Functional Inks Research Articles

Improving Definition of Screen-Printed Functional Materials for Sensing Application.

Screen printing is one of the most used techniques for developing printed electronics. It stands out for its simplicity, scalability, and effectivity. Specifically, the manufacturing of hybrid integrated circuits has promoted the development of the technique, and the photovoltaic industry has enhanced the printing process by developing high-performance metallization pastes and high-end screens. In recent years, fine lines of 50 μm or smaller are about to be adopted in mass production, and screen printing has to compete with digital printing techniques such as inkjet printing, which can reach narrower lines. In this sense, this work is focused on testing the printing resolution of a high-performance stainless-steel screen with commercial conductive inks and functional lab-made inks based on reduced graphene oxide using an interdigitated structure. We achieved electrically conductive functional patterns with a minimum printing resolution of 40 μm for all inks.

A 3D‐Printed Printhead: Custom Inkjet Dispenser for Medium Viscosity Fluids

Inkjet is a versatile non‐contact printing technique that uses droplets of ink jetted from nozzles to print on a variety of substrates. Although some modern piezoelectric printheads support high viscosity fluids (>10 cP), they are currently expensive and may not be suited for many benchtop experiments in research environment. Here, the design, fabrication by 3D‐printing (no cleanroom processes or silicon micromachining are involved) and testing of a multi‐purpose single‐nozzle piezoelectric dispenser/printhead for medium‐viscosity fluids (10–60 cP) is reported. Custom control electronics are developed to drive the printhead. The design features a 200 μm diameter nozzle, suitable for nL drop generation with solutions containing nm sized particles, thus allowing the deposition of μm thick layers of functional inks in a single printing pass. The inkjet dispenser is first tested with glycerol solutions, resulting in drops of 1.9–3.6 nL with typical drop velocity of 0.5–1.3 m s−1. The developed piezoelectric dispensing system is then demonstrated in the printing of a two‐layer capacitive sensor, using silver nanoparticle ink (conductive ink) and a transparent encapsulant ink (dielectric ink). The reported piezoelectric dispenser design can be optimized for many research purposes, allowing the dispensing of functional chemicals, materials, and biomolecules.

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.

Enhanced Resolution, Throughput, and Stability of Aerosol Jet Printing via In Line Heating

AbstractAerosol jet printing offers high resolution, broad materials compatibility, and digital patterning for flexible, conformal, and hybrid electronics. However, limited throughput, instability, and complex optimization requirements inhibit translation to industrial applications. An in‐line heater integrated on a custom printer is demonstrated to modulate droplet evaporation in the aerosol phase, thereby decoupling the deposition rate of functional solids and liquid ink to enable taller, narrower features with aspect ratios reaching 0.29 for a single line. Heating the printhead from room temperature to 80 °C reduced the sensitivity of resolution to deposition rate by ≈90%, improving reliability. With this strategy, increasing the linear deposition rate by 10x results in a modest increase of 27% in line width, compared to a four‐fold increase without heating, permitting higher throughput without sacrificing print quality. Providing a control for in‐line drying independent of ink formulation enables rapid, straightforward design of new materials and processes. This ability to engineer drying of droplets prior to impingement provides a versatile tool to meet complex fabrication challenges, as demonstrated here for both high aspect ratio printing and conformal patterning on rough and three‐dimensional surfaces.

Roll‐to‐Roll Compatible Topochemical Wetting Control for Metamaterial Printing

AbstractThe widespread utilization of metamaterials, despite their immense transformative potential, faces challenges regarding scalability in mass production. To address these limitations, an additive method that leverages liquid inks and selective wetting to produce scalable and cost‐effective metamaterials is presented. UV‐based imprinting lithography is utilized to fabricate surface energy‐modulated patterns, enabling precise solution patterning. This approach, unlike conventional UV‐based imprinting lithography, not only accurately produces the negative replica of the stamp topography during UV‐induced crosslinking but also transfers a hydrophobic layer onto the raised surfaces of the imprinted hydrophilic layer, resulting in 3D shapes with spatially modulated surface energy. In the second process step, a functional ink is dragged over the patterned substrate where it dewets to fill the hydrophilic recesses. This innovative process enables high‐speed metamaterial production, with ink deposition speeds up to 12 m min−1. The method accommodates a wide range of inks, including metals, dielectrics, and semiconductors, providing meticulous control over vertical structures such as pattern thickness and hetero‐multilayer formation. Additionally, it offers flexibility in creating metamaterials on free‐standing ultra‐thin sheets, introducing desirable attributes like foldability and disposability. The effectiveness of this approach is validated through the fabrication and characterization of metallic metamaterials.

Silk‐Based Functional Inks with Color‐Tunable Light Upconversion for Printing on Arbitrary Surfaces

AbstractA biocompatible and versatile silk‐based functional ink with hydrophilic upconversion nanoparticles (UCNPs) that can be printed or painted on arbitrary substrates, including biological surfaces is demonstrated. Hydrophilic UCNPs, ready to be dispersed in aqueous silk fibroin solution, are synthesized in large quantities via facile hydrothermal synthesis using citric acid. Based on a systematic study of dopant control to achieve robust red emission from citrate‐capped UCNPs, three primary colors are obtained, which serve as the basis for the expansion of the color palette. The synthesized UCNPs are homogeneously dispersed in regenerated silk fibroin solutions derived from Bombyx mori cocoons to produce water‐based functional inks. Cyan, magenta, and yellow (CMY) inks are created by combining red, green, and blue (RGB) inks. Demonstrator devices, where a variety of colorful patterns and codes are revealed only under near‐infrared irradiation, are realized through stencil printing or brush painting on flexible polymer films, fruit, leaves, silkworm cocoons, and pig skin. The patterns and codes are highly heat resistant (≈230 °C) and can be easily washed off with water after use. The printable/paintable inks and functions proposed here will provide new opportunities for applications where sustainability, biocompatibility, and intentional luminescence are simultaneously required.

Photochromic Responses and Stability of Functional Inks Applied on Sustainable Packaging Materials

Photochromism refers to a reversible colour change induced by the irradiation of photochromic materials with ultraviolet (UV) or visible light that reverts to the original colour after the light source is removed. This effect arises from chemical transformations between two isomers with different absorption spectra, involving processes like proton transfer, chemical-bond formation, and isomerisation. These photochromic inks, appearing as crystalline powders with micro-sized particles, require dissolution in a suitable matrix to achieve the colour change. Photochromic inks are used in security, as functional coatings for paper and packaging, in the fabric industry, and in other ways. This study examines the influence of varying concentrations of micro-sized photochromic pigments and different ink-coating thicknesses on the photochromic effect on sustainable paperboard substrates. Artificial ageing was performed to assess the photochromic response and lightfastness in relation to pigment concentration, ink-coating thickness, and the influence of the paperboard substrates. The results of this research could contribute to enhancing knowledge on employing photochromic inks for diverse packaging applications.

3D-Printed Carbon-Based Conformal Electromagnetic Interference Shielding Module for Integrated Electronics

Highlights3D printable functional inks incorporated with graphene and carbon nanotube nanoparticles were well-formulated by manipulating their rheological performanceThe frame with ultralight structure (0.076 g cm−3) and high-efficiency electromagnetic interference shielding (61.4 dB) was assembled3D-printed c-SE module was in situ integrated onto the electronics, affording multiple functions of electromagnetic compatibility and thermal dissipation.

Jetting and droplet formation of particle-loaded fluids

Inkjet printing is an attractive method for patterning and fabricating objects across many areas of industry. There is a growing interest in the printing of inks with high particle-loading, such as inks containing glass frit, ceramic and functional inks. However, the use of these inks is often limited due to uncertainty regarding the impact of their rheology on the printing process. Understanding of the role of complex rheology in the jetting of loaded inks is therefore needed to facilitate the wider application of inkjet printing. Here, we characterize the complex rheology and the jetting of model dispersion inks (containing 10, 15, and 23 vol. % TiO2 nanoparticles) and compared them with those without particles. The jetting of the model fluids was conducted with a commercial inkjet printhead (nozzle diameter 34 μm) and visualized with stroboscopic and ultra-high-speed imaging. For low particle concentrations, droplet formation is generally similar to those of unloaded inks, provided their Ohnesorge number and Weber number are matched, although the filament of the loaded model fluid tends to have earlier break-off, having a shorter length. The jetting reliability decreased with increase in particle-loading until reliable jetting can no longer be achieved, due to local particle–particle interactions in the ink channel and in the filament during the fast extensional thinning process. A jetting map is presented which illustrates the influence of particle-loading on the droplet formation, and indicates that the acceptable range of Ohnesorge number for jetting is reduced as the particle-loading is increased.

Sustainable bioelectronics fabrication through photo-induced swelling of green hydrogels

Electrical circuit manufacture for flexible electronics is a very specialized printing process in which electrically functional inks are printed onto a substrate. In almost all cases, the substrate assumes a passive role in ink distribution, which has been the conventional methodology used up until now. Herein we have discovered that a sodium carboxymethyl cellulose (CMCNa) hydrogel substrate demonstrates heightened susceptibility to UV photo-irradiating and because of molecular-level bond lability that leads to a macroscopic improved swelling (“writing” action). The localized photo-activated events lead to temporary 3D contours on the hydrogel substrate where conductive ink is held in valleys to allow the formation of conductive traces. A self-distribution of ink in the valleys is achieved which, moreover, is a type of mask-based photolithography or digital image generation. The process can be employed for polymeric inks such as PEDOT:PSS to obtain ink patterns without need of complex inkjet printers or other conventional printers. The drying causes recession of the temporary swollen hydrogel contours and returns the surface to flattened format. The process works at lower ink solids of 0.125 % and has shown that 1.15 J/mm2 of UV energy is capable of creating an electrically isolated conductive pattern. Initial water content of the system plays an important role in which 20 g/g of absorbed water/substrate is sufficient for acceptable pattern generation.

Print Durability and Recyclability of Label Paper Equipped with Printed RFID Antenna

Labels are a crucial component of products, offering informational content and attractive visuals; therefore, the durability of the print is an important quality requirement. On the other hand, in accordance with eco-design, the recyclability of printed labels is vital. In our research, the focus was on the assessment of the durability, recyclability, and deinkability of printed label paper equipped with printed RFID tags. The determined color fastness of electrophotographic prints affected by various environmental factors showed good resistance to dry rubbing and, in most cases, light and moist–heat treatment, confirming the applicability of digital printing on self-adhesive biodegradable paper labels. In the second part of this study, recyclability was assessed, and a comparison between the deinkability of the offset and digital prints and two conductive functional inks was conducted. Good deinkability was observed for the printed RFID antennas on both the offset and electrographic prints, with only a small deterioration in optical properties, especially when nano-silver conductive ink was used. The study highlights the importance of the selection of materials and printing techniques when considering the environmental impact of printed electronics. The results showed that INGEDE 11 is a suitable deinking method for printed RFID antennas on offset and electrographic prints.

Systematic Investigation of Novel, Controlled Low-Temperature Sintering Processes for Inkjet Printed Silver Nanoparticle Ink.

Functional inks enable manufacturing of flexible electronic devices by means of printing technology. Silver nanoparticle (Ag NP) ink is widely used for printing conductive components. A sintering process is required to obtain sufficient conductivity. Thermal sintering is the most commonly used method, but the heat must be carefully applied to avoid damaging low-temperature substrates such as polymer films. In this work, two alternative sintering methods, damp heat sintering and water sintering are systematically investigated for inkjet-printed Ag tracks on polymer substrates. Both methods allow sintering polyvinyl pyrrolidone (PVP) capped Ag NPs at 85°C. In this way, the resistance is significantly reduced to only 1.7 times that of the samples on polyimide sintered in an oven at 250°C. The microstructure of sintered Ag NPs is analyzed. Taking the states of the capping layer under different conditions into account, the explanation of the sintering mechanism of Ag NPs at low temperatures is presented. Overall, both damp heat sintering and water sintering are viable options for achieving high conductivity of printed Ag tracks. They can broaden the range of substrates available for flexible electronic device fabrication while mitigating substrate damage risks. The choice between them depends on the specific application and the substrateused.

Fully Printed Electrolyte‐Gated Transistor Formed in a 3D Polymer Reservoir with Laser Printed Drain/Source Electrodes

AbstractIn solution processed electronic devices it is crucial that the deposited inks are properly aligned and that all post‐processing steps are compliant with each other. Moreover, shorter channel lengths are highly beneficial to increase the device performance. Herein, laser printing of metals and polymer reservoirs allows to print sub‐micrometer sized channel lengths while confining functional inks into these small gaps. Therefore, a manufacturing concept and optimized material stack, suitable for combined inkjet and laser printing are proposed. A nanoparticulate indium oxide (In2O3) semiconductor is inkjet printed into and constrained by a 3D laser written polymer (pentaerythritol triacrylate, PETA) reservoir. Inside the 3D printed polymer reservoir, platinum (Pt) electrodes, that are further routed over the reservoir walls, are laser printed by a metal reduction process. The transistor fabrication is completed by a second inkjet printed layer of composite solid polymer electrolyte and an organic top‐gate layer (PEDOT:PSS). This concept does not exceed annealing temperatures higher than 100 °C, and is compatible with a range of substrates. The characterized electrolyte‐gated field‐effect transistor show a reasonable on/off‐ratio in the range of 104 with negligible leakage currents. This materials and hybrid device manufacturing scheme has believed great potential for bioelectronics, lab‐on‐a‐chip applications and others.

Droplet Printing Enabled by Cavity Collapse Ejection.

The behavior of cavity collapse in liquids is of fundamental importance in natural and industrial applications. It is still challenging to use the phenomenon of cavity collapse ejection in on-demand droplet printing technology. In this study, we investigate the cavity collapse ejection phenomenon in the submillimeter to millimeter scale and demonstrate that the cavity capillary energy is a critical factor affecting the state of the generated jet. Based on this phenomenon, we developed a droplet printing technology that can print nanoliter satellite-free droplets from a millimeter-sized nozzle, which reduces the risk of nozzle clogging. Using this printing technology, we demonstrated the printing of a nanoparticle suspension with 60% mass loading. Finally, we also showcased the printing of various inks for different applications using this technology, demonstrating the printability of cavity collapse-ejection printing technology in functional inks and showing potential to be applied in scenarios such as bioassays, the electronics industry, and additive manufacturing.

Debossed Contact Printing as a Patterning Method for Paper-Based Electronics.

The proliferation of printed electronic devices is feeding the growth of the Internet of Things, with devices deployed everywhere to collect and communicate data. At the same time, the increase in low-cost disposable devices is a cause for serious environmental concern. In particular, widely used plastic substrates such as poly(ethylene terephthalate) are persistent hazards to the environment. Paper is promising as a greener substrate for printed electronics because it is biodegradable and sourced from renewable materials as well as being low cost and compatible with roll-to-roll printing. However, the porous microstructure of paper promotes wicking of functional inks, leading to poor electrical performance and printing resolution. Hydrophobic coatings applied to the surface of paper create a planarized, printable surface, but these materials may compromise biodegradability and/or recyclability. This paper describes a new resist-free patterning method for printed paper-based electronics that takes advantage of the porous structure of paper. Debossed contact printing uses the pressure from a debossing tip to compress the porous structure of paper and create a patterned relief structure. Printing functional inks with an unpatterned roller deposits ink only on the raised regions of the relief structure. We demonstrate debossed contact printing of silver, carbon black, and conducting polymer inks and show that this new fabrication method is suitable for the fabrication of printed devices with dense features. We demonstrate the fabrication of antennas and patterned electrodes for RFID and smart wallpaper applications, respectively.

Actuation waveform auto-design of drop-on-demand inkjet for ejection droplet without satellite

Designing the actuation waveform is a key step for the piezoelectric inkjet printing technology, which controls the deposition quality of the material. However, the current design of actuation waveforms remains laborious and reliant on repetitive experimentation, resulting in a suboptimal match between the actuation waveform and ink properties, which may cause material deposition defects such as satellite. In this study, the influences of the ink physical properties, nozzle diameter, and ejection behavior on the kinetics of droplet formation were investigated. Further, the transition boundaries of three droplet formation modes for different types of inks, namely, no droplet, single droplet, and satellite droplet, were determined. An actuating waveform auto-design method for creating a single droplet without satellite was constructed based on the physical properties of the ink, which can easily achieve the formation and volume control of a single droplet. The efficacy of the method was validated through monitoring the droplet forming processes of four standard reagents. Moreover, the printing of an interdigitated electrode and micro-patterns was precisely controlled under well-formed droplets. This study could facilitate a comprehensive understanding of droplet formation dynamics, enabling the rapid creation of actuation waveforms for high-quality deposition of functional inks and enhancing the performance of piezoelectric inkjet-based fabrication equipment.

Solid Oxide Electrolysis Cells Fabrication: From Single Cells to Batch Production

Energy transition towards a net-zero emission scenario requires, primarily, a significant increase on the renewable energy production capabilities. However, the inherent intermittency of most common renewable sources, added to the limitations of full electrification in some important hard-to-abate sectors (heavy-duty transport, aviation, steel industry...), implies also the need of developing reliable solutions for energy conversion and storage. Here, hydrogen is gaining more and more popularity in the recent years as an effective solution as energy carrier, mostly for the decarbonization of the key industries and transport. Among the different technologies under development for power-to-hydrogen conversion, solid oxide electrolysis (SOE) outstands due to its high conversion efficiency, fuel flexibility (e.g. CO2 electrolysis) and possibility of working in reversible mode (the same device both as electrolyser and fuel cell).Currently behind competing low temperature electrolysis technologies (AEL, PEMEL) in terms of technology readiness, main challenges of SOE today relate to long-term degradation, heat management and design and reliable fabrication of large stacks and systems. Although many projects are lately flourishing in this line, the number of players able to demonstrate an upscaled fabrication of SOE stacks and systems is still limited. The work presented here represents the first step carried out at CENER for the future demonstration of a pilot fabrication line of SOE stacks, from the optimization of functional materials and inks to the fabrication of single cells and building of 2-10 kW stacks.In this study, a fabrication route for SOE planar cells (5x5 cm2) is proposed, including the optimization of every single step of the process: raw material pre-treatment, ink/slurry development, functional printing and sintering. Particular emphasis is placed on ensuring a reliable upscaling for batch production of cells and thus materials are processed in large quantities (~1 L/batch). In terms of functional materials, standard electrode and electrolytes are chosen in a first approach, viz. Ni-YSZ as hydrogen electrode, YSZ as electrolyte and LSM-YSZ as air electrode. For film deposition, tape casting and screen printing techniques are combined. The electrochemical characterization of the fabricated cells will be presented and compared with commercial ones, including degradation analysis.

Predicting inkjet jetting behavior for viscoelastic inks using machine learning

Abstract Inkjet printing offers significant potential for additive manufacturing technology. However, predicting jetting behavior is challenging because the rheological properties of functional inks commonly used in the industry are overlooked in printability maps that rely on the Ohnesorge and Weber numbers. We present a machine learning-based predictive model for jetting behavior that incorporates the Deborah number, the Ohnesorge number, and the waveform parameters. Ten viscoelastic inks have been prepared and their storage modulus and loss modulus measured, showing good agreement with those obtained by the theoretical Maxwell model. With the relaxation time of the viscoelastic ink obtained by analyzing the Maxwell model equations, the Deborah number could be calculated. We collected a large data set of jetting behaviors of each ink with various waveforms using drop watching system. Three distinct machine learning models were employed to build predictive models. After comparing the prediction accuracy of the machine learning models, we found that multilayer perceptron showed outstanding prediction accuracy. The final predictive model exhibited remarkable accuracy for an unknown ink based on waveform parameters, and the correlation between jetting behavior and ink properties was reasonable. Finally, we developed a printability map characterized by the Ohnesorge and Deborah numbers through the proposed predictive model for viscoelastic fluids and the chosen industrial printhead.

3D-Printed Intrinsically Stretchable Organic Electrochemical Synaptic Transistor Array.

Organic electrochemical transistors (OECTs) for skin-like bioelectronics require mechanical stretchability, softness, and cost-effective large-scale manufacturing. However, developing intrinsically stretchable OECTs using a simple and fast-response technique is challenging due to limitations in functional materials, substrate wettability, and integrated processing of multiple materials. In this regard, we propose a fabrication method devised by combining the preparation of a microstructured hydrophilic substrate, multi-material printing of functional inks with varying viscosities, and optimization of the device channel geometries. The resulting intrinsically stretchable OECT array with synaptic properties was successfully manufactured. These devices demonstrated high transconductance (22.5 mS), excellent mechanical softness (Young's modulus ∼ 2.2 MPa), and stretchability (∼30%). Notably, the device also exhibited artificial synapse functionality, mimicking the biological synapse with features such as paired-pulse depression, short-term plasticity, and long-term plasticity. This study showcases a promising strategy for fabricating intrinsically stretchable OECTs and provides valuable insights for the development of brain-computer interfaces.

Effect of printing parameters on the sensing performance of a 3D printed elastomeric pressure sensor

Additive manufacturing processes have created new opportunities for the printing of functional electronics. Commercial 3D printers fall short in providing the capabilities to directly print with functional inks, especially, with thermosetting non-Newtonian materials. Also, open-source slicing software are generally designed to generate g-code for thermoplastics. In this work, we modified an inexpensive commercial 3D printer to print functional inks for electronics. A plunger-based extrusion mechanism was developed using a stepper motor and lead screw. The heated extruder in the commercial printer was replaced by the new mechanism. Next, the system was employed to print ionic liquid-based soft pressure sensors, followed by an ultraviolet light curing process to polymerize the printed structure. Sensor ink was prepared by mixing ionic liquid into an acrylate-based monomer. To obtain proper rheology for printing, fumed silica was added to the ink. The effect of different printing parameters on the sensing performance of the printed sensors was studied and presented. The parameters studied were sensor thickness, nozzle diameter, extrusion rate, and raster angle. Multiple sets of sensors were printed by changing these parameters, and they are evaluated for their sensitivity. The sensitivity was measured in terms of relative change in the voltage output from the sensor system. The results show that varying the printing parameters significantly affects the sensing performance. The sensitivity increases with the decrease in sensor thickness, increase in nozzle diameter, and increase in print flow rate. The findings from this study can be employed to modulate the sensitivity of a sensor for different applications by changing various print parameters.