Crystal Display (LCD) projection whose pixels are disconnected. To tackle this lateral stair-stepping issue, we propose to defocus the curing image pattern by increasing the gap between the LCD screen and the resin vat. This gap intentionally blurs the disconnected pixels to create a continuous and smooth projection pattern. Experiments verified that the smoothened LCD pattern led to an average 81.2% reduction in surface roughness, which was much more effective than grayscale pixels. The gap between the LCD screen and the resin vat also enabled blowing air to dissipate the heat from LCD and the resin polymerization, reducing the part distortion and printing failure due to the thermal stress. The present approach improved the surface roughness and paved the way for vat photopolymerization in fabricating smooth micro-optics, microfluidic channels, and functional curved surfaces.

Combining Hydrophilic and Hydrophobic Materials in 3D Printing for Fabricating Microfluidic Devices with Spatial Wettability

AbstractThe fabrication of microfluidic flow cells via projection micro‐stereolithography (PμSL) has excited researchers in recent years. However, due to the inherent process properties of most commercial PμSL, microfluidic devices are fabricated in a monolithic fashion with uniform material properties across a flow cell. Yet, the large surface‐to‐volume ratio in microfluidics demands to tailor microchannel surface properties—particularly in planar microchannel arrangements—with spatial control and micron‐scale resolution to form a desired flow profile, e.g., emulsion droplets. Here, the fabrication of planar microfluidic devices by PμSLbased 3D printing with spatial control over surface properties is presented. For that, homemade photopolymer formulations being either hydrophilic or hydrophobic are designed. Adding acrylic acid to a resin containing poly(ethylene glycol) diacrylate lowers the contact angle down to 0° against water creating a superhydrophilic surface. By utilizing 1H,1H,2H,2H‐perfluorodecyl acrylate, a photopolymer formulation allowing for 3D‐printing a hydrophobic microchannel surface with a contact angle >120° against water is obtained. Combining these two materials, microfluidic flow cells with spatially defined wettability are 3D‐printed for emulsion formation. Finally, the resin vat of the commercial PμSL printer is switched during the printing process for fabricating multimaterial geometries, as exemplarily applied for realizing a hydrophobic‐hydrophilic‐hydrophobic device for forming O/W/O double emulsions.

Continuous 3D printing from one single droplet

3D printing has become one of the most promising methods to construct delicate 3D structures. However, precision and material utilization efficiency are limited. Here, we propose a one-droplet 3D printing strategy to fabricate controllable 3D structures from a single droplet ascribing to the receding property of the three-phase contact line (TCL) of the resin droplet. The well-controlled dewetting force of liquid resin on the cured structure results in the minimization of liquid residue and the high wet and net material utilization efficiency in forming a droplet into a 3D structure. Additionally, extra curing induced protruding or stepped sidewalls under high printing speed, which require high UV intensity, can be prevented. The critical is the free contact surface property of the droplet system with the introduction of the receding TCL, which increased the inner droplet liquid circulation and reduces the adhesion properties among the liquid resin, cured resin, and resin vat.

Constrained Window Design in Projection Stereolithography for Continuous Three-Dimensional Printing.

Although continuous projection Stereolithography (SL) printing is one of the next-generation ultra-fast additive manufacturing (AM) processes, its current constrained window design of the resin vat is suffering from many challenges, such as the complex control mechanism and limited hardware lifetime. In this article, we investigate a novel constrained window design, namely Island Window (IW), for the continuous three-dimensional (3D) printing by using the projection SL process. The proposed IW window has a highly oxygen-permeable polydimethylsiloxane membrane onto the laser-machined acrylic vat frame, which allows the formation of an effective liquid interface (>200 μm oxygen inhibition layer) to enable the continuous projection SL process. Experimental results verified the feasibility of the window design for extending the maximum printing time (increased by up to 73%) due to the enhanced oxygen concentration. It was also found that the maximum printing speed for producing parts with a smooth surface (Rz <30 μm) can be as high as 90 mm/h, which is comparable to the speeds in recently reported continuous SL processes. In addition, a variety of parts were successfully fabricated through continuous 3D printing by using the proposed IW design, implying tremendous promise for future low-cost, high-resolution, easy-controlled, and ultra-fast AM processes.

Additive manufacturing of Ku/K‐band waveguide filters: a comparative analysis among selective‐laser melting and stereo‐lithography

This study describes an experimental research activity on the manufacturing of passive waveguide components by means of two additive technologies, namely the selective-laser melting and the stereo-lithography processes. The former is a power-bed fusion process, for which aluminium, titanium, and steel powders have been considered in this study. The latter is based on the photopolymerisation of a resin vat. This technology has been applied in conjunction with an acrylonitrile–butadiene–styrene-like polymer and a subsequent electrolytic copper plating for the metallisation of the internal channels. Wideband filters operating in Ku/K band have been developed as benchmarks, for which detailed comparisons between simulations and measurements are reported.

Investigation of separation force for constrained-surface stereolithography process from mechanics perspective

PurposeOne of the major concerns of the constrained-surface stereolithography (SLA) process is that the built-up part may break because of the force resulting from the pulling-up process. This resultant force may become significant if the interface mechanism between the two contact surfaces (i.e. newly cured layer and the bottom of the resin vat) produces a strong bonding between them. The purpose of this paper is to characterize the separation process between the cured part and the resin vat by adopting an appropriate and simple mechanics-based model that can be used to probe the pulling-up process.Design/methodology/approachIn this paper, the time-histories of the pulling-up forces are measured using FlexiForce® force sensors. The experimental data are analyzed and used to estimate the constitutive parameters of the separation mechanism. Here, the separation mechanism is modeled based on the concept of cohesive zone model (CZM) that is well-studied in the field of fracture mechanics. By using the experimentally measured pulling-up force, this paper proposes a very efficient inverse technique to estimate the constitutive parameters for the CZM. The constitutive laws for the CZM facilitate in relating the separation force at the interface between the cured part and the resin vat in terms of the pulling-up velocity. Unlike work proposed earlier, computationally expensive full-scale finite element runs are not essential in the current work while estimating the required parameters of the constitutive laws. Instead, mechanics-based computationally efficient surrogate model is proposed to readily estimate these constitutive parameters.FindingsTwo constitutive laws are compared on the basis of their predictions of the separation force profile. Excellent match is obtained between the measured and the predicted separation force profiles.Originality/valueThis paper selects a suitable mechanics-based model that can characterize the separation process and proposes a computationally efficient scheme to estimate the required constitutive parameters. The proposed scheme can be used to reliably predict the separation force for the constrained-surface SLA process, leading to improved productivity and reliability of the SLA processes in fabricating the built-up parts.

Tilting separation analysis of bottom-up mask projection stereolithography based on cohesive zone model

Compared with top-down stereolithography, bottom-up mask projection stereolithography can reduce the start filling volume of vat and is able to build components with high-viscosity materials. For general photosensitive materials, a separation process is required to detach the cured layer from the resin vat surface in order to accomplish the fabrication of current layer. The separation process can be achieved without damaging the part by utilizing appropriate platform motions including pulling-up, tilting and shearing, and covering inert film on the vat surface. The tilting separation is used in both industrial and academic area. However, there is a limited corresponding study compared with pulling-up separation. The mechanism of tilting separation and its effects on separation force and fabrication process are not clear. In this paper, an analytical model based on cohesive zone model was formed and a specialized experimental system was built. Experimental studies on the tilting effects on cohesive stiffness and fracture energy were conducted by collecting and analyzing separation force data. The results showed that changing exposure area function or the part fabrication orientation changed the cohesive stiffness, and increasing tilting separation velocity caused different increase in fracture energy when using different inert films. The results of this investigation can be used to choose the reasonable platform motion and process parameters by considering the part geometry and the characteristics of both inert film and materials.

Raster‐based exposure through multiple parallel beams in stereolithography

A method to control raster‐based exposure in multiple‐LED photographic curing (MPC) is introduced. During the build process, beams of multiple sources, each exposing a separate area, are projected vertically onto the resin surface while the projector moves continuously across the resin vat. In the projector, 1,024 beams of light are arranged in an array of eight staggered rows. Part‐building data are written in numerical control (NC) geometric details (G)‐codes to produce a physical replica of the digital part design. The sliced 3D data are converted into bitmap data, an image consisting of tiny dots called pixels. The individual layers of a part are built up through pixels arranged in a bitmap. The formation of objects happens through curing in an orthogonal raster with a mesh of 62.5mm. To obtain even intensity in the surface, five beams are used to cure a single string. In part‐building experiments, microscopic photographs taken of parts clarified the characteristic pattern of the exposure technique.

Heat generation at photopolymerization by multiple-spot exposure with blue light

In this paper, heat generation in an acrylate monomer during photopolymerization in multiple-LED photographic curing (MPC) is studied. During the exposure process, an array of multiple beams of light are projected simultaneously on to the resin surface while the projector moves across the resin vat. In this way, exposure occurs through raster scanning. It was found that the heat generated by the polymerization process causes the resin temperature to increase in steps to a maximum of 25 °C when scanning at a velocity of 0.5 mm/s. Heat from the light source itself is not induced. At 3 mm/s, excess temperature decreases to only approximately 4.5 °C. For one layer, three-dimensional temperature profiles show that excess temperature is distributed equally, perpendicular to the scan direction. The polymerization process occurs slowly. Using spectrophotometry, the course of absorbance of the photoinitiators can be observed. At 470 nm, the absorbance increases from 0.4 to 1.2 in the liquid state. The absorbance of the photoinitiators decreases to 0.1. After 30 min of exposure, the polymerization process is complete. This necessitates post-curing after the actual build process.

Thermal-expansion and fracture toughness properties of parts made from liquid crystal stereolithography resins

Stereolithography is a layered manufacturing technique that uses a laser to selectively polymerize a thermosetting resin to form complex three-dimensional parts. This rapid prototyping technique has evolved over the years from a model maker to a rapid fabrication technique for soft-tooling and parts for in-service testing. With this evolution in applications has come increased demand for high-performance resins. Liquid crystal (LC) resins can be processed by stereolithography methods to provide parts with superior and in some cases unique properties compared with conventional resins. LC resins consist of rod-like molecules that can be aligned prior to cure resulting in anisotropic mechanical and physical properties. A magnet outside the resin vat can be used to control the molecular alignment. Alignment can be varied within a layer or from layer-to-layer to achieve desired properties much as is done with fiber reinforced plastics. Two properties that are important for many applications are toughness and thermal dimensional-stability. Initial studies showed that fracture toughness values of LC coupons were about twice that of isotropic coupons manufactured from the same resin. Also, we have demonstrated that the in-plane thermal expansion of LC parts can be minimized over a wide temperature range by rotating the molecular alignment 90° from one layer to the next.