Flexible Sensors Based on Electrohydrodynamic Jet Printing

Jiyang Lu; Tiantian Wang; Xiangxiang Li; Fuming Wu; Hui Yang; Wenping Hu

doi:10.7536/PC211217

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (9) : 1982-1995 DOI: 10.7536/PC211217

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Flexible Sensors Based on Electrohydrodynamic Jet Printing

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Abstract

Flexible sensors have potential applications in the fields of soft robotic, wearable electronics and biomedical, etc., due to their stable sensing performance under large deformation conditions, such as,bending, twisting, and stretching. Compared with traditional photolithography technology for constructing flexible sensors, printing as one of additive manufacturing technologies has the advantages of green, low⁃cost and large⁃area manufacturing. In various printing technologies, electrohydrodynamic jet (e⁃jet) printing technology enable replace traditional lithography technology to fabricate high⁃resolution flexible sensors, because its compatibility with multiple functional materials and special working mechanism. Recently, e⁃jet printing technology shows wide application prospects in the fields of miniaturized flexible sensors, such as flexible pressure sensors, flexible gas sensors and flexible electrochemical sensors. In the review, we focused on the recent developments of materials, processes and applications of e⁃jet printing technology in the field of flexible sensor. Firstly, we introduced the working principle of e⁃jet printing technology and various e⁃jet printing ink materials in detail. Then, the interface controlling methods between ink and flexible substrate in e⁃jet printing progress were discussed. Subsequently, the applications of e⁃jet printing technology for flexible pressure sensors, flexible gas sensors and flexible electrochemical sensors were provided. Finally, we presented the future challenges and opportunities of next⁃generation e⁃jet printing in high resolution flexible sensors.

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flexible sensors / microminiaturization / e⁃jet printing

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Jiyang Lu, Tiantian Wang, Xiangxiang Li, Fuming Wu, Hui Yang, Wenping Hu. Flexible Sensors Based on Electrohydrodynamic Jet Printing. Progress in Chemistry, 2022, 34(9): 1982-1995 DOI:10.7536/PC211217

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1 Introduction

2 E⁃jet printing

2.1 Principles of e⁃jet printing technique

2.2 Functional ink materials

2.3 Interface control in e⁃jet printing progress

3 Applications of e⁃jet printing in flexible sensors

3.1 flexible pressure sensors

3.2 flexible gas sensors

3.3 flexible electrochemical sensors

4 Conclusion and outlook

1 Introduction

Flexible sensors have been widely concerned by researchers at home and abroad because of their stable sensing performance under large deformation conditions such as bending, twisting, stretching and folding, so they have potential applications in soft robotics, wearable electronics, biomedical and other fields^{[1⇓⇓⇓⇓⇓⇓~8]}^[9⇓⇓~12]^{[13⇓⇓⇓~17]}^{[18⇓⇓~21]}^[22]. For example, the Japanese Someya research group has prepared a spiral sensor array composed of ultra-flexible transistors for monitoring the spatial distribution of mechanical pressure. The spiral sensor array can still work normally under a bending radius of 100 μm, demonstrating its high flexibility in smaller space scenarios^[23]. In addition, with the rapid development of flexible electronic technology, the demand for miniaturization and high resolution of flexible sensors in medical and health fields such as human health monitoring, early diagnosis of diseases and rehabilitation of neurological diseases is increasing^[13⇓~15]. High-resolution miniaturized flexible sensors can achieve long-term stable monitoring of human body signs, promote the further commercial application of flexible sensors, and become one of the research hotspots in the field of flexible sensors.

At present, the main manufacturing methods of flexible sensors include photolithography and printing^{[24⇓⇓⇓⇓⇓~30]}. Compared with traditional lithography technology, printing technology manufacturing, as an additive manufacturing, has the advantages of green, low cost and large area manufacturing, and is widely used in the preparation of flexible electronic devices^[31⇓~33]. Printing technologies are divided into two categories: "contact" and "non-contact": contact printing is the direct transfer of ink from the pattern template to the substrate, which is sensitive to the surface roughness of the substrate and is more suitable for large-area planar additive manufacturing, including screen, gravure and relief printing, among which screen printing is one of the most common contact printing methods (Figure 1A)^[34]; Non-contact printing is to complete pattern printing by ejecting ink to the substrate through a nozzle, which is insensitive to the surface roughness of the substrate and is suitable for plane and three-dimensional surface printing, including inkjet printing, laser printing and electrohydrodynamic inkjet printing (electrojet printing). Among them, inkjet printing (Figure 1b) is one of the most common methods in non-contact printing^[35,36]. However, the low resolution of inkjet printing technology can not meet the demand of miniaturization of flexible sensors, which limits its further application in biomedical and other fields. In order to meet the high precision requirements of flexible sensors, electrojet printing technology has attracted the attention of researchers, which can control the jet size of functional ink through high-voltage electric field, thus improving the resolution of printed patterns.At the same time, it has the advantages of flexible substrate compatibility, large area printing and low cost, and has potential application prospects in the field of high-resolution miniaturized flexible sensor manufacturing^[37,38]. Electrofluidic inkjet printing as a transformative technology, due to its compatibility and versatility, replaces conventional lithography and enables high-resolution and cross-scale fabrication of flexible sensors. It is worth noting that the surface energy matching between the flexible substrate and the functional ink directly affects the resolution of the printed pattern. In order to solve the above problems, the surface energy matching is realized by regulating the surface energy of the flexible substrate and the surface and interface performance of the functional ink, so as to further improve the resolution, accuracy and stability of the flexible sensor.

Electrojet printing technology (Figure 2) is to pull the ink out of the nozzle by a high-voltage electric field. When the surface tension of the ink is in balance with the applied electric field force, it will form a Taylor cone shape. Finally, the ink is ejected from the surface of the Taylor cone and printed on the substrate to build the device^[37]. In terms of materials and processes, the stable and controllable generation of Taylor cones and the precise control of the ink deposition surface interface are the key scientific and technological issues to achieve high-resolution pattern printing of flexible sensors. Therefore, designing the functional ink formula suitable for EFI printing, adjusting the surface energy matching between the ink and the flexible substrate, and exploring the optimal printing parameters to realize Taylor cone jet are the key links to realize the miniaturization, high precision and high resolution printing of flexible sensors.

In this paper, the latest research progress of flexible sensors based on focused electrospray printing is reviewed. Firstly, the working principle of electrohydrodynamic inkjet printing technology, common functional ink materials and the control of surface and interface in printing were introduced in detail. Then, the application progress of electrojet printing in the fabrication of flexible sensors, including flexible pressure sensors, flexible gas sensors and flexible electrochemical sensors, is reviewed. Finally, the future opportunities and challenges of EFI printing flexible sensors are summarized, hoping to promote the further application of EFI printing technology in soft robotics, wearable electronics, biomedical and other fields.

2 Electrohydrodynamic inkjet printing

2.1 Electrohydrodynamic inkjet printing technology

Electrohydrodynamic inkjet printing is a printing technology that uses high voltage to generate an electric field to print functional ink onto a substrate according to a pre-designed pattern, thus achieving high-resolution patterning (Figure 3)^[39]. Its main elements include a high-voltage power supply, a control pressure source, an ink chamber, a nozzle tip, a positioning system, and a conductive substrate, i.e., air back pressure, nozzle-to-substrate distance (offset height), droplet frequency, and voltage applied between the substrate and the nozzle tip^[40]. In the electrojet printing system, the ejection behavior of the functional ink is controlled by regulating the main parameters such as the air back pressure, the distance between the nozzle and the flexible substrate, the ink droplet frequency, and the voltage applied between the flexible substrate and the nozzle tip^[41]. The process is as follows: first, the ink is delivered from the ink chamber to the tip of the nozzle by air back pressure; Then, an electric field is generated by applying a voltage between the flexible substrate and the nozzle, causing a charge to accumulate on the overhanging ink droplet flowing out of the nozzle tip^[42]; The accumulation of charge on the suspended ink droplet leads to shear stress, causing the meniscus to become conical, known as the Taylor cone (Fig. 4). When the potential of the meniscus exceeds a certain value, the surface tension of the ink is overcome by the electrostatic force, causing the ink to be ejected from the surface of the Taylor cone onto the substrate^[43,44].

It is important to study the factors affecting the electrojet printing process to achieve uniform, high-resolution, and repeatable printing. Flow rate (Q) and electric field (E) are two important parameters in the process of electrojet printing, which determine the jet behavior of inkjet. At low E and low Q, ink droplets are ejected from the nozzle by gravity^[45]. By a small increase in E or Q (or E and Q), the meniscus reaches a "pulse" mode in which the resulting Taylor cone contracts after the droplet is expelled. Pulsating jets are formed when Q is less than the value required to stabilize the Taylor conical jet mode, or when the voltage is below a critical E^[46]. The pulsating jet frequency is in the kilohertz range, which causes the ink droplet to flow rapidly, preventing the meniscus from retracting to its original shape^[47]. By further increasing the electric field, a continuous flow and a stable Taylor cone are observed, which is called "cone-jet mode" (Fig. 4A). At higher electric fields, other ejection patterns such as "oblique ejection", "double ejection", and "multiple ejection" are observed (Figure 4C)^[48,49].

2.2 Functional ink used in electrospray printing

The properties of functional ink, such as conductivity, adjustable viscosity range, stability and surface energy matching with flexible substrate, are the key factors for the formation of electrojet printing, which are essential to improve the high resolution, high precision and device sensing performance of electrojet printing^[40]. Therefore, the selection of appropriate functional ink is the cornerstone of the successful fabrication of flexible sensors by electrojet printing. Functional ink is usually composed of polymer binder, additive, solvent and functional material. Nanoparticle ink is prone to aggregation, agglomeration, and deposition, and thus may lead to suboptimal sensor performance. By adding polymer binder, the intermolecular interaction between nanoparticles can be reduced, thereby improving the stability of the ink. The surface tension, viscosity and conductivity of the functional ink can be adjusted by adding a small amount of additives (usually less than 4%), thereby improving its stability. Surfactant and rheology modifier are commonly used additives in the preparation of functional ink. The solvent is used to dissolve or disperse the other three components and adjust the final viscosity of the ink. The researchers compared the effects of high boiling point and low boiling point solvents on ink, and found that high boiling point ink (over 200 ℃) can reduce the viscosity of ink and increase the fluidity to a certain extent, which can effectively prevent nozzle clogging^[50]. The blending parameters of the ink are critical to the quality of the printed image, and any clogging of the ink will result in poor printing. In 2021, Mkhize et al. Changed the overall conductivity and morphology of polyaniline by doping acid in functional ink, and prepared an ammonia sensor with high resolution, high sensitivity and high sensing performance by using electrojet printing technology^[51]. It shows how these factors affect sensing and printability, and demonstrates that the rheology of functional inks is an important factor to consider in ink formulation.

Functional materials endow ink with physical and chemical properties such as conductivity, adjustable viscosity and matching surface energy, which are the core components of electrojet printing ink and directly affect the high-precision printing of functional ink on flexible substrates.Therefore, the selection of appropriate printing materials is the key to achieve high resolution and high stability of flexible sensors, which has attracted the attention of researchers. At present, many functional materials, such as metal materials, polymer materials, metal oxide materials and carbon conductive materials, are used to prepare high-performance printable functional inks, and to prepare high-resolution flexible sensors for pressure, gas and electrochemistry.

2.2.1 Metallic material

Metal ink is one of the most common inks for the preparation of pressure, gas and other flexible sensors by electrojet printing because of its high viscosity and good conductivity. Metal ink materials mainly include gold, silver and copper, etc. The ideal metal conductive ink should have the advantages of low price, easy storage and high conductivity. In bulk resistivity, the highest metal is gold (2.44×10^-8Ω·m), followed by copper (1.72×10^-8Ω·m), and the lowest metal is silver (1.59×10^-8Ω·m). Gold is about 50 times more expensive than silver, and silver oxidizes very slowly in air. Copper is easy to oxidize, so the process of preparing ink is complex. Metal inks are formulated in the form of nanoparticles, which are uniformly dispersed and stable in solvents, such as gold nanoparticles (AuNPs), silver nanoparticles (AgNPs) and copper nanoparticles (CuNPs), which can be easily prepared into conductive inks with high conductivity and applied to electrofluidic inkjet printing technology. Compared with AuNPs and CuNPs, AgNPs have high conductivity, low cost and oxidation resistance, which are the main choice of functional ink in electrofluidic inkjet printing and are widely used in the preparation of functional materials for electrojet printing (Fig. 5)^[52⇓~54]. The Dong research team developed a flexible, stretchable, molten metal alloy ink with self-healing capabilities^[55]. Due to the high surface tension and viscosity of molten metal ink, the metal conductor with a resolution of less than 50 μm was prepared by electrojet printing technology. Metallic inks generally require a sintering process to remove the organic binder and melt (pre-melt) the material and promote the bonding of the separated nanoparticles together to form a continuous electron transport channel, so that the metallic ink exhibits true conductivity, thereby improving the conductivity and sensitivity of the electrospray printed sensor. The melting point of silver nanoparticles decreases with the increase of the ratio of surface area to volume^[56]. A molecular dynamics model was developed to predict the melting behavior of silver nanoparticles with diameters between 3.5 and 16 nm. The results show that the melting point decreases with the decrease of particle size, and the pre-melting phenomenon only occurs in particles larger than 7 nm^[57]. Jiang et al. Prepared AgNPs composite ink by combining the advantages of AgNPs ink and other inks, which greatly reduced the sintering temperature of AgNPs ink and the cost of ink^[58]. The reduction of the sintering temperature can better ensure that the flexible substrate is not damaged, and the applicability of the flexible sensor is improved.

2.2.2 Polymeric material

Polymers are widely used in the preparation of electrospray printed functional materials because of their good mechanical compliance and electrical conductivity (Fig. 6)^[51,59,60]. Polymers include conjugated polymers and electroactive polymers. Conjugated polymers are composed of alternating single and double bonds of carbon (aliphatic or aromatic) along the backbone chain. The delocalized electropolymer in the 2 p orbital is characterized by conductivity^[61]. Conjugated polymers are widely used in the preparation of functional inks for electrojet printing due to their conductive properties. Derivatives of polyacetylene, polythiophene, polyaniline, polypyrrole, and polyphenylene vinylene are common conjugated polymers^{[62⇓⇓~65]}. Among them, conductive poly (3,4 ethyldioxythiophene) (PEDOT), polypyrrole (PPy) and polyaniline (PANI) are widely used in electrofluidic inkjet printing due to their high conductivity, adjustable viscosity and good stability. In particular, doped polystyrene sulfonate (PSS) enables the conducting polymer PEDOT to have tunable conductivity. However, the width of the line printed by electrofluidic inkjet with PEDOT: PSS as the conductive material is usually higher than 90 μm. In order to improve the resolution of printing, Chang et al. Adopted a new strategy to optimize the moving speed. The smallest line width of about 27.25 ± 3.76 μm was printed directly on the flexible substrate by electrojet printing technology in a conical jet mode, which significantly improved the resolution of the printed pattern^[66]. In 2020, researchers used electrojet printing to directly print PEDOT: PSS ink solution on PDMS flexible substrate to prepare flexible pressure sensors with high sensitivity, fast response and low detection limit^[59]. In addition, some researchers used functional ink containing PEDOT: PSS to prepare strain sensors by electrojet printing technology^[67].

2.2.3 Carbon conductive material and metal oxide material

Carbon exhibits four sp³ hybrid orbitals with four unpaired electrons, which can form allotropes with good electrical characteristics and different mechanical properties. Among them, graphene and carbon nanotubes are two important conductive allotropes of carbon, which are the research hotspots of new carbon nanomaterials. Because of their remarkable mechanical strength, good chemical stability, simple processing and good conductivity, they are used in conductive patterns manufactured by printing technology (Fig. 6). Graphene, as a two-dimensional material, has the advantages of large specific surface area, good chemical stability and excellent conductivity, and has broad application prospects in high-resolution flexible sensors. Based on these advantages, Li et al. Successfully used graphene functional ink in electrojet printing, and the micro/nanostructured patterns of electrojet printing provide an effective solution for fine control of the electrojet printing process and printing precise microstructures^[70]. In addition, Wang Dazhi's research team developed a new functional ink formula, using ethyl cellulose dispersed in graphene ink and perfluorinated sulfonic acid resin (Nafion) dispersed in graphene ink, and then through electrojet printing technology, the mixed graphene functional ink was used to prepare electrochemical flexible sensors, which effectively improved the sensing performance of gas sensors^[71]. Metal oxide materials are composed of one or more oxides, tungsten trioxide (WO₃), nickel oxide (NiO), tin dioxide (SnO₂) and indium trioxide (In₂O₃) are common oxides. Among them, SnO₂ and In₂O₃ are common semiconductor oxide gas sensing materials, which are used in the preparation of flexible gas sensors due to their good stability and high sensitivity, as shown in Fig. 8. Three conditions must be met for oxide materials as components of functional inks to produce uniform patterns by electrojet printing: (1) oxide materials are prepared into stable nanoparticles; (2) controlling the fluid dynamics to obtain good dispersion of the ink; And (3) controlling the drying process of the ink to obtain good uniformity. The functional ink affects the microstructure of the electrospray printed pattern, which in turn affects many important properties of the flexible sensor. The design of new functional materials and their formulation into functional inks for electrojet printing are essential to improve the performance of flexible sensors in all aspects.

2.3 Surface and Interface Control in Electrospray Printing

During the fabrication of flexible sensors by electrofluidic inkjet printing, the resolution of the printed pattern is affected by the composition of the functional ink and the regulation of ink ejection behavior parameters. As one-dimensional materials, carbon nanotubes have excellent mechanical properties and good chemical stability. However, the poor dispersion of carbon nanotubes in the solvent reduces the quality of the ink. In order to solve this problem, Jeong et al. Blended the components of functional ink to make carbon nanotube/polystyrene sulfonate (MWCNT/PSS) composite functional ink^[73]. At the same time, this study proves that ethanol is the suitable solvent in the process of electrofluidic inkjet printing, because ethanol provides a lower surface tension than water, which helps to form Taylor cones and achieve four different jet modes. The resolution of printed pattern is not only affected by the composition of functional ink and the parameters of ink ejection behavior, but also closely related to the surface energy of flexible substrate and the surface and interface properties of functional ink, so it is of great significance to modulate the morphology of deposited ink droplet solution^[29,74]. Due to the imbalance of evaporation rate between the edge and the center of the ink, the capillary flow of the solution is caused, and finally the ring pattern is formed, which affects the resolution of the electrospray printing sensing pattern. Therefore, the formation mechanism of the "coffee ring" and the capillary flow inside the ink droplet have received extensive attention^[75]. In the process of electrospray printing flexible sensor, after the liquid functional ink droplet is pulled to the substrate by the electric field force, the solvent at the edge of the ink droplet will evaporate first, and its evaporation rate will be greater than that at the center of the droplet, which will affect the interior of the droplet.It will cause an outward capillary flow, and at the same time, it will take away the suspended particles, carry them to the edge of the droplet, and finally deposit at the edge, eventually forming a ring, which directly affects the shape and continuity of the sensing pattern deposited on the flexible substrate, resulting in the loss of performance of the flexible sensor. Therefore, it is very important to improve the deposition morphology and film uniformity of the ink by modifying the surface and interface of the substrate, regulating the surface energy of the substrate and the surface and interface properties of the functional ink, and preparing high-resolution and high-quality flexible sensors by electrojet printing. Lee's team fabricated a high-resolution flexible pressure sensor on polyethylene terephthalate film using electrojet printing and silver ink^[52]. In order to improve the quality of printed patterns, researchers hydrophilized the surface of polydimethylsiloxane (PDMS) flexible substrate to make its surface have hydrophilic groups such as — OH and — COOH, adjusted the surface energy of the flexible substrate to match the surface energy of silver ink, improved the deposition morphology of ink, and then improved the resolution and stability of the flexible sensor. Yu Haibo's research team studied the mechanism of regulating the coffee ring effect in the process of electrojet printing, and the study showed that the nanoscale coffee ring effect was produced as shown in Figure 9a, which realized the high-resolution pattern obtained by electrojet printing of silver nanoparticles (AgNPs) ink.These include the analysis of capillary flow, Marangoni flow, three-phase contact line dynamics, and interfacial interactions between particles, elucidating the mechanism of nanoparticle pattern formation after ink droplet evaporation^[76]^[77]^[71,78]^[79]^[80]. In this work, AgNPs inks with different chemical compositions were electrosprayed on different substrates, and the morphology of their deposits was observed by atomic force microscopy (Fig. 9b), and accurate statistics and analysis were carried out to controllably obtain rings with a line width of 50 nm.

In addition to controlling the coffee ring effect to obtain deposition patterns with different morphologies, the coffee ring effect can also be suppressed to obtain uniform patterns and improve the performance of the flexible sensor. Huang Yongan's research team successfully achieved high-resolution patterning of perovskite by combining high-viscosity perovskite precursor solution with polyvinylpyrrolidone (PVP) using electrojet printing technology^[75]. They added long-chain PVP to the perovskite solution to increase the viscosity and contact angle of the precursor, thereby reducing capillary flow and eliminating the coffee ring (Figure 10 a). The high concentration of PVP forms multiple small spaces during evaporation, thereby limiting ion flow and preventing the appearance of large particles. Electrojet printing technology can pattern perovskite solution with high viscosity, and finally print various complex patterns on flexible polyimide substrate (Fig. 10B). This work not only shows the good compatibility of electrojet printing technology with flexible substrates, but also is suitable for large area and long time printing, which shows its potential in the preparation of flexible sensors.

3 Application of EFI Printing Flexible Sensor

With the rapid development of flexible electronic technology, the demand for miniaturization and high resolution of flexible sensors such as pressure, gas and electrochemistry is increasing in the fields of medical health and environmental monitoring, such as human health monitoring, early diagnosis of diseases and rehabilitation of neurological diseases. In order to meet the requirement of high resolution of flexible sensors, electrojet printing technology has potential application prospects in the field of high-resolution and miniaturized flexible sensor manufacturing because it can control the jet size of functional ink through high-voltage electric field, thus improving the resolution of printed patterns. For example, Lee's research team has used electrospray printing technology to prepare miniature pressure sensors, which are expected to be used in the field of human health detection^[52]. This section summarizes the research progress of EFI printed flexible sensors, including flexible pressure sensors, flexible gas sensors and flexible electrochemical sensors.

3.1 EFI printing pressure transducer

Pressure sensors, also known as tactile sensors, have attracted much attention in the field of wearable electronics^[81,82]. Flexible pressure sensor adheres to human skin closely because of its flexibility, durability and high biocompatibility, so as to realize real-time monitoring of physiological health indicators such as heart rate and respiratory rate^[59]. In order to prepare high-performance skin pressure sensors, researchers usually design hybrid micro/nanostructures based on effective signal transduction mechanisms to detect pressure signals by converting external stimuli into electrical signals. There are two conduction mechanisms for converting these pressure signals into electrical signals: piezoresistive and piezocapacitive^[82]. The most common is the piezoresistive type, which is conducted by converting a change in force into a change in resistance. Electrojet printing technology has been used to print micro-nano structure patterns of different materials in the past five years because of its advantages suitable for 100 nm printing resolution^[83]. Based on the fabrication of micro pressure sensor by electrojet printing technology, the parameters of pressure sensor, such as sensitivity, linearity, hysteresis time, response time and recovery time, have been further studied. Mirza's group fabricated a piezoresistive sensor array for robotic skin on a polyimide flexible substrate using electrospray printing and PEDOT: PSS ink dissolved in dimethyl sulfoxide (DMSO)^[84]. The average temperature sensitivity is 175 mV/℃ and the signal-to-noise ratio is 20.6. In addition to polymer inks, metallic inks have also been successfully used in electrospray printed flexible pressure sensors. In 2017, Lee's research team used electrojet printing technology and silver ink to prepare a capacitive touch sensor with comb-like interdigital structure on polyethylene terephthalate film^[52]. First of all, the research group carried out a printing test, and the line width of single-layer printing was 10 μm, to prove the ability of electrojet printing technology to print micro-patterns. However, the results showed that the conductivity of single-layer printing was not high enough, and then the conductivity of the printed pattern was improved by overlapping printing and multi-layer printing. In this work, a high-resolution sensor was fabricated and tested by this method, and the effects of the number of electrodes and the length of electrodes on the output capacitance of the flexible sensor were studied. The study shows that the output capacitance increases from 2 pF to 7 pF when the number of electrodes increases from two to six. Electrode length has a similar effect on the output capacitance, increasing from 2 pF to 5 pF as the electrode length increases (from 5 mm to 12.5 mm). Finger touch also has an effect on the sensor capacitance. A single-finger touch causes a capacitance change of approximately 0.9 pF in the sensor. In order to improve the sensitivity of the sensor, in 2020, researchers prepared a flexible pressure sensor with high sensitivity, fast response and low detection limit by electrospray printing^[59]. The experimental results show that the flexible pressure sensor shows good performance in the application of finger joint movement and word pronunciation recognition (Fig. 10B), and the size of the pressure sensor is smaller than that of the finger.It meets the current demand for microminiaturization of flexible sensors in the medical field, which indicates that it has great application potential in human motion monitoring and personal health monitoring. The specific fabrication process of the flexible pressure sensor is as follows: First, the researchers hydrophilized the surface of polydimethylsiloxane (PDMS) with a thickness of 200 μm to make the surface have hydrophilic groups such as — OH and — COOH. Then, they used the self-developed NFEDW equipment (Fig. 12) to directly print the prepared PEDOT: PSS/GR/SWCNTs ink solution on the pretreated PDMS substrate to fabricate serpentine pressure-sensitive units. After natural drying, the two ends of the flexible pressure-sensitive unit were coated with a layer of silver paste, and the two copper electrodes were pasted in the silver paste respectively. After the silver paste was cured, two copper electrodes were fixed at both ends of the electrospray printed pressure-sensitive unit. Finally, another PDMS film of the same size was used to encapsulate the pressure-sensitive unit and the copper electrode. It can be seen that the quality of the printed pattern is improved by the interface control of the flexible substrate PDMS. The change of sensing performance caused by mechanical deformation makes it difficult to accurately measure the pressure value under dynamic deformation. To meet this challenge, Dong's research team developed flexible, stretchable, molten metal alloy inks with self-healing ability to print high-resolution conductive patterns^[55].

In this work, high resolution printing of low melting point metal alloys is achieved by using electrojet printing technology, which enables the preparation of metal conductors with a resolution of less than 50 μm at low cost. Molten metal is continuously ejected from a Taylor cone in the form of a fine jet/filament due to the high surface tension and viscosity of the molten metal ink during the printing process. Electrospray printed 2D patterned conductors maintain a stable electrical response after hundreds of bending cycles and over a wide range of tensile strains (0 ~ 70%). The conductor remained conductive in the bent state, and then the researchers tested the resistance of the Christmas tree-shaped conductor after bending for diameters of 10 mm and 5 mm (Figure 13). At the same time, they carried out tensile tests on metal conductors with horseshoe patterns, showing good flexibility and tensile properties. One highlight of this work is that due to the low melting point of the metal alloy ink, the printed conductor can show self-healing ability (Figure 13). It only needs to heat the device above the eutectic temperature of the metal ink and apply slight pressure to recover from the failure. The healed catheter can still conduct electricity when bent by hand. This work creates a conductive path with metallic conductivity, excellent flexibility and stretchability by preparing micro-metallic conductors through electrospray printing, demonstrating the great advantages of reusability in applications. And can be use for directionally manufacturing flexible and stretchable device.

3.2 Electrojet printed gas sensor

In recent years, air pollution is endangering human health, and more and more attention has been paid to air quality monitoring^[85,86]. Gas sensor is one of the important electronic devices for detecting harmful inorganic gases (such as NO₂, NO, NH₃, H₂, CO₂, etc.) and organic gases (methane, triethylamine, benzene, etc.)^[87⇓~89]. In addition, gas sensors have shown potential applications in different fields such as industrial and agricultural processing, aerospace, military and medical diagnostics^[90]. Gas sensors based on electrojet printing technology have been widely concerned in sensing technology because of their easy manufacture, simple process, low cost and miniaturization. Myers' group fabricated a nitric oxide sensor by electrospray printing^[91]. The method involves printing carbon and metal-based inks, converting Ag to Ag: AgCl as a reference electrode, and depositing a nitric oxide selective layer on the working electrode, followed by encapsulation to define the working area. The work introduces a combination of electrode chemistries in which nitric oxide concentration can be effectively measured using an Ag/Graphene/Au working electrode, with the magnitude of the oxidation peak increasing as the concentration of nitric oxide increases. The sensor can selectively detect nitric oxide at nanomolar levels and above by different voltammetric detection methods (cyclic voltammetry, square wave voltammetry, and amperometric detection). Although the sensitivity is typically maintained at submicromolar, the accuracy and repeatability problems remain unsolved due to the inhomogeneity of the graphene film, which results in a relatively imprecise concentration gradient between devices. In recent years, researchers have considered using ink material doping to improve the sensitivity of gas sensors, but the ink blending parameters are crucial to the quality of printed patterns, and any clogging of ink will lead to poor printing results, which increases the difficulty of research. In 2021, Mkize et al. Used electrospray printing to prepare an ammonia sensor with high resolution, high sensitivity and high sensing performance^[51]. In this work, a miniaturized gas sensor was fabricated by printing microscale patterns (10 μm particles and about 100 μm line width) much smaller than the nozzle size using the mode of continuous inkjet. Conductive polyaniline becomes more resistive in the presence of ammonia as shown in fig. 14 a. When exposure to ammonia is stopped, the sensor returns to its original baseline. Depending on the ammonia concentration exposed, different peak heights can be observed. As shown in Fig. 14 B, the sensitivity of the flexible sensor can reach the 6.9%ppm^-1, and the detection limit is calculated to be 0.74 ppm.

This shows sensing and calibration plots using HCl-doped polyaniline and can be used to create microsensors and batch printing. They were the first to observe that doping acid affects bulk polyaniline conductivity and morphology, and the first to show how these factors affect sensing and printability, while demonstrating that ink rheology is an important factor that should be considered for ink doping. The development of this work can be integrated to wearable devices, demonstrating its promising applications in the field of wearable electronics. In recent years, in order to improve the sensitivity, selectivity, stability and other properties of gas sensors, many researchers have focused on the selection of functional materials for functional inks. Choi's research group developed a new ink formulation by first producing several metal oxide (SnO₂, In₂O₃, WO₃, and NiO) nanofibers by electrospinning, and then breaking them into smaller fragments by ultrasonication and dissolving them in an organic solvent to be used as printing ink^[72]. Microscale patterns of electrospun nanofibers with a minimum diameter of less than 50 μm were prepared by electrojet printing with optimized printing parameters, which provides a possibility for the preparation of gas sensor arrays. Using this technology, the research group has subsequently integrated gas sensor arrays that can be used to detect toxic gases such as NO₂, CO, and H₂S. In addition, nanofiber microscale patterns printed by electrospray can be applied to suspended MEMS platforms without causing any structural damage (Figure 15). These works meet the demand for miniaturization of flexible sensors in the fields of environmental monitoring and healthcare diagnosis.

3.3 Electrochemical sensor

In recent years, with the rapid development of wearable electronics, there is an urgent need for low-cost, high-sensitivity and miniaturized electrochemical sensors that can monitor and detect various biological and chemical compounds in health care and other fields^[92]. Among them, the use of electrojet printing manufacturing technology to prepare highly sensitive and miniaturized electrochemical sensors has become a research hotspot of wearable sensors^[53,93]. Wang Dazhi's research group prepared carbon-based three-electrode plates by combining carbon materials with lift-off process^[94]. The researchers used the lift-off process to form the metal pattern, and then used the electrojet printing technology to print the carbon pattern on the metal layer. Finally, an electrochemical sensor was developed using a well-fabricated three-electrode plate, which can be used to detect potassium ferricyanide and nicotinamide adenine dinucleotide samples. The experiment showed that the graphene-modified carbon electrode obtained a lower anodic peak potential than the bare carbon electrode. In the electrochemical measurement of potassium ferricyanide, the graphene modified electrode can reduce the potential distance between the anodic peak and the cathodic peak. This means that graphene nanosheets can improve the effective area and electron conduction ability of the reaction, thus making the electrochemical reaction easier to occur. However, because this work is to prepare electrochemical sensors by EFI printing technology combined with lift-off process, it increases the cost and difficulty of manufacturing, which is not conducive to practical application, and can not well meet the demand trend of miniaturization of flexible sensors. Another work of the research group tried to prepare graphene microscale structure patterns only by electrojet printing technology, which met the needs of miniaturization of printed patterns, reduced the production cost and reduced the difficulty of production^[71]. In this study, platinum/graphene composite electrodes were prepared by electrospray printing and tested electrochemically. A highlight of this work is the development of new ink formulations utilizing ethyl cellulose dispersion into graphene ink and perfluorosulfonic acid resin (Nafion) dispersion into graphene ink. They found that the graphene microscale structure using ethyl cellulose dispersed graphene ink exhibited the feature of centrally arranged graphene surrounded by evaporating solution trajectories. However, the graphene structure using Nafion-dispersed graphene ink exhibits the feature of uniform arrangement. They believe that this phenomenon is due to the fact that the resistivity of the graphene structure printed by ethyl cellulose dispersed graphene ink is much lower than that of Nafion dispersed graphene ink. The researchers tested the platinum/graphene composite electrode electrochemically and found that the platinum/graphene composite electrode had a more sensitive electrochemical sensing response than the pure platinum electrode. However, in practical applications, in addition to controlling the low cost and ensuring the simple manufacturing steps, the sensitivity of the micro-sensor is also required to be relatively high. The graphene-based sensor uses electrojet printing technology to further study the formula of ink to obtain higher resolution and accuracy of printing patterns, and to improve the sensitivity of the sensor. In 2020, Wang Dazhi et al. Prepared micro-scale graphene patterns using highly concentrated graphene ink by drop-on-demand (DoD) electrojet printing mode (Fig. 16)^[69]. The ink droplet of highly concentrated graphene ink is much smaller than the inner diameter of the nozzle, which can effectively avoid nozzle blockage. As shown in fig. 16e, through different times of printing, it is found that the line width of the graphene line increases and the resistivity decreases with the increase of the number of times of printing. The resistivity of a single print (4.2 MΩ · cm) is about four times higher than that of 20 prints (1.1 MΩ · cm). The resistivity reduction at more printing times is mainly due to the enhanced intimate contact between the graphene flakes inside the graphene structure, which can facilitate electrical transport. Figure 16F shows the Raman spectrum of the once printed graphene microstructure, and the typical D, G, and 2D bands are observed around 1350, 1580, and 2700 cm^-1, respectively, indicating the presence of multilayer graphene and higher graphene quality than the reported reduced graphene oxide. In this study, different microscale graphene patterns were directly printed by DoD electrojet printing technology by controlling the pulse voltage width and frequency. The researchers printed the graphene layer directly on the platinum microelectrode to form a graphene/platinum (G/Pt) composite microelectrode. Electrochemical measurements showed that the peak current of the G/Pt composite microelectrode was more than two times larger than that of the bare Pt microelectrode. This indicates that the sensitivity of the microsensor is significantly improved, which provides a new idea for the preparation of high-performance electrochemical sensor devices. These efforts are expected to address the urgent need for low-cost, highly sensitive, miniaturized electrochemical sensors capable of monitoring and detecting various biological and chemical compounds in health care and other fields.

4 Conclusion and prospect

Electrojet printing technology opens up a new way for low-cost, green manufacturing and microminiaturization of flexible electronic products, and has potential applications in soft robotics, wearable electronics, biomedical robotics and other fields. In this paper, the latest research progress of flexible sensors based on focused electrospray printing is reviewed. Firstly, the working principle of electrohydrodynamic inkjet printing technology, common functional ink materials and the control of surface and interface in printing were introduced in detail. Then, the application progress of electrojet printing in the fabrication of flexible sensors, including flexible pressure sensors, flexible gas sensors and flexible electrochemical sensors, is reviewed.

Although electrojet printing technology has the advantages of flexible substrate compatibility, high resolution, micro-pattern printing and green development in the field of high-resolution flexible sensor printing and manufacturing, electrojet printing high-performance flexible sensors still face many challenges and opportunities. For example, the efficiency of EFI printing flexible sensors is low, and improving the printing time of devices is still an urgent problem to be solved. In addition, most flexible sensors printed by electrospray can not provide reliable results, and improving the repeatability of devices is the key to future development. Moreover, due to the long-term effect of external mechanical stress, the long-term stability of flexible sensors is not high, and they are prone to aging. However, the process of EFI printing flexible sensors is also vulnerable to environmental factors such as temperature and humidity, which also affects the performance of flexible sensors. These challenges hinder its further commercial applications in soft robotics, wearable electronics, biomedical and other fields. Therefore, by designing and preparing high-performance ink materials and flexible substrate materials, the surface and interface properties of the ink and the substrate are regulated, and the printing parameters in the printing process are optimized.It is a great opportunity and challenge for the next generation of electrojet printing technology to realize the high efficiency, high resolution and large area manufacturing of flexible electronic devices by using the multi-nozzle printing and multi-field cooperative technology of electrojet printing.

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