Photopolymer Material Research Articles

Harnessing structural hierarchy and multi-material approaches to improve crushing performance of re-entrant honeycombs

This study proposes to improve the crushing performance of re-entrant honeycombs by considering both mechanistic and material ingredients. First, a set of novel hierarchical configurations possessing hexachiral auxetic subordinate cells was designed by replacing the solid cell walls of the conventional re-entrant honeycomb. Then the voids of the modified re-entrant honeycombs and the proposed hierarchical metamaterials were filled with a flexible material by using a multi-material 3D printer. Parametric numerical analyses were performed using validated Finite Element models to identify optimal metamaterial architectures. A set of samples, including traditional and developed configurations, were 3D printed and subjected to uniaxial compression loading. The deformation mechanism, compressive strength, and energy absorption of the samples were compared at different strain levels. The results show that introducing hexachiral hierarchies along the ribs prolongs the duration of plastic deformation and keeps the auxeticity of the samples at larger strains. This provides a stable and long post-yield plateau in the compressive stress-strain curve that brings considerable improvement in energy absorption and compressive strength. In addition, multi-material approaches with the combination of rigid photopolymer and flexible rubber-like material also contribute to stabilizing the deformation mechanism of both conventional and developed hierarchical re-entrant auxetic configurations. The proposed hierarchical auxetic structures present more specific compressive strength and specific energy absorption (up to ~150 %) compared to conventional re-entrant auxetic honeycombs. Overall, the proposed hybrid designed strategies can be leveraged to manipulate the collapse mechanism and improve crushing performance.

Formation and properties of volume and relief holographic gratings in photopolymer materials

Formation and properties of volume and relief holographic gratings in photopolymer materials

Multiplexed digital volume reflection holograms generated from digital transmission holograms.

Interference from co-propagation of the object and reference beams can be digitally recorded for a digital transmission hologram (DTH). Volume holograms, as in display holography, which have been traditionally recorded in bulk photopolymer or photorefractive materials using a counter-propagating object and writing beams, are read out using multispectral light and offer the advantage of excellent wavelength selectivity. In this work, the reconstruction from a single digital volume reflection hologram (DVRH) and wavelength multiplexed DVRHs derived from respective single and multi-wavelength DTHs is investigated, using coupled wave theory and an angular spectral approach. The dependence of the diffraction efficiency on volume grating thickness, wavelength, and incident angle of the reading beam is studied.

Additive Manufactured Strain Sensor Using Stereolithography Method with Photopolymer Material.

As a result of the developments in additive manufacturing (AM) technology, 3D printing is transforming from a method used only in rapid prototyping to a technique used to produce large-scale equipment. This study presents the fabrication and experimental studies of a 3D-printed strain sensor that can be used directly in soft applications. Photopolymer-based conductive and flexible ultraviolet (UV) resin materials are used in the fabrication of the sensor. A Stereolithography (SLA)-based printer is preferred for 3D fabrication. The bottom base of the sensor, which consists of two parts, is produced from flexible UV resin, while the channels that should be conductive are produced from conductive UV resin. In total, a strain sensor with a thickness of 2 mm was produced. Experimental studies were carried out under loading and unloading conditions to observe the hysteresis effect of the sensor. The results showed a close linear relationship between the strain sensor and the measured resistance value. In addition, tensile test specimens were produced to observe the behavior of conductive and non-conductive materials. The tensile strength values obtained from the test results will provide information about the sensor placement. In addition, the flexible structure of the strain sensor will ensure its usability in many soft applications.

Curing-dependent thermo-viscoelastic and shrinkage behaviour of photopolymers

This study investigates the influence of curing on the thermo-viscoelastic properties and chemical shrinkage of a commercially available photopolymer resin used in SLA and DLP 3D printing and presents a modelling approach for predicting material properties. UV rheometer and DMA measurements were performed to investigate the viscoelastic properties of the photopolymer material as a function of temperature, degree of cure, and frequency. Time–temperature and time-cure superposition principles were applied to the experimental results. In addition, the chemical shrinkage behaviour was studied by the UV rheometer as a function of the degree of cure and temperature, and a model equation was proposed based on these parameters. Our experimental findings have led to the development of constitutive equations for the complete material behaviour.

A new technique for 3D printing dielectric structures using aerosol-jettable photopolymers

In this article, we report the development of a new method for the 3D printing of dielectrics. An aerosol-jet printer is used to deposit overlapping layers of photopolymer material under ultraviolet floodlight in the assembly of ramping microstructures in situ without the need for supporting structures. Printing is conducted using an in-house photodielectric ink, the development of which is presented with an emphasis on dielectric and mechanical bulk material characterization. Low dielectric loss at the X-band and structural strength are demonstrated, followed by print characterization wherein the driving mechanisms of the new method are explored, tied to print conditions, and related to specific material properties. Finally, a complex structure in the form of a 3D flower is printed to demonstrate the controlled and repeatable performance of the proposed technique.

Holographic recordings without compromise

AbstractWidely tunable continuous‐wave optical parametric oscillator (OPO) systems help meet the demands on holographic optical elements (HOEs) for many use cases, including head‐up displays and augmented reality devices. The wide tuning range allows for direct adjustment of the replay wavelength with little to no need for modification of the holographic recording geometry. HOEs fabricated using these OPO systems in conjunction with advanced photopolymer materials offer precisely controlled replay properties and exceptional diffraction efficiency.

Polarization-insensitive liquid crystal Fresnel lens based on self-assembly polymer gravels and chiral dopant.

A polarization-insensitive liquid crystal (LC) Fresnel lens is developed with binary LC configurations of 90°-twisted nematic (TN) and vertically-aligned (VA) domains in the adjacent zones. A LC mixture comprised of nematic host, photopolymer and chiral material is initially filled into the VA cell with orthogonal rubbing treatment. After the ultraviolet irradiation on the filled LC cell through a photomask with Fresnel zone plate pattern, the interactions among orthogonal rubbing treatment, self-assembly polymer gravels, and chiral material induce the 90°-TN structure in the odd zones, whereas the initial VA structures are maintained in the even zones. The fabricated LC Fresnel lens with binary configuration emerges a maximum diffraction efficiency of around 35% at a voltage of 2.3 V, close to the theoretical diffraction limit of around 41%. The diffractive focus of the LC Fresnel lens is polarization-insensitive at the voltage above 2 V. When the voltage reaches 10 V, the diffractive focus vanishes. The numerical calculation confirms that the polarization-insensitive property appears in the primary focus of the LC Fresnel lens. This work reports a simple method to develop a highly efficient, polarization-insensitive, and electrically tunable LC Fresnel lens which is favorable for imaging system.

Transmission functions of transmissive multilayer inhomogeneous holographic photopolymer liquid crystal diffraction structures

A new analytical model of diffraction of quasimonochromatic light beams on spatially inhomogeneous multilayer diffraction structures formed in photopolymer material with nematic liquid crystals having smooth optical inhomogeneity in the thickness of layers is presented. It is shown that when an applied electric field isapplied to diffraction layers containing a photopolymer composition with a high proportion of liquid crystalline component, a transformation of the selective response with a significant shift in angular selectivity occurs

Optical fiber temperature sensor based on Fabry-Perot interferometer with photopolymer material

We propose and demonstrate a new method for fabricating a Fabry-Perot interferometer sensor. This work uses photopolymerizable materials to construct Fabry-Perot interference structures, making the fabrication process more straightforward and less expensive than other methods. We put a 532 nm laser into the single-mode fiber (SMF) to fabricate photopolymer probes on the face of the fiber. The end face of the probe and the end face of the SMF will form a Fabry-Perot interferometer (FPI). At the same time, to demonstrate its ability to be easily fabricated for sensing various parameters, we incorporated PDMS into the photopolymer material and conducted temperature sensing experiments. The relationship between the concentration of the PDMS and the temperature sensitivity is studied. The experimental results show that the higher the concentration of PDMS, the higher the temperature sensitivity. In the case of ensuring other capabilities, the sensitivity has reached − 1.18 nm/°C in the temperature range of 20–110 °C. We propose and demonstrate a new method for fabricating a Fabry-Perot interferometer sensor. This work uses photopolymerizable materials to construct Fabry-Perot interference structures, making the fabrication process more straightforward and less expensive than other methods. We put a 532 nm laser into the single-mode fiber (SMF) to fabricate photopolymer probes on the face of the fiber. The end face of the probe and the end face of the SMF will form a Fabry-Perot interferometer (FPI). At the same time, to demonstrate its ability to be easily fabricated for sensing various parameters, we incorporated PDMS into the photopolymer material and conducted temperature sensing experiments. The relationship between the concentration of the PDMS and the testing sensitivity is studied. The experimental results show that the higher the concentration of PDMS, the higher the testing sensitivity. In the case of ensuring other capabilities, the sensitivity has reached − 1.20507 nm/°C in the temperature range of 20–80 °C. • This work uses a free-radical photopolymerizable formulation is sensitive between 450 and 550 nm and converted through a free-radical cross-linking polymerization under green light irradiation, which can easily fabricate waveguide structure on the fiber end face. • This photopolymeriazble formulation allows us to add different sensitive materials for the purpose of making different sensors. • We add PDMS into the photopolymeriazble formulation to measure the temperature. In the temperature range of 20–80 °C, the sensitivity reached − 1.20507 nm/°C and demonstrates good stability.

Knowledge domain and hotspots analysis concerning applications of two-photon polymerization in biomedical field: A bibliometric and visualized study.

Objective: Two-photon polymerization (TPP) utilizes an optical nonlinear absorption process to initiate the polymerization of photopolymerizable materials. To date, it is the only technique capable of fabricating complex 3D microstructures with finely adjusted geometry on the cell and sub-cell scales. TPP shows a very promising potential in biomedical applications related to high-resolution features, including drug delivery, tissue engineering, microfluidic devices, and so forth. Therefore, it is of high significance to grasp the global scientific achievements in this field. An analysis of publications concerning the applications of TPP in the biomedical field was performed, and the knowledge domain, research hotspots, frontiers, and research directions in this topic were identified according to the research results. Methods: The publications concerning TPP applications in biomedical field were retrieved from WoSCC between 2003 and 2022, Bibliometrics and visual analysis employing CiteSpace software and R-language package Bibliometrix were performed in this study. Results: A total of 415 publications regarding the TPP applications in the biomedical field were retrieved from WoSCC, including 377 articles, and 38 review articles. The studies pertaining to the biomedical applications of TPP began back in 2003 and showed an upward trend constantly. Especially in the recent 5 years, studies of TPP in biomedical field have increased rapidly, with the number of publications from 2017 to 2021 accounting for 52.29% of the total. In terms of output, China was the leading country and Chinese Acad Sci, Tech Inst Phys and Chem was the leading institution. The United States showed the closest cooperation with other countries. ACS applied materials and interfaces was the most prolific journal (n = 13), followed by Biofabrication (n = 11) and Optics express (n = 10). The journals having the top cited papers were Biomaterials, Advanced materials, and Applied physic letters. The most productive author was Aleksandr Ovsianikov (27 articles). Meanwhile, researchers who had close cooperation with other researchers were also prolific authors. “cell behavior”, " (tissue engineering) scaffolds”, “biomaterials,” and “hydrogel” were the main co-occurrence keywords and “additional manufacturing”, “3D printing,” and “microstructures” were the recent burst keywords. The Keyword clusters, “stem cells,” and “mucosal delivery”, appeared recently. A paper reporting unprecedented high-resolution bull models fabricated by TPP was the most locally cited reference (cited 60 times). “Magnetic actuation” and “additive manufacturing” were recently co-cited reference clusters and an article concerning ultracompact compound lens systems manufactured by TPP was the latest burst reference. Conclusion: The applications of TPP in biomedical field is an interdisciplinary research topic and the development of this field requires the active collaboration of researchers and experts from all relevant disciplines. Bringing up a better utilization of TPP as an additive manufacturing technology to better serve the biomedical development has always been the research focus in this field. Research on stem cells behaviors and mucosal delivery based on microstructures fabricated using TPP were becoming new hotspots. And it can be predicted that using TPP as a sourcing technique to fabricate biomedical-related structures and devices is a new research direction. In addition, the research of functional polymers, such as magnetic-driven polymers, was the frontier topic of TPP biomedical applications.

Heat Transfer Analysis of 3D Printed Wax Injection Mold Used in Investment Casting.

Investment casting is one of the precise casting methods where disposable wax patterns made in wax injection molds are used to make a casting mold. The production capacity of precision foundry is determined by the time taken for producing wax patterns, which depends on the time taken for wax solidification. Wax injection molds are usually made of aluminum or copper alloys with the use of expensive and time-consuming computer numerical control (CNC) processing, which makes low-volume production unprofitable. To reduce these costs, the authors present a heat transfer analysis of a 3D printed wax injection mold. Due to the low thermal conductivity of the photopolymer resin, the influence of different cooling channels’ shapes was investigated to improve the time of the manufacturing process. Transient thermal analysis was performed using COMSOL software based on the finite element method (FEM) and included a simulation of wax injection mold cooling with cold air (−23 °C), water, and without cooling. The analysis showed that use of cooling channels in the case of photopolymer material significantly reduces the solidification time of the sample (about 10 s shorter), and that under certain conditions, it is possible to obtain better cooling than obtained with the aluminum reference wax injection mold (after approximately 25–30 s). This approach allows to reduce the production costs of low-volume castings.

Ultrasound characterization of the viscoelastic properties of additively manufactured photopolymer materials.

Photopolymer-based additive manufacturing has received increasing attention in the field of acoustics over the past decade, specifically towards the design of tissue-mimicking phantoms and passive components for ultrasound imaging and therapy. While these applications rely on an accurate characterization of the longitudinal bulk properties of the materials, emerging applications involving periodic micro-architectured media also require the knowledge of the transverse bulk properties to achieve the desired acoustic behavior. However, a robust knowledge of these properties is still lacking for such attenuating materials. Here, we report on the longitudinal and transverse bulk properties, i.e., frequency-dependent phase velocities and attenuations, of photopolymer materials, which were characterized in the MHz regime using a double through-transmission method in oblique incidence. Samples were fabricated using two different printing technologies (stereolithography and polyjet) to assess the impact of two important factors of the manufacturing process: curing and material mixing. Overall, the experimentally observed dispersion and attenuation could be satisfactorily modeled using a power law attenuation to identify a reduced number of intrinsic ultrasound parameters. As a result, these parameters, and especially those reflecting transverse bulk properties, were shown to be very sensitive to slight variations of the manufacturing process.

Research on the rheological properties of bitumen modified with waste photopolymer

Recently, the use of waste materials in bitumen modification has become very important in terms of protecting natural resources, reducing environmental pollution, and increasing efficiency. Today, thousands of tons of photopolymer plates are produced every year for printing works and they become waste directly after use. Moreover, the products produced by 3D printers using photopolymer materials have the potential to become waste over time. Photopolymer (PhP) seems to be a suitable material for bitumen modification due to the precious polymers it contains. In this study, PhP obtained from waste solid sheet plates was ground and added to bitumen at 4 different rates. The performance of PhP modified bitumen was compared to pure bitumen and styrene–butadienestyrene (SBS) modification. Softening point, penetration, viscosity, dynamic shear rheometer (DSR), multi-stress creep recovery (MSCR) bending beam rheometer (BBR), storage stability tests and Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis were conducted. In conclusion, it was determined that PhP offered superior properties compared to pure binder with its high softening point, viscosity, complex modulus and low phase angle values. The 9 % PhP modification was similar in many respects to the 5 %SBS modification. Addition of 7 % PhP to pure bitumen (PG 58) made the binder capable of handling extremely heavy traffic conditions at 64 °C. The PhP modification behaved more flexibly than SBS at low temperatures and was somewhat more stable in terms of storage. There was physical changes in the structure even though there was no chemical reaction in the binder with the use of PhP and SBS.

Thermal analysis and shrinkage characterization of the photopolymers for DLP additive manufacturing processes

This paper proposes an experimental investigation of a commercial photopolymer resin followed by material modelling and manufacturing system characterization. We focus on the effect of the degree of cure and temperature on the material properties of the photopolymer materials. UV curing properties of the liquid resin are assessed with the thickness measurement by optical tomography. Besides, the specific heat capacity is determined for the almost completely cured and uncured samples with DSC measurements. Photo-DSC experiments are performed to investigate the curing reaction and modelling of the evolution of the degree of cure depending on the light intensity and temperature. In addition, chemical shrinkage behaviour is captured as function of the degree of cure by the high-precision balance set-up. As a result of our experimental studies, model equations are proposed to describe the material behaviour.

Enhancement of holographic performance and stability of photopolymer materials by introducing epoxy resin and dyes

The use of a composite photopolymer system together with a photoinitiator have a significant effect on the optical properties of a volume phase holographic grating. This paper reports on the performance of a low refractive index monomer combined with a high refractive index epoxy resin. Performance improvements include the compatibility between the cross-linked network structure of the matrix and the recording medium. The effects of the exposure conditions and the internal composition of the materials on holographic performance were investigated. The results showed that the optimal refractive index modulation depends on the ratio of the epoxy resin and the pre-polymerization temperature. The use of Irgacure 784 as a photoinitiator gave better performance than Rose Bengal (RB) in terms of diffraction efficiency (∼92.5 %), transmittance (∼98 %) and slight shrinkage (0.314 %) was observed. The use of a dual-photoinitiator gave improvements in the radical generation rate and the stability, permitted recordings at an exposure energy of 50 mJ/cm2 and afforded high sensitivity (3.9 × 10-2 cm2/J) as a result of the synergy between the two photoinitiators. In other words, due to the highly reactive Irgacure 784 and the inhibiting polymerization characteristics of RB, enhancement of holographic performance and good stability were realized. It was shown in the photocuring and aging experiments that the photopolymer possessed anti-aging properties. The photopolymer can record multiplexed images at 532 nm and polarization holograms, demonstrating that it has the capability to perform high-density data recording.

Digital Light 3D Printed Bioresorbable and NIR-Responsive Devices with Photothermal and Shape-Memory Functions.

Digital light processing (DLP) 3D printing is a promising technique for the rapid manufacturing of customized medical devices with high precision. To be successfully translated to a clinical setting, challenges in the development of suitable photopolymerizable materials have yet to be overcome. Besides biocompatibility, it is often desirable for the printed devices to be biodegradable, elastic, and with a therapeutic function. Here, a multifunctional DLP printed material system based on the composite of gold nanorods and polyester copolymer is reported. The material demonstrates robust near‐infrared (NIR) responsiveness, allowing rapid and stable photothermal effect leading to the time‐dependent cell death. NIR light‐triggerable shape transformation is demonstrated, resulting in a facilitated insertion and expansion of DLP printed stent ex vivo. The proposed strategy opens a promising avenue for the design of multifunctional therapeutic devices based on nanoparticle–polymer composites.

Investigation into Thermomechanical Response of Polymer Composite Materials Produced through Additive Manufacturing Technologies.

This paper presents the static mechanical behavior and the dynamic thermomechanical properties of four market-available reinforced and non-reinforced thermoplastics and photopolymer materials used as precursors in different additive manufacturing technologies. This article proposes a characterization approach to further address development of aeronautic secondary structures via 3D-printed composite materials replacing conventional manufactured carbon fiber reinforced polymer (CFRP) composites. Different 3D printing materials, technologies, printing directions, and parameters were investigated. Experimental results showed that carbon-reinforced ONYX_R material exhibits a transition point at 114 °C, a 600 MPa tensile strength, and an average tensile strain of 2.5%, comparable with conventional CFRP composites manufactured via autoclave, making it a suitable candidate for replacing CFRP composites, in the aim of taking advantage of 3D printing technologies. ONYX material exhibits higher stiffness than Acrylonitrile-Butadiene-Styrene Copolymer (ABS), or conventional Nylon 6/6 polyamide, the flexural modulus being 2.5 GPa; nevertheless, the 27 °C determined transition temperature limits its stability at higher temperature. Daylight High Tensile (further called HTS) resin exhibits a tensile strength and strain increase when shifting the printing direction from transversal to longitudinal, while no effect was observed in HighTemp DL400 resin (further called HTP).

Construction of a decellularized spinal cord matrix/GelMA composite scaffold and its effects on neuronal differentiation of neural stem cells

Spinal cord injury (SCI) leads to severe loss of motor and sensory functions, and the rehabilitation of SCI is a worldwide problem. Tissue-engineered scaffolds offer new hope for SCI patients, while the newly developed materials encountered a challenge in modeling the microenvironment around the lesion site. We constructed a new composite scaffold by mixing decellularized spinal cord extracellular matrix (dECM) with gelatin methacryloyl (GelMA). The dECM, as a natural biological material, retained a large number of proteins and growth factors related to neurogenesis. GelMA was a photopolymerizable material, harbored a polymer network structure, soft texture, certain shape and plenty of water. The viability, proliferation, and differentiation of neural stem cells (NSCs) on the composite scaffold were evaluated by cell count kit-8 (CCK8), Live/Dead assay, phalloidin staining, 5-Ethynyl-2’-deoxyurdine (EdU), immunofluorescence staining and western blot. The Live/Dead assay, phalloidin staining, EdU, and CCK8 assay showed that the composite scaffold had good biocompatibility and provided better support for proliferation of NSCs. Results of immunocytochemistry and western blot showed that the composite scaffolds promoted the specific differentiation of NSCs into neuron cells. Together, this dECM/GelMA composite scaffold can be used as a cell culture coating, the isolated NSCs seeded on the surface of composite scaffold expressed neuronal markers and assumed neuronal morphology. Our work provided a new method that would be widely used in tissue engineering of SCI.

UV Curing Effect(s) on Colorimetric Properties of Test Specimens Created via Stereolithography

This paper presents the results of observing colorimetric change obtained by the impact of UV curing. The test specimens were made by stereolithographic process from photopolymer materials with the addition of cyan, magenta, and yellow dye. To better understand the impact of UV curing on colorimetric permeability properties, specimens were made in five different thicknesses (1 mm, 2 mm, 3 mm, 4 mm, and 5 mm). The UV curing time was divided into eight intervals of 15 minutes, at the end of which spectral and colorimetric values of each specimen were measured. The difference in color change was defined and shown using the CIELab color system. The conclusion of the conducted tests indicates that specimens with smaller thickness have a greater color change. The correlation between the time interval of UV radiation and the direction of color change was determined.