Inkjet Research Articles

Flexible and Stretchable Printable Conductive Inks for Wearable Textile Applications

As wearable electronic devices become increasingly integrated into our daily routines, there is a growing demand for soft, flexible, and comfortable devices that can seamlessly deliver electronic functionalities. Electronic textiles (e-textiles) combine the electronic capabilities of devices such as sensors, actuators, energy storage, and communication tools with the comfort and flexibility inherent in traditional textiles. The rising interest in E-textile and sensor applications has thrust the field of printed electronics (PE) into the spotlight. Printed electronics is a rapidly expanding technology that allows the construction of electronic devices on affordable, flexible substrates, including paper and textiles. This is achieved through printing techniques, such as screen printing, 3D printing, gravure printing, offset printing, flexography, and inkjet printing, which are traditionally used in various industries like graphic arts, textiles, and polymers. This paper provides a comprehensive overview of printable conductive inks, with a focus on their role in designing textile-based wearable conductive devices for E-textile applications. Within this scope, it was examined the properties of conductive inks, presented the various printing methods used to fabricate wearable textile materials for potential use in wearable textile devices, and analyzed their performance characteristics. Lastly, it was addressed the key challenges faced in this field and identify future research directions. The aim of this paper is to contribute to the advancement of cost-effective functional conductive inks and formulations, promoting their integration into E-textile applications.

Enzyme Immobilization by Inkjet Printing on Reagentless Biosensors for Electrochemical Phosphate Detection.

Enzyme-based biosensors commonly utilize the drop-casting method for their surface modification. However, the drawbacks of this technique, such as low reproducibility, coffee ring effects, and challenges in mass production, hinder its application. To overcome these limitations, we propose a novel surface functionalization strategy of enzyme crosslinking via inkjet printing for reagentless enzyme-based biosensors. This method includes printing three functional layers onto a screen-printed electrode: the enzyme layer, crosslinking layer, and protective layer. Nanomaterials and substrates are preloaded together during our inkjet printing. Inkjet-printed electrodes feature a uniform enzyme deposition, ensuring high reproducibility and superior electrochemical performance compared to traditional drop-casted ones. The resultant biosensors display high sensitivity, as well as a broad linear response in the physiological range of the serum phosphate. This enzyme crosslinking method has the potential to extend into various enzyme-based biosensors through altering functional layer components.

Electrohydrodynamic Printing‐Based Heterointegration of Quantum Dots on Suspended Nanophotonic Cavities

AbstractNanophotonic structures are a foundation for the growing field of light‐based quantum networks and devices enabled by their ability to couple with and manipulate photons. Colloidal quantum dots (QDs) are uniquely suited to complement this range of devices due to their solution‐processability, broad tuneability, and near‐unity photoluminescence quantum yields in some cases. To bridge the gap between them, electrohydrodynamic inkjet (EHDIJ) printing serves as a highly precise and scalable nanomanufacturing method for deterministic positioning and deposition of attoliter‐scale QD droplets. This includes heterointegration in devices that are challenging to create by conventional subtractive semiconductor processing, such as QDs emitters coupled to substrate‐decoupled nanoscale resonant structures. In this work, the first successful application of EHDIJ printing for the integration of these colloidal QDs into suspended nanophotonic cavities is demonstrated, achieving selective single‐cavity deposition for cavity pairs as close as 100 nm apart. These results motivate the development of future suspended hetero‐integrated devices that utilize EHDIJ printing as a sustainable, additive, and scalable method for quantum photonics nanomanufacturing.

Inkjet printed IGZO memristors with volatile and non-volatile switching

Solution-based memristors deposited by inkjet printing technique have a strong technological potential based on their scalability, low cost, environmentally friendlier processing by being an efficient technique with minimal material waste. Indium-gallium-zinc oxide (IGZO), an oxide semiconductor material, shows promising resistive switching properties. In this work, a printed Ag/IGZO/ITO memristor has been fabricated. The IGZO thickness influences both memory window and switching voltage of the devices. The devices show both volatile counter8wise (c8w) and non-volatile 8wise (8w) switching at low operating voltage. The 8w switching has a SET and RESET voltage lower than 2 V and − 5 V, respectively, a retention up to 105 s and a memory window up to 100, whereas the c8w switching shows volatile characteristics with a low threshold voltage (Vth < − 0.65 V) and a characteristic time (τ) of 0.75 ± 0.12 ms when a single pulse of − 0.65 V with width of 0.1 ms is applied. The characteristic time alters depending on the number of pulses. These volatile characteristics allowed them to be tested on different 4-bit pulse sequences, as an initial proof of concept for temporal signal processing applications.

From Unregulated Networks to Designed Microstructures: Introducing Heterogeneity at Different Length Scales in Photopolymers for Additive Manufacturing.

Photopolymers have been optimized as protective and decorative coating materials for decades. However, with the rise of additive manufacturing technologies, vat photopolymerization has unlocked the use of photopolymers for three-dimensional objects with new material requirements. Thus, the originally highly cross-linked, amorphous architecture of photopolymers cannot match the expectations for modern materials anymore, revealing the largely unanswered question of how diverse properties can be achieved in photopolymers. Herein, we review how microstructural features in soft matter materials should be designed and implemented to obtain high performance materials. We then translate these findings into chemical design suggestions for enhanced printable photopolymers. Based on this analysis, we have found microstructural heterogenization to be the most powerful tool to tune photopolymer performance. By combining the chemical toolbox for photopolymerization and the analytical toolbox for microstructural characterization, we examine current strategies for physical heterogenization (fillers, inkjet printing) and chemical heterogenization (semicrystalline polymers, block copolymers, interpenetrating networks, photopolymerization induced phase separation) of photopolymers and put them into a material scientific context to develop a roadmap for improving and diversifying photopolymers' performance.

Inkjet Printing on Hydrophobic Surface: Practical Implementation of Stacked Coin Strategy

While inkjet printing on many hydrophilic surfaces can be achieved through control of drop spacing and droplet deposition delay alone, the same for hydrophobic substrates prove to be challenging. The low surface energies of hydrophobic surfaces lead to dewetting, unwanted coalescence of wet drops, and bulging, preventing intact and uniform lines of low‐viscosity ink to form. In this paper, we have adapted the stacked coin strategy, a well‐established and successful technique for hydrophilic surfaces, for use on hydrophobic surfaces. Stacked coin morphology occurs when the time taken to deposit a subsequent overlapping droplet in a pattern is longer than the evaporation time of the prior droplet. On hydrophobic surfaces, it is considerably more challenging and the parameter window for successful printing is smaller than on hydrophilic surfaces, thus requiring in‐depth study to implement this methodology on hydrophobic surfaces. We conducted experiments using a custom‐built inkjet printer with variable stage speed and stage temperature, where silver nanoparticle ink was used to print on Teflon‐AF substrates. Our study identified the following regimes: isolated droplets, isolated multi‐droplets, broken line, true stacked coin, and delamination. The relationship between substrate temperature, drop spacing and droplet deposition delay controls the printability window of true stacked coin morphology and its surrounding regimes. The key to the success of this strategy on hydrophobic surfaces is rapid drying of individual droplets to minimize movement due to hydrophilic coalescence of overlapping dried and wet droplets. Our study identified 180⁰C as the critical temperature for instantaneous drying of the studied ink, and a maximum drop spacing of 20µm.This article is protected by copyright. All rights reserved.

Depth from defocus technique: a simple calibration-free approach for dispersion size measurement

Particle size measurement is crucial in various applications, be it sizing droplets in inkjet printing or respiratory events, tracking particulate ejection in hypersonic impacts or detecting floating target markers in free-surface flows. Such systems are characterised by extracting quantitative information like size, position, velocity and number density of the dispersed particles, which is typically non-trivial. The existing methods like phase Doppler or digital holography offer precise estimates at the expense of complicated systems, demanding significant expertise. We present a novel volumetric measurement approach for estimating the size and position of dispersed spherical particles that utilises a unique ‘Depth from Defocus’ (DFD) technique with a single camera. The calibration-free sizing enables in situ examination of hard to measure systems, including naturally occurring phenomena like pathogenic aerosols, pollen dispersion or raindrops. The efficacy of the technique is demonstrated for diverse sparse dispersions, including dots, glass beads and spray droplets. The simple optical configuration and semi-autonomous calibration procedure make the method readily deployable and accessible, with a scope of applicability across vast research horizons.

Structural Effects of 3D Inkjet-Printed Ni(O)-YSZ Pillared Electrodes on Performances of Solid Oxide Electrochemical Reactors.

Increasing densities of reaction sites for gaseous reactants in solid oxide electrochemical reactors (SOERs), is a key strategy for achieving enhanced performance in either fuel cell or electrolysis modes. Fabrication of 3D structured components in SOERs can enhance those densities of reaction sites, which is achieved by 3D inkjet printing with high reproducibility, having developed inks with appropriate properties. First, the effects of pillar geometries on SOER performances are predicted through numerical simulations, enabling subsequent 3D printing to focus on the more effective geometries. Herein, the study reports the results of experimental validation of those predictions by evaluating the electrochemical performances of cells with various heights of 3D inkjet-printed Ni(O)- yttria stabilized zirconia (YSZ) pillars and YSZ pillars. Those measurements prove that increasing pillar heights generally increases SOER peak power densities in fuel cell mode and increased current densities at the thermoneutral potential (1.285V) in steam electrolysis mode, as predicted by simulations. With increasing pillar heights, more limitations in performance enhancement are found with YSZ electrolyte pillars than with Ni-YSZ pillars, again as predicted by simulations. The subsequent microstructural analysis of Ni-YSZ pillars proves the suitability of the Ni(O)-YSZ composite particle ink formulation and the reliability of 3D printing.

Deep Learning-Based Inkjet Droplet Detection for Jetting Characterizations and Multijet Synchronization.

Inkjet printing is a powerful direct material writing process. It can be used to deposit microfluidic droplets in designated patterns at submicrometer resolution, which reduces materials usage. Nonetheless, predicting jetting characterizations is not easy because of the intrinsic complexity of the ink-nozzle-air interactions. Thus, inkjet processes are monitored by skilled engineers to ensure process reliability. This is a bottleneck in industry, resulting in high labor costs for multiple nozzles. To address this, we present a deep learning-based method for jetting characterizations. Inkjet printing is recorded by an in situ CCD camera and each droplet is detected by YOLOv5, a 1-stage detector using a convolutional neural network (CNN). The precision, recall, and mean average precision (mAP) at a 0.5 intersection over the union (IoU) threshold of the trained model were 0.86, 0.89, and 0.90, respectively. Each regression result for a detected droplet is accumulated in chronological order for each class of droplet and nozzle. The quantified information includes velocity, diameter, length, and translation, which can be used to synchronize multinozzle jetting and, eventually, the printed patterns. This demonstrates the feasibility of autonomous real-time process testing for large-scale electronics manufacturing, such as the high-resolution patterning of biosensor electrodes and QD display pixels while exploiting big data obtained from jetting characterizations.

Opportunities, Challenges, and Strategies for Scalable Deposition of Metal Halide Perovskite Solar Cells and Modules

Hybrid organic‐inorganic perovskite solar cells (PSCs) have rapidly advanced in the new generation of photovoltaic devices. As the demand for energy continues to grow, the pursuit of more stable, highly efficient, and cost‐effective solar cells has intensified in both academic research and the industry. Consequently, the development of scalable fabrication techniques that yield a uniform and dense perovskite absorber layer with optimal crystallization plays a crucial role to enhance stability and higher efficiency of perovskite solar modules. This review provides a comprehensive summary of recent advancements, comparison, and future prospects of scalable deposition techniques for perovskite photovoltaics. We discuss various techniques, including solution‐based and physical methods such as blade coating, inkjet printing (IJP), screen printing, slot‐die coating, physical vapor deposition, and spray coating that have been employed for fabrication of perovskite modules. The advantages and challenges associated with these techniques, such as contactless and maskless deposition, scalability, and compatibility with roll‐to‐roll processes, have been thoroughly examined. Finally, the integration of multiple subcells in perovskite solar modules is explored using different scalable deposition techniques.

Inkjet Printed Potentiometric Sensors for Nitrate Detection Directly in Soil enabled by a Hydrophilic Passivation Layer

AbstractAgricultural intensification has increased the use of chemical fertilizers, promoting plant growth and crop yield. Excessive use of nitrogen fertilizers leads to nutrient loss and low nitrogen use efficiency. Management of nitrogen fertilizer input requires close to real‐time information about the soil nitrate concentration. While there is extensive work developing nitrate ion sensing solutions for liquid media, few allow for in‐soil measurements. This study introduces inkjet‐printed potentiometric sensors, containing 2 electrodes, the reference electrode (RE) and the nitrate‐selective film‐encapsulated working electrode (WE). The interaction between the nitrate‐sensitive membrane and soil nitrate ions causes a change in potential across the RE and WE. Additionally, a hydrophilic Polyvinylidene Fluoride (PVDF) layer ensures the long‐term functionality of the sensor in wet soil environments by protecting it from charged soil particles while simultaneously allowing water to flow from the soil toward the sensor electrodes. The sensors are tested in sand and silt loam soil, demonstrating their versatility across soil types. The potential change can be related to the nitrate concentration in soil, with typical sensitivities of 45–55 mV decade−1. Overall, the use of the PVDF layer allows for direct sensing in moist soil environments, which is critical for developing soil nitrate sensors.

Micropatterned Hydrogel‐Elastomer Hybrids for Flexible Wet‐Style Superhydrophobic Antifogging Tapes

AbstractTransparent materials widely used in applications such as eyewear, solar cells, and optical devices, often suffer from environmental challenges including fogging, surface contamination, and mechanical deformation. However, a universal strategy that resolves all these issues in a single platform remains to be resolved. Herein, a monolithic micropillar structure based on polydimethylsiloxane (PDMS) elastomer is prepared followed by subsequent assembly of hydrogel micropatterns in between the micropillar array using inkjet printing to achieve flexible wet‐style superhydrophobic antifogging tapes that are applicable to various transparent materials and curvatures. The monolithic micropillar structure with low‐surface‐energy characteristics facilitates easy removal of surface contaminants by rinsing with water while granting mechanical durability. In addition, micropatterning of hydrogels via inkjet printing not only enables to maintain optical transparency in a fogging environment but also makes the fabrication procedure more scalable. Furthermore, the strong adhesion between the hydrogel and the elastomer is shown to provide enhanced stability under repeated stretching cycles and long‐term exposure to water while the presence of a loosely cross‐linked PDMS at the bottom of the tape made it applicable to a wide range of transparent substrates without using additional adhesives.

Electric Field-Driven Jetting and Water-Assisted Transfer Printing for High-Resolution Electronics on Complex Curved Surfaces

High-resolution electronics on complex curved surfaces have wide applications in fields such as biometric health monitoring, intelligent aircraft skins, conformal displays, and biomimetics. However, current manufacturing processes can only adapt to limited curvature, posing a significant challenge for achieving high-resolution fabrication of electronics on complex curved surfaces. In this study, we propose a novel fabrication strategy that combines electric field-driven jetting and water-assisted transfer printing techniques to achieve the fabrication of high-resolution electronics on complex curved surfaces. The electric field-driven jetting enables the fabrication of high-resolution 2D electronics on sacrificial layer substrates. After dissolving the sacrificial layer, it is observed that the 2D electronics form a self-supporting structure with a certain rigidity and flexibility. During the water-assisted transfer printing process, this self-supporting structure undergoes stretching deformation with excellent conformity of the electronics to curved surfaces while effectively minimizing wrinkles. Finally, we successfully demonstrate the manufacture of 25 μm high-resolution electronics on highly curved surfaces (nautilus shell) and complex (scallop shell, stone) surfaces. The integrity of transferred circuit patterns and consistency of conductors are verified through infrared thermography analysis, confirming the feasibility of this manufacturing strategy. In addition, a protective film with strong adhesive properties is sprayed onto the transferred curved circuits to enhance their adhesion and resistance to extreme environments such as acids and alkalis. Our proposed technique provides a simple and effective new strategy for the fabrication of high-resolution electronics on complex curved surfaces.

Preparation of high-resolution micro/nano dot array by electrohydrodynamic jet printing with enhanced uniformity

The high-resolution array is the basic structure of most kinds of microelectronics. Electrohydrodynamic jet (E-Jet) printing technology is widely applied in manufacturing array structures with high resolution, high material compatibility and multi-modal printing. It is still challenging to acquire high uniformity of printed array with micro-nanometer resolution, which greatly influences the performance and lifetime of the microelectronics. In this paper, to improve the uniformity of the printed array, the influence of each parameter on the uniformity of the E-jet printed dot array is studied on the cobuilt NEJ-E/P200 experimental platform, finding the applied voltage plays the most important role in maintaining the uniformity of the printed array. By appropriately adjusting the printing parameters, the dot arrays with different resolutions from 500 pixels per inch (PPI) to 17,000 PPI are successfully printed. For arrays below and over 10,000 PPI, the deviations of the uniformity are within 5% and 10% respectively. In this work, the dot array over 15,000 PPI is first implemented using E-jet printing. The conclusions acquired by experimental analysis of dot array printing process are of great importance in high resolution array printing as it provides practical guidance for parameters adjustment.

Advancements in Food Printing Technologies and Their Potential Culinary Applications: A Contemporary Exploration

Food printing is a cutting-edge manufacturing technique that uses advanced printing methods such as binder jetting, extrusion-based printing, and inkjet printing to build an object layer by layer to achieve the required shape of food items such as chocolate and cheese. 3DFP (3-dimensional food printing) has the potential to combine delicate and easily degradable bioactive compounds and other functional elements into functional 3DFP food products, contributing greatly to the development of nutritious food. Many nations make different types of 3D food printers nowadays, creating specialty meals like space food, restaurants, elderly food, and floating food. Numerous benefits of 3DFP include the development of individualized food items with regard to taste and nutrition, the decentralisation of food production, the decrease of food waste, and commercial innovation. Based on the benefits of customizing current food to one’s taste and use, three-dimensional food printing technology can be applied to a variety of food categories. One of the reasons for the increase in research into this technology is the ability to produce modified products that are tailored to suit the taste preferences and specific nutritional demands of consumers. In this review, the industrial situation of 3DFP technology was examined along with recommendations for expanding the market for 3D-printed food in the new typical age.

Evaporation of binary liquids from a capillary tube

Evaporation of multi-component liquid mixtures in confined geometries, such as capillaries, is crucial in applications such as microfluidics, two-phase cooling devices and inkjet printing. Predicting the behaviour of such systems becomes challenging because evaporation triggers complex spatio-temporal changes in the composition of the mixture. These changes in composition, in turn, affect evaporation. In the present work, we study the evaporation of aqueous glycerol solutions contained as a liquid column in a capillary tube. Experiments and direct numerical simulations show three evaporation regimes characterised by different temporal evolutions of the normalised mass transfer rate (or Sherwood number $Sh$ ), namely $Sh (\tilde{t} ) = 1$ , $Sh \sim 1/\sqrt {\tilde{t} }$ and $Sh \sim \exp (-\tilde{t} )$ , where $\tilde {t}$ is a normalised time. We present a simplistic analytical model that shows that the evaporation dynamics can be expressed by the classical relation $Sh = \exp ( \tilde{t} )\,\mathrm {erfc} ( \sqrt {\tilde{t} })$ . For small and medium $\tilde{t}$ , this expression results in the first and second of the three observed scaling regimes, respectively. This analytical model is formulated in the limit of pure diffusion and when the penetration depth $\delta (t)$ of the diffusion front is much smaller than the length $L(t)$ of the liquid column. When $\delta \approx L$ , finite-length effects lead to $Sh \sim \exp (-\tilde{t} )$ , i.e. the third regime. Finally, we extend our analytical model to incorporate the effect of advection and determine the conditions under which this effect is important. Our results provide fundamental insights into the physics of selective evaporation from a multi-component liquid column.

All Solution Processable OTFT-Based on Direct-Written Printing Method Towards Flexible Electronics Applications

Development of organic electronics, particularly organic thin film transistors (OTFTs), have been centred of discussion among researchers due to their potential uses in flexible electronics applications. Conventionally, an inkjet printing method has been deployed to fabricate the OTFTs due to its simplicity in fabrication steps and applicability to diverse substrates and solution-processable materials. Nonetheless, this technique has a major drawback which requires low viscosity materials to prevent clogging issue at the printer’s nozzle. This in return limits the material selections and requires additional steps i.e. modification of the selected materials to fit the printer’s requirement or in other words, to avoid clogging at the nozzle. Therefore, this paper proposes a method to fabricate a bottom gate bottom contact (BGBC) OTFT by using a direct-write printing technique which is compatible with a commercial conductive ink that can be directly used without any further modification. This technique does not compromise the fabricated devices overall performance and can fabricate the devices up to micrometre scale. The proposed OTFT achieved a saturation mobility of 4.28x10-5 cm2/Vs, a threshold voltage of -0.4 V, an on/off current ratio of 102, and a subthreshold swing of 10 V/dec with overall fabrication temperature is less than 150 ℃, hence, makes it suitable for flexible electronics applications.

Development, Prospect and Challenge for Additive Manufacturing of Polymer Materials

As a manufacturing technology that fabricates three-dimensional objects by stacking materials layer by layer, additive manufacturing can be divided into laser fused deposition, electron beam fused deposition, inkjet printing and so on based on different additive manufacturing processes. In recent years, remarkable progress has been made in the additive manufacture of polymer materials. Firstly, the additive manufacturing equipment and process parameters suitable for polymer materials have been successfully developed, which realized the accurate manufacturing of complex structures. Secondly, by optimizing the formula and post-treatment of polymer materials, the performance and durability of additive products are improved. In addition, the researchers also explored the application potential of additive manufacturing of polymer materials in medical, aerospace and other fields. However, the additive manufacturing of polymer materials still faces some challenges. One of them is that the rheological properties of polymer materials have a great impact on the progress of additive manufacturing, which needs further research and improvement. Meanwhile, the heat and stress generated in additive manufacturing may lead to the deformation and degradation of polymer materials, which requires effective thermal management. Finally, compared with the traditional cutting process, the additive manufacturing cost of polymer materials is still high, so it is necessary to reduce the cost and promote production efficiency. In a word, additive manufacturing of polymer materials has broad prospects and application potential, but further research and improvement are still needed to overcome related challenges.

Self-pinning colloids on rough surface

Self-pinning colloids are integral to multiple applications from ink-jet printing to diagnostic platforms. The commercial surfaces involved in these applications are inherently rough and to mimic the same, we probe the self-pinning mechanistic using a model system of colloidal (sessile) droplet desiccating over a homogeneously rough surface having root-mean-square roughness, Sqnm∼0.36nm;(0<Sq<1). Further, the interplay of the particle diameter dnm∼460;30)with the substrate roughness induced length-scale on the evaporation modes of these desiccating colloidal droplets has been investigated with extensive experiments. The second parametric variation outlined the effect of varied particle concentration (ϕ,ww%∼0.05,0.1,1) on the pinning-de-pinning transition of the contact line during evaporation. These are characterised and quantified by contact angle goniometry and a general map of evaporation modes is proposed for such evaporating colloidal droplets induced by self-pinning of the dispersed colloids.

New insights into head-on bouncing of unequal-size droplets on a wetting surface

The impact of a liquid droplet with another droplet or onto a solid surface are important basic processes that occur in many applications such as agricultural sprays and inkjet printing, and in nature such as pathogens transport by raindrops. We investigated the head-on collision of unequal-size droplets of the same liquid on wetting surfaces using the direct numerical simulations technique at different size ratios. The unsteady Navier–Stokes equations are solved and the liquid–gas interface is tracked using the geometric volume-of-fluid method. The numerical model is validated by comparing simulation results of two extreme cases of droplets bouncing with the experimental data from previous studies and the agreement is quite accurate. The validated model is employed to simulate droplets bouncing at several size ratios at different Weber numbers and Ohnesorge number. Two distinct regimes are identified, namely, the inertial regime, where the restitution coefficient is a constant value close to 0.3, the viscous regime, where the restitution coefficient declines. To understand the bouncing behaviour, the velocity field is analysed and an energy budget calculation is performed. The distribution of the sessile droplet energy is found to be important and the sessile droplet surface energy is calculated by its deformation characteristics such as crater depth. Finally, a scaling analysis is performed to rationalize the insensitivity of the coefficient of restitution in the inertial regime, and its decline in the viscous regime, at large size ratios.