Recent developments in graphene conductive ink: Preparation, printing technology and application

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.

Printed electronics (PE) is one of the most dynamic technologies in the world. It proposes low-cost electronic network production in flexible substrates by numerous printing techniques, (screen printing, gravure, offset, flexographic, and inkjet printing), used in various industries. In PE, ink pigments are replaced by metallic particles or precursors that transmit electrical conductivity to the printed patterns such as carbon, polymers and conductive pigments. Conductive inks play an important role in printed electronics, and despite the number of conductive ink types available on the market, there are still issues to be addressed. Some of these restrictions include the use of toxic chemical reagents and solvents and complicated manufacturing protocols, which often make the industrialization of conductive inks an even more distant goal. In particular, conductive inks based on silver nanoparticles, Graphene and PEDOT:PSS are widely studied thanks to their high electrical conductivity. On the other hand, there is still work to be done to show the interest of inks based on phthalocyanine pigments, in particular copper phthalocyanine. Nevertheless, problems related to stability, dispersion and annealing temperature often limit the application of these four types of fillers. In this review, we present general information on available conductive fillers used for the formulation of conductive inks, focusing on metallic particles, carbon fillers, pigments and polymers. The influence and technical requirements of the regularly used printing techniques, as well as the post-processing treatments to achieve the targeted performance in the obtained inks have been discussed. In addition, the surface characteristics of the various types of extensible and flexible substrates used in portable electronics are described. Moreover, some types of printed flexible electronic components as well as notable applications of electronic textiles in various sectors are exhibited. Next, the major challenges for the manufacturing of printed flexible electronics and recommendations for future research are discussed in this review

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

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

Conductive inks have been widely investigated in recent years due to their popularity in printed electronics (PE) and flexible electronics (FE).

MXene inks are promising alternatives for conventional conductive inks in printing electronics. However, the formulation of MXene inks is challenging due to the physicochemical properties of the few solvents in which MXenes can be dispersed. Furthermore, conventional MXene dispersions form high‐viscosity gels at low concentrations, making their ink formulation even more difficult. Here, a novel co‐solvent‐based approach is reported for dispersing MXenes in polar organic solvents that have excellent physicochemical properties as a carrier solvent for ink formulation but are not suitable for dispersing MXenes. Water is used as a dispersing agent and the second component of the co‐solvent system. The surfactant‐like role of water in dispersing MXenes in such solvents is also confirmed using molecular dynamics simulations. Using this strategy, the sol‐gel transition is significantly upshifted, enabling the formulation of highly concentrated inks with single‐ or few‐layered large‐flake MXenes. Numerous types of electronic components such as interconnects, resistors, transparent flexible conductive electrodes, and micro‐supercapacitors are fabricated using high‐throughput coating and printing techniques such as gravure printing, showcasing the immense potential of MXene inks for room‐temperature printing of electronics.

Carbon nanotubes (CNT) with prominent electrical and mechanical properties are ideal candidates for flexible wearable devices. However, their poor dispersity in solvents greatly limits their applications as a conductive ink in the fabrication of wearable sensors. Herein, we demonstrate a kind of CNT-based conductive dispersion with high dispersity and adhesiveness using cellulose derivatives as the solvent, in which γ-aminopropyl triethoxy silane as a cross-linking agent reacts with cellulose to form copolymer networks, and simultaneously it also acts as an initiator to induce the self-polymerization of dopamine. Based on the conductive CNT ink, we also demonstrated textile-based strain sensors by stencil printing and sponge-based pressure sensors by the dipping method. The textile-based strain sensors could respond to external stimuli promptly. Then, the strain sensors were encapsulated via polydimethylsiloxane with the expansion of working ranges from less than 20 to nearly 70%. The encapsulated textile sensors exhibited excellent sensing performance as wearable strain sensors to monitor human motions including smile, throat vibration, finger folding, wrist bending, and elbow twisting. The sponge sensors hold high sensitivity and excellent durability as well. The conductive CNT-based ink provides an alternative idea in the development of flexible wearable devices.

Inkjet printing (IJP) is a highly versatile and useful tool for product development, prototyping, and even small-scale manufacturing in the printed electronics industry. This chapter discusses a number of printed electronic (PE) applications. PE applications make use of inkjet-printed electrically conductive structures. At first sight, IJP might not seem the most obvious method for the manufacturing of these structures and as a result, it is rather new for PE manufacturing. The potential of IJP for PE applications has been demonstrated recently in a variety of academic and industrial research laboratories. These examples are, however, usually achieved on a sheet-to-sheet (S2 S) basis and only on a small scale. Various types of functional electronic materials can be processed into functional structures and devices by IJP. Examples include conductive metallic inks, organic semiconductors, insulators, and many others.

For dispersion stability, printable graphene inks commonly employ solvents with limited surface tensions or incorporate dispersant aids that require high-temperature post-processing, restricting printability and substrate compatibility. Here, printable graphene inks are introduced with low-surface-tension solvents and mild-temperature post-processing using polypropylene carbonate (PPC). Graphene is produced by liquid-phase exfoliation with PPC, and the exfoliated graphene/PPC is used to generate printable inks. As a dispersant aid, PPC improves graphene exfoliation, dispersion stability, and redispersability in solvents with low surface tensions (<30 mJ m–2), facilitating the formulation of desirable inks for efficient aerosol jet printing on diverse substrates. Moreover, the low decomposition temperature of PPC eases its thermal removal from printed graphene, allowing high electrical conductivity with a mild post-processing temperature of 220 °C. Consequently, the graphene inks enable the fabrication of fully-printed graphene micro-supercapacitors on heat-sensitive paper substrates, exhibiting high areal capacitances, cycling stability, and mechanical resilience against bending deformation.

This study proposes a graphene conductive ink printed textile‐based microstrip antenna capable of switching between S‐band (3.03 GHz, mode) and C‐band (5.17 GHz, mode and 6.13 GHz, mode). The graphene conductive ink printed textile shows surface resistance of , conductivity of and excellent microstructural characteristics, which makes it suitable to be used as a metal sheet to fabricate the antenna structure. To overcome the low‐conductivity issue of graphene conductive ink, a multilayered substrate approach is utilised to improve radiation performances of the antenna. A peak realised gain of 2.09 dBi and radiation efficiency of 74% is achieved at the dominant mode (3.03 GHz). The frequency reconfigurability is introduced by exciting the higher order modes, and through two different feeding locations. The excitation of ports is controlled through a radio frequency (RF) pin diode switch integrated microstrip transmission line. Subsequently, the proposed structure is further designed using copper foil of thickness () and a comparative analysis of graphene‐based and copper‐based antennas is presented. The proposed antenna performs substantially well in terms of peak realised gain and radiation efficiency compared with that of the existing graphene‐based antenna and the performance deviation with respect to the copper based antenna is lowered significantly.

Personalized and pervasive healthcare devices help seamlessly integrate healthcare and wellness into the daily life, independent of time and space. Silicon Integrated Circuit (IC) has been used in many advanced healthcare applications due to the compact size and ultra-low power consumption. Meanwhile, printed electronics (PE) is considered as a promising approach enabling cost-effective manufacturing of thin, flexible, and light-weight devices. A hybrid integration of IC and PE provides a new solution for the future wearable healthcare devices. In this chapter, firstly a customized bio-sensing IC is demonstrated, which can detect and process various bio-signals; secondly, the feasibility and performance of using inkjet printing technology as enabling technology has been examined for the fabrication of flexible bio-sensing devices. Finally, a wearable and flexible Bio-Patch is presented by leveraging hybrid integration of PE and bio-sensing IC. In-vivo test results show that the flexible Bio-Patch provides high quality ECG signal comparable with the one gained by bedside ECG machine.

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.

Synthesis of Ag/RGO composite as effective conductive ink filler for flexible inkjet printing electronics

Towards the fabrication of fully printed flexible P(VDF-TrFE)-based piezoelectric sensors

Cotton fabrics were used in this study to create electrically conductive pathways through printing with conductive materials. Graphite and a mixture of graphite oxide with manganese oxide were utilized as conductive inks in screen printing. To create the conductive ink, conductive materials were mixed with other substances such as thickener, moisturizer, and water. The viscosity of the ink was 10 mPa.s. After printing on the cotton fabric samples, the effect of conductive concentration on electrical conductivity was examined. It was found that a 5% concentration of graphite achieved the highest conductivity (1.75 × 10-5 S.cm-1). To further improve conductivity, multiple layers of conductive ink were printed on the fabric. Three layers were applied, with the first and third containing graphite and the second containing graphite oxide and manganese oxide (battery core). The conductivity values reached 6.5 × 10-3 S.cm-1. This conductive fabric was used in the creation of electronic smart clothing to measure the human body temperature index. The conductive ink printed on the fabric served as a link between electronic components, transmitting electrical signals. In addition, a smart product using conductive yarns was created as a comparison to the conductive ink. Results showed that the sensor signal was transmitted well and readings were consistent.