Electrohydrodynamic Printing of Functional Inks for Flexible Electronics: Mechanisms, Materials, and Applications

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Electrohydrodynamic (EHD) jet printing is a promising direct writing method to produce micro- and nano-scale dots due to its easy manipulation, high resolution, and low cost. The effect of printing conditions on the diameter of printed dots was widely studied by both experiments and simulations. However, positional precision is also important for EHD printing. There is no published work on numerical simulation for the trajectory analysis of the ejected droplet. In the present work, a finite element model was established to investigate the droplet trajectory. The influencing factors, such as nozzle size, nozzle angle, applied voltage, ink density, and charge number in one droplet, were considered during numerical simulation. The influence of influencing factors on the electric intensity, droplet speed, and deposition direction was analyzed. The proposed simulation model provides a useful tool to analyze the droplet formation process and optimize the printing parameters to improve the positional precision of EHD printing.

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.

Electrohydrodynamic jet printing for desired print diameter

Droplet volume prediction methods in electrohydrodynamic jet printing based on multi-source data fusion

Organic printable dielectric layers are required for next‐generation electronic circuits, particularly those used as gate insulators (GIs) in organic field‐effect transistors (OFETs). Herein, optimized electrohydrodynamic (EHD) jet printing with an electrostatic force‐assisted mode is suggested for the fabrication of polyvinyl alcohol (PVA)‐based dielectric patterns after careful consideration of the ink system. The PVA molecules maintain good solubility in polar solvents, even when a crosslinking agent is added, which enables a continuous PVA jet stream with a width smaller than the diameter of the nozzle to be stably obtained. Subsequent EHD printing produces uniform PVA patterns with a smooth surface morphology suitable for the crystal growth of overlying organic semiconductors. The addition of a crosslinking agent in PVA results in direct GI patterns that exhibit superior insulating properties with high dielectric strength as compared with PVA without crosslinking agents. The OFETs with PVA‐based GIs prepared with EHD printing show stable operation with low gate leakage currents. In addition, the feasibility of EHD‐printed PVA layers is demonstrated for application as GIs in organic complementary logic gates.

Electrohydrodynamic (EHD) printing is a promising technique for additive manufacturing of high-resolution features with low cost and high efficiency. One of the major issues in EHD printing is nozzle clogging, which often occurs in the printing of colloidal inks with high concentrations. We proposed a new EHD printing nozzle configuration with dual channels to enable ink flow circulation to resolve the above issue. In this study, we focused on the meniscus dynamics and jetting characteristics of this new EHD printing process. Various jetting modes, i.e. continuous jet, pulsating jet, and pulsating droplet, as a function of the electric field intensity and nozzle configuration, were discussed. Dot patterns on a flexible substrate have been demonstrated via the new EHD printing process when operated at the pulsating droplet mode.

CFD-based numerical modeling to predict the dimensions of printed droplets in electrohydrodynamic inkjet printing

Electrohydrodynamic (EHD) jet printing has drawn attention in various fields because it can be used as a high-resolution and low-cost direct patterning tool. EHD printing uses a fluidic supplier to maintain the extruded meniscus by pushing the ink out of the nozzle tip. The electric field is then used to pull the meniscus down to the substrate to produce high-resolution patterns. Two modes of EHD printing have been used for fine patterning: continuous near-field electrospinning (NFES) and dot-based drop-on-demand (DOD) EHD printing. According to the printing modes, the requirements for the printing equipment and ink viscosity will differ. Even though two different modes can be implemented with a single EHD printer, the realization methods significantly differ in terms of ink, fluidic system, and driving voltage. Consequently, without a proper understanding of the jetting requirements and limitations, it is difficult to obtain the desired results. The purpose of this paper is to present a guideline so that inexperienced researchers can reduce the trial and error efforts to use the EHD jet for their specific research and development purposes. To demonstrate the fine-patterning implementation, we use Ag nanoparticle ink for the conductive patterning in the protocol. In addition, we also present the generalized printing guidelines that can be used for other types of ink for various fine-patterning applications.

Introduction EHD inkjet printing was applied in the fabrication of micro gas sensors recently [1-3]. In most cases, the evaporation of the ink solvent is slow, which makes thickness and uniformity of the film difficult to control. This work introduces an IR laser heating source to the EHD printing system of previous work [2], which accelerates the evaporation of the as-deposited ink solvent on the MHP, so that thick films and even columnar structures can be deposited without the use of special ink solvent.This paper reports a novel in-situ infrared (IR) laser aided electrohydrodynamic (EHD) inkjet printing technique which can deposit hierarchical micro/nano-structured gas-sensitive materials on micro-hotplate (MHP) for micro gas sensor fabrication. Gas-sensitive materials can be deposited quickly on an MHP to form flatten thin film, thick film, and 3D columnar structures. Method The diagram of the printing system is shown in Figure 1. An 808 nm IR laser source with adjustable output power is mounted on the EHD inkjet printing system, focusing at the printing spot with a diameter of about 1 mm. The freestanding MHP with metal electrodes embedded in SiO2 membrane can be heated up to 200oC in seconds by adsorption of the IR power. After EHD printing of the ink onto the 100 µm * 100 µm area of the MHP, the IR laser is turned on with optimized output power and time interval to heat the MHP to a temperature that the solvent evaporated rapidly and the dry gas-sensitive materials are uniformly covered on the surface of the MHP. By repeating the EHD printing and IR heating process, thickness of the film increases regularly and thick film can be prepared. In the preparation of columnar structure, by shrinking the amount of ink droplet and accelerating the IR heating, a tip appears on the surface of the deposited materials. The electric field intensity is larger at the tip since it shortened the distance to the needle, therefore, the electric force drives the ink droplet to deposit on the tip and the tip continues to grow, forming a 3D columnar structure. Results and Conclusions The temperature of the MHP changes with the power of the IR laser in a linear relationship (Figure 2). The flower-like tin oxide powder doped with 3 at% palladium was prepared as the hierarchical-structured gas-sensitive material with a hydrothermal method (Figure 3). The ink was prepared by mixing the powder, DI water and surfactant in a proper proportion, and it was EHD printed on the surface of the MHP (Figure 4). Without IR heating, the solvent did not evaporate completely even after 600 s at room temperature, making it not suitable to print again. By turning on the IR laser with 450 mW for 5 s, the ink was dried and uniform thin film with thickness dfilm<1 µm was coated on the MHP as shown in figure 5 (b). Repeat the printing-heating cycle for 4 times only took 100 s, and thick film of dfilm ≈ 40 µm was obtained as shown in Figure 5 (c). Figure 6 compares the power consumption of the MHP before and after printing, which shows that the MHP with thick gas sensitive film remains good thermal isolation property. The printed 3D columnar structure with height of about 120 µm is shown in figure 5 (d). Increasing the power of the laser can accelerate the evaporation. However, when the IR laser power is more than 500 mW, the droplet on the MHP boil violently and nanomaterials splashes all around.For the first time, the IR aided EHD printing allows the printing of flatten thick gas sensitive film composed of hierarchical-structured nanomaterial on MHP. Also, the technique can further be applied in fabrication of other MEMS devices.This work is supported by the Nature Science Foundation of China (61874018 and 61274076), the Fundamental Research Funds for the Central Universities, and the Nature Science Foundation of Liaoning (20180550923).

Electrohydrodynamic (EHD) jet printing represents a novel micro/nano-scale additive manufacturing process that utilises a high-voltage induced electric field between the nozzle and the substrate to print micro/nanoscale structures. EHD printing is particularly advantageous for the fabrication on flexible or non-flat substrates and of large aspect ratio micro/nanostructures and composite multi-material structures. Despite this, EHD printing has yet to be fully industrialised due to its low throughput, which is primarily caused by the limitations of serial additive printing technology. The parallel multi-nozzle array-based process has become the most promising option for EHD printing to achieve large-scale printing by increasing the number of nozzles to realise multichannel parallel printing. This paper reviews the recent development of multi-nozzle EHD printing technology, analyses jet motion with multi-nozzle, explains the origins of the electric field crosstalk effect under multi-nozzle and discusses several widely used methods for overcoming it. This work also summarises the impact of different process parameters on multi-nozzle EHD printing and describes the current manufacturing process using multi-nozzle as well as the method by which they can be realised independently. In addition, it presents an additional significant utilisation of multi-nozzle printing aside from enhancing single-nozzle production efficiency, which is the production of composite phase change materials through multi-nozzle. Finally, the future direction of multi-nozzle EHD printing development is discussed and envisioned.

Electrohydrodynamic (EHD) jet printing is an emerging technique in the field of additive manufacturing. Due to its versatility in the inks it can print, and most importantly, the printing resolution it can achieve, it is rapidly gaining favor for application in the manufacture of electronic devices, sensors, and displays among others. Although it is an affordable and accessible manufacturing process, it does require excellent operational understanding to achieve high resolution printing of up to 50 nm as reported in literature. In this review, three main aspects are considered, namely, the ink properties, the printer system itself (including design, nozzle dimensions, applied potential, and others), and the substrate onto which printing is being carried out. Knowing how all these factors can be manipulated and brought together allows the users of EHD printing to achieve extraordinary resolution and consistent results. The review is concluded with a brief discussion on where one can see the potential for development in this field of research.

Three-dimensional (3D) microneedle arrays (MAs) have shown remarkable performances for a wide range of biomedical applications. Achieving advanced customizable 3D MAs for personalized research and treatment remain a formidable challenge. In this paper, we have developed a high-resolution electrohydrodynamic (EHD) 3D printing process for fabricating customizable 3D MAs with economical and biocompatible molten alloy. The critical printing parameters (i.e., voltage and pressure) on the printing process for both two-dimensional (2D) and 3D features are characterized, and an optimal set of printing parameters was obtained for printing 3D MAs. We have also studied the effect of the tip-nozzle separation speed on the final tip dimension, which will directly influence MAs' insertion performance and functions. With the optimal process parameters, we successfully EHD printed customizable 3D MAs with varying spacing distances and shank heights. A 3 × 3 customized 3D MAs configuration with various heights ranging from 0.8 mm to 1 mm and a spacing distance as small as 350 μm were successfully fabricated, in which the diameter of each individual microneedle was as small as 100 μm. A series of tests were conducted to evaluate the printed 3D MAs. The experimental results demonstrated that the printed 3D MAs exhibit good mechanical strength for implanting and good electrical properties for electrophysiological sensing and stimulation. All results show the potential applications of the EHD printing technique in fabricating cost-effective, customizable, high-performance MAs for biomedical applications.

EHD (Electrohydrodynamic) printing is a photomask-free and direct writing technique for alternative fabrication of high resolution micro- and nano-structures with low cost and simple equipment. It is critical to understand the motion trajectory of the ejected droplet under electric filed so as to precisely control the printing process. However, there is no work which analyzed this issue previously. Thus in this paper, a finite element model was established to study the electric field distribution near the nozzle and analyze the motion trajectory of the ejected droplet during EHD printing. By using established finite element method, the electric field distribution in the nozzle and near the nozzle tip was investigated. The influence of printing distance and applied potential on the maximum electric field intensity was analyzed. Based on the electric field distribution study, the droplet trajectory analysis was carried out to confirm the motion velocity and direction of the ejected droplets.

High-resolution 3D printing, particularly electrohydrodynamic (EHD) printing, represents a transformative approach for advanced manufacturing applications, including wearable electronics, bioelectronics, and soft robotics. Despite its potential, EHD printing faces challenges such as complex waveform control, limited material compatibility, satellite droplet formation, and continuous charge accumulation. To address these issues, the use of pulse-width modulation (PWM) control is proposed to enhance EHD printing performance. The influence of duty cycles and pulse subdivisions on EHD printing was systematically investigated through experiments and simulations, analyzing their effects on jetting dynamics, droplet formation, charge accumulation, and line quality. The results demonstrate that PWM modulation significantly improves jetting stability, reduces droplet diameter by up to 25%, minimizes satellite droplet formation, and effectively mitigates charge accumulation. Furthermore, PWM control was shown to facilitate the production of high-quality patterns. Notably, the proposed PWM approach is compatible with existing waveform control setups, offering enhanced precision and stability without requiring substantial modifications. These findings underscore the potential of PWM-controlled EHD printing for achieving high-resolution, versatile manufacturing in electronics and functional device production.