Reactive Silver Inks for Patterning High-Conductivity Features at Mild Temperatures

An Approach to a Silver Conductive Ink for Inkjet Printer Technology

Silver-based metal-organic decomposition inks composed of silver salts, complexing agents and volatile solvents are now the subject of much research due to the simplicity and variability of their preparation, their high stability and their relatively low sintering temperature. The use of this type of ink in inkjet printing allows for improved cost-effective and environmentally friendly technology for the production of electrical devices, including flexible electronics. An approach to producing a silver salt-based reactive ink for jet printing has been developed. The test images were printed with an inkjet printer onto polyimide substrates, and two-stage thermal sintering was carried out at temperatures of 60 °C and 100-180 °C. The structure and electrical properties of the obtained conductive lines were investigated. As a result, under optimal conditions an electrically conductive film with low surface resistance of approximately 3 Ω/square can be formed.

One-Step Synthesis of Silver Nanoparticles Embedded in Biobased Polyamide 56 Nanofibers with High Antibacterial Activity

Embedded silver nanoparticles (Ag NPs) within nanofibers represent a highly promising alternative to common antimicrobial materials, due to the combined effective biocidal properties of Ag NPs with the biocompatibility and environmental friendliness of biobased polymers. In this study, we presented a novel one-step route to fabricate biobased polyamide 56 (PA56) nanofibers embedded with uniform Ag NPs. The process involved mixing reactive silver ammonia with PA56 solutions and then using formic acid as a reducing agent. Continuous electrospinning resulted in solvent evaporation, yielding Ag NPs highly dispersed within PA56 nanonet fibrous structures (PA56/Ag). Characterization assays confirmed the successful impregnation of Ag NPs in PA56 nanofibers, with an average size of about 32.4 nm. PA56/Ag nanofibers also displayed suitable morphology, mechanical properties, and good biocompatibility in vitro. Moreover, their antimicrobial effectiveness was evaluated against Staphylococcus aureus and Escherichia coli. Collectively, the proposed PA56/Ag nanofibers possess desirable characteristics suitable for antimicrobial applications.

The Influence of Ink Chemistry on the Microstructure Evolution and GHz RF Response of Printed Ag Transmission Lines

High-frequency transmission is limited to the skin depth in metals. Because poor conductivity cannot be compensated for by increasing the conductor thickness as with DC, optimal transport properties are prerequisites for radio frequency (RF) use. Structural and chemical analyses of transmission lines printed using a traditional ink consisting of Ag nanoflakes in a dispersing phase revealed that optimized thermal treatments yielded thorough burnout of the binder, significant grain growth, elimination of the pore volume, and electrical responses that were comparable to values obtained for thermally evaporated, fully dense Ag controls. Specifically, a low DC resistivity of 2.3 μΩ·cm (1.4× bulk Ag) and RF transmission coefficients of 0.87 and 0.75 at 5 GHz and 10 GHz, respectively, were measured in the nanoflake Ag prints. Conversely, in transmission lines printed from a metal-organic decomposition ink, residual chemical contamination impeded diffusion and densification, yielding greater porosity, small grains that are pinned, and a degraded RF response. Reasonably good porosity approximations were obtained from a model based on percolation theory. The results indicate that contaminants at interfaces and pore surfaces impede diffusion, pore elimination, and full densification, and further, alter carrier dynamics and degrade RF response.

Flexible Embedded Metal Meshes by Sputter-Free Crack Lithography for Transparent Electrodes and Electromagnetic Interference Shielding

A facile and novel fabrication method is demonstrated for creating flexible poly(ethylene terephthalate) (PET)-embedded silver meshes using crack lithography, reactive ion etching (RIE), and reactive silver ink. The crack width and spacing in a waterborne acrylic emulsion polymer are controlled by the thickness of the polymer and the applied stress due to heating and evaporation. Our innovative fabrication technique eliminates the need for sputtering and ensures stronger adhesion of the metal meshes to the PET substrate. Crack trench depths over 5 μm and line widths under 5 μm have been achieved. As a transparent electrode, our flexible embedded Ag meshes exhibit a visible transmission of 91.3% and sheet resistance of 0.54 Ω/sq as well as 93.7% and 1.4 Ω/sq. This performance corresponds to figures of merit (σDC/σOP) of 7500 and 4070, respectively. For transparent electromagnetic interference (EMI) shielding, the metal meshes achieve a shielding efficiency (SE) of 42 dB with 91.3% visible transmission and an EMI SE of 37.4 dB with 93.7% visible transmission. We demonstrate the highest transparent electrode performance of crack lithography approaches in the literature and the highest flexible transparent EMI shielding performance of all fabrication approaches in the literature. These metal meshes may have applications in transparent electrodes, EMI shielding, solar cells, and organic light-emitting diodes.

Substrate-embedded metal meshes for ITO-free organic light emitting diodes

Organic light-emitting diodes (OLEDs) have great potential for use in large-area display and lighting applications, but their widespread adoption for large areas is hindered by the high cost and insufficient performance of indium tin oxide (ITO) anodes. In this study, we introduce an alternative anode material - a silver mesh embedded in glass - to facilitate production of large-area OLEDs. We present a facile, scalable manufacturing technique to create high aspect ratio micromeshes embedded in glass to provide the planar geometry needed for OLED layers. Our phosphorescent green OLEDs achieve a current efficiency of 51.4 cd/A at 1000 cd/m2 and reach a slightly higher external quantum efficiency compared to a standard ITO/glass reference sample. Notably, these advancements are achieved without any impact on the viewing angle of the OLEDs. These findings represent a promising advancement towards ITO-free, high-efficiency OLEDs for various high performance, large-area applications, such as lighting and displays.

Copper nanodot-embedded nitrogen and fluorine co-doped porous carbon nanofibers as advanced electrocatalysts for rechargeable zinc-air batteries

Porous carbon-based electrocatalysts for cathodes in zinc-air batteries (ZABs) are limited by their low catalytic activity and poor electronic conductivity, making it difficult for them to be quickly commercialized. To solve these problems of ZABs, copper nanodot-embedded N, F co-doped porous carbon nanofibers (CuNDs@NFPCNFs) are prepared to enhance the electronic conductivity and catalytic activity in this study. The CuNDs@NFPCNFs exhibit excellent oxygen reduction reaction (ORR) performance based on experimental and density functional theory (DFT) simulation results. The copper nanodots (CuNDs) and N, F co-doped carbon nanofibers (NFPCNFs) synergistically enhance the electrocatalytic activity. The CuNDs in the NFPCNFs also enhance the electronic conductivity to facilitate electron transfer during the ORR. The open porous structure of the NFPCNFs promotes the fast diffusion of dissolved oxygen and the formation of abundant gas-liquid-solid interfaces, leading to enhanced ORR activity. Finally, the CuNDs@NFPCNFs show excellent ORR performance, maintaining 92.5% of the catalytic activity after a long-term ORR test of 20000 s. The CuNDs@NFPCNFs also demonstrate super stable charge-discharge cycling for over 400 h, a high specific capacity of 771.3 mAh g^-1 and an excellent power density of 204.9 mW cm^-2 as a cathode electrode in ZABs. This work is expected to provide reference and guidance for research on the mechanism of action of metal nanodot-enhanced carbon materials for ORR electrocatalyst design.

Unraveling the Steric Link to Copper Precursor Decomposition: A Multi-Faceted Study for the Printing of Flexible Electronics

The field of printed electronics strives for lower processing temperatures to move toward flexible substrates that have vast potential: from wearable medical devices to animal tagging. Typically, ink formulations are optimized using mass screening and elimination of failures; as such, there are no comprehensive studies on the fundamental chemistry at play. Herein, findings which describe the steric link to decomposition profile: combining density functional theory, crystallography, thermal decomposition, mass spectrometry, and inkjet printing, are reported. Through the reaction of copper(II) formate with excess alkanolamines of varying steric bulk, tris-co-ordinated copper precursor ions: "[CuL₃ ]," each with a formate counter-ion (1-3) are isolated and their thermal decomposition mass spectrometry profiles are collected to assess their suitability for use in inks (I^1-3 ). Spin coating and inkjet printing of I^1,2 provides an easily up-scalable method toward the deposition of highly conductive copper device interconnects (ρ = 4.7-5.3 × 10^-7 Ω m; ≈30% bulk) onto paper and polyimide substrates and forms functioning circuits that can power light-emitting diodes. The connection among ligand bulk, coordination number, and improved decomposition profile supports fundamental understanding which will direct future design.

Insulated Interlaced Surface Electrodes for Bacterial Inactivation and Detachment

Effective and stable antibiofouling surfaces and interfaces have long been of research interest. In this study, we designed, fabricated, and evaluated a surface coated with insulated interlaced electrodes for bacterial fouling reduction. The electrodes were printed Ag filaments of 100 μm width and 400 μm spacing over an area of 2 × 2 cm². The insulating Ag electrode coating material was polydimethylsiloxane (PDMS) or thermoplastic polyurethane (TPU) with a thickness of 10 to 40 μm. To evaluate the antibiofouling potential, E. coli inactivation after 2 min contact with the electrified surface and P. fluorescens detachment after 15 and 40 h growth were examined. The extent of bacterial inactivation was related to the insulating material, coating thickness, and applied voltage (magnitude and AC vs DC). A high bacterial inactivation (>98%) was achieved after only 2 min of treatment at 50 V AC and 10 kHz using a 10 μm TPU coating. P. fluorescens detachment after 15 and 40 h incubation in the absence of applied potential was completed with simultaneous cross-flow rinsing and AC application. Higher AC voltages and longer cross-flow rinsing times resulted in greater bacterial detachment with bacterial coverage able to be reduced to <1% after only 2 min of rinsing at 50 V AC and 10 kHz. Theoretical electric field analysis indicated that at 10 V the field strength penetrating the aqueous solution is nonuniform (∼16,000-20,000 V m^-1 for the 20 μm TPU) and suggests that dielectrophoresis plays a key role in bacterial detachment. The bacterial inactivation and detachment trends observed in this study indicate that this technique has merit for future antibiofouling surface development.

Development and Optimization of 3D-Printed Flexible Electronic Coatings: A New Generation of Smart Heating Fabrics for Automobile Applications

Textile-based Joule heaters in combination with multifunctional materials, fabrication tactics, and optimized designs have changed the paradigm of futuristic intelligent clothing systems, particularly in the automobile field. In the design of heating systems integrated into a car seat, conductive coatings via 3D printing are expected to have further benefits over conventional rigid electrical elements such as a tailored shape and increased comfort, feasibility, stretchability, and compactness. In this regard, we report on a novel heating technique for car seat fabrics based on the use of smart conductive coatings. For easier processes and integration, an extrusion 3D printer is employed to achieve multilayered thin films coated on the surface of the fabric substrate. The developed heater device consists of two principal copper electrodes (so-called power buses) and three identical heating resistors made of carbon composites. Connections between the copper power bus and the carbon resistors are made by means of sub-divide the electrodes, which is critical for electrical-thermal coupling. Finite element models (FEM) are developed to predict the heating behavior of the tested substrates under different designs. It is pointed out that the most optimized design solves important drawbacks of the initial design in terms of temperature regularity and overheating. Full characterizations of the electrical and thermal properties, together with morphological analyses via SEM images, are conducted on different coated samples, making it possible to identify the relevant physical parameters of the materials as well as confirm the printing quality. It is discovered through a combination of FEM and experimental evaluations that the printed coating patterns have a crucial impact on the energy conversion and heating performance. Our first prototype, thanks to many design optimizations, entirely meets the specifications required by the automobile industry. Accordingly, multifunctional materials together with printing technology could offer an efficient heating method for the smart textile industry with significantly improved comfort for both the designer and user.

3D printed electronics with nanomaterials

A large variety of printing, deposition and writing techniques have been incorporated to fabricate electronic devices in the last decades. This approach, printed electronics, has gained great interest in research and practical applications and is successfully fuelling the growth in materials science and technology. On the other hand, a new player is emerging, additive manufacturing, called 3D printing, introducing a new capability to create geometrically complex constructs with low cost and minimal material waste. Having such tremendous technology in our hands, it was just a matter of time to combine advances of printed electronics technology for the fabrication of unique 3D structural electronics. Nanomaterial patterning with additive manufacturing techniques can enable harnessing their nanoscale properties and the fabrication of active structures with unique electrical, mechanical, optical, thermal, magnetic and biological properties. In this paper, we will briefly review the properties of selected nanomaterials suitable for electronic applications and look closer at the current achievements in the synergistic integration of nanomaterials with additive manufacturing technologies to fabricate 3D printed structural electronics. The focus is fixed strictly on techniques allowing as much as possible fabrication of spatial 3D objects, or at least conformal ones on 3D printed substrates, while only selected techniques are adaptable for 3D printing of electronics. Advances in the fabrication of conductive paths and circuits, passive components, antennas, active and photonic components, energy devices, microelectromechanical systems and sensors are presented. Finally, perspectives for development with new nanomaterials, multimaterial and hybrid techniques, bioelectronics, integration with discrete components and 4D-printing are briefly discussed.

Advanced supramolecular design for direct ink writing of soft materials

The exciting advancements in 3D-printing of soft materials are changing the landscape of materials development and fabrication. Among various 3D-printers that are designed for soft materials fabrication, the direct ink writing (DIW) system is particularly attractive for chemists and materials scientists due to the mild fabrication conditions, compatibility with a wide range of organic and inorganic materials, and the ease of multi-materials 3D-printing. Inks for DIW need to possess suitable viscoelastic properties to allow for smooth extrusion and be self-supportive after printing, but molecularly facilitating 3D printability to functional materials remains nontrivial. While supramolecular binding motifs have been increasingly used for 3D-printing, these inks are largely optimized empirically for DIW. Hence, this review aims to establish a clear connection between the molecular understanding of the supramolecularly bound motifs and their viscoelastic properties at bulk. Herein, extrudable (but not self-supportive) and 3D-printable (self-supportive) polymeric materials that utilize noncovalent interactions, including hydrogen bonding, host-guest inclusion, metal-ligand coordination, micro-crystallization, and van der Waals interaction, have been discussed in detail. In particular, the rheological distinctions between extrudable and 3D-printable inks have been discussed from a supramolecular design perspective. Examples shown in this review also highlight the exciting macroscale functions amplified from the molecular design. Challenges associated with the hierarchical control and characterization of supramolecularly designed DIW inks are also outlined. The perspective of utilizing supramolecular binding motifs in soft materials DIW printing has been discussed. This review serves to connect researchers across disciplines to develop innovative solutions that connect top-down 3D-printing and bottom-up supramolecular design to accelerate the development of 3D-print soft materials for a sustainable future.

A Fluorinated Polyimide Based Nano Silver Paste with High Thermal Resistance and Outstanding Thixotropic Performance

Because of high conductivity, acceptable cost and good screen-printing process performance, silver pastes have been extensively used for making flexible electronics. However, there are few reported articles focusing on high heat resistance solidified silver pastes and their rheological properties. In this paper, a fluorinated polyamic acids (FPAA) is synthesized by polymerization of the 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and 3,4'-diaminodiphenylether as monomers in the diethylene glycol monobutyl. The nano silver pastes are prepared by mixing the obtained FPAA resin with nano silver powder. The agglomerated particles caused by nano silver powder are divided and the dispersion of nano silver pastes are improved by three-roll grinding process with low roll gaps. The obtained nano silver pastes possess excellent thermal resistance with 5% weight loss temperature higher than 500 °C. The volume resistivity of cured nano silver paste achieves 4.52 × 10^-7 Ω·m, when the silver content is 83% and the curing temperature is 300 °C. Additionally, the nano silver pastes have high thixotropic performance, which contributes to fabricate the fine pattern with high resolution. Finally, the conductive pattern with high resolution is prepared by printing silver nano pastes onto PI (Kapton-H) film. The excellent comprehensive properties, including good electrical conductivity, outstanding heat resistance and high thixotropy, make it a potential application in flexible electronics manufacturing, especially in high-temperature fields.

All-atmospheric fabrication of Ag-Cu core-shell nanowire transparent electrodes with Haacke figure of merit >600 × 10-3 Ω-1

Transparent conducting electrodes (TCEs) are essential components in devices such as touch screens, smart windows, and photovoltaics. Metal nanowire networks are promising next-generation TCEs, but best-performing examples rely on expensive metal catalysts (palladium or platinum), vacuum processing, or transfer processes that cannot be scaled. This work demonstrates a metal nanowire TCE fabrication process that focuses on high performance and simple fabrication. Here we combined direct and plating metallization processes on electrospun nanowires. We first directly metallize silver nanowires using reactive silver ink. The silver catalyzes subsequent copper plating to produce Ag-Cu core-shell nanowires and eliminates nanowire junction resistances. The process allows for tunable transmission and sheet resistance properties by adjusting electrospinning and plating time. We demonstrate state-of-the-art, low-haze TCEs using an all-atmospheric process with sheet resistances of 0.33 Ω sq^-1 and visible light transmittances of 86% (including the substrate), leading to a Haacke figure of merit of 652 × 10^-3 Ω^-1. The core-shell nanowire electrode also demonstrates high chemical and bending durability.

One-step metallization of weft-knitted fabrics for wearable biaxial strain sensors

One-step direct patterning of high definition conductive tracks in textiles is realized through laser direct writing in combination with a silver organometallic ink developed in-house. Photoreduction, nano-crystallization, and sintering are accomplished in one pass under the irradiation of a CW green laser light (λ = 532 nm) at moderate intensities (I ≥ 95 mW/mm²). By tailoring the surface tension and viscosity of the ink, high-definition conductive tracks are formed in weft-knitted polyester-Spandex composite fabrics, well-following the laser's profile with negligible coffee stain effect. Length resistance as low as 4 Ω/cm is measured and anisotropy of the gauge factor as high as 25 is achieved. The metallized fabric exhibits reversible and hysteresis-free electromechanical responses subject to high strains. Durability assessment qualifies that the as-metallized strain sensors are able to sustain their performance for over 5000 stretch/release cycles, demonstrating its potential applications in biaxial strain sensing and interactive smart textiles.

Reactive silver inks for antiviral, repellent medical textiles with ultrasonic bleach washing durability compared to silver nanoparticles

Medical textiles are subject to particularly harsh disinfection procedures in healthcare settings where exposure risks are high. This work demonstrates a fabric treatment consisting of a reactive silver ink and low surface energy PDMS polymer that provides for superhydrophobicity and antiviral properties against enveloped herpes simplex virus stocks even after extended ultrasonic bleach washing. The antiviral properties of reactive silver ink has not been previously reported or compared with silver nanoparticles. The fabric treatment exhibits high static contact angles and low contact angle hysteresis with water, even after 300 minutes of ultrasonic bleach washing. Similarly, after this bleach washing treatment, the fabric treatment shows reductions of infectious virus quantities by about 2 logs compared to controls for enveloped viruses. The use of silver ink provides for better antiviral efficacy and durability compared to silver nanoparticles due to the use of reactive ionic silver, which demonstrates more conformal coverage of fabric microfibers and better adhesion. This study provides insights for improving the wash durability of antiviral silver fabric treatments and demonstrates a bleach wash durable, repellent antiviral treatment for reusable, functional personal protective equipment applications.

Sweat analysis with a wearable sensing platform based on laser-induced graphene

The scientific community has shown increasing interest in laser scribing for the direct fabrication of conductive graphene-based tracks on different substrates. This can enable novel routes for the noninvasive analysis of biofluids (such as sweat or other noninvasive matrices), whose results can provide the rapid evaluation of a person's health status. Here, we present a wearable sensing platform based on laser induced graphene (LIG) porous electrodes scribed on a flexible polyimide sheet, which samples sweat through a paper sampler. The device is fully laser manufactured and features a two layer design with LIG-based vertical interconnect accesses. A detailed characterization of the LIG electrodes including pore size, surface groups, surface area in comparison to electroactive surface area, and the reduction behavior of different LIG types was performed. The bare LIG electrodes can detect the electrochemical oxidation of both uric acid and tyrosine. Further modification of the surface of the LIG working electrode with an indoaniline derivative [4-((4-aminophenyl)imino)-2,6-dimethoxycyclohexa-2,5-dien-1-one] enables the voltammetric measurement of pH with an almost ideal sensitivity and without interference from other analytes. Finally, electrochemical impedance spectroscopy was used to measure the concentrations of ions through the analysis of the sweat impedance. The device was successfully tested in a real case scenario, worn on the skin during a sports session. In vitro tests proved the non-cytotoxic effect of the device on the A549 cell line.

Room-Temperature Annealing-Free Gold Printing via Anion-Assisted Photochemical Deposition

Metal patterning via additive manufacturing has been phasing-in to broad applications in many medical, electronics, aerospace, and automotive industries. While previous efforts have produced various promising metal-patterning strategies, their complexity and high cost have limited their practical application in rapid production and prototyping. Herein, a one-step gold printing technique based on anion-assisted photochemical deposition (APD), which can directly print highly conductive gold patterns (1.08 × 10⁷ S m^-1 ) under ambient conditions without post-annealing treatment, is introduced. Uniquely, the APD uses specific ion effects with projection lithography to pattern Au nanoparticles and simultaneously sinter them into tunable porous gold structures. The significant influence of kosmotropic or chaotropic anions in the precursor ink on tuning the morphologies and conductivities of the printed patterns by employing a series of different ions, including Cl^- ions, in the printing process is presented. Additionally, the resistance stabilities and the electrochemical properties of the APD-printed gold patterns are carefully investigated. The high conductivity and excellent conformability of the printed Au electrodes are demonstrated with reliable performance in electrophysiological signal delivery and acquisition for biomedical applications. This work exploits the potential of photochemical-deposition-based metal patterning in flexible electronic manufacturing.

Facile Synthesis of Ag/Carbon Quantum Dots/Graphene Composites for Highly Conductive Water-Based Inks

The development of graphene conductive inks with a high conductivity and dispersion stability in water poses considerable challenges. Herein, a highly conductive Ag/carbon quantum dots (CQDs)/graphene (G) composite with good dispersity and stability in water was prepared for the first time through the in situ photoreduction of AgNO3 and deposition of Ag onto graphene nanosheets obtained via CQD-assisted liquid-phase exfoliation. Ag nanoparticles with an average size of ∼1.88 nm were uniformly dispersed on graphene nanosheets. The Ag/CQDs/G composite exhibited good dispersity and stability in water for 30 days. The formation mechanism of the Ag/CQDs/G composites was also discussed. CQDs played a vital role in coordinating with Ag+ and reducing it under visible light conditions. The addition of only 1.58 wt % of Ag NPs to the CQDs/G film resulted in a significant decrease in the electrical resistivity by approximately 89.5%, reaching a value of 0.054 Ω cm for a 40 μm thick Ag/CQDs/G film. A low resistivity of 2.15 × 10-3 Ω cm for the Ag/CQDs/G film was achieved after rolling compression with a compression ratio of 78%. The Ag/CQDs/G film exhibited good conductivity and durability when bent, rolled, or twisted. Moreover, the resistivity of the film displayed a slight deviation after 5000 bending cycles, indicating its outstanding stability. This study provides an efficient strategy for preparing graphene-based conductive composites with good dispersibility and stability in water as well as novel high-performance conductive inks for application in flexible printed electronics.

Laser-Assisted Nanotexturing and Silver Immobilization on Titanium Implant Surfaces to Enhance Bone Cell Mineralization and Antimicrobial Properties

Despite the great advancement and wide use of titanium (Ti) and Ti-based alloys in different orthopedic implants, device-related infections remain the major complication in modern orthopedic and trauma surgery. Most of these infections are often caused by both poor antibacterial and osteoinductive properties of the implant surface. Here, we have demonstrated a facile two-step laser nanotexturing and immobilization of silver onto the titanium implants to improve both cellular integration and antibacterial properties of Ti surfaces. The required threshold laser processing power for effective nanotexturing and osseointegration was systematically determined by the level of osteoblast cells mineralized on the laser nanotextured Ti (LN-Ti) surfaces using a neodymium-doped yttrium aluminum garnet laser (Nd:YAG, wavelength of 1.06 μm). Laser processing powers above 24 W resulted in the formation of hierarchical nanoporous structures (average pore 190 nm) on the Ti surface with a 2.5-fold increase in osseointegration as compared to the pristine Ti surface. Immobilization of silver nanoparticles onto the LN-Ti surface was conducted by dip coating in an aqueous silver ionic solution and subsequently converted to silver nanoparticles (AgNPs) by using a low power laser-assisted photocatalytic reduction process. Structural and surface morphology analysis via XRD and SEM revealed a uniform distribution of Ag and the formation of an AgTi-alloy interface on the Ti surface. The antibacterial efficacy of the LN-Ti with laser immobilized silver (LN-Ti/LI-Ag) was tested against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. The LN-Ti/LI-Ag surface was observed to have efficient and stable antimicrobial properties for over 6 days. In addition, it was found that the LN-Ti/LI-Ag maintained a cytocompatibility and bone cell mineralization property similar to the LN-Ti surface. The differential toxicity of the LN-Ti/LI-Ag between bacterial and cellular species qualifies this approach as a promising candidate for novel rapid surface modification of biomedical metal implants.

Highly Conductive Networks of Silver Nanosheets

Although printed networks of semiconducting nanosheets have found success in a range of applications, conductive nanosheet networks are limited by low conductivities (<10⁶ S m^-1 ). Here, dispersions of silver nanosheets (AgNS) that can be printed into highly conductive networks are described. Using a commercial thermal inkjet printer, AgNS patterns with unannealed conductivities of up to (6.0 ± 1.1) × 10⁶  S m^-1 are printed. These networks can form electromagnetic interference shields with record shielding effectiveness of >60 dB in the microwave region at thicknesses <200 nm. High resolution patterns with line widths down to 10 µm are also printed using an aerosol-jet printer which, when annealed at 200 °C, display conductivity >10⁷  S m^-1 . Unlike conventional Ag-nanoparticle inks, the 2D geometry of AgNS yields smooth, short-free interfaces between electrode and active layer when used as the top electrode in vertical nanosheet heterostructures. This shows that all-printed vertical heterostructures of AgNS/WS₂ /AgNS, where the top electrode is a mesh grid, function as photodetectors demonstrating that such structures can be used in optoelectronic applications that usually require transparent conductors.

Deposition of metallic silver from versatile amidinate precursors for use in functional materials

Silver (Ag) amidinate metal organic decomposition precursors of the type: [Ag2((ArN)2C(H))2] (Ar = 2,6-dimethylphenyl (1), 2,6-diethylphenyl (2) and 2,6-diisopropylphenyl (3)) have been used for the first time in the deposition of Ag films on glass with multiple functionalities with potential application in optical/biological sensors or for use in electronic circuitry. Precursors 1–3 were isolated from the reaction of silver acetate with the appropriate ligand in a 1:2 stoichiometry and were characterized by 1H and 13C{1H} NMR, thermal gravimetric analysis and single crystal X-ray diffraction for 2. Single-layer depositions at 200 °C on glass substrates via spin coating produced transparent (>90% transmittance) coatings, with well-defined Ag nanoparticles. Multi-layer depositions at 200 °C on glass had a metallic lustre and were found to be conductive ( ρ = 0.916–1.83 × 10−6 Ωm). All films were strongly adhered and displayed excellent coverage of the substrate. Ag films deposited from 1 to 3 were analysed by grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray analysis and scanning electron microscopy, with optical properties determined by UV-Vis spectroscopy.

Metallized Skeleton of Polymer Foam Based on Metal-Organic Decomposition for High-Performance EMI Shielding

Highly conductive polymer foam with light weight, flexibility, and high-performance electromagnetic interference (EMI) shielding is highly desired in the fields of aerospace, communication, and high-power electronic equipment, especially in the board-level packaging. However, traditional technology for preparing conductive polymer foam such as electroless plating and electroplating involves serious pollution, a complex fabrication process, and high cost. It is urgent to develop a facile method for the fabrication of highly conductive polymer foam. Herein, we demonstrated a lightweight and flexible silver-wrapped melamine foam (Ag@ME) via in situ sintering of metal-organic decomposition (MOD) at a low temperature (200 °C) on the ME skeleton modified with poly(ethylene imine). The Ag@ME with a continuous 3D conductive network exhibits good compressibility, an excellent conductivity of 158.4 S/m, and a remarkable EMI shielding effectiveness of 63 dB in the broad frequency of 8.2-40 GHz covering X-, Ku-, K-, and Ka-bands, while the volume content is only 2.03 vol %. The attenuation mechanism of Ag@ME for EM waves is systematically investigated by both EM simulation and experimental analysis. Moreover, the practical EMI shielding application of Ag@ME in board-level packaging is demonstrated and it shows outstanding near-field shielding performance. This novel strategy for fabrication of highly conductive polymer foam with low cost and non-pollution could potentially promote the practical applications of Ag@ME in the field of EMI shielding.

Enhanced Thermal Conducting Behavior of Pressurized Graphene-Silver Flake Composites

Modern electronics continue to shrink down the sizes while becoming more and more powerful. To improve heat dissipation of electronics, fillers used in the semiconductor packaging process need to possess both high electrical and thermal conductivity. Graphene is known to improve thermal conductivity but suffers from van der Waals interactions and thus poor processibility. In this study, we wrapped silver microflakes with graphene sheets, which can enable intercoupling of phonon- and electron-based thermal transport, to improve the thermal conductivity. Using just 1.55 wt % graphene for wrapping can achieve a 2.64-times greater thermal diffusivity (equivalent to 254.196 ± 10.123 W/m·K) over pristine silver flakes. Graphene-wrapped silver flakes minimize the increase of electrical resistivity, which is one-order higher (1.4 × 10^-3 Ω·cm) than the pristine flakes (5.7 × 10^-4 Ω·cm). Trace contents of wrapped graphene (<1.55 wt %) were found to be enough to bridge the void between Ag flakes, and this enhances the thermal conductivity. Graphene loading at 3.76 wt % (beyond the threshold of 1.55 wt %) results in the significant graphene aggregation that decreases thermal diffusivity to as low as 16% of the pristine Ag filler. This work recognizes that suitable amounts of graphene wrapping can enhance heat dissipation, but too much graphene results in unwanted aggregation that hinders thermal conducting performance.

MODs vs. NPs: Vying for the Future of Printed Electronics

This Minireview compares two distinct ink types, namely metal-organic decomposition (MOD) and nanoparticle (NP) formulations, for use in the printing of some of the most conductive elements: silver, copper and aluminium. Printing of highly conductive features has found purpose across a broad array of electronics and as processing times and temperatures reduce, the avenues of application expand to low-cost flexible substrates, materials for wearable devices and beyond. Printing techniques such as screen, aerosol jet and inkjet printing are scalable, solution-based processes that historically have employed NP formulations to achieve low resistivity coatings printed at high resolution. Since the turn of the century, the rise in MOD inks has vastly extended the range of potentially applicable compounds that can be printed, whilst simultaneously addressing shelf life and sintering issues. A brief introduction to the field and requirements of an ink will be presented followed by a detailed discussion of a wide array of synthetic routes to both MOD and NP inks. Unindustrialized materials will be discussed, with the challenges and outlook considered for the market leaders: silver and copper, in comparison with the emerging field of aluminium inks.

Microstructures in All-Inkjet-Printed Textile Capacitors with Bilayer Interfaces of Polymer Dielectrics and Metal-Organic Decomposition Silver Electrodes

Soft printed electronics exhibit unique structures and flexibilities suited for a plethora of wearable applications. However, forming scalable, reliable multilayered electronic devices with heterogeneous material interfaces on soft substrates, especially on porous and anisotropic structures, is highly challenging. In this study, we demonstrate an all-inkjet-printed textile capacitor using a multilayered structure of bilayer polymer dielectrics and particle-free metal-organic decomposition (MOD) silver electrodes. Understanding the inherent porous/anisotropic microstructure of textiles and their surface energy relationship was an important process step for successful planarization. The MOD silver ink formed a foundational conductive layer through the uniform encapsulation of individual fibers without blocking fiber interstices. Urethane-acrylate and poly(4-vinylphenol)-based bilayers were able to form a planarized dielectric layer on polyethylene terephthalate textiles. A unique chemical interaction at the interfaces of bilayer dielectrics performed a significant role in insulating porous textile substrates resulting in high chemical and mechanical durability. In this work, we demonstrate how textiles' unique microstructures and bilayer dielectric layer designs benefit reliability and scalability in the inkjet process as well as the use in wearable electronics with electromechanical performance.

3D Particle Free Printing of Biocompatible Conductive Hydrogel Platforms for Neuron Growth and Electrophysiological Recording

Electrically conductive 3D periodic microscaffolds are fabricated using a particle-free direct ink writing approach for use as neuronal growth and electrophysiological recording platforms. A poly (2-hydroxyethyl methacrylate) (pHEMA)/pyrrole ink, followed by chemical in situ polymerization of pyrrole, enables hydrogel printing through nozzles as small as 1 μm. These conductive hydrogels can pattern complex 2D and 3D structures and have good biocompatibility with test cell cultures (~94.5% viability after 7 days). Hydrogel arrays promote extensive neurite outgrowth of cultured Aplysia californica pedal ganglion neurons. This platform allows extracellular electrophysiological recording of steady-state and stimulated electrical neuronal activities. In summation, this 3D conductive ink printing process enables preparation of biocompatible and micron-sized structures to create customized in vitro electrophysiological recording platforms.

Effect of nanofiller morphology on the electrical conductivity of polymer nanocomposites

Conductive polymers and nanocomposites have attracted great attention in industry and academia for their tremendous potential applications. Most of the research was focused on the type and amount of nano-additives used and fewer on their morphology which is critical in forming the conductive network. In this paper, a detailed investigation of the effect nanomaterial’s morphology was carried out to study their electrical conductivity properties. Silver nanowire (AgNW) nanocomposite and silver nanoparticle (AgNP) nanocomposite were fabricated. The morphology, crystallinity, and orientation of various silver nanofillers were characterized. AgNW based nanocomposites have shown a lower percolation threshold. A conductive unit based model was established to explain the evolution of the conductive network and aggregation. The aggregation geometry of nanofiller appeared as a dominant factor in altering the percolation behavior.

Inflight fiber printing toward array and 3D optoelectronic and sensing architectures

Scalability and device integration have been prevailing issues limiting our ability in harnessing the potential of small-diameter conducting fibers. We report inflight fiber printing (iFP), a one-step process that integrates conducting fiber production and fiber-to-circuit connection. Inorganic (silver) or organic {PEDOT:PSS [poly(3,4-ethylenedioxythiophene) polystyrene sulfonate]} fibers with 1- to 3-μm diameters are fabricated, with the fiber arrays exhibiting more than 95% transmittance (350 to 750 nm). The high surface area-to-volume ratio, permissiveness, and transparency of the fiber arrays were exploited to construct sensing and optoelectronic architectures. We show the PEDOT:PSS fibers as a cell-interfaced impedimetric sensor, a three-dimensional (3D) moisture flow sensor, and noncontact, wearable/portable respiratory sensors. The capability to design suspended fibers, networks of homo cross-junctions and hetero cross-junctions, and coupling iFP fibers with 3D-printed parts paves the way to additive manufacturing of fiber-based 3D devices with multilatitude functions and superior spatiotemporal resolution, beyond conventional film-based device architectures.

Low-temperature sintering of silver patterns on polyimide substrate printed with particle-free ink

In this study, a transparent particle-free reactive silver ink was used to fabricate conductive patterns on a flexible substrate. Thermal annealing and plasma irradiation at low temperature were utilized to improve the conductivity of the as-printed patterns. The effects of sintering process parameters on the microstructure and resistivity of the patterns were investigated. Under the optimized processing conditions, the resistivity of the pattern reached 1.2 × 10 ^-7 Ω · m by thermal sintering, while it was 8 × 10 ^-8 Ω · m after plasma sintering. Combined with these two sintering techniques, the resistivity was reduced to 6 × 10^-8 Ω · m, close to that of bulk silver. This work provides an alternative solution for the fabrication of highly conductive feature patterns on common flexible substrates.

Approaching the Practical Conductivity Limits of Aerosol Jet Printed Silver

Previous efforts to directly write conductive metals have been narrowly focused on nanoparticle ink suspensions that require aggressive sintering (>200 °C) and result in low-density, small-grained agglomerates with electrical conductivities <25% of bulk metal. Here, we demonstrate aerosol jet printing of a reactive ink solution and characterize high-density (93%) printed silver traces having near-bulk conductivity and grain sizes greater than the electron mean free path, while only requiring a low-temperature (80 °C) treatment. We have developed a predictive electronic transport model which correlates the microstructure to the measured conductivity and identifies a strategy to approach the practical conductivity limit for printed metals. Our analysis of how grain boundaries and tortuosity contribute to electrical resistivity provides insight into the basic materials science that governs how an ink formulator or process developer might approach improving the conductivity. Transmission line measurements validate that electrical properties are preserved up to 20 GHz, which demonstrates the utility of this technique for printed RF components. This work reveals a new method of producing robust printed electronics that retain the advantages of rapid prototyping and three-dimensional fabrication while achieving the performance necessary for success within the aerospace and communications industries.

Micro-Computed Tomography to Assess the Processing Quality of the Additive Manufacturing of Spiral Microcoils

Additive manufactured polymers and metallized microstructures are widely used in the production of electronic components. However, such three-dimensional printed metallized electrical components inevitably have processing errors that affect their performance. It is vital to understand defects' effects on the performance of the final product. In this study, we simulated micro-computed tomography (CT) data. A spiral cavity is printed by stereolithography and spiral inductors with different numbers of turns are fabricated by injecting Gallium Indium liquid metal (EGaIn). Through the theoretical simulation of the spiral inductor and the characterization of the electrical performance, we found that the relative error in the simulation of 2.5-turn, 4.5-turn, and 6.5-turn spiral inductors is +30.6%, +13%, and +6%, respectively, compared with the experimental data. The CT data are obtained by a CT scanning microcoil and a reconstruction model with real structural features is established based on the data. The results show that the relative error between the reconstructed model with real defects and the experimental data is +10.4%, -3.7%, and -1.5%, respectively, which is closer to the experimental data. According to the CT data simulation that provides a more accurate theoretical prediction, the actual effect of a defect on the final product can be assessed. The difference between the experimental results and the theoretical simulation can be inferred from the reconstruction model.

A Hybrid Process for Printing Pure and High Conductivity Nanocrystalline Copper and Nickel on Flexible Polymeric Substrates

Printing functional devices on flexible substrates requires printing of high conductivity metallic patterns. To prevent deformation and damage of the polymeric substrate, the processing (printing) and post-processing (annealing) temperature of the metal patterns must be lower than the glass transition temperature of the substrate. Here, a hybrid process including deposition of a sacrificial blanket thin film, followed by room environment nozzle-based electrodeposition, and subsequent etching of the blanket film is demonstrated to print pure and nanocrystalline metallic (Ni and Cu) patterns on flexible substrates (PI and PET). Microscopy and spectroscopy showed that the printed metal is nanocrystalline, solid with no porosity and with low impurities. Electrical resistivity close to the bulk (~2-time) was obtained without any thermal annealing. Mechanical characterization confirmed excellent cyclic strength of the deposited metal, with limited degradation under high cyclic flexure. Several devices including radio frequency identification (RFID) tag, heater, strain gauge, and temperature sensor are demonstrated.

Reactive Conductive Ink Capable of In Situ and Rapid Synthesis of Conductive Patterns Suitable for Inkjet Printing

We report a fabrication method of the conductive pattern based on in situ reactive silver precursor inks by inkjet printing. The reactive silver precursor inks were prepared with ethylene glycol and deionized water mixture as the solvent, and silver nitrate as silver source. Sodium borohydride solution as the reducing agent was first coated on photographic paper by screen printing process, and then dried at 50 °C for 4 h. Furthermore, the reactive silver precursor inks were printed on a photographic paper coated with sodium borohydride using inkjet printing to form silver nanoparticles in situ due to redox reaction, and thus a conductive pattern was obtained. The effects of the reactive silver precursor ink concentration and printing layer number and treatment temperature on the electrical properties and microstructures of the printed patterns were investigated systematically. The size range of in situ-formed silver nanoparticles was 50–90 nm. When the reactive silver precursor ink concentration was 0.13 g/mL, the five-layer printed pattern exhibited a sheet resistance of 4.6 Ω/γ after drying at room temperature for 2 h; furthermore, the sheet resistance of the printed pattern decreased to 1.4 Ω/γ after drying at 130 °C for 2 h. In addition, the display function circuit was printed on the photographic paper to realize the display of the numbers 0–99. It provides new research ideas for the development of environmentally friendly and low-cost flexible paper-based circuits.

Room-temperature, atmospheric-pressure microsputtering of dense, electrically conductive, sub-100 nm gold films

In this study, we investigate the effects of deposition parameters (i.e. gas flow rates, bias voltages, and nozzle-to-substrate separation) on the microstructure of <100 nm thick gold deposits made with a room-temperature, atmospheric-pressure, ion-drag microsputterer. Without resorting to the use of vacuum or substrate heating, optimization of the printing process yields dense, continuous deposits (96.5% coverage) with low electrical resistivity (45 μΩ cm). Using statistical analysis, we developed a simple model that provides insight into the dynamics of such a printing method; based on this model, we identify electrostatic effects as the most important factor that influences the deposition process.

Microstructure and Resistivity Analysis of Silver Nanoparticle-Based Crystalline Conductive Films Synthesized using PEG Surfactant

Silver nanoparticle-based crystalline conductive films were synthesized using a simple and environmentally friendly method centered on chemical reduction. A stoichiometric balance of three different molecular weights of polyethylene glycol (PEG) was used as a capping agent. Resistivity, and its correlation with temperature and the particle size of nanoparticle films, was probed. The silver nanoparticles were characterized using thermogravimetric analysis (TGA) and field emission scanning electron microscopy (FESEM). Further silver films deposited on a glass substrate were characterized by FESEM, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and resistivity measurements. Particle size distribution and room temperature electrical conductivity were also investigated. The high conductivity of sintered films suggested applications for the ink-jet printing of electronic circuitry on thermally sensitive substrates.

Magnetic Field Patterning of Nickel Nanowire Film Realized by Printed Precursor Inks

This paper demonstrates an easily prepared novel material and approach to producing aligned nickel (Ni) nanowires having unique and customizable structures on a variety of substrates for electronic and magnetic applications. This is a new approach to producing printed metallic Ni structures from precursor materials, and it provides a novel technique for nanowire formation during reduction. This homogeneous solution can be printed in ambient conditions, and it forms aligned elemental Ni nanowires over large areas upon heating in the presence of a magnetic field. The use of templates or subsequent purification are not required. This technique is very flexible, and allows the preparation of unique patterns of nanowires which provides opportunities to produce structures with enhanced anisotropic electrical and magnetic properties. An example of this is the unique fabrication of aligned nanowire grids by overlaying layers of nanowires oriented at different angles with respect to each other. The resistivity of printed and cured films was found to be as low as 560 µΩ∙cm. The saturation magnetization was measured to be 30 emu∙g⁻¹, which is comparable to bulk Ni. Magnetic anisotropy was induced with an axis along the direction of the applied magnetic field, giving soft magnetic properties.

Inkjet Printing of Reactive Silver Ink on Textiles

Inkjet printing of functional inks on textiles to embed passive electronics devices and sensors is a novel approach in the space of wearable electronic textiles. However, achieving functionality such as conductivity by inkjet printing on textiles is challenged by the porosity and surface roughness of textiles. Nanoparticle-based conductive inks frequently cause blockage/clogging of inkjet printer nozzles, making it a less than ideal method for applying these functional materials. It is also very challenging to create a conformal conductive coating and achieve electrically conductive percolation with the inkjet printing of metal nanoparticle inks on rough and porous textile and paper substrates. Herein, a novel reliable and conformal inkjet printing process is demonstrated for printing particle-free reactive silver ink on uncoated polyester textile knit, woven, and nonwoven fabrics. The particle-free functional ink can conformally coat individual fibers to create a conductive network within the textile structure without changing the feel, texture, durability, and mechanical behavior of the textile. It was found that the conductivity and the resolution of the inkjet-printed tracks are directly related with the packing and the tightness of fabric structures and fiber sizes of the fabrics. It is noteworthy that the electrical conductivity of the inkjet-printed conductive coating on pristine polyethylene terephthalate fibers is improved by an order of magnitude by in situ heat-curing of the textile surface during printing as the in situ heat-curing process minimizes the wicking of the ink into the textile structures. A minimum sheet resistance of 0.2 ± 0.025 and 0.9 ± 0.02 Ω/□ on polyester woven and polyester knit fabrics is achieved, respectively. These findings aim to advance E-textile product design through integration of inkjet printing as a low-cost, scalable, and automated manufacturing process.