Photopolymerization Technology Research Articles

Field-Controlled Microrobots Fabricated by Photopolymerization.

Field-controlled microrobots have attracted extensive research in the biological and medical fields due to the prominent characteristics including high flexibility, small size, strong controllability, remote manipulation, and minimal damage to living organisms. However, the fabrication of these field-controlled microrobots with complex and high-precision 2- or 3-dimensional structures remains challenging. The photopolymerization technology is often chosen to fabricate field-controlled microrobots due to its fast-printing velocity, high accuracy, and high surface quality. This review categorizes the photopolymerization technologies utilized in the fabrication of field-controlled microrobots into stereolithography, digital light processing, and 2-photon polymerization. Furthermore, the photopolymerized microrobots actuated by different field forces and their functions are introduced. Finally, we conclude the future development and potential applications of photopolymerization for the fabrication of field-controlled microrobots.

Near-infrared light induced curing of electrically conductive adhesive assisted by upconversion particles

We report a highly-efficient method for preparing an electrically conductive adhesive (ECA) by using the upconversion nanoparticle-assisted near infrared photopolymerization (UCAP) technology.

Additive manufacturing and characterization of microstructure evolution of Inconel 718 superalloy produced by vat photopolymerization

Vat photopolymerization is a potential method to prepare metal parts. This research reports the first manufacturing of nickel-based superalloy Inconel 718 components by vat photopolymerization. The photocurable pastes with a high-volume loading of metal powder of 55 % were prepared, and it has the advantages of excellent stability and fluidity at a constant shear rate, which is suitable for fabricating green body. Inconel 718 complex green parts with high precision were successfully printed by vat photopolymerization. Subsequently, the green body was debinded to remove resin and sintered at different temperatures to obtain metal parts and retain their specific characteristics. The microstructure evolution of the printed parts during sintering was studied comprehensively, and the sintering mechanism was discussed in detail. The microstructure and phase compositions were characterized by SEM, EDX, and XRD, respectively. The results indicate that oxides were formed in the sintered parts, hindering further densification of Inconel 718 metal components. Nickel-based superalloy with complex structures was obtained using vat photopolymerization technology.

VAT photopolymerization 3D printing optimization of high aspect ratio structures for additive manufacturing of chips towards biomedical applications

Organ-on-chip and Lab-on-chip are microfluidic devices widely applied in the biomedical field. They are traditionally produced by soft lithography: starting from a mold fabricated by optical photolithography, a Polydimethylsiloxane (PDMS) device is obtained by casting and baking. While this technique offers the possibility to produce features with high resolution, it is not flexible enough to respond to the necessity of customization and prototyping. In this study, we propose as alternative the production of devices by digital light processing (DLP), a vat photopolymerization technology, in combination with a commercially available, biocompatible resin. Studying the process factors by a statistical methodology called Design of Experiment (DoE), we were able to achieve small features with high aspect ratio (60). DoE method allowed us to have a deep understanding of the process without the need of any physical inspection of the involved phenomena, and to generate empirical models, correlating the process factors to the dimensions of the final printed object. We proved that this optimization was beneficial also in terms of transparency (evaluated by UV-Vis spectrophotometry), and mechanical strength (evaluated by a compression test) of the printed resin. Finally, a proof-of-concept microfluidic device was fabricated, sealed to a PDMS membrane through an oxygen plasma treatment, and tested against leakage on a microfluidic circuit for one week. As result, we proved that DLP printing is not only a suitable method to develop microfluidic devices, but if correctly optimized it can also reproduce small features in the order of tens of micrometers rapidly.

Fabrication of high-strength Si/SiC composites with low shrinkage rates via vat photopolymerization technology and liquid silicon infiltration

Vat photopolymerization (VPP) technology has shown great application potential in the manufacture of high-precision complex silicon carbide (SiC) based ceramic components. However, in comparison with the conventional fabrication technologies, the SiC ceramics produced by VPP technology have insufficient strengths or relatively larger shrinkage rates to meet the requirements of practical applications. Herein, we develop a novel bimodal SiC material recipe for VPP technology and prepare Si/SiC composites by the consequent liquid silicon infiltration (LSI) method. The addition of fine SiC particles improves the material’s mechanical properties significantly. The final products possess low shrinkage rates (3.52 ± 0.26% in the x-direction, 3.73 ± 0.39% in the y-direction, and 4.69 ± 0.35% in the z-direction) as well as high flexural strength (363.23 ± 14.21 MPa) and low open porosity of 0.097%. The material compositions and fabrication strategies proposed in this work demonstrate an efficient way for the additive manufacturing of high-precision complex Si/SiC ceramic parts (e.g. lightweight space mirrors and structural ceramic parts). • A novel material recipe is developed for vat photopolymerization-fabricated Si/SiC composites. • The final Si/SiC composites have high strength (>350 MPa) and low shrinkage rates (< 5%). • A large-size reflective mirror ( Ф 172 mm) is manufactured in this study.

Antifouling and Antibacterial Surfaces Grafted with Sulfur-Containing Copolymers

Antifouling and antibacterial surfaces that can prevent nonspecific biological adhesion are important to support a myriad of biomedical applications. In this study, we have used an innovative photopolymerization technology to develop sulfur-containing polymer-grafted antifouling and antibacterial surfaces. The relationship between the hydrophilic property and the capability to resist protein and macrophage adsorption of the surface copolymer brushes was investigated. The sulfide monomer incorporated into the surface copolymer brushes can be further ionized to carry positive charges and impart antibacterial activity, leading to surfaces with dual antifouling and antibacterial functions. We believe that the reported sulfur-containing polymer brushes can be considered an emerging and important polymer for antifouling and antibacterial applications.

Improvement in the storage stability of free radical photocurable materials under sunlight based on the cis → trans photoisomerization of pyrrole chalcone photoinitiator

A novel pyrrole chalcone photoinitiator, (E)-3-((1H-pyrrol-2-yl)methylene)chroman-4-one (E-PCO), was synthesized and characterized. The photoisomerization progress of E-PCO converting into a mixture of E-PCO/Z-PCO was examined by analysing the time-dependent evolution using 1H NMR spectroscopy and UV–vis absorption spectroscopy. The results demonstrated that E-PCO efficiently initiated LED free radial photopolymerization with the additive TEOA, while the photoequilibrium of E-PCO/Z-PCO ineffectively initiated photopolymerization, endowing photocurable materials containing E-PCO/Z-PCO with high storage stability to sunlight. Moreover, Z-PCO of the photoequilibrium was proven to isomerize back to E-PCO fully upon LED@465 nm irradiation so that photocurable materials containing E-PCO/Z-PCO could restore its photoreactivity for polymerization after illumination. Therefore, an approach for improving photocurable materials' storage stability to sunlight by controlling the initiating system's photoreactivity was proposed in this research, which has notable potential for outdoor photopolymerization technology.

Large-molecular-weight acyldiphenylphosphine oxides as low-mobility type I photoinitiator for radical polymerization

In recent years, the light source for photopolymerization technology is evolving from energy-hungry mercury lamps to the safer light-emitting diodes (LEDs), and it is urgent to develop new photoinitiator adapted to the photopolymerization system with LED light source. Some acyldiphenylphosphine oxide photoinitiators have been designed and developed for LED light source, but their application is severely limited in the fields of food packing, medical equipment, environmentally-friendly product etc. due to their high mobility in photocured polymers. In this study, eight acyldiphenylphosphine oxide molecules (DAPO-1 ∼ DAPO-8) were synthesized by introducing large molecular weight groups and their UV–visible absorption, photodegradation quantum yield, photopolymerization properties, mobility and solubility in monomer were determined. Thereinto, photoinitiators show lower mobility than TPO, the photoinitiator commonly used in industry, as well as good solubility in monomers and excellent photopolymerization performance. Therefore, they can serve as potential substitutes for commercial TPO, especially in the application of low-migration photocurable system.

A Narrowband 3-D Printed Invar Spherical Dual-Mode Filter With High Thermal Stability for OMUXs

A 3-D printed narrowband bandpass filter based on spherical dual-mode resonators is presented in this article. It is designed for output multiplexers (OMUXs) using high-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> spherical dual-mode resonators. Realization is by laser powder bed fusion (L-PBF) technology of Invar alloy chosen for its low coefficient of thermal expansion (CTE). Using PBF circumvents the alloy&#x2019;s manufacturability issues associated with its hardness in machining and free forming. Compared with polymer-based vat photopolymerization technology, PBF allows for direct metal manufacture of complex monolithic microwave components with better thermal-mechanical properties and higher power-handling capability. Using Invar can further help achieve high temperature stability of the filter in high-power operation. To demonstrate the proposed solution, detailed thermal-RF test at different temperatures was carried out. The experimental results of a 0.47&#x0025; fourth-order silver-plated Invar filter with two transmission zeros verify the design and manufacturing. An insertion loss of 1 dB and an effective temperature coefficient of less than 2 ppm/K were achieved.

One-pot efficient preparation of microcapsules based on photopolymerization for self-healing coatings

Self-healing microcapsules are currently one of the most promising functional materials due to their excellent performance. The preparation of self-healing microcapsules via photopolymerization is quite attractive considering its efficiency and environmental-friendliness. However, the influence of the preparation process and shell-formation materials on the formation and the morphology of microcapsule have not been systematically researched. This work combines photopolymerization technology and solvent evaporation to prepare microcapsule, and comprehensively explores the influence of process conditions and shell-formation materials on the microcapsules. Two preparation processes, UV-SR (photopolymerization first and then solvent removal) and SR-UV (solvent removal first and then photopolymerization), have been employed to prepare microcapsules. The SR-UV method is more suitable for preparing microcapsules with high core content and spherical core-shell morphology. The particle size distribution of microcapsules is related to the shear strength during initial emulsion preparation, and the size of microcapsules can be effectively controlled. At the same time, the effect of the shell-formation materials is also systematically studied. The method is universal for the preparation of microcapsules based on commercial UV-curable prepolymers, but the introduction of UV-curable monomers may lead to the transformation of microcapsules from core-shell structure to multi-core structure. Microcapsules can be uniformly dispersed in water-borne epoxy, and the coating based on microcapsules exhibits excellent self-healing properties.

Design, inverted vat photopolymerization 3D printing, and initial characterization of a miniature force sensor for localized in vivo tissue measurements

BackgroundTissue healthiness could be assessed by evaluating its viscoelastic properties through localized contact reaction force measurements to obtain quantitative time history information. To evaluate these properties for hard to reach and confined areas of the human body, miniature force sensors with size constraints and appropriate load capabilities are needed. This research article reports on the design, fabrication, integration, characterization, and in vivo experimentation of a uniaxial miniature force sensor on a human forearm.MethodsThe strain gauge based sensor components were designed to meet dimensional constraints (diameter ≤3.5mm), safety factor (≥3) and performance specifications (maximum applied load, resolution, sensitivity, and accuracy). The sensing element was fabricated using traditional machining. Inverted vat photopolymerization technology was used to prototype complex components on a Form3 printer; micro-component orientation for fabrication challenges were overcome through experimentation. The sensor performance was characterized using dead weights and a LabVIEW based custom developed data acquisition system. The operational performance was evaluated by in vivo measurements on a human forearm; the relaxation data were used to calculate the Voigt model viscoelastic coefficient.ResultsThe three dimensional (3D) printed components exhibited good dimensional accuracy (maximum deviation of 183μm). The assembled sensor exhibited linear behavior (regression coefficient of R2=0.999) and met desired performance specifications of 3.4 safety factor, 1.2N load capacity, 18mN resolution, and 3.13% accuracy. The in vivo experimentally obtained relaxation data were analyzed using the Voigt model yielding a viscoelastic coefficient τ=12.38sec and a curve-fit regression coefficient of R2=0.992.ConclusionsThis research presented the successful design, use of 3D printing for component fabrication, integration, characterization, and analysis of initial in vivo collected measurements with excellent performance for a miniature force sensor for the assessment of tissue viscoelastic properties. Through this research certain limitations were identified, however the initial sensor performance was promising and encouraging to continue the work to improve the sensor. This micro-force sensor could be used to obtain tissue quantitative data to assess tissue healthiness for medical care over extended time periods.

Visible-Light-Sensitive Triazine-Coated Silica Nanoparticles: A Dual Role Approach to Polymer Nanocomposite Materials with Enhanced Properties.

Nanocomposite materials are of great interest because of their superior properties. Besides the traditional synthesis methods that require high temperatures or toxic solvents, photopolymerization technology provides a simple, low-cost, and environmentally friendly route in preparing nanocomposites. In this research, the preparation of blue-light-sensitive triazine derivative-coated silica nanoparticles is presented. The resulting triazine-coated silica nanoparticles can play a dual role, i.e., acting as both photoinitiators to trigger photopolymerization reactions under the irradiation of LED@410 nm and fillers to endow the produced photopolymer nanocomposite materials with enhanced properties. Specifically, the triazine-coated silica nanoparticles can successfully induce free radical polymerization of trimethylolpropane triacrylate efficiently under the irradiation of LED@410 nm and demonstrate comparable photoinitiation ability to the triazine derivative-based photoinitiator. The effects of different loading amounts of triazine-coated silica nanoparticles toward the photopolymerization kinetics are also evaluated. By coating with the triazine derivative, the nanoparticles show good dispersion in the polymer matrix and significantly reduce the shrinkage of the samples during the photopolymerization. Moreover, the photocured nanocomposites exhibit enhanced migration stability and mechanical properties when an optimal amount of triazine-coated silica nanoparticles is added in the formulation.

Effect of monomers with different functionalities on stability, rheology, and curing behavior of ceramic suspensions

Vat photopolymerization (VP) technology is a popular manufacturing technology for ceramic parts with complex shapes and high dimensional accuracy. However, there is still a lack of systematic research on how to design suitable monomer formulations for ceramic powder with different characteristics. In this study, the effect of monomers with different functionalities on the stability, rheology, and curing behavior of different ceramic suspensions was investigated intensively. The results showed that the high-functionality monomer facilitates improved stability and cure depth. Whereas, it has a negative impact on viscosity and excess width. For ceramic powders with a low refractive index and light absorbance, a relatively high cure depth can be obtained by using a monomer with low functionality. Moreover, the cure depth and viscosity of aluminum nitride (AlN) suspensions with different monomer combinations were also studied. Finally, the alumina (Al2O3), zirconia (ZrO2), AlN, and silicon nitride (Si3N4) parts were successfully fabricated.

Development of software and hardware components to the Internet of an additive manufacturing system based on vat photopolymerization technology

The global competition and shifts in consumer’s demand are forcing companies to enhance their productivity by reducing the manufacturing time, cost and increasing the value of final merchandise. Recent engineering researchers find that the additive manufacturing (AM) process could be an innovative solution due to its advantages in building complex shapes using different materials. However, the lack of developing tools to automate AM equipment has limited applying this technique to fabricate products within a production system. Thus, an internet of additive manufacturing(IoAM) system for the Vat photopolymerization process is proposed. The IoAM system is composed of three components which are described. The first one is the machine hardware integrated by all the various mechanical parts of a conventional Bottom-Up DLP 3D printer managed by a control board based on IoT technology. The second element is a web interface that controls and monitors a single or an array of machines from one or multiple operators and devices. The last module is firmware, which is the intermediary between the software and hardware. Besides, it is responsible for executing the adaptive control for the printing parameters, which has allowed to speed up the manufacturing process. The geometric calibration method to overcome distortion of photomask produced by DLP and the energy calibration algorithm to solve the non-uniform energy distribution have permitted to fabricate objects with high resolution around 30um in the XY axis and 50 μm as a layer thickness.

Photopolymerization of Bio-Based Polymers in a Biomedical Engineering Perspective.

Photopolymerization is an effective method to covalently cross-link polymer chains that can be shaped into several biomedical products and devices. Additionally, polymerization reaction may induce a fluid-solid phase transformation under physiological conditions and is ideal for in vivo cross-linking of injectable polymers. The photoinitiator is a key ingredient able to absorb the energy at a specific light wavelength and create radicals that convert the liquid monomer solution into polymers. The combination of photopolymerizable polymers, containing appropriate photoinitiators, and effective curing based on dedicated light sources offers the possibility to implement photopolymerization technology in 3D bioprinting systems. Hence, cell-laden structures with high cell viability and proliferation, high accuracy in production, and good control of scaffold geometry can be biofabricated. In this review, we provide an overview of photopolymerization technology, focusing our efforts on natural polymers, the chemistry involved, and their combination with appropriate photoinitiators to be used within 3D bioprinting and manufacturing of biomedical devices. The reviewed articles showed the impact of different factors that influence the success of the photopolymerization process and the final properties of the cross-linked materials.

The effect of time on mechanical properties of biocompatible photopolymer resins used for fabrication of clear dental aligners

Clear dental aligners are used for treating orthodontic anomalies (misaligned teeth, inappropriate contact between upper and lower teeth etc.), minor irregularities and bruxism. Using additive manufacturing technologies clear dental aligners are made of biocompatible photopolymer, using a vat photopolymerization technology. One of problems in application is the change of aligner material properties after production, including strength and elongation at failure. This can cause different sequence of tooth displacement which will not correspond to the planned therapy. In this paper three types of material testing are conducted i.e., tensile, compressive and three-point bending testing on specimens of 1 (24 h), 3 (72 h), 5 (120 h) and 7 (168 h) days old. Mechanical properties, such as tensile, compressive and flexural strength and strain at failure are monitored in order to show the effect of time on biocompatible photopolymer resin.

Heavy metal removal from aqueous solutions by chitosan-based magnetic composite flocculants

In this study, three magnetic flocculants with different chelating groups, namely, carboxymethyl chitosan-modified Fe3O4 flocculant (MC), acrylamide-grafted magnetic carboxymethyl chitosan flocculant (MCM), and 2-acrylamide-2-methylpropanesulfonic acid copolyacrylamide-grafted magnetic carboxymethyl chitosan flocculant (MCAA) were prepared, synthesized, and characterized by photopolymerization technology. They were applied to the flocculation removal of Cr(III), Co(II), and Pb(II). The effect of flocculation condition on the removal performance of Cr(III), Co(II), and Pb(II) was studied. Characterization results show that the three magnetic carboxymethyl chitosan-based flocculants have been successfully prepared with good magnetic induction properties. Flocculation results show that the removal rates of MC, MCM, and MCAA on Cr(III) are 51.79%, 82.33%, and 91.42%, respectively, under the conditions of 80 mg/L flocculant, pH value of 6, reaction time of 1.5 hr, G value of 200 s−1, and precipitation magnetic field strength of 120 mT. The removal rates of Co(II) by MC, MCM, and MCAA are 54.33%, 84.99%, and 90.49%, respectively. The removal rates of Pb(II) by MC, MCM, and MCAA are 61.54%, 91.32%, and 95.74%, respectively. MCAA shows good flocculation performance in composite heavy metal-simulated wastewater. The magnetic carboxymethyl chitosan-based flocculant shows excellent flocculation performance in removing soluble heavy metals. This research provides guidance and ideas for the development of efficient and low-cost flocculation technology to remove heavy metals in wastewater.

A Review of Vat Photopolymerization Technology: Materials, Applications, Challenges, and Future Trends of 3D Printing.

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.

Evaluating mechanical properties of a photosensitive acrylic resin for additive manufacturing with the addition of organophilic clays

Developing new materials for additive manufacturing (AM) technologies is an important topic, usually targeting cost reduction or better physical properties, or both. This research aims to evaluate the mechanical properties and thermal stability of a non-commercial basic photosensitive acrylic material by the addition of organophilic clays. The organophilic clays Cloisite 20® and organophilic bentonite clay modified with cetyltrimethyl ammonium bromide (BN-CT) were dispersed—0.5, 1.0 and 1.5 (wt%)—in the monomer trimethylolpropane triacrylate, using two dispersion methods. The samples were obtained by the AM principle, similar to the vat photopolymerization technology, in which the deposition and curing were performed layer by layer with Ultraviolet-C radiation. The formulations were checked for viscosity, shrinkage, warping, and strength (tensile and bending test), and analyzed using a scanning electron microscope (SEM), an energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD), and thermogravimetry (TGA). The characterization of the formulations obtained from the two organophilic clays, Cloisite 20® and BN-CT, by the two methods of dispersion, in principle, indicated the formation of a material with an intercalated hybrid morphology structure. The formulations showed improvements in tensile strength (15% for Cloisite 20® and 18% for BN-CT) and flexural strength (14% for Cloisite 20® and 10% BN-CT). The formations´ thermal stability (degradation) was not affected by the addition of the clays; therefore, the cheaper BN-CT would be preferred to improve their mechanical properties.

Photopolymerization of Macroscale Black 3D Objects Using Near-Infrared Photochemistry

The efficiency of the photopolymerization technology significantly decreases when the color of materials blackens, which is contributed by the limitations of light penetration. Herein, we demonstrate rapid generation of black 3D objects up to the centimeter level in size based on melanin using near-infrared (NIR) photochemistry. Melanin, of a low absorption coefficient in the NIR range, allows thorough penetration of the 980 nm light to induce emission from upconversion nanoparticles (UCNPs) for initiating UCNP-assisted photopolymerization (UCAP). A model that describes light-attenuation gradients and dose-dependent kinetics in UCAP-guided NIR photochemistry is developed. Notably, the established model for the UCAP concept provides sufficient vertical light penetration to form scale-predictable black materials and instructs 3D printing applications. The critical control parameters were evaluated, and it was shown that complex macroscale black objects can be processed within dozens of minutes. The modeling methodologies integrated with rich functional fillers will further extend the versatility of UCAP technology in device design and manufacturing.