Digital Light Processing 3D Printing of Polymer Composites Based on Tunable Curing Resins with Photoswitchable Molecules

Digital Light Processing 3D Printing of Polymer Composites Based on Tunable Curing Resins with Photoswitchable Molecules | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Digital Light Processing 3D Printing of Polymer Composites Based on Tunable Curing Resins with Photoswitchable Molecules Saiful Islam Sagor, Anasheh Khecho, Erina Baynojir Joyee This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6736596/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study presents a novel additive manufacturing (AM) technique, Photo switchable Direct Light Processing (P-DLP), which utilizes a dynamic mask imaging photoinitiation approach to mitigate light scattering effects caused by filler particles like Silicon Carbide (SiC) in composite printing. Traditional vat photopolymerization methods, while known for their high precision, face significant challenges in balancing speed and resolution, requiring extensive support structures and dealing with material instability during fabrication. The P-DLP technique overcomes these limitations by employing a dynamic masking system, where ultraviolet (UV) light initiates photopolymerization, and visible (blue) light selectively inhibits undesired polymerization. This mechanism allows for precise control over the curing process, enabling the fabrication of complex, high-resolution structures while minimizing scattering-induced distortions. A key aspect of this research is the development of refined resin formulations that integrate azobenzene as a photo switchable molecule, enhancing the controllability of polymerization kinetics. UV-Vis spectrophotometry results showed that azobenzene extended the absorption spectrum into the blue region, with higher concentrations significantly increasing absorbance in the 380–500 nm range, confirming its potential as a photoinhibitor. Though the decreased tensile strength and elastic modulus due to agglomeration and chain disruption, the proposed P-DLP with dual wavelength light demonstrated effective curing of layers by inhibiting undesired curing in boundary and void regions, enabling high-resolution patterning with reduced overcuring artifacts. The advancements introduced in P-DLP make it particularly suited for applications requiring high precision and material integrity, such as optics, medical implants, and soft robotics. This approach represents a significant breakthrough in composite AM, addressing fundamental challenges of conventional methods and enabling faster, more accurate production of intricately detailed components across diverse industrial and biomedical applications. Digital Light Processing (DLP) Polymer composite Silicon Carbide Azobenzene Isomerization Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction Composite materials have become an essential class of materials in advanced engineering applications due to their capacity to integrate distinct material properties into a single, optimized system. Typically composed of a polymer matrix and reinforcing filler particles, composites offer enhanced mechanical, thermal, and functional properties that outperform conventional single-phase materials. Key advantages include high strength-to-weight ratio, enhanced durability, and the ability to tailor performance attributes to specific design requirements [ 1 ]. These features make composites especially attractive for high-performance fields such as aerospace, automotive, defense, and increasingly, biomedical engineering. In the biomedical domain, composite materials are employed in applications ranging from structural scaffolds and prosthetics to implants and wearable electronics, where their biocompatibility, strength, and chemical resistivity are particularly beneficial. To meet the demand for customized and complex biomedical devices, additive manufacturing (AM) has emerged as a critical fabrication strategy. AM technologies allow for the layer-by-layer construction of patient-specific structures with high geometric complexity and minimal material waste [ 2 , 3 ]. Among AM techniques, vat photopolymerization (VP) has gained widespread attention for its superior resolution, surface finish, and ability to process intricate geometries. VP process can be divided in two main categories, stereolithography (SLA) [ 4 ], which uses a point-wise scanning laser to cure resin, and digital light processing (DLP) [ 5 ], which projects entire layer patterns via spatially modulated light. Compared to SLA, DLP offers higher throughput and spatial control through digital mask manipulation, making it more suitable for high-speed, precision fabrication. Researchers have extensively leveraged DLP based VP for fabricating composite structures by dispersing micro- or nanoscale fillers into photosensitive resins. These reinforcements can significantly improve the mechanical and functional performance of printed parts. However, the addition of other components to pure photosensitive monomers introduces light scattering issues, affecting the polymerization process. These challenges arise from the complex interactions between light and material, including refraction, scattering, and absorption kinetics [ 6 ]. Filler particles disrupt the uniform transmission of curing light, causing light scattering, refraction, and absorption within the resin. This leads to non-uniform energy distribution, limited curing depth, poor layer adhesion, and defects in the printed geometry. Especially, in DLP systems, where entire cross-sections are cured simultaneously, the presence of scattering particles can severely impair print fidelity and structural performance. Such inconsistencies can lead to overcuring or under curing, ultimately impacting the mechanical properties and dimensional accuracy of the final printed part [ 7 ]. To mitigate these issues, researchers have proposed various approaches. Within DLP systems, different manufacturing configurations have been explored to control and project light for curing [ 8 ]. Researchers have explored various solutions, including optimizing exposure parameters and light energy distribution. For mask based DLP, controlling the exposure time of different pixels helps regulate overcuring by adjusting the light energy distribution in the exposed area [ 9 ]. Additionally, the mask-division method, proposed in [ 9 ], suggests segmenting light exposure to reduce scattering effects in the boundary region. Digital micromirror device (DMD)-based projectors are widely used in commercial and high-performance research settings due to their ability to reflect high-intensity UV light with excellent spatial resolution [ 10 ]. These systems utilize an array of microscopic mirrors that rapidly tilt to direct light pixel-by-pixel. Liquid crystal display (LCD)-based DLP systems, by contrast, use a transmissive liquid crystal panel to modulate light in a planar manner [ 11 ]. While LCD-based systems offer a cost-effective alternative, they suffer from limitations in UV light transmission due to the absorption and scattering properties of the liquid crystal material, which can reduce curing efficiency and resolution. Besides these, researchers have also explored LED-array based segmented projection systems, where spatially controlled exposure is achieved by illuminating subregions sequentially [ 12 ]. Wu et al. used a LED based light projection system to reduce the overcuring and scattering artifacts by varying local light intensity [ 9 ]. In another study Zhou et al., have used a hybrid DLP-laser systems that combine wide-area projection with precise laser reinforcement for improved interlayer bonding in composites [ 13 ]. Most of the described approaches still rely on material-dependent parameters such as particle size, shape, and refractive index, meaning that any change in filler composition will need re-optimization. This limits adaptability and real-time control over polymerization. Besides manufacturing based configuration of DLP systems, some studies have developed index-matched resin formulations to reduce the refractive mismatch between fillers and the surrounding medium. The use of UV absorbers to localize light penetration has also been explored, although this can introduce vertical gradients in curing, affecting layer uniformity. For example, Xie and He demonstrated that exceeding 0.5 wt% of graphene oxide in LCD-based VP systems led to significant light attenuation, preventing effective polymerization. Similar limitations have been observed in SLA systems using nonwoven glass or carbon fiber mats, where optical obstruction restricts feature resolution [ 14 ]. To optimize VP 3D printing for high spatial resolution and defined architectures, a deeper understanding of the molecular structure of photopolymerizable inks and the kinetics of the photopolymerization reaction is required [ 15 ]. Further ink development is necessary to achieve the resolution and speed needed to meet the increasing demand for rapid fabrication and mass production [ 12 ]. A major gap remains in existing strategies that lack the ability to actively and spatially regulate curing behavior in real time. Most methods rely on passive materials design or static light modulation, which are often insufficient for compensating the dynamic optical effects introduced by high filler loadings. This constraint motivates the need for a more adaptive and tunable curing mechanism that can respond to complex light-material interactions. In this study, we propose the use of multi-wavelength DLP systems [ 16 ] that incorporate dual-wavelength control, where one wavelength activates polymerization and the other inhibits it, this strategy offers real-time spatial regulation of curing behavior. This wavelength-specific chemical control represents a significant advancement for printing with light-scattering fillers, offering more reliable curing depth and structural fidelity in composite materials. The use of voxel-based exposure in DMD systems allows for highly programmable and accurate photopolymerization; however, in composite systems with light-scattering fillers, even the highest-resolution projection may suffer from uneven curing. In such cases, incorporating a dual-wavelength approach provides an innovative solution by enabling not only light-pattern control but also wavelength-specific chemical control of polymerization. This introduces an extra dimension of spatial precision where polymerization can be selectively activated or inhibited in targeted regions depending on the wavelength applied. Our study aims to translate this concept into a practical solution by integrating wavelength-specific control directly into the materials and optical system design. Building upon this concept, we propose a materials- and optics-based strategy to realize such control in practice. We developed a novel DLP-based approach utilizing a photoswitchable polymer resin system incorporating azobenzene, a well-characterized photochromic molecule. Azobenzene undergoes reversible photoisomerization when exposed to different wavelengths of light, transitioning between trans and cis states [ 17 ]. This molecular switching mechanism can be harnessed to regulate local polymerization activity through light wavelength selection. In our dual-wavelength DLP platform, blue light serves as a polymerization initiator, while UV light functions as a photoinhibitor. This dual-wavelength control enables spatially resolved, pixel-by-pixel modulation of the curing process, allowing us to counteract the non-uniform curing effects caused by light-scattering fillers such as SiC. In this study, we further develop and evaluate a series of azobenzene-functionalized composite inks containing varying concentrations of polymer matrix, silicon carbide (SiC), and photoinhibitory components. We investigate the optical behavior of these formulations using UV-Vis spectrophotometry to understand how light absorption and transmittance change with filler content. We then fabricate test structures using a custom-built DLP prototype and assess their mechanical properties through tensile testing, quantifying the influence of photoinhibition and filler loading. Additionally, we implement a mask-based spatial light modulation technique, comparing conventional uniform exposure with contour-specific mask designs to enhance boundary resolution and reduce overcuring. This work introduces a dynamic, wavelength-selective strategy for DLP-based composite printing that addresses the longstanding limitations imposed by optically dense filler systems. By integrating smart resin chemistry with programmable light control, our method provides a versatile and scalable framework for manufacturing high-fidelity, mechanically robust composite structures. The proposed approach holds broad implications for applications in biomedical devices, photonic materials, soft robotics, and precision engineering, where functional composites and geometric complexity are increasingly in demand. 2. Materials and Methodology 2.1 Material Preparation To test the concept of curing property manipulation described in the introduction part, a composite resin was prepared for testing. In this study, for testing the optical properties, seven different ink compositions were prepared by using an acrylic based photocurable polymer resin (3DM ABS, 3D Materials Inc.) as base matrix. This resin contains acrylic acid esters, acrylated monomers, oligomers, and TPO-solid (Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide) and BAPO (Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide) as the photoinitiator [ 18 ]. Acrylate-based photopolymer resins are among the most commonly utilized formulations in different applications, including bone cement, contact lenses, and orthopedics applications [ 19 ]. When exposed to UV radiation, the photoinitiator absorbs the light energy and produces free radicals as illustrated in Fig. 1 (a). These free radicals trigger the polymerization process of the acrylate materials, allowing them to cure within seconds. Along with methacrylate resin, SiC powder (US Nano, Houston, TX, USA) with an average particle size of 40 µm and Azobenzene (with a purity of 98%, Sigma Aldrich, USA), as photoswitchable molecule, were mixed in different ratios to prepare the ink resin suspension. The chemical structures of these base components of the prepared ink samples are shown in Fig. 1 (b). Photoexcitation of N = N bond in Azobenzene molecules under exposure to specific wavelengths of light enable reversible photoisomerization [ 20 – 25 ]. These characteristics make Azobenzene a suitable candidate for wavelength selective modulation of the photopolymerization process. The composite inks with varying concentrations of SiC particles and azobenzene were prepared as detailed in Table 1 . The selected concentration ranges were determined based on both experimental feasibility and the need to evaluate the effects of each component on light transmission and curing performance. A range (0%-3%) of SiC concentrations was tested to assess its influence on optical properties, specifically transmittance and absorbance. It was observed that SiC loadings above 3 wt% resulted in the composite becoming visually opaque, effectively blocking light transmission and rendering spectrophotometric analysis infeasible. Azobenzene concentrations were varied to evaluate their minimal amount of inhibitory effect on polymerization under dual-wavelength exposure. It was found that concentrations above 1wt% significantly suppressed the curing process under visible light, likely due to excessive light absorption or photoinhibition effects. Therefore, material concentrations were systematically varied to balance optical clarity, curing responsiveness, and measurable spectroscopic behavior, enabling the formulation of resins suitable for dual-wavelength DLP printing. Table 1 Composition of the photopolymer ink used in this study. Composite Azobenzene (wt %) SiC (wt %) Resin (wt %) C0 0 0 100 C1 0 1 99 C2 0 3 97 C3 1 0 99 C4 3 0 97 C5 0.5 1 98.5 C6 1 3 96 Figure 1 (c–e) illustrates the preparation of the composite suspension, which followed two sequential steps. First, an ultrasonic sonicator (Qsonica Q700, 700 W, 20 kHz, USA) was used for 10 minutes to disperse the particles within the resin suspension. The samples were placed in a glass beaker and submerged in chilled water to dissipate heat generated during sonication. The sonicator operated at 20 kHz with a pulse cycle of 5 seconds to effectively agitate and break up any particle agglomerates. Following sonication, the suspension was mixed using a temperature-controlled magnetic stirrer (Cheshire Enterprise, LLC, USA) at 500 rpm for 5 minutes. This ensured uniform distribution of the particles and helped maintain suspension homogeneity during printing. 2.2 Spectrophotometric Analysis of Photoswitchable Ink Spectrophotometry determines how light interacts with matter through reflection, refraction, scattering, absorbance, and transmittance by quantifying the amount of light being absorbed, reflected, or transmitted at a particular wavelength [ 26 ]. Absorbance is a dimensionless quantity that varies from zero for completely transparent samples (where transmittance is 100%) to an infinitely large value for fully opaque samples (where transmittance is 0%) [ 27 ]. In our study, absorbance of the developed inks was examined using a UV-Visible spectrometer (Goyojo, China). This spectrophotometer operates at 110 V, 50/60 Hz, with a 6 nm Tungsten Lamp and a wavelength range of 350–1020 nm to determine the optical properties. In a spectrophotometer, absorbance is determined by directing a collimated beam of light at a specific wavelength through a flat, parallel-sided material that is perpendicular to the beam as illustrated in Fig. 2 . The intensity of the light entering the sample (I₀) is greater than the intensity of the light that exits (I) because some of the energy is absorbed by the molecules within the sample and this absorbed energy values are shown in the spectrophotometer as absorbance. The relationship of emitted light (I ₀ ) and transmitted light (I) with absorbance (A) is determined by the following Eq. ( 1 ) [ 27 ]: $$\:A\:=\:-\:{log}_{10}\:\left(\frac{I}{I₀}\:\right)\:$$ 1 In this study, the spectrophotometer was first turned on and allowed to warm up for 30 minutes. A blank solution of distilled water was then used to calibrate the instrument. After calibration, the samples were placed in 1 cm wide transparent glass cuvettes, wiped with alcohol to eliminate any interference in light transmission and inserted into the chamber cell. The absorbance values of pure resin and composites with different concentrations were calculated using Eq. ( 1 ) across a wavelength range of 350–700 nm. Subsequently, absorption coefficients were calculated using Beer-Lambert’s law, which establishes a relationship between wavelength and absorbance. This was done based on the following Eq. ( 2 ) [ 28 ]: $$\:\alpha\:\:=\:2.303\:\times\:\:\frac{A}{d}$$ 2 Where, α is the absorbance coefficient (cm − 1 ), A is the absorbance value, d is the thickness of the samples in cm. 2.3 DLP Curing with Conventional Mask A custom DLP 3D printer prototype was developed to test the developed inks. This setup includes an imaging unit, a resin vat with a transparent bottom surface, a linear actuator to lift the build platform, and two linear stages that allow the resin vat to slide. Different parts of the prototype are shown in Fig. 3 (a). A visible light projector was used as the imaging unit equipped with 1024 × 768 microarray and an envelope of 42.7 × 32 mm, having modified lenses to project images according to the mask over an adjustable working distance. The projection settings including focus, brightness and contrast were adjusted to produce a clear projection image on the transparent plane of resin vat. The 3D printer was designed with the projector positioned vertically relative to the resin tank, eliminating the need for reflecting mirrors [ 29 ]. This design choice reduces the size of the printer and also offers greater flexibility in defining the printing area. A micro controller was used to move both the vat and platform according to a 2D mask image. To reduce the separation force of the cured part from the tank surface, the bottom of the resin vat was coated with polydimethylsiloxane (PDMS) film due to its inherent property of anti-sticking and durability [ 30 ]. A digital mask planning and process control slicing software was developed using C + + shown in Fig. 4 (b). This system integrates geometry slicing, image generation, suspension deposition control and motion coordination. The software generates image masks based on the computer aided design (CAD), as shown in Fig. 4 (a) and projects them onto the bottom of the build platform through the resin tank. At the start of the bottom-up DLP printing process, the build platform lowers into the photopolymerizable liquid resin until it is positioned one layer thickness above the PDMS layer. The layer thickness is controlled by maintaining the distance between the resin vat floor and the build platform. The software converts the CAD drawing into 2D mask images, which are projected from the light source onto the bottom of the build platform through the resin tank. This exposure cures the entire 2D layer, causing the photopolymerized resin to adhere to the build plate and forming the first layer as shown in Fig. 3 (b). After curing one layer, the build platform moves upward with one layer thickness which allows uncured resin to refill the space. This process is repeated for the second layer, which adheres to the first and continues layer by layer until the complete 3D part is printed. The completed part, printed with pure resin is shown in Fig. 4 (c). These steps were followed to print the same structures (Fig. 4 a) using the formulated inks listed in Table 1 . 2.4 DLP Curing with Core and Contour Mask One of the primary limitations of DLP method for composite printing is the geometric accuracy errors that occur in boundary regions due to light diffraction. As light propagates, it reflects off the Digital Micromirror Device (DMD) that leads to diffraction and an unintended increase in the spot size [ 31 ]. This effect is further exacerbated when printing SiC-polymer composites. The scattering properties of the SiC particle contribute to additional light diffusion. Consequently, this diffracted, unintended exposure extends beyond the target areas which reduce printing accuracy in the XY plane of the fabricated structure. To mitigate these inaccuracies, it is essential to control the curing process at the boundary regions by controlling either the light power intensity or the exposure time. One effective approach to control the optical power involves adjusting the grayscale values of the projected mask images. To investigate the relationship between grayscale values and optical power at the curing position, an experiment was conducted using an optical photodiode power sensor (S120VC, Thorlabs, Fig. 5 (a). The optical power sensor was placed between the resin vat and the light source to precisely measure the light intensity at different grayscale values. The results indicate a linear correlation between grayscale values and optical power, with a grayscale value of 255 corresponding to maximum light intensity. To ensure control over light exposure in boundary regions, the conventional projection masks were divided into two separate masks: a core mask and a contour mask. In the first exposure cycle, the core mask was fully activated with a grayscale value of 255, ensuring maximum light intensity to cure the inner region completely. At the same time, the contour mask was set to a grayscale value of zero that ensured the contour region was inactive and prevented the activation of the photoinitiator at the contour region. The exposure time was set to 10 seconds to ensure that the light with the highest intensity fully cured the core region. Following the core curing procedure, a second exposure cycle was initiated, where the contour mask was partially active with an optimized grayscale value of 180. At this time the core mask remained deactivated. The optimal grayscale value for the contour region was determined through an iterative optimization approach, leveraging parametric tuning and experimental calibration. A range of grayscale values (150–200) with 2-units increment was tested, and their effects on boundary integrity and polymerization depth were evaluated. The optimization process employed a gradient-based adjustment strategy, as shown in Fig. 6 , where the exposure intensity was incrementally modified based on the resulting boundary sharpness and material conversion efficiency. Image processing techniques were used to analyze the cured regions, ensuring uniformity and minimal overcuring effects. The selected optimized grayscale value of 180, employing around 60% relative light intensity, was found to provide sufficient curing depth while maintaining dimensional accuracy. At an optimized relative light intensity of 60%, the curing strength was sufficient to create a well-defined rectangular layer, as shown in Fig. 5 (b). However, residual uncured areas were observed across the cross-section, indicating incomplete polymerization. This suggests that while the exposure was sufficient for defining the boundary, internal diffusion limitations or variations in light absorption may have contributed to incomplete crosslinking. When the relative intensity fell below this threshold, curing was insufficient to maintain the intended thickness, leading to edge distortions and discontinuities. 3. Results and discussions In our experiment, seven sample groups were designed, fabricated, and tested using the proposed P-DLP process introduced in Section 1 . All groups were prepared following the material preparation procedure described in Section 2 . For mechanical testing, each sample was printed using the same CAD model used in Section 3 . 3.1 Spectral Absorption Analysis of Functionalized Inks Figure 7 illustrates the spectrophotometric behavior of seven different samples prepared and tested to analyze their response under various light wavelengths. Figure 7 a shows the absorption characteristics of pure resins (C0) containing TPO and BAPO photoinitiators, which strongly absorb UV light in the 380–405 nm range. For inks C1 and C2 (Fig. 7 b), adding 1% and 3% SiC increases UV absorbance, though the effect remains small at these concentrations. Figure 7 b shows that as the SiC concentration increases, absorbance in the 350–450 nm range also rises. This suggests that SiC influences light interaction through both absorption and scattering. Since SiC is known for its light-scattering properties and low transparency, higher concentrations of SiC lead to increased scattering and energy dissipation of UV light [ 32 , 33 ]. As a result, the spectrophotometric measurements show higher absorbance at elevated SiC concentrations. In the case of inks C3 and C4, the addition of azobenzene extends the absorbance into the blue light spectrum. This is due to photoisomerization, where azobenzene molecules switch between trans and cis forms when exposed to light, enhancing their light absorption ability as shown in Fig. 7 c [ 34 ]. A comparison between 1% and 3% azobenzene concentrations reveals that absorbance increases with higher concentration, consistent with the Beer-Lambert Law. Additionally, at higher concentrations, a red shift and a plateau in absorbance suggest intermolecular interactions between azobenzene molecules under blue light exposure [ 35 , 36 ]. This behavior indicates that azobenzene could function as an effective photo-inhibitor in ink formulations, improving control over the curing process. For inks C5 and C6 in Fig. 7 d, which contain both SiC and azobenzene, a combined effect on absorbance is observed. When azobenzene and SiC present at 0.5% and 1% concentration respectively, absorbance significantly increases in the 380–530 nm range, likely due to azobenzene’s light-responsive properties [ 37 ]. Increasing the azobenzene and SiC concentration to 1% and 3% respectively, results in a plateau in absorbance between 380–450 nm. This suggests a balance between SiC-induced light scattering and azobenzene absorption. The interaction of these components across the UV and blue spectrum suggests that azobenzene absorbs scattered light, particularly near the boundaries of the printed geometry. This property could help mitigate overcuring caused by SiC’s scattering effects, improving the precision of the curing process in DLP printing. 3.2 Tensile Testing of Printed Structures Tensile testing is a destructive mechanical test used to determine key properties such as tensile strength, yield strength, ductility, and elastic modulus of a material. It involves applying a uniaxial tensile force to a specimen until failure for generating stress-strain curve that characterizes the material's behavior under load. This test is essential for understanding how materials deform and ultimately fail under tensile forces, which is critical for ensuring structural reliability and performance of the composite [ 38 ]. In this study, the tensile test was performed with a Mark-10 F505-EM test frame, as shown in Fig. 8.3D printed samples with composites C0, C1, C2, C5, and C6 were carefully secured in the grips of a tensile testing apparatus. The bottom grip remained stationary, while the upper grip applied an increasing tensile load. As the tensile load increased, each sample progressively elongated until it eventually failed. To ensure consistency and reduce experimental error, two identical samples of each composite were tested. Throughout this process, the force applied and the corresponding deformation were accurately recorded. From Fig. 8 , it is evident that the pure acrylate-based resin (C0) exhibited the highest ultimate tensile strength (~ 47 MPa) along with the highest elastic modulus (32 MPa), indicating a well-balanced combination of stiffness and strength. The addition of SiC as a filler had a noticeable impact on the mechanical properties. The C1 composite, containing 1% SiC, showed relatively moderate tensile strength (~ 45 MPa) and exhibited a slight reduction in stiffness (30 MPa), suggesting minimal reduction in mechanical performance compared to the C0. However, increasing the SiC content to 3% in the C3 composite led to a significant decrease in both ultimate tensile strength (~ 25 MPa) and elastic modulus (19.17 MPa), resulting more brittleness in the printed part. This degradation can be attributed to the tendency of SiC particles to agglomerate at higher concentrations, which causes non-uniform dispersion within the polymer matrix. Such aggregation forms micro-clusters act as stress concentrators and weakens the composite by promoting crack initiation and propagation under load. Consequently, this localized stress concentration undermines the tensile properties of the material [ 39 ]. Moreover, since SiC is an inherently brittle ceramic, its incorporation with polymer matrix exacerbates brittleness and compromise mechanical performance [ 40 ]. Furthermore, the incorporation of azobenzene in addition to SiC fillers negatively influenced the mechanical characteristics of the composites. The C5 composite, comprising 0.5% azobenzene and 1% SiC, demonstrated lower tensile strength (~ 21 MPa) and a reduced elastic modulus (18.55 MPa). The samples were brittle and they had undergone up to 4% of strain before breaking. The effect was more pronounced in the C6 composite, which contained 1% azobenzene and 3% SiC, resulting in the poorest mechanical performance among all tested samples—exhibiting the lowest ultimate tensile strength (~ 12 MPa) and a comparable elastic modulus (19.99 MPa). The microscopic image of the C6 sample’s fractured surface was taken using Scanning Electron Microscope (SEM, JEOLJSM-6480 CO., LTD, Japan) and it shows a rough, irregular edge, indicating a brittle fracture mode, supporting the observation of poor ductility and early failure. The reversible photoisomerization of azobenzene molecules upon UV and visible light exposure alters the molecular geometry from a linear (trans) to a bent (cis) configuration, inducing local disruptions within the polymer matrix. This conjugated structure hinder the mobility of polymer chains and reduce the material’s ability to deform under tensile stress and contributes to decreased tensile strength and elongation [ 41 ].Therefore, to preserve the mechanical integrity of such composite systems, the concentration of azobenzene must be carefully controlled to balance its functional benefits with structural performance. 3.3 Evaluation of Masking Segmentation in DLP Printing After evaluating the optical and mechanical performance of the formulated inks, printing tests were conducted to further assess their processability. To test the effectiveness of the proposed masking segmentation technique, a dog bone-inspired 3D model was designed, as illustrated in Fig. 4 (a). The part was fabricated using the segmented mask projection method outlined in Section 3.3 . It was observed that (Fig. 9 a), the cured part exhibited an unintended separation line between the core and contour regions, indicating a weaker interfacial bond that compromised the structural integrity of the printed component. This issue arose due to the curing sequence used in the segmented mask projection process. In the initial exposure cycle, the core region was fully activated at maximum light intensity, while the contour region remained completely inactive. In the subsequent exposure cycle, the contour zone was partially exposed using the optimized grayscale value of 180, found from the approach described in Fig. 6 , whereas the core region was not re-exposed. Since these regions were polymerized independently under distinct light exposure conditions, the linker monomers at the interface may not have undergone complete crosslinking which lead to poor adhesion and the formation of the visible separation line. These findings highlight the need for further optimization of the segmentation strategy to ensure uniform polymerization and enhance interfacial bonding. This simultaneous curing strategy resulted in a more uniform polymerization process, reducing the chance of insufficient crosslinking at the interface. Additionally, to mitigate the risk of overcuring, the exposure time per cycle was reduced to 5 seconds, which is half the duration used in the initial approach. As shown in Fig. 9 (b), the modified approach eliminated the separation line, indicating improved interfacial adhesion and structural integrity. However, a slight overcuring effect was observed in the top-left portion of the sample, suggesting that further optimization of exposure time may be necessary to refine the process and achieve optimal geometric precision. 3.4 Concept of Dual Light DLP Printing with Azobenzene Composite To evaluate the feasibility of the proposed dual-wavelength DLP printing method using photoswitchable composite resins (C4), experiments were conducted using two different wavelengths. Initially, composite samples (C4) were tested separately under UV and blue light exposures. A simple 17 mm × 17 mm square-shaped 2D mask was projected in each test, with an exposure time of 10 seconds. A DLP UV projector from LightCrafter 4500, Texas Instruments, USA was used for initial UV exposure. A significant over curing beyond the intended masked region was observed due to scattered ultraviolet light (Fig. 10 a). In contrast, exposing the same resin sample (C4) to the blue light using an RGB DLP projector from Texas Instrument, under the same conditions resulted in no visible polymerization (Fig. 10 b). This outcome confirms the effective photoinhibitory property of azobenzene under blue illumination, indicating its potential role in controlling unintended polymerization. Next, we tested the above described concept with our new developed prototype, using simultaneous projection of both UV (DLP® LightCrafter™ 4500, 405 nm, Texas Instrument) and blue light (DLP® LightCrafter™ 4500, RGB, Texas Instrument) sources. As shown in Fig. 10 c, the two lights were arranged orthogonally at a 90-degree angle. A reflective dichroic mirror was used to direct both light sources onto the resin at the same time, allowing different regions to receive different wavelengths. For this test, the exposure time was increased to 20 seconds to observe the curing behavior more clearly. The area exposed to UV light showed signs of overcuring, while the region marked with a blue dotted line in the center remained in a gel-like (Fig. 10 c), under cured state. This indicated delayed or inhibited polymerization under blue light in that region, despite the extended exposure. Based on these observations, a thin-wall cylindrical structure featuring multiple internal voids, inspired from the structure of lotus root (Fig. 11 a) was cured using the dual light DLP prototype to demonstrate the feasibility of the proposed dual-wavelength DLP process. The developed prototype currently shows some alignment issues between the UV and blue light sources (Fig. 11 b). Specifically, in certain areas, the blocking mask projected by the blue light source does not completely overlap with the contour mask from the UV source. Due to this misalignment, some regions lack proper exposure of the blocking mask, potentially resulting in overcuring. As illustrated in Fig. 11 (b) for the lotus root-inspired structure, one side of the circular geometry shows a slight gap where the blue blocking mask fails to merge accurately with the UV contour mask. In contrast, on the opposite side, the blocking and contour masks are properly aligned, allowing precise exposure of the blocking mask on both the peripheral and internal void regions. This mask projection approach involved two sequential primary exposure steps. In the first, UV light was used to selectively cure the core and contour regions at optimized intensities, following a segmentation method similar to that described in Section 4.3. Specifically, in the first step, full-intensity UV light was applied to the core, while reduced intensity was used for the contour. In the second step, both regions received partial-intensity UV exposure at optimized gray scale value to further refine polymerization, as shown in Fig. 11 (c). Simultaneously, a secondary exposure with blue light was applied using a blocking mask positioned around the outer boundary of the UV-cured region and along the internal voids (Fig. 11 d). Figure 11 (e) presents a successfully cured layer printed using the proposed dual-wavelength DLP approach. As shown in Fig. 11 (e, right), the circular portion maintained a well-defined shape on the side where the blocking mask and the UV contour mask aligned correctly. In contrast, the opposite side exhibited signs of overcuring caused by SiC-induced light scattering, attributed to the misalignment of the masks. This misalignment prevented proper blocking mask exposure, resulting in geometric distortion, the formation of cure bleed areas, and loss of the intended circular shape. These results validate the effectiveness of the blocking mask in leveraging azobenzene’s photoinhibitory capability, preventing unintended polymerization due to scattered UV light. Also, it is observed that the regions intended to remain void were successfully protected and preserved in their uncured state, maintaining the designed porous geometry. This demonstrates the printability of our proposed method by the integration of this dual-wavelength strategy shown in Fig. 10 (c), emphasizing its spatial precision and ability to minimize scattering-induced curing errors common in conventional DLP processes. Conclusion In summary, we demonstrated the feasibility of effectively controlling DLP 3D printing through the incorporation of azobenzene composites in the functional ink, utilizing azobenzene's photoinhibitory properties under blue light. Through comprehensive spectral absorption analysis and printing experiments, the photoinhibitory capability of azobenzene within functionalized inks was successfully validated. Additionally, this research addressed critical challenges associated with mask DLP 3D printing, such as visible separation lines on printed surfaces, through optimized light-intensity-controlled exposure in the core and contour region. Also, a novel blocking mask concept using the photoinhibitory effect of azobenzene under blue light in dual-wavelength DLP printing is proposed and applied for reducing issues related to overcuring. Our future research will further investigate the practical feasibility of dual-wavelength printing with blocking mask by properly aligning the blocking mask with core and contour mask, optimizing azobenzene concentration, exposure parameters and enhancing resin formulations to facilitate the fabrication of precise composite structures with enhanced mechanical strength for diverse industrial and biomedical applications. Declarations Author contributions Saiful Islam Sagor: Conceptualization, methodology, experimental design, data collection, and manuscript preparation Anasheh Khecho: Characterization, manuscript preparation and review. Erina Baynojir Joyee: Conceptualization, supervision, project administration formal analysis. Funding sources This material is based upon work supported by the National Science Foundation under Grant No. 2301462. References Phiri R (2024) Advances in lightweight composite structures and manufacturing technologies: A comprehensive review. Heliyon Rahatuzzaman M (2024) Design, fabrication, and characterization of 3D-printed ABS and PLA scaffolds potentially for tissue engineering. Results in Engineering Rehman M Additive manufacturing for biomedical applications: a review on classification, energy consumption, and its appreciable role since COVID–19 pandemic. Progress in Additive Manufacturing 2023 Zhang F (2021) The recent development of vat photopolymerization: A review. Additive Manufacturing Wang G (2023) Recent progress in additive manufacturing of ceramic dental restorations. J Mater Res Technol Narongdej P (2025) Vat photopolymerization (VP) of solvent-free carbon Nanoparticle-Acrylic nanocomposites. Composites Part A You S (2020) Mitigating Scattering Effects in Light-Based Three-Dimensional Printing Using Machine Learning. Journal of Manufacturing Science and Engineering Salas A (2022) Chemistry in light–induced 3D printing. ChemTexts Wu X (2019) Influence of boundary masks on dimensions and surface roughness using segmented exposure in ceramic 3D printing. Ceramics International Liu X (2024) Area-Exposure Additive Manufacturing Techniques and Devices: A Mini-Review. Additive Manufacturing Frontiers Safaee S (2022) Field-assisted additive manufacturing of polymeric composites. Additive Manufacturing Hosseinabadi HG (2023) Ink material selection and optical design considerations in DLP 3D printing. Zhou C (2015) A Novel Low-Cost Stereolithography Process Based on Vector Scanning and Mask Projection for High-Accuracy, High-Speed, High-Throughput, and Large-Area Fabrication. Journal of Computing and Information Science in Engineering Xie J (2020) Study on the liquid crystal display mask photo-curing of photosensitive resin reinforced with graphene oxide. Applied Polymer Wiley Green BJ (2019) Modification of mechanical properties and resolution of printed stereolithographic objects through RAFT agent incorporation. Additive Manufacturing Thijssen Q (2023) From pixels to voxels: A mechanistic perspective on volumetric 3D-printing. Prog Polym Sci Airinei A (2023) Computational and experimental investigation of photoresponsive behavior of 4,4′-dihydroxyazobenzene diglycidyl ether. Results in Chemistry Pan Y ADDITIVE MANUFACTURING OF MAGNETIC FIELD-RESPONSIVE SMART POLYMER COMPOSITES , in Proceedings of the ASME (2016) International Manufacturing Science and Engineering Conference MSEC2016 . 2016: Blacksburg, Virginia, USA Deb ÁS-AaS Acrylic-Based Materials for Biomedical and Bioengineering Applications Beharry AA (2011) Azobenzene photoswitches for biomolecules. Chem Soc Rev Slanska M (2024) Azobenzene-Based Photoswitchable Substrates for Advanced Mechanistic Studies of Model Haloalkane Dehalogenase Enzyme Family. Catalysis Courtine C Photoswitchable assembly of long-lived azobenzenes in water using visible light. Fedele C New tricks and emerging applications from contemporary azobenzene research. Petrikat RI A Photoswitchable Metallocycle Based on Azobenzene: Synthesis, Characterization, and Ultrafast Dynamics. Tan EMM (2015) Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy. Nat Commun Eyring M (2013) Spectroscopy in Forensic Science. Morris R (2015) Spectrophotometry Al-Azzawi ZM (2022) Influence of Nano Silicon Carbide (SiC) Embedded in Poly(Vinyl Alcohol)(PVA) Lattice on the Optical Properties. Silicon Pan Y (2012) Smooth surface fabrication in mask projection based stereolithography. J Manuf Process Khecho A (2025) DLP-based additive manufacturing of hollow 3D structures with surface activated silicone carbide-polymer composite. Composites Part B Wang Y (2024) Optimize projected mask images for improving three-dimensional printing accuracy for digital light processing based vat photopolymerization. Additive Manufacturing Semenov AV (2013) Light Scattering in Nanocrystalline SiliconCarbide (ncSiC) Films. Journal of Surface Investigation Naftaly M (2016) Silicon carbide—a high-transparency nonlinear material for THz applications. Optical Society of America Lu P (2021) Wavelength-selective light-matter interactions in polymer science. Matter Konrad DB (2020) Computational Design and Synthesis of a Deeply Red-Shifted and Bistable Azobenzene. Journal of the American Chemical Society Kienzler MA (2013) A Red-Shifted, Fast-Relaxing Azobenzene Photoswitch for Visible Light Control of an Ionotropic Glutamate Receptor. Journal of the American Chemical Society Purkait MK (2018) Photoresponsive Membranes. Interface Science and Technology Chawla KK (2005) Mechanical Properties: Tensile Properties. Fu S-Y (2008) Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites. Part B, Composites Behera A (2023) The Consequence of SiC Filler Content on the Mechanical, Thermal, and Structural Properties of a Jute/Kevlar Reinforced Epoxy Hybrid Composite. Silicon Wang DH (2014) Impact of Backbone Rigidity on the Photomechanical Response of Glassy, Azobenzene-Functionalized Polyimides. Macromolecules Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6736596","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":461206497,"identity":"9255545c-eb76-4c7b-a4a6-4534e9510f75","order_by":0,"name":"Saiful Islam Sagor","email":"","orcid":"","institution":"UNC Charlotte","correspondingAuthor":false,"prefix":"","firstName":"Saiful","middleName":"Islam","lastName":"Sagor","suffix":""},{"id":461206498,"identity":"3590ef90-85b2-4499-a893-34a72edec5d6","order_by":1,"name":"Anasheh Khecho","email":"","orcid":"","institution":"UNC Charlotte","correspondingAuthor":false,"prefix":"","firstName":"Anasheh","middleName":"","lastName":"Khecho","suffix":""},{"id":461206499,"identity":"bf71abac-08be-456b-b70d-7d963a9ae043","order_by":2,"name":"Erina Baynojir Joyee","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7UlEQVRIiWNgGAWjYFCCAyDChsGAgYHxAFTIgBgtaWBlBxgSDIjRAgaHSdDCz3j4mMSHivPy5uxnHxz4+OOPHAN78zYJfFokG46lSc44c9twZ0+6wcEZCQbGDDzHyvBqMThwxkyat+12gsGBNIbDPAkGiQ0SOWZ4tdiDtPz9dy7B4PwzsJb6Bvk3+LUYMAC1MDYcSDC4AbElgUGCB78WiQPHki17jiUbbrjxjOHgjDRjwzaetGILfFr4Zxw+eONHjZ28wfk0xgcfbOTk+dkPb7yBTwvQGhZUZ7DhVQ62poH5A0FFo2AUjIJRMLIBAFHnTYCHSY0gAAAAAElFTkSuQmCC","orcid":"","institution":"UNC Charlotte","correspondingAuthor":true,"prefix":"","firstName":"Erina","middleName":"Baynojir","lastName":"Joyee","suffix":""}],"badges":[],"createdAt":"2025-05-24 04:14:23","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-6736596/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6736596/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83596460,"identity":"766a6a4a-856b-4a53-b7b7-18b371821ccb","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":188754,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Photopolymerization process of methacrylate-based polymer resin;\u003cstrong\u003e \u003c/strong\u003e(b)\u003cstrong\u003e \u003c/strong\u003eMolecular structure of Azobenzene, Photopolymer Resin and atomic arrangement of SiC particles; (c-e) Preparation steps of the Azobenzene-SiC-polymer suspension.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/83812f700b8e6f56f92648f2.png"},{"id":83596457,"identity":"523799c8-cc3e-48bd-a8e5-92b457424649","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":36351,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of spectrophotometry\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/01e27d7bd8a5f0cafdeb3250.png"},{"id":83597245,"identity":"ba565272-2b8a-4425-b1ee-e1e69bd56f88","added_by":"auto","created_at":"2025-05-29 08:09:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":797963,"visible":true,"origin":"","legend":"\u003cp\u003e(a)\u003cstrong\u003e \u003c/strong\u003eExperimental setup of the DLP prototype; (b) Bottom-up layer by layer printing process in DLP printing.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/b33be4795bc2179240534f97.png"},{"id":83596464,"identity":"9eded309-8c2f-4442-ac01-969d7c954b72","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1101223,"visible":true,"origin":"","legend":"\u003cp\u003e(a) CAD model of the design mask; (b) Process software; (c) Printing process of the layers; (d) Cured part using polymer resin.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/1484e361ae15a55e85de0c6c.png"},{"id":83597557,"identity":"0d6c3a66-7cc9-4480-9444-03c65f319639","added_by":"auto","created_at":"2025-05-29 08:17:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":173610,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The relationship curve between the grayscale of projected images and optical power of projected light; (b) Curing thickness under different relative light intensity.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/d6b38e1b7db1aba7fb60dcd7.png"},{"id":83597241,"identity":"6e1d3b6d-928b-4245-b258-70acf68d736a","added_by":"auto","created_at":"2025-05-29 08:09:07","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":98420,"visible":true,"origin":"","legend":"\u003cp\u003eThe flowchart of finding the optimum grayscale value of contour mask\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/914489623a3b3d4a96d11021.png"},{"id":83596461,"identity":"5565e1f3-22d3-4cdd-b1e2-0f642dd28204","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":247330,"visible":true,"origin":"","legend":"\u003cp\u003eAbsorption coefficient spectra of composite formulations across UV and visible wavelength ranges for (a) Pure resin; (b) C1 and C2; (c) C3 and C4; (d) C5 and C6.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/ccfcc57cfefa903ecb3ac1ae.png"},{"id":83596469,"identity":"58f56c15-4a1c-4698-8464-6bc6e34e9fbb","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":717730,"visible":true,"origin":"","legend":"\u003cp\u003eStress vs Strain curve of 3D printed samples with different SiC and azobenzene compositions.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/64f74da63a3bda0cf098c26e.png"},{"id":83596467,"identity":"2fb9e38d-6dd2-4e28-ac36-6a590bbd6f98","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":830987,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Sliced layers of the CAD model used for mask generation; (b) Conventional single-layer mask projection; (c) Initial segmented mask approach showing separate activation of core and contour regions; (d) Modified segmentation strategy with simultaneous partial activation of both core and contour; (e) Printed part from the initial approach showing a visible separation line between core and contour; (f) Printed part from the modified approach showing a smooth transition and improved interfacial bonding\u003cstrong\u003e. \u003c/strong\u003eTo address this issue, an alternative masking segmentation approach was implemented. In this modified method, both the contour and core regions were exposed simultaneously but at different light intensities. During the first exposure cycle, the core region was fully activated at maximum intensity, while the contour region was partially activated at 60% intensity, instead of remaining inactive as in the previous approach. In the second exposure cycle, the core region was not completely deactivated but was instead maintained in a partially active state with an optimized grayscale value of 180, similar to the contour region.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/0ffaf39e81715431b017fcc2.png"},{"id":83596465,"identity":"31114048-d1c6-4be8-9362-ba0a9f74b32c","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":670446,"visible":true,"origin":"","legend":"\u003cp\u003eInvestigation of photopolymerization of C-4 (a) under UV light, (b) under blue light; (c) Proposed mask projection system during dual light DLP printing.\u003c/p\u003e","description":"","filename":"image10.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/3c5255bbacf79cb442b9c73c.png"},{"id":83596468,"identity":"355ba779-d706-4273-9761-6287e3848e2a","added_by":"auto","created_at":"2025-05-29 08:01:07","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1125216,"visible":true,"origin":"","legend":"\u003cp\u003e(a) CAD model of the lotus root-inspired structure; (b) Light source setup of the dual wavelength DLP system, showing aligned vs. misaligned regions of UV and blue light sources; (c) Sequential segmented exposures under UV light for core and contour regions; (d) Blue light projection using a blocking mask for photoinhibition in targeted zones; (e) Optical image of the printed structure showing cure bleed regions due to light misalignment (left) and a uniform edge resulting from proper alignment (right), highlighting the effect on internal void preservation.\u003c/p\u003e","description":"","filename":"image11.png","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/2704b7826869fbd6cc0c5ae9.png"},{"id":83598198,"identity":"5143f2cf-0e15-483e-b200-a342a2fc90dc","added_by":"auto","created_at":"2025-05-29 08:25:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6420886,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6736596/v1/c6443caa-63c0-433b-85d1-e126eb971e27.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eDigital Light Processing 3D Printing of Polymer Composites Based on Tunable Curing Resins with Photoswitchable Molecules\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eComposite materials have become an essential class of materials in advanced engineering applications due to their capacity to integrate distinct material properties into a single, optimized system. Typically composed of a polymer matrix and reinforcing filler particles, composites offer enhanced mechanical, thermal, and functional properties that outperform conventional single-phase materials. Key advantages include high strength-to-weight ratio, enhanced durability, and the ability to tailor performance attributes to specific design requirements [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. These features make composites especially attractive for high-performance fields such as aerospace, automotive, defense, and increasingly, biomedical engineering. In the biomedical domain, composite materials are employed in applications ranging from structural scaffolds and prosthetics to implants and wearable electronics, where their biocompatibility, strength, and chemical resistivity are particularly beneficial.\u003c/p\u003e \u003cp\u003eTo meet the demand for customized and complex biomedical devices, additive manufacturing (AM) has emerged as a critical fabrication strategy. AM technologies allow for the layer-by-layer construction of patient-specific structures with high geometric complexity and minimal material waste [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Among AM techniques, vat photopolymerization (VP) has gained widespread attention for its superior resolution, surface finish, and ability to process intricate geometries. VP process can be divided in two main categories, stereolithography (SLA) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], which uses a point-wise scanning laser to cure resin, and digital light processing (DLP) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], which projects entire layer patterns via spatially modulated light. Compared to SLA, DLP offers higher throughput and spatial control through digital mask manipulation, making it more suitable for high-speed, precision fabrication.\u003c/p\u003e \u003cp\u003eResearchers have extensively leveraged DLP based VP for fabricating composite structures by dispersing micro- or nanoscale fillers into photosensitive resins. These reinforcements can significantly improve the mechanical and functional performance of printed parts. However, the addition of other components to pure photosensitive monomers introduces light scattering issues, affecting the polymerization process. These challenges arise from the complex interactions between light and material, including refraction, scattering, and absorption kinetics [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Filler particles disrupt the uniform transmission of curing light, causing light scattering, refraction, and absorption within the resin. This leads to non-uniform energy distribution, limited curing depth, poor layer adhesion, and defects in the printed geometry. Especially, in DLP systems, where entire cross-sections are cured simultaneously, the presence of scattering particles can severely impair print fidelity and structural performance. Such inconsistencies can lead to overcuring or under curing, ultimately impacting the mechanical properties and dimensional accuracy of the final printed part [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo mitigate these issues, researchers have proposed various approaches. Within DLP systems, different manufacturing configurations have been explored to control and project light for curing [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Researchers have explored various solutions, including optimizing exposure parameters and light energy distribution. For mask based DLP, controlling the exposure time of different pixels helps regulate overcuring by adjusting the light energy distribution in the exposed area [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Additionally, the mask-division method, proposed in [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], suggests segmenting light exposure to reduce scattering effects in the boundary region. Digital micromirror device (DMD)-based projectors are widely used in commercial and high-performance research settings due to their ability to reflect high-intensity UV light with excellent spatial resolution [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. These systems utilize an array of microscopic mirrors that rapidly tilt to direct light pixel-by-pixel. Liquid crystal display (LCD)-based DLP systems, by contrast, use a transmissive liquid crystal panel to modulate light in a planar manner [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. While LCD-based systems offer a cost-effective alternative, they suffer from limitations in UV light transmission due to the absorption and scattering properties of the liquid crystal material, which can reduce curing efficiency and resolution. Besides these, researchers have also explored LED-array based segmented projection systems, where spatially controlled exposure is achieved by illuminating subregions sequentially [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Wu et al. used a LED based light projection system to reduce the overcuring and scattering artifacts by varying local light intensity [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. In another study Zhou et al., have used a hybrid DLP-laser systems that combine wide-area projection with precise laser reinforcement for improved interlayer bonding in composites [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMost of the described approaches still rely on material-dependent parameters such as particle size, shape, and refractive index, meaning that any change in filler composition will need re-optimization. This limits adaptability and real-time control over polymerization. Besides manufacturing based configuration of DLP systems, some studies have developed index-matched resin formulations to reduce the refractive mismatch between fillers and the surrounding medium. The use of UV absorbers to localize light penetration has also been explored, although this can introduce vertical gradients in curing, affecting layer uniformity. For example, Xie and He demonstrated that exceeding 0.5 wt% of graphene oxide in LCD-based VP systems led to significant light attenuation, preventing effective polymerization. Similar limitations have been observed in SLA systems using nonwoven glass or carbon fiber mats, where optical obstruction restricts feature resolution [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo optimize VP 3D printing for high spatial resolution and defined architectures, a deeper understanding of the molecular structure of photopolymerizable inks and the kinetics of the photopolymerization reaction is required [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Further ink development is necessary to achieve the resolution and speed needed to meet the increasing demand for rapid fabrication and mass production [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. A major gap remains in existing strategies that lack the ability to actively and spatially regulate curing behavior in real time. Most methods rely on passive materials design or static light modulation, which are often insufficient for compensating the dynamic optical effects introduced by high filler loadings. This constraint motivates the need for a more adaptive and tunable curing mechanism that can respond to complex light-material interactions.\u003c/p\u003e \u003cp\u003eIn this study, we propose the use of multi-wavelength DLP systems [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] that incorporate dual-wavelength control, where one wavelength activates polymerization and the other inhibits it, this strategy offers real-time spatial regulation of curing behavior. This wavelength-specific chemical control represents a significant advancement for printing with light-scattering fillers, offering more reliable curing depth and structural fidelity in composite materials. The use of voxel-based exposure in DMD systems allows for highly programmable and accurate photopolymerization; however, in composite systems with light-scattering fillers, even the highest-resolution projection may suffer from uneven curing. In such cases, incorporating a dual-wavelength approach provides an innovative solution by enabling not only light-pattern control but also wavelength-specific chemical control of polymerization. This introduces an extra dimension of spatial precision where polymerization can be selectively activated or inhibited in targeted regions depending on the wavelength applied.\u003c/p\u003e \u003cp\u003eOur study aims to translate this concept into a practical solution by integrating wavelength-specific control directly into the materials and optical system design. Building upon this concept, we propose a materials- and optics-based strategy to realize such control in practice. We developed a novel DLP-based approach utilizing a photoswitchable polymer resin system incorporating azobenzene, a well-characterized photochromic molecule. Azobenzene undergoes reversible photoisomerization when exposed to different wavelengths of light, transitioning between trans and cis states [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. This molecular switching mechanism can be harnessed to regulate local polymerization activity through light wavelength selection. In our dual-wavelength DLP platform, blue light serves as a polymerization initiator, while UV light functions as a photoinhibitor. This dual-wavelength control enables spatially resolved, pixel-by-pixel modulation of the curing process, allowing us to counteract the non-uniform curing effects caused by light-scattering fillers such as SiC.\u003c/p\u003e \u003cp\u003eIn this study, we further develop and evaluate a series of azobenzene-functionalized composite inks containing varying concentrations of polymer matrix, silicon carbide (SiC), and photoinhibitory components. We investigate the optical behavior of these formulations using UV-Vis spectrophotometry to understand how light absorption and transmittance change with filler content. We then fabricate test structures using a custom-built DLP prototype and assess their mechanical properties through tensile testing, quantifying the influence of photoinhibition and filler loading. Additionally, we implement a mask-based spatial light modulation technique, comparing conventional uniform exposure with contour-specific mask designs to enhance boundary resolution and reduce overcuring.\u003c/p\u003e \u003cp\u003eThis work introduces a dynamic, wavelength-selective strategy for DLP-based composite printing that addresses the longstanding limitations imposed by optically dense filler systems. By integrating smart resin chemistry with programmable light control, our method provides a versatile and scalable framework for manufacturing high-fidelity, mechanically robust composite structures. The proposed approach holds broad implications for applications in biomedical devices, photonic materials, soft robotics, and precision engineering, where functional composites and geometric complexity are increasingly in demand.\u003c/p\u003e"},{"header":"2. Materials and Methodology","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Material Preparation\u003c/h2\u003e \u003cp\u003eTo test the concept of curing property manipulation described in the introduction part, a composite resin was prepared for testing. In this study, for testing the optical properties, seven different ink compositions were prepared by using an acrylic based photocurable polymer resin (3DM ABS, 3D Materials Inc.) as base matrix. This resin contains acrylic acid esters, acrylated monomers, oligomers, and TPO-solid (Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide) and BAPO (Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide) as the photoinitiator [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Acrylate-based photopolymer resins are among the most commonly utilized formulations in different applications, including bone cement, contact lenses, and orthopedics applications [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. When exposed to UV radiation, the photoinitiator absorbs the light energy and produces free radicals as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). These free radicals trigger the polymerization process of the acrylate materials, allowing them to cure within seconds. Along with methacrylate resin, SiC powder (US Nano, Houston, TX, USA) with an average particle size of 40 \u0026micro;m and Azobenzene (with a purity of 98%, Sigma Aldrich, USA), as photoswitchable molecule, were mixed in different ratios to prepare the ink resin suspension. The chemical structures of these base components of the prepared ink samples are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). Photoexcitation of N\u0026thinsp;=\u0026thinsp;N bond in Azobenzene molecules under exposure to specific wavelengths of light enable reversible photoisomerization [\u003cspan additionalcitationids=\"CR21 CR22 CR23 CR24\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. These characteristics make Azobenzene a suitable candidate for wavelength selective modulation of the photopolymerization process. The composite inks with varying concentrations of SiC particles and azobenzene were prepared as detailed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The selected concentration ranges were determined based on both experimental feasibility and the need to evaluate the effects of each component on light transmission and curing performance. A range (0%-3%) of SiC concentrations was tested to assess its influence on optical properties, specifically transmittance and absorbance. It was observed that SiC loadings above 3 wt% resulted in the composite becoming visually opaque, effectively blocking light transmission and rendering spectrophotometric analysis infeasible.\u003c/p\u003e \u003cp\u003eAzobenzene concentrations were varied to evaluate their minimal amount of inhibitory effect on polymerization under dual-wavelength exposure. It was found that concentrations above 1wt% significantly suppressed the curing process under visible light, likely due to excessive light absorption or photoinhibition effects. Therefore, material concentrations were systematically varied to balance optical clarity, curing responsiveness, and measurable spectroscopic behavior, enabling the formulation of resins suitable for dual-wavelength DLP printing.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComposition of the photopolymer ink used in this study.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComposite\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAzobenzene (wt %)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSiC (wt %)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eResin (wt %)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e97\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e99\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e97\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e98.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e96\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c\u0026ndash;e) illustrates the preparation of the composite suspension, which followed two sequential steps. First, an ultrasonic sonicator (Qsonica Q700, 700 W, 20 kHz, USA) was used for 10 minutes to disperse the particles within the resin suspension. The samples were placed in a glass beaker and submerged in chilled water to dissipate heat generated during sonication. The sonicator operated at 20 kHz with a pulse cycle of 5 seconds to effectively agitate and break up any particle agglomerates. Following sonication, the suspension was mixed using a temperature-controlled magnetic stirrer (Cheshire Enterprise, LLC, USA) at 500 rpm for 5 minutes. This ensured uniform distribution of the particles and helped maintain suspension homogeneity during printing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Spectrophotometric Analysis of Photoswitchable Ink\u003c/h2\u003e \u003cp\u003eSpectrophotometry determines how light interacts with matter through reflection, refraction, scattering, absorbance, and transmittance by quantifying the amount of light being absorbed, reflected, or transmitted at a particular wavelength [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Absorbance is a dimensionless quantity that varies from zero for completely transparent samples (where transmittance is 100%) to an infinitely large value for fully opaque samples (where transmittance is 0%) [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. In our study, absorbance of the developed inks was examined using a UV-Visible spectrometer (Goyojo, China). This spectrophotometer operates at 110 V, 50/60 Hz, with a 6 nm Tungsten Lamp and a wavelength range of 350\u0026ndash;1020 nm to determine the optical properties. In a spectrophotometer, absorbance is determined by directing a collimated beam of light at a specific wavelength through a flat, parallel-sided material that is perpendicular to the beam as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The intensity of the light entering the sample (I₀) is greater than the intensity of the light that exits (I) because some of the energy is absorbed by the molecules within the sample and this absorbed energy values are shown in the spectrophotometer as absorbance. The relationship of emitted light (I\u003csub\u003e₀\u003c/sub\u003e) and transmitted light (I) with absorbance (A) is determined by the following Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:A\\:=\\:-\\:{log}_{10}\\:\\left(\\frac{I}{I₀}\\:\\right)\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIn this study, the spectrophotometer was first turned on and allowed to warm up for 30 minutes. A blank solution of distilled water was then used to calibrate the instrument. After calibration, the samples were placed in 1 cm wide transparent glass cuvettes, wiped with alcohol to eliminate any interference in light transmission and inserted into the chamber cell. The absorbance values of pure resin and composites with different concentrations were calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) across a wavelength range of 350\u0026ndash;700 nm.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSubsequently, absorption coefficients were calculated using Beer-Lambert\u0026rsquo;s law, which establishes a relationship between wavelength and absorbance. This was done based on the following Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\alpha\\:\\:=\\:2.303\\:\\times\\:\\:\\frac{A}{d}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere, α is the absorbance coefficient (cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), A is the absorbance value, d is the thickness of the samples in cm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 DLP Curing with Conventional Mask\u003c/h2\u003e \u003cp\u003eA custom DLP 3D printer prototype was developed to test the developed inks. This setup includes an imaging unit, a resin vat with a transparent bottom surface, a linear actuator to lift the build platform, and two linear stages that allow the resin vat to slide. Different parts of the prototype are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a). A visible light projector was used as the imaging unit equipped with 1024 \u0026times; 768 microarray and an envelope of 42.7 \u0026times; 32 mm, having modified lenses to project images according to the mask over an adjustable working distance. The projection settings including focus, brightness and contrast were adjusted to produce a clear projection image on the transparent plane of resin vat. The 3D printer was designed with the projector positioned vertically relative to the resin tank, eliminating the need for reflecting mirrors [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. This design choice reduces the size of the printer and also offers greater flexibility in defining the printing area. A micro controller was used to move both the vat and platform according to a 2D mask image. To reduce the separation force of the cured part from the tank surface, the bottom of the resin vat was coated with polydimethylsiloxane (PDMS) film due to its inherent property of anti-sticking and durability [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA digital mask planning and process control slicing software was developed using C\u0026thinsp;+\u0026thinsp;+\u0026thinsp;shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b). This system integrates geometry slicing, image generation, suspension deposition control and motion coordination. The software generates image masks based on the computer aided design (CAD), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) and projects them onto the bottom of the build platform through the resin tank.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt the start of the bottom-up DLP printing process, the build platform lowers into the photopolymerizable liquid resin until it is positioned one layer thickness above the PDMS layer. The layer thickness is controlled by maintaining the distance between the resin vat floor and the build platform. The software converts the CAD drawing into 2D mask images, which are projected from the light source onto the bottom of the build platform through the resin tank. This exposure cures the entire 2D layer, causing the photopolymerized resin to adhere to the build plate and forming the first layer as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). After curing one layer, the build platform moves upward with one layer thickness which allows uncured resin to refill the space. This process is repeated for the second layer, which adheres to the first and continues layer by layer until the complete 3D part is printed. The completed part, printed with pure resin is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c). These steps were followed to print the same structures (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) using the formulated inks listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 DLP Curing with Core and Contour Mask\u003c/h2\u003e \u003cp\u003eOne of the primary limitations of DLP method for composite printing is the geometric accuracy errors that occur in boundary regions due to light diffraction. As light propagates, it reflects off the Digital Micromirror Device (DMD) that leads to diffraction and an unintended increase in the spot size [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. This effect is further exacerbated when printing SiC-polymer composites. The scattering properties of the SiC particle contribute to additional light diffusion. Consequently, this diffracted, unintended exposure extends beyond the target areas which reduce printing accuracy in the XY plane of the fabricated structure. To mitigate these inaccuracies, it is essential to control the curing process at the boundary regions by controlling either the light power intensity or the exposure time. One effective approach to control the optical power involves adjusting the grayscale values of the projected mask images.\u003c/p\u003e \u003cp\u003eTo investigate the relationship between grayscale values and optical power at the curing position, an experiment was conducted using an optical photodiode power sensor (S120VC, Thorlabs, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). The optical power sensor was placed between the resin vat and the light source to precisely measure the light intensity at different grayscale values. The results indicate a linear correlation between grayscale values and optical power, with a grayscale value of 255 corresponding to maximum light intensity. To ensure control over light exposure in boundary regions, the conventional projection masks were divided into two separate masks: a core mask and a contour mask. In the first exposure cycle, the core mask was fully activated with a grayscale value of 255, ensuring maximum light intensity to cure the inner region completely. At the same time, the contour mask was set to a grayscale value of zero that ensured the contour region was inactive and prevented the activation of the photoinitiator at the contour region. The exposure time was set to 10 seconds to ensure that the light with the highest intensity fully cured the core region.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFollowing the core curing procedure, a second exposure cycle was initiated, where the contour mask was partially active with an optimized grayscale value of 180. At this time the core mask remained deactivated. The optimal grayscale value for the contour region was determined through an iterative optimization approach, leveraging parametric tuning and experimental calibration. A range of grayscale values (150\u0026ndash;200) with 2-units increment was tested, and their effects on boundary integrity and polymerization depth were evaluated. The optimization process employed a gradient-based adjustment strategy, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, where the exposure intensity was incrementally modified based on the resulting boundary sharpness and material conversion efficiency. Image processing techniques were used to analyze the cured regions, ensuring uniformity and minimal overcuring effects. The selected optimized grayscale value of 180, employing around 60% relative light intensity, was found to provide sufficient curing depth while maintaining dimensional accuracy. At an optimized relative light intensity of 60%, the curing strength was sufficient to create a well-defined rectangular layer, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b). However, residual uncured areas were observed across the cross-section, indicating incomplete polymerization. This suggests that while the exposure was sufficient for defining the boundary, internal diffusion limitations or variations in light absorption may have contributed to incomplete crosslinking. When the relative intensity fell below this threshold, curing was insufficient to maintain the intended thickness, leading to edge distortions and discontinuities.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussions","content":"\u003cp\u003eIn our experiment, seven sample groups were designed, fabricated, and tested using the proposed P-DLP process introduced in Section \u003cspan refid=\"Sec1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. All groups were prepared following the material preparation procedure described in Section \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. For mechanical testing, each sample was printed using the same CAD model used in Section \u003cspan refid=\"Sec7\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Spectral Absorption Analysis of Functionalized Inks\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e illustrates the spectrophotometric behavior of seven different samples prepared and tested to analyze their response under various light wavelengths. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea shows the absorption characteristics of pure resins (C0) containing TPO and BAPO photoinitiators, which strongly absorb UV light in the 380–405 nm range. For inks C1 and C2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), adding 1% and 3% SiC increases UV absorbance, though the effect remains small at these concentrations. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb shows that as the SiC concentration increases, absorbance in the 350–450 nm range also rises. This suggests that SiC influences light interaction through both absorption and scattering. Since SiC is known for its light-scattering properties and low transparency, higher concentrations of SiC lead to increased scattering and energy dissipation of UV light [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. As a result, the spectrophotometric measurements show higher absorbance at elevated SiC concentrations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the case of inks C3 and C4, the addition of azobenzene extends the absorbance into the blue light spectrum. This is due to photoisomerization, where azobenzene molecules switch between trans and cis forms when exposed to light, enhancing their light absorption ability as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. A comparison between 1% and 3% azobenzene concentrations reveals that absorbance increases with higher concentration, consistent with the Beer-Lambert Law. Additionally, at higher concentrations, a red shift and a plateau in absorbance suggest intermolecular interactions between azobenzene molecules under blue light exposure [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. This behavior indicates that azobenzene could function as an effective photo-inhibitor in ink formulations, improving control over the curing process. For inks C5 and C6 in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed, which contain both SiC and azobenzene, a combined effect on absorbance is observed. When azobenzene and SiC present at 0.5% and 1% concentration respectively, absorbance significantly increases in the 380–530 nm range, likely due to azobenzene’s light-responsive properties [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. Increasing the azobenzene and SiC concentration to 1% and 3% respectively, results in a plateau in absorbance between 380–450 nm. This suggests a balance between SiC-induced light scattering and azobenzene absorption. The interaction of these components across the UV and blue spectrum suggests that azobenzene absorbs scattered light, particularly near the boundaries of the printed geometry. This property could help mitigate overcuring caused by SiC’s scattering effects, improving the precision of the curing process in DLP printing.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Tensile Testing of Printed Structures\u003c/h2\u003e \u003cp\u003eTensile testing is a destructive mechanical test used to determine key properties such as tensile strength, yield strength, ductility, and elastic modulus of a material. It involves applying a uniaxial tensile force to a specimen until failure for generating stress-strain curve that characterizes the material's behavior under load. This test is essential for understanding how materials deform and ultimately fail under tensile forces, which is critical for ensuring structural reliability and performance of the composite [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. In this study, the tensile test was performed with a Mark-10 F505-EM test frame, as shown in Fig.\u0026nbsp;8.3D printed samples with composites C0, C1, C2, C5, and C6 were carefully secured in the grips of a tensile testing apparatus. The bottom grip remained stationary, while the upper grip applied an increasing tensile load. As the tensile load increased, each sample progressively elongated until it eventually failed. To ensure consistency and reduce experimental error, two identical samples of each composite were tested. Throughout this process, the force applied and the corresponding deformation were accurately recorded.\u003c/p\u003e \u003cp\u003eFrom Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, it is evident that the pure acrylate-based resin (C0) exhibited the highest ultimate tensile strength (~ 47 MPa) along with the highest elastic modulus (32 MPa), indicating a well-balanced combination of stiffness and strength. The addition of SiC as a filler had a noticeable impact on the mechanical properties. The C1 composite, containing 1% SiC, showed relatively moderate tensile strength (~ 45 MPa) and exhibited a slight reduction in stiffness (30 MPa), suggesting minimal reduction in mechanical performance compared to the C0. However, increasing the SiC content to 3% in the C3 composite led to a significant decrease in both ultimate tensile strength (~ 25 MPa) and elastic modulus (19.17 MPa), resulting more brittleness in the printed part. This degradation can be attributed to the tendency of SiC particles to agglomerate at higher concentrations, which causes non-uniform dispersion within the polymer matrix. Such aggregation forms micro-clusters act as stress concentrators and weakens the composite by promoting crack initiation and propagation under load. Consequently, this localized stress concentration undermines the tensile properties of the material [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Moreover, since SiC is an inherently brittle ceramic, its incorporation with polymer matrix exacerbates brittleness and compromise mechanical performance [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurthermore, the incorporation of azobenzene in addition to SiC fillers negatively influenced the mechanical characteristics of the composites. The C5 composite, comprising 0.5% azobenzene and 1% SiC, demonstrated lower tensile strength (~ 21 MPa) and a reduced elastic modulus (18.55 MPa). The samples were brittle and they had undergone up to 4% of strain before breaking. The effect was more pronounced in the C6 composite, which contained 1% azobenzene and 3% SiC, resulting in the poorest mechanical performance among all tested samples—exhibiting the lowest ultimate tensile strength (~ 12 MPa) and a comparable elastic modulus (19.99 MPa). The microscopic image of the C6 sample’s fractured surface was taken using Scanning Electron Microscope (SEM, JEOLJSM-6480 CO., LTD, Japan) and it shows a rough, irregular edge, indicating a brittle fracture mode, supporting the observation of poor ductility and early failure. The reversible photoisomerization of azobenzene molecules upon UV and visible light exposure alters the molecular geometry from a linear (trans) to a bent (cis) configuration, inducing local disruptions within the polymer matrix. This conjugated structure hinder the mobility of polymer chains and reduce the material’s ability to deform under tensile stress and contributes to decreased tensile strength and elongation [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].Therefore, to preserve the mechanical integrity of such composite systems, the concentration of azobenzene must be carefully controlled to balance its functional benefits with structural performance.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Evaluation of Masking Segmentation in DLP Printing\u003c/h2\u003e \u003cp\u003eAfter evaluating the optical and mechanical performance of the formulated inks, printing tests were conducted to further assess their processability. To test the effectiveness of the proposed masking segmentation technique, a dog bone-inspired 3D model was designed, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a). The part was fabricated using the segmented mask projection method outlined in Section \u003cspan refid=\"Sec10\" class=\"InternalRef\"\u003e3.3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eIt was observed that (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea), the cured part exhibited an unintended separation line between the core and contour regions, indicating a weaker interfacial bond that compromised the structural integrity of the printed component. This issue arose due to the curing sequence used in the segmented mask projection process. In the initial exposure cycle, the core region was fully activated at maximum light intensity, while the contour region remained completely inactive. In the subsequent exposure cycle, the contour zone was partially exposed using the optimized grayscale value of 180, found from the approach described in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, whereas the core region was not re-exposed. Since these regions were polymerized independently under distinct light exposure conditions, the linker monomers at the interface may not have undergone complete crosslinking which lead to poor adhesion and the formation of the visible separation line. These findings highlight the need for further optimization of the segmentation strategy to ensure uniform polymerization and enhance interfacial bonding.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis simultaneous curing strategy resulted in a more uniform polymerization process, reducing the chance of insufficient crosslinking at the interface. Additionally, to mitigate the risk of overcuring, the exposure time per cycle was reduced to 5 seconds, which is half the duration used in the initial approach. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(b), the modified approach eliminated the separation line, indicating improved interfacial adhesion and structural integrity. However, a slight overcuring effect was observed in the top-left portion of the sample, suggesting that further optimization of exposure time may be necessary to refine the process and achieve optimal geometric precision.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Concept of Dual Light DLP Printing with Azobenzene Composite\u003c/h2\u003e \u003cp\u003eTo evaluate the feasibility of the proposed dual-wavelength DLP printing method using photoswitchable composite resins (C4), experiments were conducted using two different wavelengths. Initially, composite samples (C4) were tested separately under UV and blue light exposures. A simple 17 mm × 17 mm square-shaped 2D mask was projected in each test, with an exposure time of 10 seconds. A DLP UV projector from LightCrafter 4500, Texas Instruments, USA was used for initial UV exposure. A significant over curing beyond the intended masked region was observed due to scattered ultraviolet light (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea). In contrast, exposing the same resin sample (C4) to the blue light using an RGB DLP projector from Texas Instrument, under the same conditions resulted in no visible polymerization (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eb). This outcome confirms the effective photoinhibitory property of azobenzene under blue illumination, indicating its potential role in controlling unintended polymerization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNext, we tested the above described concept with our new developed prototype, using simultaneous projection of both UV (DLP® LightCrafter™ 4500, 405 nm, Texas Instrument) and blue light (DLP® LightCrafter™ 4500, RGB, Texas Instrument) sources. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ec, the two lights were arranged orthogonally at a 90-degree angle. A reflective dichroic mirror was used to direct both light sources onto the resin at the same time, allowing different regions to receive different wavelengths. For this test, the exposure time was increased to 20 seconds to observe the curing behavior more clearly. The area exposed to UV light showed signs of overcuring, while the region marked with a blue dotted line in the center remained in a gel-like (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ec), under cured state. This indicated delayed or inhibited polymerization under blue light in that region, despite the extended exposure.\u003c/p\u003e \u003cp\u003eBased on these observations, a thin-wall cylindrical structure featuring multiple internal voids, inspired from the structure of lotus root (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea) was cured using the dual light DLP prototype to demonstrate the feasibility of the proposed dual-wavelength DLP process. The developed prototype currently shows some alignment issues between the UV and blue light sources (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb). Specifically, in certain areas, the blocking mask projected by the blue light source does not completely overlap with the contour mask from the UV source. Due to this misalignment, some regions lack proper exposure of the blocking mask, potentially resulting in overcuring. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(b) for the lotus root-inspired structure, one side of the circular geometry shows a slight gap where the blue blocking mask fails to merge accurately with the UV contour mask. In contrast, on the opposite side, the blocking and contour masks are properly aligned, allowing precise exposure of the blocking mask on both the peripheral and internal void regions. This mask projection approach involved two sequential primary exposure steps. In the first, UV light was used to selectively cure the core and contour regions at optimized intensities, following a segmentation method similar to that described in Section 4.3. Specifically, in the first step, full-intensity UV light was applied to the core, while reduced intensity was used for the contour. In the second step, both regions received partial-intensity UV exposure at optimized gray scale value to further refine polymerization, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(c). Simultaneously, a secondary exposure with blue light was applied using a blocking mask positioned around the outer boundary of the UV-cured region and along the internal voids (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(e) presents a successfully cured layer printed using the proposed dual-wavelength DLP approach. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(e, right), the circular portion maintained a well-defined shape on the side where the blocking mask and the UV contour mask aligned correctly. In contrast, the opposite side exhibited signs of overcuring caused by SiC-induced light scattering, attributed to the misalignment of the masks. This misalignment prevented proper blocking mask exposure, resulting in geometric distortion, the formation of cure bleed areas, and loss of the intended circular shape. These results validate the effectiveness of the blocking mask in leveraging azobenzene’s photoinhibitory capability, preventing unintended polymerization due to scattered UV light. Also, it is observed that the regions intended to remain void were successfully protected and preserved in their uncured state, maintaining the designed porous geometry. This demonstrates the printability of our proposed method by the integration of this dual-wavelength strategy shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(c), emphasizing its spatial precision and ability to minimize scattering-induced curing errors common in conventional DLP processes.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we demonstrated the feasibility of effectively controlling DLP 3D printing through the incorporation of azobenzene composites in the functional ink, utilizing azobenzene's photoinhibitory properties under blue light. Through comprehensive spectral absorption analysis and printing experiments, the photoinhibitory capability of azobenzene within functionalized inks was successfully validated. Additionally, this research addressed critical challenges associated with mask DLP 3D printing, such as visible separation lines on printed surfaces, through optimized light-intensity-controlled exposure in the core and contour region. Also, a novel blocking mask concept using the photoinhibitory effect of azobenzene under blue light in dual-wavelength DLP printing is proposed and applied for reducing issues related to overcuring. Our future research will further investigate the practical feasibility of dual-wavelength printing with blocking mask by properly aligning the blocking mask with core and contour mask, optimizing azobenzene concentration, exposure parameters and enhancing resin formulations to facilitate the fabrication of precise composite structures with enhanced mechanical strength for diverse industrial and biomedical applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSaiful Islam Sagor: Conceptualization, methodology, experimental design, data collection, and manuscript preparation\u003c/p\u003e\n\u003cp\u003eAnasheh Khecho: Characterization, manuscript preparation and review.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eErina Baynojir Joyee: Conceptualization, supervision, project administration formal analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding sources\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis material is based upon work supported by the National Science Foundation under Grant No. 2301462.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003ePhiri R (2024) Advances in lightweight composite structures and manufacturing technologies: A comprehensive review. 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Part B, Composites\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBehera A (2023) \u003cem\u003eThe Consequence of SiC Filler Content on the Mechanical, Thermal, and Structural Properties of a Jute/Kevlar Reinforced Epoxy Hybrid Composite.\u003c/em\u003e Silicon\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang DH (2014) Impact of Backbone Rigidity on the Photomechanical Response of Glassy, Azobenzene-Functionalized Polyimides. Macromolecules\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Digital Light Processing (DLP), Polymer composite, Silicon Carbide, Azobenzene, Isomerization","lastPublishedDoi":"10.21203/rs.3.rs-6736596/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6736596/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study presents a novel additive manufacturing (AM) technique, Photo switchable Direct Light Processing (P-DLP), which utilizes a dynamic mask imaging photoinitiation approach to mitigate light scattering effects caused by filler particles like Silicon Carbide (SiC) in composite printing. Traditional vat photopolymerization methods, while known for their high precision, face significant challenges in balancing speed and resolution, requiring extensive support structures and dealing with material instability during fabrication. The P-DLP technique overcomes these limitations by employing a dynamic masking system, where ultraviolet (UV) light initiates photopolymerization, and visible (blue) light selectively inhibits undesired polymerization. This mechanism allows for precise control over the curing process, enabling the fabrication of complex, high-resolution structures while minimizing scattering-induced distortions. A key aspect of this research is the development of refined resin formulations that integrate azobenzene as a photo switchable molecule, enhancing the controllability of polymerization kinetics. UV-Vis spectrophotometry results showed that azobenzene extended the absorption spectrum into the blue region, with higher concentrations significantly increasing absorbance in the 380\u0026ndash;500 nm range, confirming its potential as a photoinhibitor. Though the decreased tensile strength and elastic modulus due to agglomeration and chain disruption, the proposed P-DLP with dual wavelength light demonstrated effective curing of layers by inhibiting undesired curing in boundary and void regions, enabling high-resolution patterning with reduced overcuring artifacts. The advancements introduced in P-DLP make it particularly suited for applications requiring high precision and material integrity, such as optics, medical implants, and soft robotics. This approach represents a significant breakthrough in composite AM, addressing fundamental challenges of conventional methods and enabling faster, more accurate production of intricately detailed components across diverse industrial and biomedical applications.\u003c/p\u003e","manuscriptTitle":"Digital Light Processing 3D Printing of Polymer Composites Based on Tunable Curing Resins with Photoswitchable Molecules","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-29 08:01:02","doi":"10.21203/rs.3.rs-6736596/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ce079d56-3bfa-4ecb-8967-c81c9c5020cf","owner":[],"postedDate":"May 29th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-05-29T08:01:03+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-29 08:01:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6736596","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6736596","identity":"rs-6736596","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}