Metallic Inks Research Articles

Improved fatigue resistance in transfer-printed flexible circuits embedded in polymer substrates with low melting temperatures

Flexible electronics are of great interest and importance due to their applications in a range of fields, from wearable electronics to solar cells. Whilst resolutions of printed flexible electronics have been improving in recent years, there remain problems with mechanical fatigue and substrate cost, curtailing the use of such devices and resulting in increased cost and waste products. Here we present a novel method for improving the fatigue resistance of printed flexible electronics by a factor of ∼40 by sintering the electronics prior to transferring them into low-cost polymer substrates, such that they remain embedded. This method is demonstrated using circuits printed using silver nanoparticulate ink with an aerosol jet printer, and could be applicable to multiple different metallic inks. At the same time, this method can be used to transfer print circuits into polymers with low melting temperatures, without the need for otherwise detrimentally high sintering temperatures required for ink curing.

Design, fabrication, and hypervelocity impact testing of screen-printed flexible micrometeoroid and orbital debris impact sensors for long-duration spacecraft health monitoring

Micrometeoroid and orbital debris (MMOD) are a key risk for spacecraft damage that could compromise missions. Detecting and evaluating MMOD damage is therefore a crucial component in the health monitoring of spacecraft, especially for long duration, deep space expeditions. In this work, we developed a passive sensor system fabricated from conductive metal ink screen-printed on flexible Kapton, using roll-to-roll manufacturing suitable for low-cost fabrication of large areas of sensors, as a MMOD sensor for a spacecraft shield. The sensor is integrated into a low density, two-wall Whipple shield comprising of thin aluminum sheets sandwiching a polyimide foam. The shield with the sensors were tested with hypervelocity impacts at approximately 7 km/s using different particle diameters. Data collected from the sensors were successfully used to determine the impact size, impact location, and predict the impact energy of the damage.

Voidless metal lines sintered with intense pulsed light and their applications as transparent metal-mesh electrodes

An optimal metal ink that can be sintered without void generation by intense pulsed light (IPL) was systematically formulated. This ink is a hybrid ink in which silver (Ag) nanoparticle ink and copper (Cu) precursor ink are mixed in an optimal weight ratio, and the reduced Cu fills the space between Ag nanoparticles during the sintering process. The optimization process of the ink encompasses the shape and size of nanoparticles, the type of solvent, and the mixing ratio of the Cu precursor ink. This optimized ink was filled in the engraved trenches and IPL-sintered to fabricate flexible transparent metal-mesh electrodes. If there are voids in the metal lines after IPL sintering, the metal lines may shrink under condition of high temperature and high humidity for a long time, which may deteriorate the stability of the metal electrode. It was confirmed that the hybrid ink optimized in this study hardly shrinks due to no voids inside the metal lines after IPL sintering and has excellent environmental and mechanical stability. The metal-mesh electrode produced using this ink showed superior properties compared to other competing transparent electrodes such as indium-tin oxide or Ag nanowires when used for transparent heaters and electromagnetic shielding applications.

Bio‐Inspired Differential Capillary Migration of Aqueous Liquid Metal Ink for Rapid Fabrication of High‐Precision Monolayer and Multilayer Circuits

AbstractManipulating liquid metal inks to create conductive microstructures has attracted widespread interest as liquid metal microstructures are turning into influential components in flexible electronics. However, it is challenging to prevent the issues with low precision, low efficiency, and residue caused by sedimentation, free diffusion, and the Marangoni effect. Inspired by the water transport in plants, the wetting‐induced assembly method based on the differential capillary effect for liquid metal ink is created to realize the facile and rapid manufacture of liquid metal conductive microstructures. The single‐micron accuracy circuits with a minimum of ≈4 µm straight lines are fabricated to a centimeter scale. This method can also be extended to the preparation of multilayer circuits (minimum 5 µm through hole). The resulting entirely flexible stretchable circuits make it possible to construct highly stretchable devices, such as flexible transparent conductors and stretching sensors. Transparent conductors exhibit excellent mechanical (maximum ≈750% tensile rupture limit) and optoelectronic properties (the transmittance reaches ≈87% and the sheet resistance is ≈0.5 Ω/□)|making them suitable for optically‐clear electromagnetic shielding. This study offers a fresh and plain approach to solving the assembly problem of liquid metal inks, paving the way for the creation of flexible electronic devices

A Surface Conformal Laser‐Assisted Alloying Reaction for 3D‐Printable Solid/Liquid Biphasic Conductors

Recently, electronics research has made major advances toward a new platform technology facilitating form factor‐free devices. 3D printing techniques have attracted significant attention in the context of fabricating arbitrarily shaped circuits. Herein, a 3D‐printable metallic ink comprising multidimensional eutectic gallium indium (EGaIn)/Ag hierarchical particles is proposed to fabricate arbitrarily designable solid/liquid biphasic conductors that can be inherently self‐healed/chip bonded and do not suffer from liquid flood out due to their liquid and solid nature, respectively. The EGaIn/Ag hierarchical particles are designed to have plasmonic optical absorption at the visible green–red wavelength regime, which is elucidated by an optical simulation study, and also enable the direct transfer of thermal energy, generated in the vicinity of the Ag nanoparticles, to the surface of the EGaIn particles. The 3D surface conformal green laser irradiation process activates the evolution of the biphasic conductive layer from the as‐printed insulating particulate one. The chemical/physical evolution is elucidated along with a photothermal simulation study for clarifying the suppression of undesirable side reactions. It is demonstrated that the biphasic conductors formed by successive 3D printing and the surface conformal green laser irradiation process exhibit electrical properties that have thus far been unexplored in solid metallic conductors.

Flexible strain sensor with self-healing function for human motion monitoring

Flexible strain sensors with highly similar effects to human skin have been given great attention due to their potential application in personal health monitoring, human–computer interaction systems and artificial electronic skin fields. In particular, the self-healing properties of the sensors are important for their long-term and repeated use during the actual operation. Herein, a flexible strain sensor with complete self-healing function is proposed by combining self-healable PDMS film with rich hydrogen bonds and conductive ink based on recoverable liquid metal. By adjusting the contents of different components of self-healing PDMS film and the relative mass fraction of the liquid metal ink in the strain sensor, the tensile stress and resistance of flexible sensor can be changed to match different usage scenarios. The sensor can achieve a maximum tensile stress of 0.83 MPa and an elongation at break of 843%. After self-healing for 24 h at room temperature, its tensile stress can revert to 82% of the original value, while the electrical connection can instantaneously recover to initial situation after fracture surface contacts. This hints its potential advantage as wearable sensors for motion monitoring of the human body and developable applications in medical monitoring, recyclable electronics and artificial skin.

Room Temperature Electronic Functionalization of Thermally Sensitive Substrates by Inkjet Printing of a Reactive Silver‐Based MOD Ink

AbstractThe developments in inkjet printing technology and the printed electronics industry in the past two decades have provided cost‐effective, environment‐friendly, and reliable alternates to traditional methods of fabricating electrical devices. However, most commercial metallic inks require high sintering temperatures to form desired functional patterns, which limits the applications of printed electronics in scenarios that require electrical devices on thermally sensitive substrates, like biomaterials or bio‐synthetic composite materials. This study provides the synthetic route of a novel silver‐based metal organic decomposition (MOD) ink which is used to form highly conductive silver films on the thermally sensitive skin‐inspired silk/epoxy composite substrates by directly inkjet printing with accurate pattern control, whilst self‐decomposing and sintering at room temperature. The fabricated silver patterns on the thermally sensitive silk/epoxy composite substrate are highly conductive with conductivity of 4.65 × 104 S m−1. These silver patterns also show impressive malleability as bulk silver films, which can be further developed into motion sensors for wearable devices or medical applications. Our strategy provides a general platform for electronic functionalization without temperature constraints. The particle‐free, reactive silver‐precursor, and lower sintering temperature of the ink also widen the choices of substrates, as exemplified herein with outstanding printing quality and high electrical conductivity (1.20 × 106 S m−1) also achieve on paper.

Recent progress in liquid metal printing and its applications.

This paper focuses on the latest research printing technology and broad application for flexible liquid metal (LM) materials. Through the newest template printing method, centrifugal force assisted method, pen lithography technology, and laser method, the precision of liquid metal printing on the devices was improved to 10 nm. The development of novel liquid metal inks, such as PVA-LM ink and ethanol/PDMS/LM double emulsion ink, have further enhanced the recovery, rapid printing, high conductivity, and strain resistance. At the same time, liquid metals also show promise in the application of biochemical sensors, photocatalysts, composite materials, driving machines, and electrode materials. Liquid metals have been applied to biomedical, pressure/gas, and electrochemical sensors. The sensitivity, biostability, and electrochemical performance of these LM sensors were improved rapidly. They could continue to be used in healthy respiratory, heartbeat monitoring, and dopamine detection. Meanwhile, the applications of liquid metal droplets in catalytic-assisted MoS2 deposition, catalytic growth of two-dimensional (2D) lamellar, catalytic free radical polymerization, catalytic hydrogen absorption/dehydrogenation, photo/electrocatalysis, and other fields were also summarized. Through improving liquid metal composites, magnetic, thermal, electrical, and tensile enhancement alloys, and shape memory alloys with excellent properties could also be prepared. Finally, the applications of liquid metal in micro-motors, intelligent robot feet, nanorobots, self-actuation, and electrode materials were also summarized. This paper comprehensively summarizes the practical application of liquid metals in different fields, which helps understand LMs development trends, and lays a foundation for subsequent research.

Sinter‐Based Additive Manufacturing of Graded Porous Titanium Scaffolds by Multi‐Inks 3D Extrusion

A 3D printing approach to design and produce cellular scaffolds with a precise tunable pore architecture, in terms of size, fraction, and interconnectivity is reported. Different metallic inks are formulated by mixing hydrogel with Ti–6Al–4V atomized powders of various sizes. After 3D printing by direct‐ink writing (DIW) followed by debinding and sintering, the fraction and size of macropores (, designed by computer‐aided design (CAD)) and micropores (, remaining after sintering), the roughness and the microstructure are determined by high‐resolution X‐ray tomography and electron microscopy, and correlated to the initial powder size. It is shown that playing with initial powder size allows designing different pore architectures, from interconnected micropores to fully dense filaments. These phenomena are combined with a multi‐inks DIW approach to fabricate architectured structures with graded microporosity. This new route is promising for the production of functional materials, such as biomedical scaffolds or implants, with tunable osseointegration, stiffness, and strength. The micropores could also be loaded with active molecules and positioned according to release needs.

Shellac-paper composite as a green substrate for printed electronics

Printed electronic (PE) devices that sense and communicate data will become ubiquitous as the Internet of things continues to grow. Devices that are low cost and disposable will revolutionize areas such as smart packaging, but a major challenge in this field is the reliance on plastic substrates such as polyethylene terephthalate. Plastics discarded in landfills degrade to form micro- and nanoplastics that are hazardous to humans, animals, and aquatic systems. Replacing plastics with paper substrates is a greener approach due to the biodegradability, recyclability, low cost, and compatibility with roll-to-roll printing. However, the porous microstructure of paper promotes the wicking of functional inks, which adversely affects printability and electrical performance. Furthermore, truly sustainable PE must support the separation of electronic materials, particularly metallic inks, from the paper substrate at the end of life. This important step is necessary to avoid contamination of recycled paper and/or waste streams and enable the recovery of electronic materials. Here, we describe the use of shellac—a green and sustainable material—as a multifunctional component of green, paper-based PE. Shellac is a cost-effective biopolymer widely used as a protective coating due to its beneficial properties (hardness, UV resistance, and high moisture- and gas-barrier properties); nonetheless, shellac has not been significantly explored in PE. We show that shellac has great potential in green PE by using it to coat paper substrates to create planarized, printable surfaces. At the end of life, shellac acts as a sacrificial layer. Immersing the printed device in methanol dissolves the shellac layer, enabling the separation of PE materials from the paper substrate.

Avalanche Coalescence of Liquid Metal Particles for Uniform Flexible and Stretchable Electrodes

AbstractSelf‐sinterable gallium‐based liquid metal (LM) ink for a handy drop‐casting coating method is developed. The fabricated LM electrode is coffee‐ring‐free and bilayer‐free, which guarantees high electrical conductivity, wettability, flexibility, and stretchability. The ink consists of Galinstan, ethanol‐water mixture, hydrochloric acid, and polyvinylpyrrolidone. The binary mixture provides excellent wetting conditions and suppresses a coffee‐ring stain from Marangoni mixing. The acid removes an oxide layer on the LM interface. The colloidal interaction forces by self‐assembled polyvinylpyrrolidone induce avalanche coalescences of the particles at the end of evaporation, which produces a thin and uniform dried conductive layer. The acid and polymer provide a bilayer‐free structure and electrification without any post sintering. The drop‐casted self‐transformed LM electrode showed excellent electrical resistance of O (0.1–1 Ω) on folding and stretching conditions. It is believed that this method can serve as flexible and stretchable electrodes, on various substances for soft robots and wearable devices, without any complicated post‐process.

Tightly Coupled Ultra-Wideband Phased-Array Implemented by Three-Dimensional Inkjet Printing Technique

In order to enhance the gains from antennas suitable for airplane-mounted platforms, a tightly coupled antenna array is investigated in this paper. Specifically, a three-dimensional (3-D) inkjet printing technique is used to implement the conformal characteristics needed for the array. Both the radiators and substrate of the antenna array have been fabricated by combining the fused deposition modeling and microdroplet injection molding technologies, based on an existing 3-D printer. Here, through a unique combination of 3-D and 2-D inkjet printing of dielectric material and metallic ink, respectively, we demonstrate a monolithically integrated design for a nonplanar antenna for the first time. The antenna measurements herein show the complete characterization of this new process in terms of minimum feature size and achievable conductivities. This antenna configuration offers a high-gain performance with a low-cost and rapid fabrication technique by using 3-D printing techniques. To check our design, the voltage standing wave ratio and radiation patterns were tested after adding the newly designed feed structure. The results show that the design process is very efficient. Both the antenna element and the array demonstrate positive properties and are in very good agreement with the specially mounted platform.

Direct Ink Writing of AISI 316L Dense Parts and Porous Lattices

The focus of the present work is the development of a metallic ink that possesses controlled rheological properties: by keeping printing parameters constant both AISI 316L dense parts and porous scaffolds have been produced. Shrinkage, porosity, and mechanical properties have been studied to evaluate the link between the ink rheological properties and the final part. Depending on binder composition, linear shrinkage ranging from 9% to 22% and porosity from 34 to 7 vol% are measured. Tensile strength for specimens sintered at 1240 °C reach the value of 444 MPa and elongation at a break of 12.3%. These values are still far from additively manufactured AISI 316L parts with powder bed fusion technologies, but represent an improvement compared to previously reported data in the literature for AISI 316L parts 3D printed by DIW. Porous scaffolds with a spanning distance of 1.2 mm are printed and sintered. Porosity of 74 vol% and compression strength of 74 MPa are measured for this set of samples showing how the produced ink represents a valuable alternative to pastes already present in the literature.

Aerosol Jet Printing and Interconnection Technologies on Additive Manufactured Substrates

Nowadays, digital printing technologies such as inkjet and aerosol jet printing are gaining more importance since they have proven to be suitable for the assembly of complex microsystems. This also applies to medical technology applications like hearing aids where patient-specific solutions are required. However, assembly is more challenging than with conventional printed circuit boards in terms of material compatibility between substrate, interconnect material and printed ink. This paper describes how aerosol jet printing of nano metal inks and subsequent assembly processes are utilized to connect electrical components on 3D substrates fabricated by Digital Light Processing (DLP). Conventional assembly technologies such as soldering and conductive adhesive bonding were investigated and characterized. For this purpose, curing methods and substrate pretreatments for different inks were optimized. Furthermore, the usage of electroless plating on printed metal tracks for improved solderability was investigated. Finally, a 3D ear mold substrate was used to build up a technology demonstrator by means of conductive adhesives.

3D-printable, lightweight, and electrically conductive metal inks based on evaporable emulsion templates jammed with natural rheology modifiers

Conductive metal inks with 3D-printable rheological properties have gained considerable attention, owing to their potential for manufacturing 3D electronics. Typically, such inks are formulated with high volume fractions of metal particles to achieve both rheological and electrical percolation. However, this leads to a high product cost and weight, making this approach potentially undesirable for practical application. In this study, naturally occurring ingredients, i.e., bee pollen microparticles (BPMPs) and citric acids (CAs), are used to produce a jammed hexane-in-aqueous suspension-type emulsion with controllable viscoelasticity as a template for conductive metal particles. Correspondingly, it is possible to develop 3D-printable, lightweight, and conductive inks. The BPMPs and CAs, as rheology modifiers, facilitate the 3D printability of the ink. After drying, the ink forms 3D networks without macroscopic discontinuities. Hexanes co-dispersed with BPMPs and CAs in the aqueous continuous phase improve the ink rheological processability and create internal macropores within the 3D-printed structure upon evaporation under ambient conditions, decreasing the product density. A conductive copper ink based on the emulsion template shows excellent 3D printability and electrical percolation at low metal loadings (<10 vol%); moreover, the printed ink with the optimized formulation has a remarkably low density (<2 g ∙ cm−3).

Direct-ink writing and compression behavior by in situ micro-tomography of architectured 316L scaffolds with a two-scale porosity

Metallic scaffolds with 3D macroporous architecture were produced by direct-ink writing, an additive manufacturing process, which consists in: (i) ink formulation (ii) 3D layerwise extrusion (iii) debinding and sintering. An easy-handling and CMR-free metallic ink composed of aqueous gel and stainless steel powder was optimized to allow 3D extrusion when keeping stiffness and viscosity for design adequation. Evolution upon sintering under vacuum, resulting in a second-scale micronic porosity, was followed by microscopy and 3D X-ray tomography. The ductile behavior was assessed by in situ compressive test under X-Ray tomography, showing a predominant role of macropores compared to micropores in the filaments. This design method with the same hydrogel can be applied to generate other architectured structures of sinterable alloys.

High Operation Frequency and Strain Tolerance of Fully Printed Oxide Thin Film Transistors and Circuits on PET Substrates.

The major limitations of solution-processed oxide electronics include high process temperatures and the absence of necessary strain tolerance that would be essential for flexible electronic applications. Here, a combination of low temperature (<100°C) curable indium oxide nanoparticle ink and a conductive silver nanoink, which are used to fabricate fully-printed narrow-channel thin film transistors (TFTs) on polyethylene terephthalate (PET) substrates, is proposed. The metal ink is printed onto the In2 O3 nanoparticulate channel to narrow the effective channel lengths down to the thickness of the In2 O3 layer and thereby obtain near-vertical transport across the semiconductor layer. The TFTs thus prepared show On/Off ratio ≈106 and simultaneous maximum current density of 172 µA µm-1 . Next, the depletion-load inverters fabricated on PET substrates demonstrate signal gain >200 and operation frequency >300kHz at low operation voltage of VDD = 2V. In addition, the near-vertical transport across the semiconductor layer is found to be largely strain tolerant with insignificant change in the TFT and inverter performance observed under bending fatigue tests performed down to a bending radius of 1.5mm, which translates to a strain value of 5%. The devices are also found to be robust against atmospheric exposure when remeasured after a month.

A Review on Antibacterial Biomaterials in Biomedical Applications: From Materials Perspective to Bioinks Design.

In tissue engineering, three-dimensional (3D) printing is an emerging approach to producing functioning tissue constructs to repair wounds and repair or replace sick tissue/organs. It allows for precise control of materials and other components in the tissue constructs in an automated way, potentially permitting great throughput production. An ink made using one or multiple biomaterials can be 3D printed into tissue constructs by the printing process; though promising in tissue engineering, the printed constructs have also been reported to have the ability to lead to the emergence of unforeseen illnesses and failure due to biomaterial-related infections. Numerous approaches and/or strategies have been developed to combat biomaterial-related infections, and among them, natural biomaterials, surface treatment of biomaterials, and incorporating inorganic agents have been widely employed for the construct fabrication by 3D printing. Despite various attempts to synthesize and/or optimize the inks for 3D printing, the incidence of infection in the implanted tissue constructs remains one of the most significant issues. For the first time, here we present an overview of inks with antibacterial properties for 3D printing, focusing on the principles and strategies to accomplish biomaterials with anti-infective properties, and the synthesis of metallic ion-containing ink, chitosan-containing inks, and other antibacterial inks. Related discussions regarding the mechanics of biofilm formation and antibacterial performance are also presented, along with future perspectives of the importance of developing printable inks.

Plasma enhanced inkjet printing of particle-free silver ink on polyester fabric for electronic devices

Most formulations of metal inks comprise of a suspension of nanoparticles however, these suffer when printed by the formation of unwanted agglomeration. The exploration and optimisation of particle-free silver-complex ink is causing a strong demand for inkjet printing of these formulations over nanoparticle-based suspension inks. This is due to the enhanced printability and rapid conversion via thermal reduction into a conductive material, which can be utilised in electronics manufacture. We developed a silver-complex ink for ‘smart-clothing’ through inkjet printing. The high-quality printing - characterised by no satellite droplet formation and fast speed - is demonstrated upon polyester fabric by the formation of electrical circuitry using a thermal reduction process. Fabric printing is limited by good metal coverage and adhesion, which we demonstrate and improve on in the work by the application of a low temperature, atmospheric air plasma pre-treatment to the polyester surface, which improves printed silver density and coverage using a plasma device which is easy to operate and economic. Printed silver layer reduction and film crystallinity is characterised from high resolution scanning electron microscopy, and spectroscopy (Ultraviolet-visible and Raman) detailing growth mechanisms for high track feature conductivity, producing a low sheet resistivity of 1.378 ± 0.001 Ω/□ and by the lighting of a 1.9 V, 250 mA Light Emitting Device, highlighting its application for conductive features processing. • Conductive films on PET fabric. • Ink jet printing particle-free silver-complex ink. • Conductive thin Ag films, 1.378 ± 0.001 Ω/□. • Enhancement of printed density and coverage by atmospheric plasma pre-treatment.

Aerosol Jet® printing 3D capabilities for metal and polymeric inks

Aerosol Jet® printing (AJ®P) is a direct writing printing technology which deposits functional aerosolized solutions on free-form substrates. Its potential has been widely adopted for two-dimensional (2D) microscale constructs in printed electronics (PE), and it is rapidly growing toward surface structuring and biological interfaces. However, limited research has been devoted to its exploitation as a three-dimensional (3D) printing technique. This paper investigates 3D AJ®P capabilities of three inks along with a comparison of their abilities and limitations by employing three AJ®P 3D strategies (continuous jet deposition, layer-by-layer, point-wise). In particular, 3D microstructures of increasing complexity based on a silver nanoparticle (AgNPs)-, a poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS)-, and a collagen-based ink are here investigated at various aspect ratios and resolutions. Results show the possibility to print not only arrays of micro-pillars of different aspect ratios (AgNPs-ARs ∼ 20, PEDOT:PSS-ARs ∼ 7, collagen-ARs ∼ 3), but also dense and complex (but low reproducible) leaf- or flake-like structures (especially in AgNPs), and lattice units (collagen). This study demonstrates that the fabrication of 3D AJ® printed microstructures firmly depend on the printing parameters and the ink (co–)solvents fast-drying phenomena during printing. Moreover, it provides guidelines about ink development and print strategies for 3D AJ®P micro-structuring, opening its adoption in a vast range of applications in life science (tissue engineering, bioelectronic interfaces), electronics, and micromanufacturing.