Printable graphene inks with polypropylene carbonate for low-surface-tension solvents and mild-temperature post-processing

Internet of Things (IoT) has become one of the trending technologies in recent years. With the wide applications and rapid development of wireless communication and IoT, there are growing number of requirements on novel device design, such as flexible antennas and tunable antennas. The demand of transparent and flexible wireless communicational devices is ever-increasing for a wide range of scenarios, such as noninvasive healthcare, real-time wearable electronics, etc. Besides, a highly integrated antenna with tunable bandwidth and low energy consumption is in pressing demands in order to reduce the number of antennas and achieve better signal strength and higher speed signal transmission. Graphene materials have drawn much attention due to its extraordinary optical properties, high electrical and thermal conductivity, and excellent mechanical properties. All of these properties underline that graphene can be considered as a highly competitive material for wearable communication devices. Moreover, the electrical resistance of graphene and graphene oxide could be simply changed by external electric fields, applied strain and environment, which alters the electrical characteristics of the graphene-based microwave devices. Accordingly, graphene materials have a great potential in various reconfigurable or tunable devices. Our group fabricated the first graphene-based flexible and transparent wireless antenna with 2D large single-crystalline graphene and 1D AgNW hybrid material based on the features of graphene, such as high carrier mobility, transparency and flexibility, etc. The antenna exhibited a 75% light transmittance and an ultrawide bandwidth ranging from 5.6 to 12.8 GHz, combining with outstanding durability and stability with an almost negligible variation of reflection coefficients over 1000 bending cycles. Further research confirmed that the existence of graphene harmonized the compatibility between antenna and integrated chips, hence improved the total performance of the radio frequency identification (RFID) tags. Graphene/AgNW-based RFID tag was fabricated with an operating UHF band of 850–960 MHz and a reading distance of 130 cm, showing a great commercial potential. The excellent performance and unique properties of graphene/AgNW-based antenna and RFID tag came from the synergistic effect between 1D and 2D nanomaterials, providing a new vision for the design of next generation wireless electronics. Meanwhile, a slot antenna with tunable bandwidth was proposed. The antenna consisted of a planar slot antenna with graphene ink printed at the gap between the ground and a small patch. The graphene ink was made from the mixture of graphene powder and graphene oxide aqueous dispersion. Graphene oxide sheets functioned as effective surfactants rendering assistance to form uniform printable inks, displayed alterable electrical resistance by optimization of mixing proportions between graphene and graphene oxide. Dried graphene inks displayed tunable electrical resistance under applied DC voltages. The variation in the resistance of graphene ink affected radiation of the slot antenna, resulting in a shift of operating bandwidth. The proposed antenna achieved a shifting bandwidth of 0.54 from 6 to 5.46 GHz after applying DC voltage less than 15 V. Such cost-effective, easily acquired, and printable graphene inks integrated the dual advantages of graphene and graphene oxide. It could be further applied in many other fields of printable electronic devices.

Electronic printing is an interdisciplinary technology of traditional printing technology and microelectronics manu- facturing. The development of electronic printing has mostly benefited from the progress of nanomaterials research. Graphene, a novel two-dimensional carbon nanomaterial, has shown excellent electrical, thermal, optical properties in flexible electrical devices. Graphene with traditional metals or polymer materials together could act as the main conductive components in conductive ink. Printing of graphene ink represents a cost-effective deposition technique to obtain patterned conductive graphene films, and further assemble them into functional electrical devices. Therefore, electronic printing based on graphene conductive ink is one of the recent research hotspots. In this paper, we review recent developments and advances in research of graphene conductive ink. This review begins with an introduction of different preparation strategies for graphene conductive inks. For printing, the three major pathways for producing graphene sheets are oxidation-reduction, solvent exfoliation and electrochemical expansion of graphite. Different preparation strategies for conductive inks are classified into three major methods: Graphene inks stabilized by surfactants or functional groups, and graphene-based composite conductive ink. The detailed review of graphene conductive ink preparation is discussed with specific examples. Preparation of graphene conductive inks of high concentrations, stability and printing adaptability is the key issue in electronic printing. Subsequently, an introduction of common printing methods and principles is given. Five printing methods are discussed in this review, including inkjet printing, gravure printing, transfer printing, direct writing and three- dimensional (3D) printing. Printing is kind of additive manufacturing, by depositing graphene onto substrates of various materials, sizes, flexibility and roughness for conductive pattern. Different printing techniques have unique requirements of ink rheological properties. The inkjet printing is becoming the most common technique employed in both academic research and industrial application. The realization of rapid, accurate, simple and controllable printing has important influence on the application of graphene conductive ink. Finally, applications of printed graphene conductive ink in flexible functional devices, including basic electrical circuits, energy storage devices and mechanical/chemical sensing devices, are envisioned. Basic electrical circuits, like flexible conductive patterns, field-effect transistors and radio-frequency circuits, play an important role in the fabrication of wearable devices. Printing also offers a cheap, scalable method of fabricating energy storage devices, including supercapacitor and lithium battery. The unique structure of graphene makes possible the fabrication of different kinds of sensors, including strain, temperature, chemical, electrochemical, photo-electricity sensors and biosensors. An outlook of potential future trends in printing graphene conductive ink research and technology is followed. In summary, printing graphene conductive ink has made many significant advances in a wide range of applications. However, the industrial-level application is still limited, and the preparation and application of graphene conductive ink still need further study. A number of key issues should be solved, including stability of graphene ink, electronic conductivity of printed circuit, limited printing resolution, etc. Overall, electronic printing technology based on graphene conductive ink is not meant as a replacement for microelectronic manufacturing engineering, but instead provides an opportunity to produce large-area flexible electronic devices at low cost. Electronic printing of graphene conductive ink will result in a diverse range of novel applications in many fields, and it calls for more research in the future.

With the development of wearable electronics, the use of engineered functional inks with printing technologies has attracted attention owing to its potential for applications in low‐cost, high‐throughput, and high‐performance devices. However, the improvement in conductivity and stretchability in the mass production of inks is still a challenge for practical use in wearable applications. Herein, a scalable and efficient fluid dynamics process that produces highly stretchable, conductive, and printable inks containing a high concentration of graphene is reported. The resulting inks, in which the uniform incorporation of exfoliated graphene flakes into a viscoelastic thermoplastic polyurethane is employed, facilitated the screen‐printing process, resulting in high conductivity and excellent electromechanical stability. The electrochemical analysis of a stretchable sodium ion sensor based on a serpentine‐structured pattern results in excellent electrochemical sensing performance even under strong fatigue tests performed by repeated stretching (300% strain) and release cycles. To demonstrate the practical use of the proposed stretchable conductor, on‐body tests are carried out in real‐time to monitor the sweat produced by a volunteer during simultaneous physical stretching and stationary cycling. These functional graphene inks have attractive performance and offer exciting potential for a wide range of flexible and wearable electronic applications.

Graphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications

A series of inkjet printing processes have been studied using graphene-based inks. Under optimized conditions, using water-soluble single-layered graphene oxide (GO) and few-layered graphene oxide (FGO), various high image quality patterns could be printed on diverse flexible substrates, including paper, poly(ethylene terephthalate) (PET) and polyimide (PI), with a simple and low-cost inkjet printing technique. The graphene-based patterns printed on plastic substrates demonstrated a high electrical conductivity after thermal reduction, and more importantly, they retained the same conductivity over severe bending cycles. Accordingly, flexible electric circuits and a hydrogen peroxide chemical sensor were fabricated and showed excellent performances, demonstrating the applications of this simple and practical inkjet printing technique using graphene inks. The results show that graphene materials-which can be easily produced on a large scale and possess outstanding electronic properties-have great potential for the convenient fabrication of flexible and low-cost graphene-based electronic devices, by using a simple inkjet printing technique. Open image in new window

Stable-jetting water-based graphene oxide (GO) and graphene (GR) inks without any surfactant or stabilizer are prepared from an unstable-jetting water-based starting solvent, with many thermal bubble inkjet satellite drops, by simply increasing the material concentration. The concentration-dependent thermal bubble inkjet droplet generation process is studied in detail. To overcome the low concentration properties of water-based thermal bubble inkjet inks, the substrate temperature is tuned below 60 °C to achieve high-quality print lines. Due to the difference in hydrophilicity and hydrophobicity of the 2D materials, the printed GO lines show a different forming mechanism from that of the GR lines. The printed GO lines are reduced by thermal annealing and by ascorbic acid, respectively. The reduced GO lines exhibit electrical conductivity of the same order of magnitude as that of the GR lines.

Achieving high efficiency in graphene production and printing process simultaneously is challenging, but it needs to be addressed as it is critical for realizing the commercial viability of printed graphene devices. This study successfully substantiates these requirements by significantly improving the efficiency of graphene production and subsequently developing an inkjet-printable graphene ink that enables the rapid formation of the percolation network of graphene flakes. The integration of a flow coil reactor into an ultrasonic bath results in scalable and rapid graphene production, with graphene productivity up to three orders of magnitude higher than conventional liquid-phase exfoliation (LPE), offering the potential that ultrasonic LPE can benefit the scalability and simplicity of graphene production. In addition, the graphene ink, optimized by ink formulation, has a stable high graphene concentration of 3.5 g L–1, resulting in the formation of stable percolation networks of graphene flakes only after two printing passes under optimized printing conditions. The printed graphene patterns are also confirmed to be conformable to various substrates and durable against repeated stretching and bending stress. By ensuring high efficiency in graphene production and inkjet-printable ink preparation, this study would promote the commercialization of graphene production and the resulting printed graphene devices.

Solvent evaporation induced fabrication of porous polycaprolactone scaffold via low-temperature 3D printing for regeneration medicine researches

Colombia is one of the world's leading exporters of coal and coke, with significant reserves of high-quality coal. Most of the coal exported from Colombia is used for power generation, but there is also a class of coal that is suitable for making coke. Coke is a raw material required for making steel, and it is produced by heating coal to high temperatures in the absence of air. Colombia is the third-largest global exporter of coke, with a significant portion of its exports going to the steel industry in countries such as Brazil, Mexico, and the United States. The country's high-quality coal reserves and proximity to major markets make it an important player in the global coal and coke trade. On the other hand, graphene and its derivative Graphene Oxide (GO) have unique properties that make them suitable for a wide range of commercial applications. Graphene has exceptional mechanical strength and high electrical conductivity, which make it an attractive material for a variety of electronic and structural applications. For example, graphene-based materials are being developed for use in flexible electronic devices, sensors, and high-strength composites. GO, on the other hand, is highly resistive but still retains exceptional mechanical strength. This makes it useful in applications where electrical conductivity is not necessary but mechanical strength and durability are important. Graphene production using current techniques can be expensive and inefficient, which limits its widespread adoption for commercial applications. However, new production methods, such as Liquid Phase Exfoliation (LPE), are being developed to address these challenges. LPE is a method for producing graphene and graphene oxide that involves using a liquid solvent to break apart graphite into individual graphene sheets. This method is more efficient and cost-effective than traditional methods such as mechanical exfoliation and chemical vapor deposition. In recent years, there has been increasing interest in using high-rank coal from Colombia as a raw material for graphene production using LPE. This is because high-rank coal from Colombia is known to have a high carbon content and low impurity levels, which makes it an ideal raw material for graphene production. Researchers have successfully produced GO using the LPE method and high-rank coal from Boavita, Colombia. This has the potential to significantly increase the supply of graphene and graphene oxide, making it more accessible for commercial applications. Additionally, using coal as a raw material for graphene production has the potential to create a new market for coal, which could benefit the Colombianeconomy. In order to synthesize GO, it is important to establish a suitable protocol for the grinding procedure and particle size selection. (i. more than 0.15 mm, ii. 0.15 mm to 0.05 mm, and iii. less than 0.05 mm) were defined. To compare the yield, the mineral matter removal procedure was carried out with a basic leaching bath. Coal oxidation was performed using the modified Hummers process, and GO was then obtained using LPE. This method has the following advantages:•It is feasible to produce GO from high-rank coal with acceptable quality and particulate size smaller than 0.15 mm, yields that are close to 5%, and flakes with fewer than 15 layers.•This approach also could eliminate dependence on graphite as the carbon feedstock for graphene production.•It is an alternative to manufacture GO from coal dust collected from mines.

Event Abstract Back to Event Engineered biological structures by 3D printing of dual hydrogel inks Hagit Gilon1*, Yinfeng He1, Felicity Rose2, Jing Yang2 and Ricky D. Wildman1 1 University of Nottingham, Additive Manufacturing & 3D Printing, United Kingdom 2 University of Nottingham, Centre for Biomolecular Sciences, United Kingdom Introduction: Tissue engineering aims to replicate the architecture and function of human tissue for both treatment of disease and toxicity evaluation. One promising method of achieving this is 3D bioprinting, which aims to selectively print or deposit the constituent parts of structure in such as way to reproduce the main functions. 3D bioprinting has been mainly preformed using one of three major additive manufacturing techniques: inkjet, micro extrusion and laser assisted[1],[2]. Micro valve based printing offers a compromise between the fast printing of inkjet and the good handling of water based inks of micro extrusion. It could therefore potentially prove to be important for certain bioprinting applications but it has not been thoroughly tested so far. Mimicking blood vessels architecture is a major goal in tissue engineering as they are essential to provide the cells with a supply of oxygen and nutrients, especially for complex tissue architectures. Another example are attempts to reproduce stem cell crypt morphology in the large intestine[3], due to the importance of the intestine for functions such as drug administration, nutrients transport and cell differentiation. In this work, we use a Picodot jetting valve to ink jet two hydrogel based inks, one providing structure and the other a fugitive ink that is removed to create vascular and crypts-like structures[4],[5]. Materials and Methods: Ink formulations: 10%w/v gelatine (Type A, 300 bloom) ink solution was prepared by dissolving gelatine powder in DI water and was kept at 370C during printing process. Gelatine methacrylate (GelMA) was synthesized following a previously reported procedure[6]. GelMA ink was prepared by dissolving 15%w/v GelMA and 0.3% w/v photoinitiator (Irgacure 2959) in Phosphate Buffer Saline. 3D Printing system: Structures of intestinal crypts and vascular structures were fabricated by a jetting valve system Picodot LV (Nordson EFD). The Picodot print head jetting is synchronized with a 3-axis high performance linear stage (Aerotech). The ink was jetted through a 50 μm nozzle by an applied air pressure of 30 psi and a rod movement driven by two piezoelectric actuators. Two gelatine-based inks were used. Firstly, gelatine was used as a fugitive ink which was removed in a final step to create the vascular spaces, and secondly, photopolymerizable GelMA was used for the structural part of the scaffold. For printing intestinal crypts structures, we printed an array of pillars using gelatine as a sacrificial template. For vascular structure, we printed a branched 3D network of gelatine. In both cases, GelMA was casted onto the printed template and the gelatine was washed by DI water at 370 C. The vascular structure was perfused with red dye to prove the structure was able to support perfusion. Results and Discussion: Crypts structures were obtained after eliminating the sacrificial ink. The obtained vascular structure was perfused with red dye to test flow feasibility. In addition, red dye diffusion throughout the GelMA scaffold was observed. Conclusion: We demonstrated the feasibility of a simple fabrication process for recreating intestinal crypts and vascular structures using Picodot LV jetting valve embedded in a linear stage using gelatine as a fugitive ink and GelMA as a scaffold. As a next step, we plan to use cell laden GelMA for casting and monitor the viability of the cells.

The selection of materials and technologies for green printed electronics design is a fundamental and time-consuming task. This paper represents a rigorous experimental overview in which different printing technologies, ink formulations, and paper-based substrates are examined and analyzed. Three printing techniques are investigated: inkjet printing, screen printing, and direct ink writing. Regarding the inks, formulations based on carbon and silver have been chosen as conductive materials. Initially, the electrical properties of the selected inks have been characterized on a conventional substrate in printed electronics such as polyethylene terephthalate. Later, the printing conditions are optimized for various paper-based substrates, including commercial papers and substrates based on cellulose nanofibers (CNF). CNF are also used as a coating for commercial papers and their influence on the printing quality is evaluated. The substrates are also characterized in terms of morphology, wettability, and thermal stability. This study facilitates the benchmarking tasks for researchers developing new devices and contributes toward the eco-design of flexible green printed electronics.

Aqueous graphene inks were obtained by liquid-phase exfoliation of graphite and bonded to biodegradable polymers. Consequently, green and eco-friendly graphene inks are proposed that are free of VOCs and harmful organic compounds. Three graphene inks indicated as GGe, GTr, and GTw were prepared with corresponding optimized biopolymer concentrations of 0.1 mg.mL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> of gelatin, 1 mg.mL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> triton X-100, and 1.5 mg.mL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> tween-20. The flake diameter distribution for the graphene inks ranged from 100 nm to 800 nm as measured by dynamic light scattering (DLS). GTr is made of smaller flake diameters followed by GTw and GGe has the largest flakes. Graphene films were printed using aerosol jet technique with a 500 nm thickness and their sheet resistance (sR) was of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.6\ \mathrm{k}\Omega/\square$</tex> for GTr, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5\ \mathrm{k}\Omega/\square$</tex> for GTw, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$14\ \mathrm{k}\Omega/\square$</tex> for GGe. The low sheet resistance for GTr was assigned to better ink wettability and adhesion on top PET substrates. These electrical resistances are suitable for the fabrication of green chemiresistive flexible sensors.

Based on the advantages of Graphene(Gr), such as low cost, high conductivity, low oxidation resistance and high chemical stability, Gr ink is prepared: Gr dispersion was prepared in isopropyl alcohol and water by adding polyvinylpyrrolidone (PVP) and sodium dodecyl sulfonate (SDS).By studying the stability of Gr dispersions, the optimal dispersants and proportions were further screened, and different concentrations of Gr inks were prepared on the basis of the optimal dispersants and proportions. The particle size, viscosity, surface tension, density, pH, conductivity and other physical and chemical properties of Gr ink were tested. The experimental results showed that (1) The dispersion effect of PVP was better than that of SDS. With the increase of PVP ratio, the stability time of the dispersion solution first increased and then stabilized, and the optimal dispersion ratio was obtained as W Gr : W PVP = 2:1. (2) The concentration of Gr is positively correlated with the physical and chemical properties such as density, viscosity and particle size. With the increase of graphene concentration, the density, viscosity and particle size all increase. The surface tension is greatly affected by solvent properties. (3) After calculation and comparison of ink rheological properties, isopropyl alcohol (IPA) as solvent group, Gr 35 mg/mL and 50 mg/mL, meet the requirements of commercial printing; IPA-H 2 O was the solvent group. Except for the Gr concentration of 50 mg/mL, serious shear conversion behavior appeared. The Gr inks with other concentrations had good compatibility with the nozzle, which was suitable for inkjet 3D printing technology.

The rheology of printing inks. III. Studies of simple dispersions

Simple and scalable production of micro-supercapacitors (MSCs) is crucial to address the energy requirements of miniature electronics. Although significant advancements have been achieved in fabricating MSCs through solution-based printing techniques, the realization of high-performance MSCs remains a challenge. In this paper, graphene-based MSCs with a high power density were prepared through screen printing of aqueous conductive inks with appropriate rheological properties. High electrical conductivity (2.04 × 104 S∙m-1) and low equivalent series resistance (46.7 Ω) benefiting from the dense conductive network consisting of the mesoporous structure formed by graphene with carbon black dispersed as linkers, as well as the narrow finger width and interspace (200 µm) originating from the excellent printability, prompted the fully printed MSCs to deliver high capacitance (9.15 mF∙cm-2), energy density (1.30 µWh∙cm-2) and ultrahigh power density (89.9 mW∙cm-2). Notably, the resulting MSCs can effectively operate at scan rates up to 200 V∙s-1, which surpasses conventional supercapacitors by two orders of magnitude. In addition, the MSCs demonstrate excellent cycling stability (91.6% capacity retention and ~100% Coulombic efficiency after 10,000 cycles) and extraordinary mechanical properties (92.2% capacity retention after 5000 bending cycles), indicating their broad application prospects in flexible wearable/portable electronic systems.