Aqueous Ink Research Articles

Multi-material additive manufacturing of functionally graded carbide ceramics via active, in-line mixing

Advanced ceramics are required in many applications including armor, engine components, and wear parts for abrasive, corrosive, and high temperature environments. Heterogeneous structuring in these materials has the potential to unlock extrinsic mechanisms that improve damage tolerance, of vital importance for structural functionality. However, traditional ceramic powder processing and forming techniques limit the design space to simple geometries with chemically homogenous microstructures. Thus, additive manufacturing by direct ink writing (DIW) was applied to fabricate multi-phase carbide specimens with tailored composition and mesostructure. A custom DIW system was developed to allow simultaneous extrusion and mixing of multiple inks, comprised of ceramic particulate suspensions, through a single nozzle. Boron carbide (B4C) and silicon carbide (SiC) were chosen for this study due to their excellent mechanical properties. Aqueous B4C and SiC inks were loaded to 47.5 vol% ceramic content and showed yield-pseudoplastic behavior. The carbide inks were characterized and modified to exhibit similar rheological behavior (yield stress and viscosity), and were used to produce B4C–SiC parts with either discrete or continuous composition variation. Specimens were hot pressed at 35 MPa and 1950 °C, yielding near full density with hardness (Knoop) values of 20–23 GPa. Tailored heterogeneity, achieved via active in-line mixing, is revealed through microstructural characterization. Cracking observed in the specimen with discretely varied composition is the result of thermally-induced residual stress, and is elucidated through analytical calculations.

Inkjet-printed Ti3C2Tx MXene electrodes for multimodal cutaneous biosensing

Among the existing two-dimensional materials, MXenes, i.e. transition metal carbides, nitrides and/or carbonitrides, stand out for their excellent electrochemical properties. Due to their high charge storage capacity, metal-like conductivity, biocompatibility as well as hydrophilicity, Ti3C2Tx MXene-based inks hold great potential for scalable production of skin conformable electronics via direct printing methods. Herein, we develop an aqueous MXene ink and inkjet-print MXene films on freestanding, flexible, and conducting polymer-based substrates. These skin-adherent MXene electrodes detect electrocardiography signals with high signal-to-noise ratio while exhibiting preserved electrical performance after 1000 cycles of bending with a 50 d long shelf life in ambient conditions. We show that printed MXene films can be further functionalized to perform as multifunctional biosensing units. When integrated with a sodium (Na+) ion selective membrane, MXene electrodes detect Na+ in artificial sweat with a sensitivity of 40 mV per decade. When the films are functionalized with antibodies, they generate an electrical signal in response to a pro-inflammatory cytokine protein (interferon gamma) with a sensitivity of 3.9 mV per decade. Our findings demonstrate how inkjet-printed MXene films simplify the fabrication of next-generation wearable electronic platforms that comprise multimodal sensors.

Direct Ink Writing of Fully Bio-Based Liquid Crystalline Lignin/Hydroxypropyl Cellulose Aqueous Inks: Optimization of Formulations and Printing Parameters

Following the recent demonstration of the potential to direct ink write lyotropic blends of organosolv lignin (OSL) and hydroxypropyl cellulose (HPC), this study aims to optimize the formulations and direct ink writing parameters for fully bio-based lignin/HPC inks. A prescreening identifies the theoretical window of printability for different compositions for formulations based on OSL solutions of 45, 47.5, and 50% solid contents and OSL/HPC wt %/wt % ratios of 30/70, 40/60, and 50/50. Measurements of shear-viscosity and recovery behavior evidence the shear-thinning contribution of HPC and the viscosity recovery contribution of lignin. Shape fidelity, morphology, and mechanical properties of printed monolayers from the prescreened formulations reveal band texture morphology in printed parts and the best performance for the inks with a solid content of 49% and an OSL/HPC ratio of 50/50 (Young's modulus of 995 MPa, ultimate tensile strength of 18.5 MPa, and toughness of 2.8 MJ m-3). The ink composition has a more significant impact on final properties than the printing parameters, albeit a decreased printing speed of 5 mm s-1 and increased pressure of 3.5 bar contribute favorably to properties. Finally, multiple layer objects were successfully printed for compositions having solid contents of 49 and 50.5% and OSL/HPC ratios of 40/60 and 50/50.

Colloidal Nanosurfactants for 3D Conformal Printing of 2D van der Waals Materials.

Printing techniques using nanomaterials have emerged as a versatile tool for fast prototyping and potentially large-scale manufacturing of functional devices. Surfactants play a significant role in many printing processes due to their ability to reduce interfacial tension between ink solvents and nanoparticles and thus improve ink colloidal stability. Here, a colloidal graphene quantum dot (GQD)-based nanosurfactant is reported to stabilize various types of 2D materials in aqueous inks. In particular, a graphene ink with superior colloidal stability is demonstrated by GQD nanosurfactants via the π-π stacking interaction, leading to the printing of multiple high-resolution patterns on various substrates using a single printing pass. It is found that nanosurfactants can significantly improve the mechanical stability of the printed graphene films compared with those of conventional molecular surfactant, as evidenced by 100 taping, 100 scratching, and 1000 bending cycles. Additionally, the printed composite film exhibits improved photoconductance using UV light with 400 nm wavelength, arising from excitation across the nanosurfactant bandgap. Taking advantage of the 3D conformal aerosol jet printing technique, a series of UV sensors of heterogeneous structures are directly printed on 2D flat and 3D spherical substrates, demonstrating the potential of manufacturing geometrically versatile devices based on nanosurfactant inks.

Water-based highly conductive graphene inks for fully printed humidity sensors

Conductive graphene inks have attracted extensive research interests for printed electronics due to the excellent electrical and mechanical properties of graphene. While the traditional organic solvents used for graphene inks will result in high environmental cost, environmentally friendly solvents are required for conductive graphene inks. In this work, we demonstrate a highly conductive aqueous graphene ink printed on a paper substrate using screen printing technique. The well-tuned rheological properties of the graphene ink enable the printed lines to achieve a high resolution as fine as 45 µm in width. After being dried at 70 °C and roller compressed, the printed graphene lines present a high conductivity, up to 4.23 × 104 S m−1 at 3 µm thickness. It also shows an excellent bending stability during the roller compression process, which is compatible for large-scale manufacturing. Combining with inkjet-printed graphene oxide as the sensing layer, the graphene lines are exploited as the electrodes for a fully printed humidity sensor on the paper substrate. Such a low drying temperature, high printing resolution and low-cost printing method offers an example that two-dimensional materials are cooperatively utilized for fully printed flexible electronic devices.

Programmable Porous Polymers via Direct Bubble Writing with Surfactant-Free Inks.

Fabrication of macroporous polymers with functionally graded architecture or chemistry bears transformative potential in acoustic damping, energy storage materials, flexible electronics, and filtration but is hardly reachable with current processes. Here, we introduce thiol–ene chemistries in direct bubble writing, a recent technique for additive manufacturing of foams with locally controlled cell size, density, and macroscopic shape. Surfactant-free and solvent-free graded three-dimensional (3D) foams without drying-induced shrinkage were fabricated by direct bubble writing at an unparalleled ink viscosity of 410 cP (40 times higher than previous formulations). Functionalities including shape memory, high glass transition temperatures (>25 °C), and chemical gradients were demonstrated. These results extend direct bubble writing from aqueous inks to nonaqueous formulations at high liquid flow rates (3 mL min−1). Altogether, direct bubble writing with thiol–ene inks promises rapid one-step fabrication of functional materials with locally controlled gradients in the chemical, mechanical, and architectural domains.

Multidipping Technique for Fabrication Time Reduction and Performance Improvement of Solution‐Processed Single‐Walled Carbon Nanotube Thin‐Film Transistors

A newly-designed multi-dipping technique of semiconducting single-walled carbon nanotubes (SWCNTs) enables easy implementation of flexible/stretchable solution-processed SWCNT thin-film transistors (TFTs) in a very short time. Using commercialized aqueous SWCNT ink, repetition of deionized (DI) water rinsing during multi-dipping process makes possible the rapid deposition of a dense and high quality SWCNT network formation, thus leading to significant reduction in total fabrication time but improved electrical performances for SWCNT TFTs. The image shows deposition mechanisms for conventional one-time dipping (left) and multi-dipping techniques (right). Further details can be found in the article number 1901413 by Taehoon Kim, Yongtaek Hong and co-workers.

The rheological performance of aqueous ceramic ink described based on the modified Windhab model

The good stability is the pre-requirement for ceramic ink in ink-jetted printing technology. The aqueous ceramic ink owning a significant shear-thinning effect is proposed using guar gum as additive agent. Due to the shear-thinning effect, the ink in static state has a high viscosity, which contributes to keep the ink stable. At the same time, the ink in spray state has a very low viscosity to meet the requirement of spraying. To describe this rheological performance, the Windhab model is modified, in which the correction term ηα is added into η∞. The term ηα is related to the concentration of guar gum and the solid content of the aqueous ceramic ink. A slip model is proposed to understand the interaction between pigment particles and guar molecules.

Chemical Funneling of Colloidal Gold Nanoparticles on Printed Arrays of End-Grafted Polymers for Plasmonic Applications.

Spatially defined assembly of colloidal metallic nanoparticles is necessary for fabrication of plasmonic devices. In this study, we demonstrate high-resolution additive jet printing of end-functional polymers to serve as templates for directed self-assembly of nanoparticles into architectures with substantial plasmonic activity. The intriguing aspect of this work is the ability to form patterns of end-grafted poly(ethylene glycol) through printing on a hydrophobic layer that consists of fluoroalkylsilanes. The simultaneous dewetting of the underlying hydrophobic layer together with grafting of the printed polymer during thermal annealing enables fabrication of spatially defined binding sites for assembly of nanoparticles. The employment of electrohydrodynamic jet printing and aqueous inks together with reduction of the feature size during thermal annealing are critically important in achieving high chemical contrast patterns as small as ∼250 nm. Gold nanospheres of varying diameters selectively bind and assemble into nanostructures with reduced interparticle distances on the hydrophilic patterns of poly(ethylene glycol) surrounded with a hydrophobic background. The resulting plasmonic arrays exhibit intense and pattern-specific signals in surface-enhanced Raman scattering (SERS) spectroscopy. The localized seed-mediated growth of metallic nanostructures over the patterned gold nanospheres presents further routes for expanding the composition of the plasmonic arrays. A representative application in SERS-based surface encoding is demonstrated through large-area patterning of plasmonic structures and multiplex deposition of taggant molecules, all enabled by printing.

LixNa2-xW4O13 nanosheet for scalable electrochromic device.

The printed electronics technology can be used to efficiently construct smart devices and is dependent on functional inks containing well-dispersed active materials. Two-dimensional (2D) materials are promising functional ink candidates due to their superior properties. However, the majority 2D materials can disperse well only in organic solvents or in surfactant-assisted water solutions, which limits their applications. Herein, we report a lithium (Li)-ion exchange method to improve the dispersity of the Na2W4O13 nanosheets in pure water. The Li-ion-exchanged Na2W4O13 (LixNa2-xW4O13) nanosheets show highly stable dispersity in water with a zeta potential of -55 mV. Moreover, this aqueous ink can be sprayed on various substrates to obtain a uniform LixNa2-xW4O13 nanosheet film, exhibiting an excellent electrochromic performance. A complementary electrochromic device containing a LixNa2-xW4O13 nanosheet film as an electrochromic layer and Prussian white (PW) as an ion storage layer exhibits a large optical modulation of 75% at 700 nm, a fast switching response of less than 2 s, and outstanding cyclic stability. This Na2W4O13-based aqueous ink exhibits considerable potential for fabricating large-scale and flexible electrochromic devices, which would meet the practical application requirements.

Design of Potassium-Selective Mixed Ion/Electron Conducting Polymers.

An approach providing cation-selective poly-(3,4-ethylenedioxythiophene)(PEDOT):polyelectrolyte-mixed conductors is presented in this communication based on the structural modification of this ambivalent (ionic and electronic conductive) polymer complex. First, an 18-crown-6 moiety is integrated into the styrene sulfonate monomer structure as a specific metal cation scavenger particularly targeting K+ versus Na+ detection. This newly functionalized monomer is characterized by 1 H NMR titration to evaluate the ion selectivity. Aqueous PEDOT dispersion inks containing the polymeric ion-selective moieties are designed and their electrical and electrochemical properties analyzed. These biocompatible inks are the first proof-of-concept step towards ion selectivity in view of their interfacing with biological cells and microorgans of interest in the field of biosensors and physiology.

Inkjet-Printed Top-Gate Thin-Film Transistors Based on InGaSnO Semiconductor Layer with Improved Etching Resistance

Inkjet-printed top-gate metal oxide (MO) thin-film transistors (TFTs) with InGaSnO semiconductor layer and carbon-free aqueous gate dielectric ink are demonstrated. It is found that the InGaO semiconductor layer without Sn doping is seriously damaged after printing aqueous gate dielectric ink onto it. By doping Sn into InGaO, the acid resistance is enhanced. As a result, the printed InGaSnO semiconductor layer is almost not affected during printing the following gate dielectric layer. The TFTs based on the InGaSnO semiconductor layer exhibit higher mobility, less hysteresis, and better stability compared to those based on InGaO semiconductor layer. To the best of our knowledge, it is for the first time to investigate the interface chemical corrosivity of inkjet-printed MO-TFTs. It paves a way to overcome the solvent etching problems for the printed TFTs.

Turning Trash into Treasure: Additive Free MXene Sediment Inks for Screen-Printed Micro-Supercapacitors.

Printed functional conductive inks have triggered scalable production of smart electronics such as energy-storage devices, antennas, wearable electronics, etc. Of particular interest are highly conductive-additive-free inks devoid of costly postdeposition treatments to eliminate sacrificial components. Due to the high filler concentration required, formulation of such waste-free inks has proven quite challenging. Here, additive-free, 2D titanium carbide MXene aqueous inks with appropriate rheological properties for scalable screen printing are demonstrated. Importantly, the inks consist essentially of the sediments of unetched precursor and multilayered MXene, which are usually discarded after delamination. Screen-printed structures are presented on paper with high resolution and spatial uniformity, including micro-supercapacitors, conductive tracks, integrated circuit paths, and others. It is revealed that the delaminated nanosheets among the layered particles function as efficient conductive binders, maintaining the mechanical integrity and thus the metallic conductive network. The areal capacitance (158 mF cm-2 ) and energy density (1.64 µWh cm-2 ) of the printed micro-supercapacitors are much superior to other devices based on MXene or graphene. The ink formulation strategy of "turning trash into treasure" for screen printing highlights the potential of waste-free MXene sediment printing for scalable and sustainable production of next-generation wearable smart electronics.

Ammoniated aqueous precursor ink processed copper iodide as hole transport layer for inverted planar perovskite solar cells

The inorganic carrier transport layers are robust and stable to the environment compared to the organic hole transport layer (HTL). Here, we report on the fabrication of the halide perovskite solar cells (HaPSCs) employing CuI deposited with ammoniated aqueous solution ink as HTL and explored the material properties and device characteristics. The film morphology of CuI is found to have an influence on the HaP film growth and hence effects on the device parameters. The HaPSC with CuI has demonstrated the power conversion efficiency of ~14.21% with high reproducibility. The capacitance spectra analysis shows that the deep trap states are induced in the perovskite absorber layer. The results obtained from the temperature-dependent open circuit voltage implicates that the recombination activities in the perovskite bulk are dominant. Furthermore, the HaPSC with CuI has revealed better air stability compared to the device with PEDOT:PSS. This work suggests that the CuI processed with aqueous precursor is a promising alternative HTL to PEDOT:PSS for efficient and stable perovskite solar cells as well as in a tandem device.

Formulation of a graft polymer-containing aqueous yellow ceramic ink for digital ink-jet printing.

We herein reported the preparation of an eco-friendly aqueous yellow ceramic ink and subsequent examination of its printability for ink-jet printing applications. For this purpose, a water based ceramic ink was formulated using ceramic pigments to give a clear color during high-temperature heat treatment (>1000 °C), ultimately yielding an ink suitable for application in ceramic tile decoration. To improve the low dispersion stability of the aqueous ceramic ink compared to that of a conventional organic solvent-based ceramic ink, the pigment particles were dispersed using a graft polymer. More specifically, this graft polymer was composed of polyacrylic acid as the main chain to provide electrostatic repulsion and grafted poly(ethylene glycol) methyl ether methacrylate as the side chain to provide steric hindrance. Furthermore, additive-induced foaming issues were significantly reduced through the use of this graft polymer as a surfactant. The effects of the rheological properties of the prepared ceramic ink and the subsequent operation conditions of the piezoelectric print head were then investigated in detail to optimize the ink jettability and printability.

One-step aqueous fabrication of a silver nanowire composite transparent conductive film with high uniformity and stability

High-performance silver nanowire transparent conductive films composited with the chitosan–lactic acid were fabricated with an aqueous ink via a one-step solution process.

Highly sensitive and durable wearable strain sensors from a core-sheath nanocomposite yarn

With fiber-based wearable electronics being developed and widely used in various fields, the safety and durability of nanomaterials in fibrous system become big concerns due to the leakage of nanomaterials and possible failure in functionality. To date, it is still challenging to fabricate wearable strain sensors with sensitivity and durability. Herein, we report a simple strategy for large-scale fabrication of core-sheath nanocomposite yarn (CSCY) based wearable sensor with high sensitivity and superior durability. By dispersing aqueous CNT ink into braided composite yarns as the core (BYs-CNT) and electrospun polyurethane (PU) nanofibers as the sheath, a unique core-sheath micro-nanofibrous structure was fabricated to grant the yarn with extremely high sensing sensitivity (maximum GF up to 980), long-term stability and great durability (against washing and dilute hydrochloric acid). The as-developed CSCY strain sensor was demonstrated to monitor real-time subtle and vigorous human activities. The present work provides insights for developing reliable and stable, highly sensitive, flexible and durable wearable electronics with potential applications in different areas.

One-pot ball-milling preparation of graphene/carbon black aqueous inks for highly conductive and flexible printed electronics

Stable aqueous carbon inks, with graphene sheets (GSs) and carbon black (CB) as conductive fillers, are prepared by a simple one-pot ball-milling method. The as-prepared composite ink with 10 wt% GSs shows optimized rheological properties (viscosity and thixotropy) for screen printing. The as-printed coatings based on the above ink are uniform and dense on a polyimide substrate, and exhibit a sandwich-type conductive three dimensional network at the microscale. The resistivity of the typical composite coating is as low as 0.23±0.01 Ω cm ( 92±4 Ω sq−1 , 25 μm), which is 30% as that of a pure CB coating (0.77±0.01 Ω cm). It is noteworthy that the resistivity decreases to 0.18±0.01 Ω cm ( 72±4 Ω sq−1 , 25 μm) after a further rolling compression. The coating exhibits good mechanical flexibility, and the resistance slightly increases by 12% after 3000 bending cycles. With the CB/GSs composite coatings as a flexible conductor, fascinating luminescent bookmarks and membrane switches were fabricated, demonstrating the tremendous potential of these coatings in the commercial production of flexible electronics and devices.

Inkjet Printing of Li‐Rich Cathode Material for Thin‐Film Lithium‐Ion Microbatteries

The observed downsizing tendency of microelectronic devices leads to a higher demand in new types of miniaturized energy sources. Thin‐film Li‐ion batteries (LiBs) are promising candidates to fulfil this function. New materials and technologies should be investigated for customized production of miniaturized, high‐efficient solid‐state batteries. Herein, inkjet printing technology is considered as a promising one for the fabrication of LiBs. The modification of crystalline lattice of Li‐rich cathode material by aluminium, sodium, and potassium and their influence on power efficiency are studied in detail. Lithium‐manganese‐rich compounds are chosen as the most suitable composition of an active component for LiBs fabrication. The stable aqueous colloidal ink composition is synthesized and its rheological parameters are optimized for inkjet printing in terms of viscosity, surface tension, and contact angle. Protocols for inkjet printing for the fabrication of thin‐film cathodes with the thickness of less than 10 μm are reported. The good correlation of electrochemical properties such as average voltage, capacity, and energy between inkjet printed and conventionally fabricated electrodes confirms the feasibility of the suggested technological approach and selected cathode material composition.

Direct Inkjet Printing of Aqueous Inks to Flexible All-Solid-State Graphene Hybrid Micro-Supercapacitors.

In this article, the inkjet printing technique is demonstrated for the stacking of reduced graphene oxide (RGO) and molybdenum trioxide (MoO3) nanosheets for flexible all-solid-state micro-supercapacitors. The ammonium molybdate tetrahydrate/graphene oxide ((NH4)6Mo7O24·4H2O/GO) aqueous inks are facilely printed on polymide (PI) film and transformed to RGO/MoO3 hybrids via thermal treatments at air atmosphere. The compound inks are water-based, inkjet-printable, and nontoxic for inkjet printing to form two-dimensional crystal materials. The physical properties of aqueous inks are optimized within a printable range characterized by the Ohnesorge number of 1 < Z < 14. The inkjet-printed symmetric micro-supercapacitors (MSCs) with poly(vinyl alcohol) (PVA)-H2SO4 gel electrolyte possess a wide voltage window of 0-0.8 V, excellent flexibility, a high volumetric specific capacitance of 22.5 F cm-3 at 0.044 A cm-3, as well as good cyclic stability due to the synergistic effect of RGO and MoO3. Furthermore, the inkjet-printed composite MSCs delivered a maximum energy density of 2 mWh cm-3 and a power density of 0.018 W cm-3, and the capacity retention rate of inkjet-printed MSCs is still retained 82% even after 10 000 charge-discharge cycles, indicating good electrochemical properties. Above all, the as-designed inkjet printing technique shows potential for flexible and wearable energy storage electronics.