3D Printing of Deformable Multicolor Alternating‐Current Electroluminescent Devices Through Rational Design of Functional Inks

Abstract

The development of flexible and customizable electroluminescent devices represents a significant challenge in advanced manufacturing. This paper introduces a novel approach for fabricating highly deformable, fully 3D‐printed alternating‐current electroluminescent devices through the rational design of UV‐curable functional inks. The devices feature a unique multilayer structure including a UV‐curable thiol‐ene crosslinked emission layer (ZBS‐t‐SE) and temperature‐responsive ionic hydrogel electrodes (FFP). The ZBS‐t‐SE demonstrates exceptional mechanical properties, with a strain of 259% at 727 kPa, whereas the FFP electrodes exhibit excellent printability through controlled micelle formation, high ionic conductivity (2.5 × 10⁻² S cm⁻¹), and stable performance under repeated deformation (>3000 cycles at 200% strain). The optimized devices maintain stable operation under various deformation modes, including stretching, bending, and twisting, achieving a maximum luminance of 267.4 cd m^{− 2} at 200% strain. Furthermore, the 3D printing approach enables the fabrication of complex 3D structures with multi‐color emission through precise spatial control of functional materials, presenting a transformative strategy for next‐generation flexible electronics and display technologies.

A highly deformable alternating‐current electroluminescent device is fabricated via direct ink writing of UV‐curable functional inks. The rational design of the thiol‐ene emission layer and ionic hydrogel electrodes enables high‐resolution printing of complex 3D architectures. The device maintains stable operation under various deformation modes, including 300% strain, offering novel opportunities for stretchable displays with multicolor emission capabilities.

1. Introduction

Alternating‐current electroluminescent (ACEL) devices have attracted significant attention for their potential in display and lighting applications owing to their simple device structure and uniform light‐emission characteristics.^{[ 1} , ² , ³ , ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ^{20 ]} Conventional ACEL devices adopt a sandwich structure with electrodes and an emission layer in a 2D plate configuration, typically manufactured through manual assembly processes.^{[ 1} , ² , ³ , ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ^{20 ]} However, these devices, with rigid electrodes, exhibit limited structural versatility and mechanical properties. Recent advancements in soft electrodes, such as ionogels and ionic hydrogels, have enabled the fabrication of conformal and deformable ACEL devices, allowing for stable performance under various mechanical deformations.^{[ 6} , ⁷ , ⁸ , ⁹ , ¹⁰ , ^{11 ]} These performance characteristics render such devices suitable for application in wearable displays and soft electronics.

Notably, to fully leverage the advantages of soft and deformable characteristics, the development of ACEL devices with diverse patterns and 3D morphologies is crucial. The ability to fabricate complex 3D architectures^{[ 12} , ¹³ , ^{14 ]} can facilitate the integration of ACEL devices with curved surfaces and dynamic structures, maximizing their potential in applications such as advanced displays, signage, and interactive interfaces. Traditional fabrication techniques such as screen printing and photolithography have been used to create patterned ACEL devices. For instance, hydrogel patterning using photomask^{[ 10 ]} and electrode patterning through screen printing^{[ 15} , ^{16 ]} have been demonstrated. However, these approaches are fundamentally limited to 2D configurations and cannot achieve 3D structures.

Direct ink writing (DIW), a precision extrusion‐based 3D printing method, offers a promising solution for fabricating highly customized, intricate device architectures.^{[ 17} , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ²³ , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ^{30 ]} While other additive manufacturing techniques, such as digital light processing, have demonstrated exceptional resolution (50–100 µm) and complex geometry capabilities for elastomeric materials in recent studies,^{[ 31} , ³² , ³³ , ^{34 ]} DIW offers several unique advantages. These include precise spatial control during material deposition, compatibility with a wide range of functional materials, and the ability to create complex 3D structures in a single manufacturing process.^{[ 17} , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ²³ , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ^{30 ]} Furthermore, the layer‐by‐layer printing process enables the formation of well‐defined interfaces between functional layers, which is crucial for optimizing charge injection and transport in ACEL devices.^{[ 17} , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ^{29 ]} Although the feasibility of fully 3D‐printed ACEL devices using conducting elastomers as electrodes^{[ 18} , ¹⁹ , ²⁰ , ²¹ , ²² , ^{23 ]} has been demonstrated, the integration of printable ionic hydrogel electrodes^{[ 21 ]} with elastomeric emission layers could result in enhanced mechanical properties and unique electrical characteristics through electrical double layer formation. Recent studies have shown significant progress in hydrogel‐elastomer interfaces^{[ 35 ]} and multifunctional applications of hydrogel‐based systems,^{[ 36 ]} further expanding their potential in flexible electronics. Thus, the success of DIW‐based fabrication depends on the development of printable materials that possess both appropriate rheological properties for extrusion and the required functionality for device operation.

In this study, we introduce a novel material system for fabricating ACEL devices through DIW, using temperature‐responsive ionic hydrogel electrodes and UV‐curable elastomeric emission layers. To enable hydrogel printing, we formulate a printable ink by incorporating Pluronic F‐127 (F‐127) and Pluronic F‐127 dimethacrylate (FDMA) into the pre‐gel solution, effectively increasing its viscosity through controlled micelle formation at elevated temperatures.^{[ 30 ]} For the emission layer, we develop a 3D‐printable elastomer by combining a thiol‐functionalized polydimethylsiloxane (PDMS) oligomer with an Ecoflex precursor, achieving both precise printability and enhanced mechanical properties. This novel formulation demonstrates unique electroluminescent characteristics, including enhanced luminance under moderate strain (100–200%), while maintaining excellent color stability. The proposed approach enables the fabrication of highly deformable ACEL devices through the synergistic combination of carefully designed printable materials. The crosslinked ionic hydrogels provide excellent ionic conductivity and promote electrical double layer formation, while the UV‐curable elastomeric emission layer ensures uniform light emission and device stability. The integrated printing process enables the fabrication of complex 3D architectures while preserving the mechanical and electrical properties of each functional layer.

2. Results and Discussion

The development of 3D‐printable emission layers for ACEL devices requires precise control of both rheological properties and crosslinking mechanisms. Figure 1a presents the molecular design strategy for the UV‐curable emission layer based on thiol‐ene click chemistry. The ink for this layer is a uniform blend of PDMS, Ecoflex, thiol‐functionalized oligosiloxane (TFO), ZnS, BaTiO₃, and SiO₂. The vinyl‐terminated PDMS (M _w ≈ 25 000) forms the primary elastomeric network, offering chain mobility. Ecoflex 00–30 introduces additional vinyl groups and enhances elasticity through its unique branched structure. TFO, synthesized through controlled condensation of (3‐mercaptopropyl)trimethoxysilane (MPTMS) and dimethoxydimethylsilane (DMDMS), acts as a multifunctional crosslinker owing to its multiple thiol groups (‐SH), enabling rapid photopolymerization via thiol‐ene click reactions. The corresponding synthetic scheme and characterization data (Silicon‐29 nuclear magnetic resonance (²⁹Si‐NMR) and Fourier‐transform infrared (FT‐IR) spectra) are presented in Figure S1 (Supporting Information). ZnS:Cu,Cl phosphor (d ≈ 20 µm) provides blue electroluminescence under an alternating‐current field,^{[ 10 ]} while BaTiO₃ nanoparticles (50–100 nm) enhance the local electric field through their high dielectric constant (ɛ ≈ 1000).^{[ 37 ]} Additionally, spherical SiO₂ particles (0.2–0.3 µm) serve as rheological modifiers, forming a secondary network structure that increases the viscosity of the ink from 113 to 2193 Pa·s upon 5 wt.% addition, ensuring structural stability before UV‐curing. The photoinitiator, benzophenone (BP), is introduced to initiate the photo‐crosslinking reaction.

Figure 1.

a) Illustration of the DIW‐printed ZBS‐t‐SE, including network formation after polymerization. b) Photographs of 3D‐printed ZBS‐t‐SE with c) enlarged view and corresponding line plot. d) Comparison of diameters for different nozzle sizes after 3D printing electrodes. e) Cross‐sectional SEM images and EDS mapping images for Zn, Ba, and Si in ZBS‐t‐SE. f) Dielectric constants of Ecoflex, Ecoflex/ZnS, and Ecoflex/ZnS/BaTiO₃. g) Tensile strain–stress curves for 3D‐printed t‐SE and ZBS‐t‐SE.

The UV‐curing mechanism shown in Figure 1a is based on thiol‐ene click chemistry, which enables rapid photopolymerization and network formation. Upon exposure to UV light (365 nm, 23 mWcm⁻²), BP generates radicals that initiate the crosslinking reaction. These radicals trigger the formation of thiyl radicals (─S•) from the thiol groups (‐SH) of TFO molecules.^{[ 34} , ³⁵ , ^{36 ]} Subsequently, the thiyl radicals react with the vinyl groups (Si─CH═CH₂) in both the PDMS and Ecoflex chains through thiol‐ene addition, forming thioether bonds (─S─CH₂─CH₂─Si─). This reaction generally proceeds rapidly (<40 s) owing to the high reactivity of the thiol‐ene click reaction.^{[ 38} , ³⁹ , ^{40 ]} (Figure S2, Supporting Information) Each TFO molecule, containing multiple thiol groups, functions as a crosslinking point, connecting multiple polymer chains through thioether bonds. We denote this thiol‐ene crosslinked silicone elastomer system as t‐SE, prepared by combining PDMS and Ecoflex in an equal weight ratio.

This crosslinking process simultaneously entraps the functional fillers (ZnS:Cu,Cl, BaTiO₃, and SiO₂) within the elastomeric network while maintaining their uniform distribution. Notably, rapid photopolymerization kinetics is crucial not only for preserving the printed structural integrity but also for preventing filler aggregation that could compromise device performance. The resulting crosslinked network provides both mechanical robustness and flexibility, essential for developing stretchable electroluminescent devices.

To optimize the ink formulation, we systematically investigated the effects of the PDMS:Ecoflex ratio and SiO₂ content on rheological properties. The optimal composition was determined to be PDMS:Ecoflex = 1:1 (w/w), which provides suitable viscoelastic properties for high‐resolution printing while maintaining mechanical flexibility (Figure S3, Supporting Information). The incorporation of 5 wt.% SiO₂ nanoparticles increased the ink viscosity from 206 to 2193 Pa·s, providing the necessary yield stress for structural retention after printing (Figure S4 and Table S1, Supporting Information).

The printability of the optimized formulation was demonstrated by 3D printing mesh structures with varying geometrical complexities. Figure 1b shows a representative mesh pattern (12 mm × 12 mm) printed using the optimized ink and a 250 µm nozzle. The printed mesh exhibits well‐defined structures with uniform line widths and spacing. Higher magnification imaging (Figure 1c) reveals the high fidelity of the printed features, with clear edges and consistent cross‐sectional profiles, confirming excellent shape retention of the UV‐curable ink. The structural uniformity is maintained across the entire printed area, indicating stable flow behavior during the extrusion process.

To quantitatively evaluate the printing resolution, the relationship between nozzle diameter and printed feature dimensions was analyzed. As shown in Figure 1d, nozzles with inner diameters of 200, 250, and 500 µm produce printed line widths of 225, 289, and 536 µm, respectively. A consistent diameter offset of 35–38 µm across different nozzle sizes demonstrates precise control over the printing process. This dimensional accuracy is attributable to the optimized viscoelastic properties of the ink (2193 Pa·s), which help maintain structural integrity post‐printing, and its shear‐thinning behavior, which enables smooth extrusion under applied pressure.

The uniform distribution of functional components within the emission layer was confirmed through detailed microscopic analyses. Cross‐sectional scanning electron microscopy (SEM) imaging of the cured emission layer reveals a homogeneous matrix structure without visible aggregation or phase separation (Figure 1e). Furthermore, elemental mapping via energy‐dispersive X‐ray spectroscopy (EDS) analysis demonstrates the uniform spatial distribution of key components: Zn from ZnS phosphor particles, Ba from BaTiO₃ dielectric particles, and Si from both the silicone matrix and SiO₂ rheological modifiers. The uniform intensity distribution of these elemental signals across the analyzed area confirms stable particle dispersion (Figure 1e).

The incorporation of BaTiO₃ nanoparticles into the emission layer significantly enhances its dielectric properties, which is crucial for efficient electroluminescent performance. As shown in Figure 1f, the addition of BaTiO₃ increases the dielectric constant from 2.1 to 6.7 at a frequency of 1 kHz. This enhancement is attributable to the high intrinsic dielectric constant of BaTiO₃ and its uniform distribution within the matrix, as confirmed by EDS analysis. Moreover, frequency‐dependent dielectric measurements demonstrate stable dielectric behavior across the operating frequency range (10–100 kHz), indicating effective integration of BaTiO₃ particles into the crosslinked network structure.

Figure 1g presents tensile stress–strain curves for the neat t‐SE and fully formulated emission layer containing ZnS:Cu,Cl, BaTiO₃, and SiO₂ particles (ZBS‐t‐SE). The neat t‐SE exhibits a maximum stress of 438 kPa at 408% strain, characteristic of a highly elastic silicone matrix formed through thiol‐ene crosslinking of PDMS and Ecoflex precursors. In comparison, ZBS‐t‐SE demonstrates enhanced mechanical strength, with a maximum stress of 727 kPa at 259% strain. This mechanical behavior reflects the reinforcing effect of the incorporated particles and their interaction with the silicone matrix. Despite the reduction in maximum strain, the composite maintains sufficient elasticity (>250%) for highly deformable device applications, achieving a balance between mechanical reinforcement and stretchability (Table S2, Supporting Information).

The electrode ink formulation consists of several key components: FDMA as a crosslinking agent for micellar structures, F‐127 as a temperature‐responsive micelle‐forming agent, N‐hydroxyethyl acrylamide (HEAm) and acrylamide (Am) as co‐monomers for covalent network formation, and 1 m LiCl as an electrolyte to provide the ionic conductivity necessary for electrode function.

Figure 2a illustrates the proposed design strategy for the ionic hydrogel electrode, based on the thermal responses of FDMA and F‐127. FDMA, synthesized via the acrylation of F‐127 with methacrylic anhydride (MA) (synthetic scheme and characterization in Figure S5, Supporting Information), provides photo‐crosslinkable methacrylate groups while maintaining the amphiphilic structure essential for micelle formation. The pre‐gel ink exhibits excellent printability owing to its unique temperature‐dependent behavior, which was systematically characterized. The solution exhibits dramatic changes in viscosity with temperature, with the viscosity increasing from 0.2 Pa·s at 0 °C to over 2 × 10⁴ Pa·s at 37 °C. This sharp transition corresponds to the critical micelle temperature, above which the amphiphilic components self‐assemble into micelles. Dynamic light scattering (DLS) measurements confirm the formation of mixed micelles between FDMA and F‐127 at 37 °C, with an average size of 20.96 nm (Figure S6, Supporting Information). This micelle formation increases the viscosity of the hydrogel precursor solution, allowing it to retain the shape of the printed structures before UV‐curing (Figure S7a, Supporting Information). Notably, the temperature‐triggered viscosity increase is fully reversible, as evidenced by repeated DLS measurements showing consistent micelle formation and dissociation cycles between 3 and 37 °C (Figure S7b, Supporting Information).

Figure 2.

a) Illustration of the extrusion 3D printing procedure. b) Image of 3D‐printed FFP and its c) enlarged view. d) Comparison of diameters for different nozzle sizes after 3D printing electrodes. e) Transmittance of 3D‐printed FFP ionic hydrogels f) Tensile strain–stress curve for FFP.

Upon UV exposure, the methacrylate groups in FDMA undergo photopolymerization with HEAm and Am. The crosslinking of poly(HEAm‐co‐Am) (PHA) with FDMA leads to the formation of a chemically crosslinked network with tunable hydrophilicity. The chemical composition of the pre‐gel ink was optimized by varying the composition of FDMA and F‐127 while maintaining a constant monomer (HEAm and Am) content (5 wt.%). In the case of only FDMA (FDMA:F‐127:PHA = 20:0:5 wt.%), the hydrogel exhibits poor mechanical properties, with a maximum strain of 303%. Although FDMA forms temperature‐responsive micelles and provides chemical crosslinking through its methacrylate groups, this composition results in suboptimal mechanical performance. The balanced composition (FDMA:F‐127:PHA = 10:10:5 wt.%) results in significantly enhanced mechanical properties, with a maximum strain of 986% at a stress of 970 kPa (Figure S8, Supporting Information). This dramatic improvement is attributable to the synergistic combination of both FDMA and F‐127: Both components contribute to temperature‐responsive micelle formation for printing precision, while the methacrylate groups in FDMA enable the formation of a permanent network through photopolymerization. Equal proportions of FDMA and F‐127 (10:10 wt.%) thus lead to the optimal balance between printability and mechanical durability. In the proposed formulation, 5 wt.% of a mixture of HEAm and Am in equimolar ratios was added, as determined through prior optimization (Figure S9, Supporting Information). Am provides basic hydrophilicity through its amide groups, whereas HEAm offers additional hydrogen bonding capability through its hydroxyl groups, collectively improving the water retention and mechanical properties of the hydrogel.^{[ 10 ]} This optimized hydrogel system (FDMA:F‐127:PHA = 10:10:5 wt.%) is referred to as FFP ionic hydrogel.

The printability of the FFP ionic hydrogel was evaluated by fabricating mesh structures. Figure 2b shows a representative mesh pattern printed at 37 °C, demonstrating uniform line widths and spacing. Higher magnification imaging (Figure 2c) reveals well‐defined edges and consistent width, confirming the excellent shape fidelity of the printed features. The printing resolution was quantitatively evaluated using different nozzle sizes (Figure 2d). Nozzles with inner diameters of 200, 250, and 500 µm produce line widths of 215, 267, and 517 µm, respectively, demonstrating precise dimensional control.

Additionally, the printed FFP ionic hydrogel exhibits excellent optical transparency (96.1% at 600 nm), which is crucial for ACEL devices (Figure 2e). As the electrode layers sandwich the emission layer in the device structure, high transparency of the electrodes is essential to minimize optical losses and ensure efficient light transmission from the emission layer.

Moreover, the printed FFP also demonstrates excellent mechanical properties, achieving a maximum stress of 970 kPa at 986% strain (Figure 2f). These robust mechanical characteristics are vital for deformable devices, as they enable the electrodes to maintain stable electrical contact with the emission layer under various deformation conditions, such as bending, stretching, and twisting.

The influence of LiCl concentration on the electrical and mechanical properties of FFP was investigated as a function of LiCl concentration. Figure 3a shows that the ionic conductivity increases with LiCl concentration up to 1.0 m, reaching a maximum value of 2.5 × 10⁻² S cm⁻¹, before slightly decreasing at higher concentrations. This trend arises from two competing mechanisms: At lower concentrations (< 1.0 m), the conductivity increases owing to the higher number of available charge carriers (Li⁺ and Cl⁻ ions). However, above 1.0 m, the increased ion–ion interactions and reduced free water content hinder ion mobility, leading to a decrease in overall conductivity.^{[ 41 ]} The hydrophilic network structure of FFP provides well‐defined ion transport pathways through its hydrated domains, while the crosslinked network ensures structural stability.

Figure 3.

a) Measured ionic conductivity and b) tensile strain–stress curves for the FFP ionic hydrogel. c) Electrochemical impedance spectroscopy Nyquist plots for the FFP ionic hydrogel. Change in the electrical resistance of the FFP ionic hydrogel d) as a function of strain (square symbols) and curve‐fitting with λ² (line) e) during repeated elongation of 200% over 3000 cycles.

The mechanical properties also depend on the LiCl concentration. Stress–strain curves (Figure 3b) reveal that increasing the LiCl concentration from 0.25 to 2 m significantly affects the mechanical behavior of FFP. At lower concentrations (0.25–0.5 m), the hydrogel shows low stress at high strains, suggesting a softer network structure. As the LiCl concentration increases to 1 m, coinciding with the optimal ionic conductivity, the mechanical properties are balanced, with both reasonable stretchability and strength. However, further increase in the LiCl concentration (1.5–2 m) leads to higher stress at the same strain level, indicating a stiffer network. This concentration‐dependent mechanical behavior can be explained by the interaction between LiCl and the hydrogel network. At lower concentrations, the ions primarily enhance electrical properties without significantly affecting the network structure. As the concentration increases, Li⁺ and Cl⁻ ions begin to interact with the hydrophilic segments of the network, influencing chain mobility and the water structure within the hydrogel. At high concentrations (>1 m), the increased ion‐polymer interactions and reduced free water content result in a more rigid network structure, as evidenced by the steeper stress–strain curves. Based on these electrical and mechanical analyses, 1 m LiCl was selected as the standard electrolyte for all subsequent experiments, including the fabrication of ACEL devices.

The electrical characteristics of FFP containing 1 m LiCl were analyzed through electrochemical impedance spectroscopy. The Nyquist plot (Figure 3c) shows typical ionic conductor behavior and can be well‐fitted using an equivalent circuit model consisting of a bulk resistance (R _b) in series with a constant phase element.^{[ 42 ]}

The calculated bulk resistance of 1.01 kΩ at high frequency aligns with the direct‐current resistance measurements, confirming the consistent ionic conductivity of the hydrogel electrode. Using the formula σ = t(AR)⁻¹, where t and A represent the thickness and contact area of the hydrogel, respectively, the ionic conductivity of FDMA/F‐127/PHA in 1 m LiCl was calculated to be 2.5 × 10⁻² S m⁻¹.

The electromechanical behavior of FFP ionic hydrogel was characterized by measuring the resistance changes under various deformation conditions. As shown in Figure 3d, the resistance change (R/R₀) follows a quadratic relationship with strain (λ ²), where λ denotes the stretching ratio (1 + strain/100). This relationship indicates that the resistance change is predominantly governed by geometrical factors during deformation, suggesting that the ion transport pathways remain continuous even under large strains. This quadratic dependence matches the theoretical prediction for a conductor maintaining constant resistivity during deformation, where the resistance change is solely determined by geometric factors (i.e., length increase and cross‐sectional area reduction).^{[ 43 ]} The excellent agreement between experimental data and the λ ² model indicates that the intrinsic ionic conductivity of FFP remains unchanged during stretching up to 1000% strain. This behavior is crucial for stretchable electrode applications, as it ensures consistent electrical performance regardless of the deformation state.

The mechanical and electrical durability of FFP was evaluated through cyclic deformation tests. As shown in Figure 3e, under repeated stretching to 200% strain over 3000 cycles, the resistance change remains highly stable and reversible, demonstrating excellent fatigue resistance and reliable electrical performance under dynamic deformation. This fatigue resistance is crucial for deformable electronics, where the electrodes must maintain both mechanical integrity and electrical functionality under repeated dynamic deformation during actual device operation.

Building on the 3D printable t‐SE and FFP materials, fully 3D‐printed ACEL devices were fabricated using a sequential layer‐by‐layer printing process based on DIW. As illustrated in Figure 4a, the first step is to print the bottom FFP electrode at 37 °C using a 0.26 mm needle at a speed of 10 mm s⁻¹. In this step, temperature‐induced micelle formation enables precise pattern definition. Next, the ZBS‐t‐SE emission layer is printed at 25 °C using a 0.2 mm needle at a speed of 5 mm s⁻¹, leveraging its optimized rheological properties for high‐resolution deposition. A second FFP electrode layer is then printed at 37 °C to complete the active device structure. Finally, the device is encapsulated by printing protective t‐SE layers at the top and bottom, ensuring environmental stability during operation. Each layer is UV‐cured at a wavelength of 365 nm immediately after printing to prevent interlayer mixing and ensure structural integrity. This all‐3D printing approach enables the fabrication of complex, multilayer ACEL devices in a single, integrated process.

Figure 4.

3D printing of an ACEL device. a) Illustration of the extrusion 3D printing procedure of the ACEL device. Step (I):3D printing of electrode layer solution of FFP at 37 °C. Step (II): 3D printing of the emission layer at room temperature. Step (III): 3D printing of electrode layer solution of FFP at 37 °C. Step (IV), (V):3D printing of top and bottom device encapsulation layer at room temperature. b) Cross‐sectional optical microscopy image of 3D‐printed ACEL device. c) Capacitance changes for FFP/ZBS‐t‐SE/FFP and ZBS‐t‐SE at different frequencies. d) Tensile stress–strain curves for the ACEL device.

As shown in Figure 4b, cross‐sectional imaging reveals a distinct layer configuration with precisely controlled thicknesses: FFP electrodes (800 µm), ZBS‐t‐SE emission layer (100 µm), and t‐SE encapsulation layers (350 µm). The sharp and well‐defined boundaries between layers, without any mixing or diffusion at the interfaces, confirm successful layer‐by‐layer printing with excellent spatial control.

To investigate the electrical characteristics of the printed ACEL device, the frequency‐dependent capacitance of both the complete device structure (FFP/ZBS‐t‐SE/FFP) and the emission layer (ZBS‐t‐SE) alone was measured. As shown in Figure 4c, the emission layer shows a constant, low capacitance (≈0.0043 nF) across the considered frequency range (0–50 kHz). In contrast, the complete device exhibits significantly higher capacitance, particularly at low frequencies, reaching ≈90 nF at 1 kHz. The capacitance ratio of the emission layer to the complete device (C_emission/C_device ≈ 10⁻⁴) indicates substantial charge accumulation at the FFP electrode–emission layer interfaces. This capacitive behavior is crucial for ACEL device operation, as the electroluminescence mechanism relies on the acceleration of charge carriers by the local electric field within the emission layer.^{[ 43 ]}

The mechanical properties of the fully 3D‐printed ACEL device were evaluated through tensile testing. Figure 4d shows the stress–strain curve of the complete device structure, demonstrating excellent stretchability with a maximum strain of 300% while maintaining structural integrity. The stress–strain curve exhibits a typical elastomeric response, reaching ≈700 kPa at 300% strain. This robust mechanical performance can be attributed to the synergistic combination of the stretchable FFP electrodes and optimized ZBS‐t‐SE emission layer.

Next, the electroluminescent performance of the 3D‐printed ACEL device was evaluated. Figure 5a shows photographs of a rectangular device (10 mm × 30 mm) under ambient light (left) and operation (right), demonstrating uniform cyan‐blue emission across the printed structure. Under an applied voltage of 400 V at 30 kHz, the device exhibits a luminance of 98.5 cd m^{− 2}, comparable to that of conventionally fabricated devices (98.6 cd m^{− 2}) using identical materials (Figure S10, Supporting Information), confirming that our 3D printing process maintains the intrinsic electroluminescent properties of the materials.

Figure 5.

a) Photograph of the 3D‐printed ACEL device. b) Changes in luminance at various applied voltages and frequencies of the electric field, and corresponding curve‐fitting. c) CIE coordinates for the ACEL device at various frequencies. d) Light‐emitting performance of the ACEL device under various deformation modes (twisting and bending) and photographs of the light‐emitting operation. e) Change in the luminance of the ACEL device at different strains. f) CIE coordinates of the ACEL device at various elongations. g) Change in the luminance of the ACEL device during 100% elongation/release over 1000 cycles.

To assess the fundamental operation characteristics, we analyzed the device performance under normal conditions. The luminance increases with both applied voltage and frequency (Figure 5b), reaching 98.7 cd m^{− 2} at 400 V and 30 kHz. The relationship between luminance and electric field follows a typical ACEL behavior, characterized by stronger emission at higher frequencies.^{[ 44} , ⁴⁵ , ^{46 ]} The data can be well‐fitted with typical ACEL behavior described by the empirical equation L = L₀exp(‐β/V ^1/2), where L is the luminance, L₀ and β are constants, and V denotes the electric field strength. This exponential dependence is characteristic of impact excitation mechanisms in ACEL devices, where charge carriers are accelerated by the electric field to excite the luminescent centers.^{[ 47 ]} As shown in Figure 5c, the emission color exhibits a slight frequency dependence, as evidenced by the shift in CIE coordinates from warm cyan‐blue (0.1793, 0.3415) at 1 kHz to deeper blue (0.1541, 0.2286) at 30 kHz. This blueshift with increasing frequency is attributable to the different excitation pathways available to charge carriers at higher frequencies, although the emission remains within the blue region of the spectrum across all operating conditions.

The influence of mechanical deformation on device performance was investigated through a series of twisting, bending, and stretching tests (Figure 5d). Remarkably, the proposed device maintains its electroluminescent functionality even under significant deformation, exhibiting stable blue emission during stretching up to 300%. The initial luminance of 98 cd m^{− 2} at 0% strain increases significantly to 190.8 cd m^{− 2} at 100% strain and reaches a maximum of 267.4 cd m^{− 2} at 200% strain, before decreasing to 154.3 cd m^{− 2} at 300% strain. The reduction in electrode–emission layer interface resistance owing to enhanced contact under strain and the decreased thickness of the emission layer leads to more intense local electric fields. The subsequent decrease in luminance at higher strains (>200%) likely results from the eventual increase in electrode resistance and decrease in the phosphor density at high elongations.^{[ 46 ]}

Nevertheless, the device maintains substantial brightness even at 300% strain, with luminance still exceeding the initial value at 0% strain. As shown in Figure 5c, the corresponding CIE coordinates remain consistent across various stretching conditions, indicating stable emission characteristics. The corresponding emission spectra maintain their characteristic cyan‐blue peak ≈480 nm, albeit with intensity variations (Figure 5e). Figure 5f shows the luminance variation over 100 cycles of 100% strain deformation. The device maintains stable electroluminescent performance throughout the cycling test, with luminance fluctuating between ≈100 and 190 cd m^{− 2}. This periodic variation in luminance corresponds directly to the stretching and releasing cycles, with higher luminance at maximum strain (100%) and lower values at the relaxed state (0%), consistent with static strain measurement results. The luminance pattern exhibits no significant degradation over the 100 cycles, demonstrating the excellent electromechanical durability of our 3D‐printed ACEL device. Additionally, the fabricated ACEL devices demonstrate excellent environmental stability, maintaining consistent luminance performance across a wide range of humidity (25–85% RH) and temperature (20–60 °C) conditions, as shown in Figures S11 and S12 (Supporting Information), respectively. These results demonstrate that the fabricated device successfully combines mechanical flexibility with stable electroluminescent performance, enabled by the optimized design of both the electrode and emission layers.

To demonstrate the versatility of the proposed 3D printing approach, we fabricated various ACEL devices with complex geometries and multicolor patterns. Figure 6a shows the fabrication of a checkerboard pattern by alternating extrusion of ZnS:Cu,Cl (blue) and ZnS:Mn,Cl (orange) phosphor‐containing inks from two separate nozzles. The clear boundary between different colored regions confirms precise spatial control during multicolor patterning without mixing. The successful integration of soft components into ACEL devices is demonstrated through a tricolor striped pattern (1 mm wide) including ZnS:Cu,Cl (blue), ZnS:Mn,Cl (orange), and ZnS:Cu,Cl (green) phosphors (Figure 6b). The device maintains stable and uniform emission across all three colors even under 100% strain, demonstrating excellent mechanical compatibility of the functional layers.

Figure 6.

High‐resolution multicolor 3D‐printed ACEL devices. a) Dual‐nozzle printing process and photographs of checkerboard pattern device under ambient (off) and operating (on) conditions, demonstrating distinct blue and orange emission regions. b) Stretchable tri‐color device showing stable and uniform emission before and after stretching. c,d) 3D‐printed structures with complex geometries: (c) pyramid and (d) chess pieces (pawn and knight) showing uniform emission. e) High‐resolution printing demonstration: text “ACEL” with 500 µm feature size (magnified view showing clear line definition).

The ability to produce complex geometries was evaluated by fabricating 3D architectures. Uniform thickness control was achieved by sequentially printing each layer with identical spatial patterns but adjusted extrusion parameters. For pyramid and chess piece structures (Figure 6c,d), each layer was printed with the same 3D geometry but different thicknesses (outer FFP: 0.8 mm, ZBS‐t‐SE: 0.1 mm), resulting in uniform light emission across the entire 3D surface.

To showcase the high‐resolution printing capability of our system, we fabricated the text pattern “ACEL” with 500 µm feature resolution (Figure 6e). The magnified image of the pattern reveals sharp and well‐defined line edges without spreading or distortion, confirming the excellent printing precision of our UV‐curable functional inks. This high‐resolution patterning capability, combined with our multi‐color and 3D printing approach, demonstrates the potential for fabricating sophisticated ACEL devices with complex designs.

3. Conclusion

We developed a fully 3D printable ACEL device system through the rational design of functional inks. The emission layer was engineered using a UV‐curable thiol‐ene system (ZBS‐t‐SE), combining PDMS and Ecoflex with optimized mechanical properties, enabling high‐resolution printing and stable device operation. The electrodes were fabricated using a temperature‐responsive ionic hydrogel (FFP) that provides excellent ionic conductivity (2.5 × 10⁻² S cm⁻¹) and optical transparency (96.1% at 600 nm). Using a DIW process, we achieved precise layer‐by‐layer fabrication with controlled dimensions and interfaces, resulting in ACEL devices with stable performance. The optimized device exhibits excellent electroluminescent characteristics with luminance reaching 267.4 cd m^{− 2} at 200% strain and maintains stable operation under various deformation modes, including bending, twisting, and cyclic stretching.

Notably, the proposed 3D printing approach enables the fabrication of ACEL devices with complex 3D architectures and multicolor patterns through precise spatial control of different phosphor materials. The rapid UV‐curing process ensures structural integrity while preventing interlayer mixing, facilitating the preparation of sophisticated device designs. This versatile fabrication strategy opens new possibilities for creating customized illumination systems, particularly in applications requiring conformal integration with 3D surfaces or dynamic mechanical deformation. The capability to print multifunctional ACEL devices with controlled geometry and emission properties represents a significant advancement in the development of next‐generation flexible and wearable displays.

4. Experimental Section

Materials

Pluronic F‐127 (M _n = 12600 g·mol⁻¹), MA (≥94%), triethylamine (TEA, >99%), dichloromethane (DCM), Am, HEAm (97.0%), DMDMS (95.0%), MPTMS (95.0%), hydrochloric acid (97.0%), vinyl‐terminated PDMS, BaTiO₃ (≥99%), 2,2‐bis(hydroxymethyl)propionic acid (98.0%), and BP (99.0%) were purchased from Sigma–Aldrich (St. Louis, MO, USA). Lithium chloride (LiCl, 98.0%) was purchased from Tokyo Chemical Industry (Tokyo, Japan). Lithium phenyl‐2,4,6‐trimethylbenzoylphosphinate (LAP) was purchased from SNvia (Busan, Korea). Ecoflex 00–30 was purchased from Smooth‐On (Macungie, PA, USA). The ZnS:Cu,Cl phosphor microparticles were obtained from National EL Technology (Gyeonggi‐do, South Korea). All chemicals were used as received without further purification.

Synthesis of FDMA

A solution of Pluronic F‐127 (10 g) in anhydrous DCM (20 mL) was prepared. Subsequently, a solution of TEA in five fold molar excess was added, and the mixture was cooled to a temperature of 0–5 °C in an ice bath. A separate solution containing a small amount of anhydrous DCM and a five fold molar excess of MA was added dropwise over 1 h. The reaction proceeded for five days at room temperature. To separate the product, the reaction mixture was precipitated using cold diethyl ether, and the solid precipitate was acquired by centrifugation at 4000 rpm for 5 min. The product was dried in a vacuum at 40 °C for one day. Finally, dialysis was performed, and the product was freeze‐dried. The degree of methacrylation was determined to be >95% using proton NMR (H‐NMR): (500 MHz, D₂O, δ in PPM): 5.6337 (m, 2 H, CH₂═C(CH₃)COO⁻), 6.0591 (t, 2 H, (CH₂═C(CH₃)COO⁻), 1.8337 (t, 2 × 3 H, CH₂═C(CH₃)COO⁻), 1.0694 (m, 67 × 3 H, ─CH─CH₃), 3.5844 (m, 67 × 1 H, ─CH─CH₃), 3.4484 (d, 67 × 1 H, ─CH─CH₂─O⁻), 3.6684 (d, 2 × 98 × 4 H, ─O─CH₂─CH₂─O⁻).

Synthesis of TFO

TFO, a thiol group crosslinker, was synthesized by reacting MPTMS, DMDMS, and 0.1 N HCl aqueous solution in a molar ratio of 1:1:2.5. These reagents were mixed in a flask and subjected to vigorous stirring at 70 °C for 1 h to initiate hydrolysis. Subsequently, the mixture was heated to 80 °C under continuous N₂ gas purging for 8 h to drive the condensation reaction. Upon completion, the TFO product was isolated.

Preparation of Inks

The ZBS‐t‐SE ink was prepared by mixing PDMS and Ecoflex 00–30 (A:B = 1:1) precursor mixtures with TFO, BP, ZnS:Cu,Cl phosphor, BaTiO₃, and SiO₂ in a weight ratio of 6:2:1:0.1:5.5:1:0.8. The mixture was degassed under vacuum. All prepared inks were loaded into 10 mL syringes and stored in the dark prior to 3D printing. The FFP ink was prepared by dissolving Am (0.19 g), HEAm (277 µL), aqueous LAP (158 µL, 2wt.%), and LiCl (1 m) in deionized water, followed by degassing under vacuum. Subsequently, FDMA and F‐127 were added to the solution, and the solution was stirred overnight in a refrigerator at 3 °C to obtain the pre‐gel ink.

Fabrication of 3D‐Printed ACEL Devices

The 3D printing process was performed using an Invivo Bio 3D printer (Rokit, Seoul, South Korea). The FFP electrode ink was extruded through a 0.25 mm diameter nozzle at a printing speed of 10 mm s⁻¹ and a temperature of 37 °C. The ZBS‐t‐SE emission layer and t‐SE encapsulation layer were printed using a 0.25 mm nozzle at 5 mm s⁻¹ at room temperature. The device was fabricated by sequentially printing the bottom FFP electrode (thickness:0.8 mm), ZBS‐t‐SE emission layer (thickness:0.1 mm), and top FFP electrode. The device was then encapsulated with t‐SE layers (thickness:0.4 mm). Each layer was cured under UV radiation (365 nm) for 5 min immediately after printing, followed by an additional 10 min of curing. The ACEL devices with 3D architectures were fabricated following the same process as that of the 2D architectures. First, FFP electrode ink was used to print the structure in the desired size and shape at 10 mm s⁻¹ using a 0.6 mm nozzle. Next, the ZBS‐t‐SE ink was used to print the structure at 5 mm s⁻¹ using a 0.2 mm nozzle (thickness:0.1 mm). Finally, FFP electrode ink was applied at 10 mm s⁻¹ using a 0.6 mm nozzle to complete the structure (thickness: 0.8 mm). Each layer was cured under UV radiation (365 nm) for the same duration as for 2D architectures.

Measurements

Stress–strain curves were recorded using a universal testing machine (Shimadzu EZ‐SX, Japan) at a tensile stroke rate of 50 mm min⁻¹ for the ionic hydrogel (thickness: 0.8 mm, length: 2.0 cm, width: 0.5 cm) and Ecoflex 00–30 (thickness: 0.6 mm, length:2.0 cm, width:0.5 cm). Cyclic tensile tests were performed at a speed of 0.05 mm s⁻¹ without any waiting time. The electrical resistance was measured at room temperature using an LCR meter (Wayne Kerr 4100, UK). Repetitive stretching of the sample was performed using an x‐axis motorized system (SL2‐15, ST1, Korea). The rate and holding time were 2 mm s⁻¹ and 1 s, respectively. Electrochemical impedance spectroscopy was conducted using a ZIVE SP1 electrochemical workstation (WonATech, Korea) at a voltage of 25 mV and frequency of 10⁻⁴–10⁵ kHz. The solution viscosity was evaluated using a rotational rheometer DVNext (Brookfield, New York, USA). A solvent trap was used to prevent the evaporation of water from the solution during rheological measurements. DLS measurements were performed using a Zetasizer Nano S90 (Malvern Instruments, Malvern, UK). All solutions were diluted 5.88 times and filtered using a syringe filter (pore size of 0.45 µm). Stretchable ACEL devices were operated using a high‐voltage Pintek HA‐805 amplifier (New Taipei City, Taiwan) coupled with a Rigol DG‐1022 function generator (Beijing, China). The amplifier supplied the output voltage and frequency for the ACEL device, and the as‐generated signals were recorded using a Tektronix TDS‐2002 oscilloscope (Beaverton, Oregon, USA). The luminance of the ACEL device was measured using a Konica Minolta CS‐2000 spectroradiometer (Osaka, Japan).

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Park J., Singh S., Yoon J., 3D Printing of Deformable Multicolor Alternating‐Current Electroluminescent Devices Through Rational Design of Functional Inks. Small 2025, 21, 2502435. 10.1002/smll.202502435

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.