Electrohydrodynamically Printed Microlens Arrays with the High Filling Factor near 90%

Design, Development and Experimental Investigation of E-jet Based Additive Manufacturing Process

Electrohydrodynamic (EHD) inkjet printing has gained widespread attention in electronics, biomedicine, and materials science for its exceptional resolution and printing versatility. However, the droplet formation process is governed by complex interactions between driving waveform parameters and fluid properties, making traditional trial-and-error optimization inefficient. To address this, a hybrid approach combining numerical simulation, machine learning regression, and genetic algorithm optimization is proposed to achieve precise control of droplet diameter. A multiphysics numerical model is established in COMSOL Multiphysics to simulate the complete cycle of Taylor cone formation, jetting, and droplet deposition under pulsed electric fields. Parametric studies are conducted to investigate the influence of waveform characteristics and fluid properties on droplet size and jetting stability. Based on these simulations, a dataset of 912 samples is constructed for machine learning analysis. Among seven regression models evaluated, the artificial neural network (ANN) shows the best predictive performance and is further integrated with a genetic algorithm to optimize the driving parameters for different target droplet diameters. Experimental validation is performed using a Super Inkjet (SIJ) printing system. The results confirm the effectiveness of the proposed method: the average droplet diameter error ranges from 1.00 μm to 1.89 μm, and 84.21% of the droplets fall within ±5% of the target diameter. This study demonstrates a practical and data-driven framework for enhancing precision and process control in EHD printing.

Temperature measurement is very important for thermal control, which is required for the advancement of mechanical and electronic devices. However, current temperature sensors are limited by their inability to measure curved surfaces. To overcome this problem, several methods for printing flexible substrates were proposed. Among them, electrohydrodynamic (EHD) inkjet printing technology was adopted because it has the highest resolution. Since EHD inkjet printing technology is limited by the type of ink used, an ink with temperature-sensing properties was manufactured for use in this printer. To confirm the applicability of the prepared ink, its resistance characteristics were investigated, and the arrangement and characteristics of the particles were observed. Then, the ink was printed using the EHD inkjet approach. In addition, studies of the meniscus shapes and line widths of the printed results under various conditions confirmed the applicability of the ink to the EHD inkjet printing technology and the change in its resistance with temperature.

Fabrication of high integrated microlens arrays on a glass substrate for 3D micro-optical systems

Microlens arrays (MLAs) nowadays are critical micro-optical components and they can be applied in many application fields, such as optical communication systems and flat panel display modules. This article describes a novel approach to the fabrication of tunable, highly reliable, and uniform polydimethylsiloxane (PDMS) MLAs. A polydimethylsiloxane (PDMS) membrane is bonded to a micro-milled poly(methyl methacrylate) (PMMA) microfluidic chip and exposed to silicone oil of a specific viscosity. Molecules in the oil insert themselves into the molecular structure of the PDMS membrane, causing it to swell and subsequently form dome-shaped MLAs. From our experiments, we derived the following conclusions. First, the homogeneous swelling of the PDMS resulted in MLAs with a high numerical aperture (0.5), high uniformity illumination (CV of the illumination intensity is between 2.5%∼5.1%), and high uniformity (CV of sag height of MLAs is less than 0.05). Second, the shorter molecular chains in low-viscosity oils diffused more readily into the PDMS membrane, which increased the effects on swelling, resulting in MLAs with higher sag height and higher numerical aperture. For example, the 5 cst silicone oil resulted in sag height of 191 µm with NA of 0.50, whereas the 100 cst silicone oil resulted in sag height of 86 µm with numerical aperture of 0.33. Finally, the integrated mixer module enabled the simultaneous tuning of the 7 × 7 MLAs simply by adjusting the injection flow rates of the constituent silicone oils.

Parametric Optimization of E-Jet Based Micro Manufacturing System through Hybrid Taguchi Methodology

Paper-based microchannels are important components in paper-based microfluidic devices. Paper-based microchannels are usually manufactured by creating lines with hydrophobic patterns enclosing an area of hydrophilic paper inside. Among the many techniques for making hydrophobic lines, the printing method in general and inkjet printing in particular have become more common because of their advantages in terms of material saving, high precision, low-cost, etc. In this paper, the electrohydrodynamic (EHD) inkjet printing method, which has been studied and developed in recent years, has been used to drop glycol ether solvent-based ink onto a nitrocellulose paper. The results of the field emission scanning electron microscope (FE-SEM) show that the solvent based ink could dissolve through membrane and form a transparent hydrophobic layer on the created space. The width of the barrier lines can be controlled from about 200 μm to 260 μm, which is quite small when compared to other manufacturing methods. Based on this fabrication process, paper-based microfluidic analytical devices have been designed and fabricated for use in analytical biochemistry assays. Human chorionic gonadotropin (hCG) was selected as the biological target for this assay. Initial results have shown that the target for hCG can be quantitatively determined by these devices with a coefficient of determination (R2) of approximately 0.99. Therefore, EHD inkjet printing technology can be used for the manufacture of paper-based microfluidic analytical devices (μPADs) that may be applied in the future for analytical biochemistry assays.

A new method for producing gapless biaxial microlens array is proposed for a dual-directional light-control film with a high fill factor. Three steps in this production technique are included for formations of semiellipsoid microlens array with low fill factor to be caulked into gapless biaxial microlens array mold inserts. First, two photoresists with different melting temperatures method will be used to obtain the semiellipsoid microlens array with low fill factor. Second, the air-pressure electroforming caulking is applied to fill the gaps between adjacent lenses after the thermal reflow. Third, the secondary master mold of gapless biaxial microlens array is fabricated for hot embossing process to replicate the array pattern onto PMMA sheet. A gapless biaxial microlens array with long and short axes is developed to change the vertical and horizontal light emitting angles. Based on the simulated and experimental results, the horizontal light emitting angle can be increased to 30° and the vertical one is reduced to 15° by using the biaxial microlens array with high fill factor as the optical scattering film, to effectively enhance optical uniformity for the application of a rear projection display.

We describe a simple and effective method to fabricate a uniform plastic microlens array (MLA) with high fill-factor over a large area utilizing self-aligned diffuser lithography (SADL). In order to make an intimate contact between the photomask and the positive photoresist during 3D diffuser lithography, which is crucial for obtaining a uniform MLA mold over a large area, we fabricated a self-aligned metal mask directly on top of the positive photoresist, eliminating any air gap between the metal mask and the underlying photoresist. After replication of the developed concave MLA mold onto the poly(dimethylsiloxane) (PDMS), a standard deviation of sag (height) of the MLA was observed by laser scanning confocal lithography. The standard deviation, which indicates uniformity, was reduced by as much as a factor of 6 by applying SADL compared with that obtained from conventional diffuser lithography. Using this method, we fabricated a 7 inch MLA sheet with excellent uniformity. The proposed method can be extensively applied for fabrication of large-size MLA sheets with plastic materials thanks to its simplicity and versatility.

Electrohydrodynamic (EHD) inkjet printing has emerged as a powerful micro-/nanofabrication technique for high-resolution perovskite quantum dot (PeQD) color-conversion layers, offering precise control over pixel morphology, dimensions, and composition. This review systematically examines the mechanisms of cone-jet and electrostatic-attraction modes in EHD printing, highlighting recent advances in PeQD ink design, solvent and ligand engineering, and printing parameter optimization. Perovskite precursor and colloidal inks are discussed in detail, emphasizing strategies to enhance droplet ejection stability, suppress coffee-ring effects, and achieve uniform, high-luminescence pixels. Ligand exchange, dual-ligand passivation, and core−shell or polymer encapsulation are shown to effectively mitigate ion migration, surface defects, and environmental degradation, thereby improving photoluminescence efficiency and stability. Multi-channel and multi-nozzle EHD printing systems enable dynamic halide composition control and parallel RGB pixel deposition, facilitating ultrahigh-resolution patterning down to submicron feature sizes. Finally, the review highlights future directions, including synergistic PeQD material synthesis, advanced ink formulation, scalable high-throughput printing, and integration of PeQD color-conversion pixels into full-color micro-LED displays with minimal crosstalk and robust operational stability. These developments collectively demonstrate the immense potential of EHD inkjet printing for next-generation high-performance display technologies.

In this study, we fabricated a nanostructure on the surface of the micro-lens array (MLA), which is one of the light extraction technologies of organic light-emitting diodes (OLEDs), by performing the Reactive Ion -Etching (RIE) process. The MLA consists of a lensed area and a lens-less bottom (flat film area). We performed a systematic analysis to find ways to improve the light extraction efficiency of the MLA surface and flat film area. By controlling the RIE process time and type of gas plasma, nanostructures were formed on the surface of the MLA. O2 and CF4 gas plasmas resulted in nanostructures with tall heights and high aspect ratios, whereas CHF3 and Ar gas plasmas resulted in nanostructures with small heights and low aspect ratios. Furthermore, it was found that the nanostructures were not covered over the entire area, and the extent to which the nanostructures were distributed varied depending on the process time. As the RIE process time increases, the nanostructure expands from the top surface of the MLA to the flat film area. This limited the light extraction efficiency improvement. At a short process time of 50 s, nanostructures were formed only on the upper surface of the MLA hemisphere, which increased the light extraction efficiency. However, at long process times over 50 s, the surface of the hemisphere of MLA was covered with vertically aligned nanostructures, which decreased the efficiency. While the flat film area was covered with nanostructures at the longest process time of ~3200 s, it was effective, but the total efficiency was further decreased by the trade-off between them. As a result, the high-aspect-ratio nanostructured MLA patterned only on the top surface of the hemispherical MLA with a 50 s O2 plasma treatment showed the highest efficiency, which was slightly higher than that of the bare MLA. We expect that if the nanostructures can be formed in a direction perpendicular to the MLA surface and the flat film area simultaneously, the light extraction efficiency would be further improved.

We present close-packed microlens arrays (MLAs) with high numerical aperture (NA) over a large range of dimensions based on glass blowing process and replication method. Polydimethylsiloxane (PDMS) MLAs with diameters from \(50~\mu \) m to 1 mm and a high NA of 0.37 (maximum achievable NA for a PDMS plano-convex lens) are demonstrated, which are difficult to achieve with the conventional methods for large microlenses. In addition, MLAs with various fill-factors up to 96.1% and different base shapes are demonstrated. By adjusting the cavity depth and the chamber pressure, the ratio of sag height and diameter can be further adjusted.

This paper discusses a run-to-run iterative learning control (ILC) algorithm for Electrohydrodynamic jet (E-jet) printing. E-jet printing is a nano-manufacturing process that uses electric field induced fluid jet printing through nano-scale nozzles for achieving better control and resolution than traditional jet-printing processes. The printing process is controlled by changing the voltage potential between the nozzle and the substrate. However, it is difficult to maintain constant operating conditions such as stand-off height during a run of the printing process. The change in operating conditions results in fluctuating jet frequency. For stabilizing the jetting frequency across a single run, we propose a proportional ILC algorithm. We determine the jetting frequency by recording the electric current pulses when ink droplets are released from the nozzle. The frequency profile obtained from current measurements is then used to shape the voltage profile across a run to compensate for changing operating conditions. Experimental results are presented to validate the proposed control method.

Microlens arrays have been widely used as an essential component in many optical systems. The conventional fabrication techniques, such as ink-jet printing, photoresist thermal reflow, grayscale photolithography, and LIGA process have some drawbacks in aspects of process complexity and high fabrication cost. This paper presents a low-temperature, rapid and cost-effective method to successfully generate SU-8 microlens arrays on biocompatible Parylene-C surfaces. Vapor-induced SU-8 dewetting has been achieved on chemically modified Parylene-C surfaces that were selectively pre-treated with O(2) or SF(6) plasma. By this technique, the SU-8 droplets are self-organized onto desired positions to form a microlens array. This method is also suitable for production of microlens array on curved surfaces. The progress of dewetting has been studied by both experiment and simulation.

We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 ± 5 deg) and array fill factor of (85 ± 3)% were obtained with 100-μm PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 × 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 ± 7)% in the light-extraction efficiency was achieved.