Optimal Ink Research Articles

Ink Formulation and Printing Parameters for Inkjet Printing of Two Dimensional Materials: A Mini Review.

Inkjet printing of two-dimensional (2D) material has been a center of interest for wearable electronics and has become a promising platform for next-generation technologies. Despite the enormous progress made in printed 2D materials, there are still challenges in finding the optimal printing conditions involving the ink formulation and printing parameters. Adequate ink formulation and printing parameters for target 2D materials rely on empirical studies and repeated trials. Therefore, it is essential to compile promising strategies for ink formulation and printing parameters. In this context, this review discusses the optimal ink formulations to prepare stable ink and steady ink jetting and then explores the critical printing parameters for fabricating printed 2D materials of a high quality. The summary and future prospects for inkjet-printed 2D materials are also addressed.

Effect of the solvent evaporation rate of silver ink on the electrohydrodynamic-printing formability of textile-based printing electronics

The enabling electrohydrodynamic (EHD) printing technology in a one-step forming, continuous, and controllable manner has gained wide attention in the field of flexible printed electronics. The evaporation characteristic of ink solvent during the EHD printing greatly affects the shape of the jet as well as the penetration and diffusion of inks on fabrics, which is crucial to the formation of high-quality printed electronics. However, few works have deeply investigated the control of ink solvent evaporation to adjust the formability of EHD printing electronics on rough and porous textiles. Here, conductive inks with different solvent evaporation rates are formulated. The effect of solvent evaporation on the motion of inks is evaluated by the contact angle over time. Furthermore, the morphology and electrical properties under different deformation of EHD-printed conductive lines are observed and measured. The results show that the morphology of conductive lines printed on fabric could be accurately controlled by the ratios of the solvent in inks, and the solvent evaporation rate has a significant inverse-parabolic effect on electrical resistance and its stability under deformation. Moreover, the serviceability of the optimal ink is demonstrated by the performance of an EHD-printed antenna for ultra-high frequency radio frequency identification tags, and its maximum reading range is 9.1 m under typical application examples. These findings will provide a guide for ink formulation and process control of EHD printing in flexible textile-based electronics.

An Inkjet-Printed PEDOT:PSS-Based Stretchable Conductor for Wearable Health Monitoring Device Applications.

A stretchable conductor is one of the key components in soft electronics that allows the seamless integration of electronic devices and sensors on elastic substrates. Its unique advantages of mechanical flexibility and stretchability have enabled a variety of wearable bioelectronic devices that can conformably adapt to curved skin surfaces for long-term health monitoring applications. Here, we report a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-based stretchable polymer blend that can be patterned using an inkjet printing process while exhibiting low sheet resistance and accommodating large mechanical deformations. We have systematically studied the effect of various types of polar solvent additives that can help induce phase separation of PEDOT and PSS grains and change the conformation of a PEDOT chain, thereby improving the electrical property of the film by facilitating charge hopping along the percolating PEDOT network. The optimal ink formulation is achieved by adding 5 wt % ethylene glycol into a pristine PEDOT:PSS aqueous solution, which results in a sheet resistance of as low as 58 Ω/□. Elasticity can also be achieved by blending the above solution with the soft polymer poly(ethylene oxide) (PEO). Thin films of PEDOT:PSS/PEO polymer blends patterned by inkjet printing exhibits a low sheet resistance of 84 Ω/□ and can resist up to 50% tensile strain with minimal changes in electrical performance. With its good conductivity and elasticity, we have further demonstrated the use of the polymer blend as stretchable interconnects and stretchable dry electrodes on a thin polydimethylsiloxane (PDMS) substrate for photoplethysmography (PPG) and electrocardiography (ECG) recording applications. This work shows the potential of using a printed stretchable conducting polymer in low-cost wearable sensor patches for smart health applications.

Advances in Screen Printing of Conductive Nanomaterials for Stretchable Electronics.

Stretchable electronics have demonstrated tremendous potential in wearable healthcare, advanced diagnostics, soft robotics, and persistent human–machine interfaces. Still, their applicability is limited by a reliance on low-throughput, high-cost fabrication methods. Traditional MEMS/NEMS metallization and off-contact direct-printing methods are not suitable at scale. In contrast, screen printing is a high-throughput, mature printing method. The recent development of conductive nanomaterial inks that are intrinsically stretchable provides an exciting opportunity for scalable fabrication of stretchable electronics. The design of screen-printed inks is constrained by strict rheological requirements during printing, substrate–ink attraction, and nanomaterial properties that determine dispersibility and percolation threshold. Here, this review provides a concise overview of these key constraints and a recent attempt to meet them. We begin with a description of the fluid dynamics governing screen printing, deduce from these properties the optimal ink rheological properties, and then describe how nanomaterials, solvents, binders, and rheological agents are combined to produce high-performing inks. Although this review emphasizes conductive interconnections, these methods are highly applicable to sensing, insulating, photovoltaic, and semiconducting materials. Finally, we conclude with a discussion on the future opportunities and challenges in screen-printing stretchable electronics and their broader applicability.

Patterning of Nano-Hydroxyapatite onto SiO2 and Electro-spun Mat Surfaces Using Dip-Pen Nanolithography

We have demonstrated the ability to controlled-pattern hydroxyapatite nanoparticles (nanoHA) onto the SiO2 and electrospun mat surfaces using dip-pen nanolithography (DPN). One of the keys to the success of this research was the formulation of nanoHA inks with well-defined physicochemical properties for specific use with dip-pen nanolithography. Studies on the effect of ink viscosity in terms of dispersion, stability, and accuracy of patterning determined the optimal formulation of ink as 3 w/v % nanoHA with 30-50% glycerol in isopropyl alcohol. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) were used to determine the dispersion of in solution. 30-50% glycerol solution yielded a dispersed nanoHA particles of size ~200 nm in ink. The stability of the inks evaluated by zeta-potential measurements and by turbidity (the tendency for nanoHA sedimentation over time) showed stability for the higher viscosity inks (30-50% glycerol formulated against 0% glycerol). Atomic force microscopy (AFM) and SEM were utilized to image the printed patterns on a SiO2 substrate and on an electrospun mat. The optimal ink consisted of 30% glycerol and 3 w/v % nanoHA with printed features ranging from 745 – 1175 nm on a SiO2 substrate. Additionally, nanoHA printed onto an electrospun scaffold showed DPN without fiber damage onto soft-matter. The development of a nanoHA ink for controlled DPN patterning will enable single cellular interactions to nanoscale patterning on many surfaces, including printed nanoHA-gradient-scaffold for interface tissue engineering.

3D-printed nanoporous ceramics: Tunable feedstock for direct ink write and projection microstereolithography

Porous ceramic materials have a wide range of potential applications for which controlling the structure across multiple length scales is desirable. Additive manufacturing (AM) of porous ceramics is therefore of interest for design flexibility. Here, a ceramic ink compatible with 2 AM techniques, projection microstereolithography (PμSL) and direct ink write (DIW), was formulated and demonstrated printed parts with a range of controllable feature sizes as well as nanoporosity resulting from partial sintering. A diacrylate polymer was mixed with 3% yttria partially stabilized zirconia (3YZ) ceramic nanoparticles having different sizes and solids loadings to find formulations that meet the printing requirements of both AM techniques. Detailed rheological studies were used to determine optimal ink formulations to use for either printing method. The resulting 3YZ structures printed with DIW and PμSL have engineered macro cavities with span lengths greater than several millimeters, wall thicknesses of 200 to 540 μm, and porosity within the wall structure on the order of 100 nm. This study revealed that through facile composition changes to the 3YZ ink, it was feasible to use the same ink base for multiple AM techniques without the need for separate cumbersome ink development processes.

Ink bridge control in the electrohydrodynamic printing with a coaxial nozzle

Abstract A coaxial nozzle, consisting of two coaxially aligned capillaries to minimize the nozzle clogging (often encountered in a single capillary EHD printing) by enabling continuous ink circulation between the inner and outer ink channels, had been recently proposed for the electrohydrodynamic (EHD) printing. The EHD printing by the coaxial nozzle, however, remains in the early development stage, particularly with respect to the control of ink flow and meniscus during the jetting process. The ink bridge, which connects both openings of the inner and outer capillaries, plays an important role in keeping the ink circulating and influencing the jetting meniscus pinned at the inner capillary tip. In this study, a closed-loop feedback control was developed to automatically regulate the volume of the ink bridge during the printing. It was found that the ink bridge volume affects the volume and shape of the jetting meniscus, which is directly correlated to the printed dot size. Comparing the coaxial printing nozzle to the single capillary nozzle, the addition of the larger outer nozzle does not decrease the printing resolution. The difference in the printed dot diameter between the two printing configurations is less than 5 %. For a given print setting and a specific ink, an optimal ink bridge volume was identified to produce the smallest dot size. With the ink-bridge shape control, the print resolution can be improved by 24.5 %. The correlation between the ink bridge volume, jetting meniscus volume, and the printed dot size also depends on the ink conductivity and the extrusion length of the inner capillary. The coaxial nozzle EHD printing, featuring the continuous ink circulation and effective ink bridge control, provides a reliable manufacturing process in the pattern generation and microfabrication.

Anode ink formulation for a fully printed flexible fuel cell stack

In fuel cells the underlying reactions take place at the catalyst layers composed of materials favoring the desired electrochemical reactions. This paper introduces a formulation process for a catalyst inkjet ink used as an anode for a fully printed flexible fuel cell stack. The optimal ink formulation was 2.5 wt% of carbon–platinum–ruthenium mixture with 0.5% Nafion concentration in a diacetone alcohol solvent vehicle. The best jetting performance was achieved when 1 wt% binder was included in the ink formulation. Anodes with resistivity of approximately 0.1 Ω cm were inkjet printed, which is close to the commercial anode resistivity of 0.05 Ω cm. The anodes were used in fuel cell stacks that were prepared by utilizing only printing methods. The best five-cell-air-breathing stack showed an open circuit potential under H2/air conditions of 3.4 V. The peak power of this stack was 120 µW cm−2 at 1.75 V, with a resistance obtained from potentiostatic impedance analysis of 295 Ohm cm2. The printed electrodes showed a performance suitable for low-performance solutions, such as powering single-use sensors.

Piezo‐electric based Ultraprecise/Ultratrace 3D Food Printing of Potato Resistant Starch

Recently, compared to traditional food production systems, 3D food printing has gained a lot of interest because of the potential benefits of being able to customized food products fabrication in colour, shape, texture, flavor and even nutrition. Successful printing of food objects in 3D food printing is critical, and high accuracy and precision food printing technology is required to reproduce delicate and complex food 3D structures. In 3D food printing, the properties of food materials, such as rheological properties, chemical properties and thermal stability, are significant for the structure accuracy and precision of printed objects. In this study, we tried to present a 3D structure layering method for potato resistant starch using drop‐on‐demand (DOD) printing method (piezoelectric system type) that can express the fine structure of food by inkjet printing technology. Nanocrystals cellulose (CNC) and sodium alginate (AS) were used to form a food 3D structure, and 3D printing optimal ink formulation ratio of CNC and AS (CNC:AS=80:20) was selected through preliminary experiments. The purpose of this study was to determine the possibility of 3D printing and printability after comparing rheological properties of mixtures with and without annealing by varying the content of potato resistant starch (PRS) in the mixture of CNC and AS. 3D food structure with various combination of potato resistant starch (RS) to a mixture of CNC and AS was printed, and compared the rheological properties of the mixture that had not been temperature‐treated and the one that had been annealed. After that, the optimal material combination for precise 3D printing was explored together with the physical properties of the printed object.Support or Funding InformationThis research was supported in part by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (2018R1D1A1B07045349), the High Value‐added Food Technology Development Program, the Ministry of Agriculture, Food and Rural Affairs (MAFRA), Republic of Korea (118059‐2) and the Ewha Womans University Research Grant of 2019.Piezo‐electric based Ultraprecise/Ultratrace 3D Food PrintingFigure 1

High‐Resolution Inkjet Printing of Quantum Dot Light‐Emitting Microdiode Arrays

AbstractThe direct printing of microscale quantum dot light‐emitting diodes (QLEDs) is a cost‐effective alternative to the placement of pre‐formed LEDs. The quality of printed QLEDs currently is limited by nonuniformities in droplet formation, wetting, and drying during inkjet printing. Here, optimal ink formulation which can suppress nonuniformities at the pixel and array levels is demonstrated. A solvent mixture is used to tune the ejected droplet size, ensure wetting, and provoke Marangoni flows that prevent coffee stain rings. Arrays of green QLED devices are printed at a resolution of 500 pixels in.−1 with a maximum luminance of ≈3000 cd m−2 and a peak current efficiency of 2.8 cd A−1. The resulting array quality is sufficient to print displays at state‐of‐the‐art resolutions.

3D-Printed Grids with Polymeric Photocatalytic System as Flexible Air Filter

Significant concerns regarding nitric oxide removal are related to the difficulty in developing a catalytic system with improved removal efficiency. With the consideration to immobilize efficient photocatalyst, fulfill multilevel hierarchy and involve excellent reusability, 3D printing methodology is adopted to explore the novel photocatalytic system as a flexible and freestanding air filter. The efficient photocatalyst - g-C3N4 serves as starting material, while the Poly-(ethylene glycol) double acrylate is used as both dispersant and matrix to form an optimal ink. By forming newly bonded composite, the band gap of g-C3N4 is reduced by 0.1 eV, led to increasing of photocatalysis efficiency. This photocatalytic system exhibits excellent NO removal capability and durability, indicating that such air filter can be a promising candidate for a real application. Simulation models are also established to study light absorption in the grid geometry as well as the influence of printing parameter like grid spacing.

Controlling the micro-structure of disperse water-based inks for ink-jet printing

The study aims to investigate the effect of additives (poly (vinyl pyrrolidone) PVP, glycerol, ethylene glycol) on the performances of disperse water-based inks for ink-jet printing and how such additives interact with water. This should help into design suitable inks for textile printing by ink-jet printing, in particular for synthetic fibers such as polyester fabrics. Thirteen kinds of inks were prepared, and the glycerol content was replaced by water and PVP (K15, K30, K60), and the solubility parameter of the additive was calculated by the group contribution method. Low field nuclear magnetic resonance (LF-NMR) reveals the existence of bound water, intra-microstructure water, and extra-microstructure water in the ink, and the intra-microstructure water is entrapped inside the micro-structure formed by additives, mainly due to the presence of PVP. The optimal ink (Y5K30) has a low linewidth relative change (0.4%) and excellent sharpness. The reason is the existence of microstructure in ink, which reduces the mobility of solvent in ink and the diffusion of ink on fabric. PVP and disperse dyes have a strong affinity and help to improve the color depth and color fastness of the fabric.

Cost-effective silver ink for printable and flexible electronics with robust mechanical performance

An ideal silver precursor ink should be highly stable, transparent, and compatible with a broad range of substrates and facile fabricating techniques to provide a uniform electrodes with bulk conductivity at mild temperatures (<100 °C) for high performance electronic applications. Herein, we report a facile chemical route to develop the robust silver precursor ink having long stability, which possesses key characteristics of an optimal ink and provides a practical realization in flexible electronics via multiple techniques. The obtained transparent silver ink offers an unambiguously unique and economical approach towards printable electronic devices. The silver electrodes are fabricated by silver ink nozzle-jet printing and ink-pen writing that demonstrate remarkable conductivities (106–107 S/m) close to bulk silver (∼107 S/m) after annealing at 70–100 °C. The adhesion and conductivity of printed silver is highly stable in various mechanical bending. High quality nano-crystalline and pin holes free silver patterns are observed on flexible PET and PI substrates. The probable mechanism for formulated silver ink is elucidated. The precursor ink formulation process can possibly eliminate the need for costlier methods such as gravure printing and paves the way to industrial manufacturing of prudent electronic devices.

Rheological behavior of titania ink and mechanical properties of titania ceramic structures by 3D direct ink writing using high solid loading titania ceramic ink

In this paper, high solid loading titania ceramic ink containing sodium hexametaphosphate and sodium alginate had been successfully prepared at room temperature. Rheological behaviors of titania ceramic ink with different solid content were investigated. The rheological results indicated that high solid loading (72 wt%) titania ceramic ink whose shear elastic modulus reached approximately 106 Pa in the linear viscoelastic region was the optimal ceramic ink for direct ink writing, due to its good shear thinning and viscoelastic behavior. High solid loading (72 wt%) titania ceramic ink was utilized to fabricate three dimensional titania ceramic parts, and a series of test methods were employed to examine the related properties of titania ceramics sintered at different temperatures including the volumetric shrinkage, porosity, density, microstructures, compressive strength and elastic modulus. The results showed that the volumetric shrinkage and density increased when increasing the sintering temperatures, and the opposite phenomenon was found in porosity of titania ceramic. When the sintering temperature was 1300 °C, the compressive strength was about 125.00 MPa. The elastic modulus of titania ceramics sintered at different temperatures (from 1000 °C to 1400 °C) was between 1.22 GPa and 6.87 GPa.

Rheological evaluation of Laponite/alginate inks for 3D extrusion-based printing

The 3D printing of soft materials is challenging mainly due to their rheological behavior. The 3D extrusion-based printing was correlated with the rheological properties for each stage of the process. A protocol to obtain an optimal ink was defined and Laponite/alginate mixtures were analyzed. All mixtures exhibited a pronounced shear-thinning behavior. Higher alginate concentrations partially hindered the rheology modifier effect of Laponite. The filament formation during extrusion and good printability were observed for Laponite concentrations of at least 5 wt%. The optimal ink was defined as a function of the viscosity profile, the filament formation ability, the flow-point and the solid-like/liquid-like behaviors. The viscosity recovery test demonstrated an instant structure recovery for the optimal ink, which did not present shear rate dependence. Jointly, an extrusion-based modular printer head was developed and tested to be compatible with open source 3D printers. Finally, the 3D printed gels were crosslinked to obtain single (SN) and double network (DN) hydrogels. In these latter, a second network precursor of poly(acrylamide) was used. As established, the rheological characterizations constitute a powerful tool to analyze the printability of soft materials.

Thin Copper Flakes for Conductive Inks Prepared by Decomposition of Copper Formate and Ultrafine Wet Milling

AbstractFabrication of devices by printing conductive interconnections on plastic substrates is of growing interest. Currently, silver flakes are wildly used, however the high cost of silver prevents their wide use in many electrical devices. A new two‐step process for synthesizing thin copper flakes, and their utilization in conductive inks, is reported. In the first step, sub‐micrometer copper particles are formed by thermal decomposition and self‐reduction of copper formate. These copper particles are then milled in a wet bead mill that results in their transformation into thin flakes with an average thickness of 48 nm. X‐ray diffraction results indicate that the copper particles undergo plastic deformation in a mechanism similar to cold rolling. The effect of various process parameters and type of dispersing agents on the morphology and electrical performance is studied. The ink formulations result in printed patterns with 22% of bulk copper conductivity. The optimal ink is used to print functioning near field communication antennas on polyimide film, which is found to have a high bending durability.

Improved fine layer patterning using soft blanket gravure printing technology

We have developed a ‘soft blanket gravure’ (SBG) printing technology that can form patterns onto curved or three-dimensional solid surfaces using functional ink. In SBG printing, a soft and thick blanket (called a ‘soft blanket’) is employed in the similar process to conventional gravure offset printing processes. We found and reported that SBG printing could completely receive silver ink from printing plates and transfer it to target substrates more effectively than conventional gravure offset printing under various conditions. In this paper, we measured the swelling ratio of the soft blanket using two different organic solvents and performed SBG printing using printing plates with groove depths of 10–100 μm and two silver inks that contained the two different solvents. SBG printing formed patterns more than 11 μm thick and the same widths as the grooves on the printing plate (50 μm) using the optimal ink solvent. We also observed that the soft blanket could receive silver ink from the grooves without cobwebbing, in contrast to a conventional blanket. These results indicate that the SBG printing technology significantly improves the printing process because the soft blanket can absorb more solvent from silver ink.

Addressing the colorimetric redundancy in 11-ink color separation

To improve color reproduction, many printers today use extra colorants, in addition to the traditional four inks (Cyan, Magenta, Yellow and Black). Adding the complementary colorants (Red, Green and Blue) increases the gamut of reproducible colors, while lighter versions of the primary inks can be added to reduce graininess and dot visibility. Using more than three inks introduces colorimetric redundancy in the color separation process, because different ink combinations can reproduce the same target color. When additional inks are introduced, this redundancy rapidly increases, and it is thus crucial to introduce additional constraints in the color separation process, to improve determinacy and to optimize different aspects of print quality. This study focuses on an analysis of the redundancy in the color separation process for an 11-ink printer. It is investigated how the extensive colorimetric redundancy can be utilized to select optimal ink combinations to meet the, sometimes contradictory, criteria of color accuracy, graininess and ink consumption. Analysis of the results of applying different criteria in the color separation process shows that the result heavily depends on the selected criterion. For example, prioritizing graininess will improve print quality by reducing dot visibility, imposing the use of lighter inks, but it will also increase ink consumption.

3D printing of photocurable poly(glycerol sebacate) elastomers

Three-dimensional (3D) printed scaffolds have great potential in biomedicine; however, it is important that we are able to design such scaffolds with a range of diverse properties towards specific applications. Here, we report the extrusion-based 3D printing of biodegradable and photocurable acrylated polyglycerol sebacate (Acr-PGS) to fabricate scaffolds with elastic properties. Two Acr-PGS macromers were synthesized with varied molecular weights and viscosity, which were then blended to obtain photocurable macromer inks with a range of viscosities. The quality of extruded and photocured scaffolds was dependent on the initial ink viscosity, with flow of printed material resulting in a loss of structural resolution or sample breaking observed with too low or too high viscosity inks, respectively. However, scaffolds with high print resolution and up to ten layers were fabricated with an optimal ink viscosity. The mechanical properties of printed scaffolds were dependent on printing density, where the scaffolds with lower printing density possessed lower moduli and failure properties than higher density scaffolds. The 3D printed scaffolds supported the culture of 3T3 fibroblasts and both spreading and proliferation were observed, indicating that 3D printed Acr-PGS scaffolds are cytocompatible. These results demonstrate that Acr-PGS is a promising material for the fabrication of elastomeric scaffolds for biomedical applications.

Color changing effects with anisotropic halftone prints on metal

We propose a color reproduction framework for creating specularly reflecting color images printed on a metallic substrate that change hue or chroma upon in-plane rotation by 90°. This framework is based on the anisotropic dot gain of line halftones when viewed under specular reflection. The proposed framework relies on a spectral prediction model specially conceived for predicting the color of non-rotated and of 90° in-plane rotated cross-halftones formed of superpositions of horizontal and vertical cyan, magenta and yellow line halftones. Desired non-rotated and rotated image colors are mapped onto the sub-gamut allowing for the desired hue or chroma shift and then, using a 6D correspondence table, converted to optimal cross-halftone ink surface coverages. The proposed recolorization and decolorization framework is especially effective for creating surprising effects such as image parts whose hues change, or gray regions that become colorful. It can be adapted to commercial printers capable of printing with cyan, magenta and yellow inks on substrates formed by an ink attracting polymer lying on top of a metallic film layer. Applications may include art, advertisement, exhibitions and document security.