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All issues Volume 4 / No 2 (2025) Natl Sci Open, 4 2 (2025) 20240016 Full HTML
Issue
Natl Sci Open
Volume 4, Number 2, 2025
Special Topic: Flexible Electronics and Micro/Nanomanufacturing
Article Number 20240016
Number of page(s) 15
Section Engineering
DOI https://doi.org/10.1360/nso/20240016
Published online 02 September 2024

© The Author(s) 2024. Published by Science Press and EDP Sciences.

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

INTRODUCTION

Flexible transparent electrodes (FTEs) can maintain high transmittance, excellent electrical conductivity and mechanical stability under bending, twisting and other morphologies at the same time, which is an important part of the composition of flexible electronic devices. FTEs have a wide range of applications in the fields of flexible touch screens [1,2], solar cells [3,4], supercapacitors [5,6], flexible electroluminescent devices [7,8], flexible sensors [9–12] and conformal antennas [13,14]. The exploration of FTEs has led to the utilization of a diverse range of conductive materials for their fabrication. These materials include conductive metal oxides (e.g., indium tin oxide (ITO)) [15,16], conductive polymers (e.g., Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)) [17–20], carbon-based materials (graphene, carbon nanotubes, etc.) [21–23], metal nanowires (gold, silver, copper, etc.) [24–27], ultrathin metal thin films [28–32], metal mesh [33–36] and other materials. The high cost of indium and inherent brittleness limit traditional ITO electrodes. Metal nanowire-based electrodes often exhibit non-uniform square resistance and significant surface roughness. In contrast, embedded metal mesh exhibits several advantages, including low surface roughness, good environmental stability, and high mechanical fatigue strength. Additionally, it can enhance conductivity while maintaining high light transmission by improving the aspect ratio of electrodes, making it a promising candidate for the next generation of flexible transparent electrodes. However, it faces technical challenges, including the fabrication of ultrafine and large aspect ratio metal meshes and embedding into flexible substrates.

Some scholars have put forward a variety of potential methods for the fabrication of embedded metal meshes. The fabrication methods can be classified into four categories, including nanoimprinting technology [37–39], photolithography technology [40–43], printing technology [44–48], and other novel composite fabrication methods [49–51]. Khan et al. [41] utilized nanoimprinting to create a microgroove substrate, followed by electrowetting-assisted scraping of the conductive silver paste into the microgroove. Subsequent sintering and spin-coating of a PEDOT:PSS conductive polymer film resulted in transparent electrodes with specific characteristics, such as a conductor width ranging from 800 nm at the bottom to 1.3 μm at the top, a square resistance of 0.4 Ω/sq, and light transmittance of 76%. Zhang et al. [43] used the same method to prepare supercapacitors with a thickness of up to 190 μm and a capacitance of 19.5 mF/cm2. Wang et al. [38] employed roll-to-roll nanoimprinting to fabricate substrates with patterned grooves. Subsequently, the silver paste was scraped into the grooves, resulting in the fabrication of transparent electrodes with a square resistance of 22.1 Ω/sq, a transmittance of 82%, and a line width of 3.5 μm with a depth of 1.6 μm. However, the scraping process employed in this method may result in the wastage of ink and the deposition of residual ink on the electrode surface. Choi et al. [40] combined photolithography and electroplating processes to fabricate transparent electrodes with 70% transmittance and 6.5 Ω/sq square resistance for flexible organic solar cells. Nevertheless, the photolithography-based fabrication method requires a vacuum environment, incurs high costs, and is limited in large-area fabrication. Li et al. [52] rapidly fabricated a high-resolution embedded metal mesh through liquid substrate electric-field-driven microscale three-dimensional (3D) printing, achieving a light transmittance of 85.97% and a square resistance of 6 Ω/sq. However, the printing-based fabrication method requiring multiple layers of repetitive printing to enhance the aspect ratio of the conductor can be time-consuming and result in relatively poor structural consistency. In a study by Kong et al. [50], the formation and control of cracks between rigid film and flexible substrate were investigated. These electrodes showed a conductivity of 1.1 × 104 S/cm with a transmittance of 86%. However, the line width exceeded 200 μm. Therefore, it is still challenging to fabricate FTEs with embedded metal mesh in a high aspect ratio efficiently and cost-effectively.

This paper proposes self-confined electrohydrodynamic printing on micro-structured substrates for the efficient and cost-effective preparation of FTEs with embedded metal mesh. This method combines the ease of preparation of high aspect ratio micro-grooves by imprinting and the high resolution of electrohydrodynamic printing, with the advantages of non-contact filling and high ink utilization. The fabricated FTE with embedded Ag/Cu metal mesh demonstrates exceptional optical transmission and conductivity with an ultralow sheet resistance of 0.08 Ω/sq at 83.4% optical transmittance. In addition, the FTE exhibits outstanding mechanical flexibility with a resistance change of less than 5% after 1000 bending cycles with a curvature radius of 2 mm and high resolution with a minimum line width of 1 μm. Furthermore, its application in transparent flexible heaters and electromagnetic shielding films has shown promising performance. The FTE is capable of exhibiting a high heating temperature (130°C) at a low input voltage (4 V) with a fast thermal response, while also providing a high electromagnetic interference (EMI) shielding efficiency (SE) of more than 29 dB in X-band.

RESULTS AND DISCUSSION

Figure 1A depicts the process of self-confined electrohydrodynamic printing on a micro-structured substrate, for fabricating high-performance transparent electrodes. The process involves three key steps. (1) Localized hydrophilic and hydrophobic treatment of the micro-structured substrate: oxygen plasma etching and wet application of hydrophobic coatings are performed sequentially to make the grooves hydrophilic inside and hydrophobic outside. (2) Ink filling into micro-grooves via electrohydrodynamic printing, then, sintering in an oven at 150°C for 40 min. (3) Selective electroplating of copper at a current density of 1.5 A/dm2. The resulting transparent electrodes with an embedded metal mesh are illustrated in Figure 1B. Notably, these electrodes exhibit both good flexibility and excellent optical transmittance. A video showcasing the filling process for self-confined electrohydrodynamic printing on a micro-structured substrate is provided in Movie S1 (Supplementary information). Figure 1C presents an optical image of the flexible transparent electrode, showcasing regular wire shapes with consistent wire widths. The microgrooves are filled with a width of 4 μm and a depth of 8 μm., as illustrated in Figure 1D.

thumbnail Figure 1

Principle and process effect of self-confined electrohydrodynamic printing on micro-structured substrates. (A) Schematics for the fabrication process of Ag/Cu mesh film; (B) FTEs with an embedded metal mesh; (C), (D) the optical and SEM images of FTEs with an embedded Ag/Cu mesh; (E) comparison of copper electroplating on the PDMS plane and in the PDMS micro-grooves; (F) comparison of FTEs made in other works [15–36].

To emphasize the significance of utilizing a micro-structured substrate, the consequences of selective electroplating of copper on both the polydimethylsiloxane (PDMS) plane and PDMS microgrooves were examined, as depicted in Figure 1E. The unevenness of the wire width is further amplified following the copper electroplating of the wires on the PDMS plane. Conversely, the widths of the wires were found to adhere strictly to the structural shape of the template following the electroplating of copper in the PDMS microgrooves. Furthermore, the incorporation of Ag/Cu electrodes within the PDMS substrate was found to enhance the adhesion between the wires and the substrate, thus preventing the detachment of metal meshes from the grooves.

To assess the optical and electrical performance of flexible transparent electrodes with metal mesh, the researchers introduce the concept of the figure of merit (FoM). This parameter is defined as the ratio of electrical conductivity to optical transparency and can be calculated using the following formula [41,53,54]:

FoM = σ dc σ opt = 188.5 R ×s ( 1 T − 1 ) ,

where σopt is the optical conductivity of the conductive film, σdc is the direct current (DC) conductivity of the film, T (%) is the transmittance at a wavelength of 550 nm, and Rs (Ω/sq) is the film square resistance.

Figure 1F presents a comparison of transparent electrodes fabricated from silver mesh, silver/copper mesh, copper mesh, silver nanowire, graphene, and ITO using various fabrication methods reported in the existing literature. It is evident that the majority of transparent electrodes have a FoM below 5000 (positioned to the right and below the FoM = 5000 line), the larger the value of FoM, the better the combined optical and electrical performance of the FTEs. In this study, the FoM value of the FTEs with a hexagonal metal mesh with a line width of 4 μm and a side length of 180 μm (Sample1) reached 15,881, and the FoM value of the square metal mesh with a line width of 6 μm and a side length of 180 μm (Sample2) reached 24,708. Thus meets the criteria for high light transmittance and electrical conductivity requirements.

Self-confined electrohydrodynamic printing refers to a process where droplets injected by electrohydrodynamic printing all flow into a groove, rather than remaining on the outside under certain undesirable conditions. These conditions include when the droplet deposition position deviates from the centerline of the groove, and when the droplet diameter is larger than the groove width. To achieve this, it is essential to consider the surface energy of the hydrophobic and hydrophilic surfaces. The lower surface energy of the more hydrophobic surface causes droplets to naturally flow from it to the hydrophilic surface due to surface tension [55,56]. The treatment method is illustrated in Figure 2A. The yellow dotted line in the figure represents the groove edge line. The micro-structured substrate underwent oxygen plasma etching to achieve a hydrophilic state with a static contact angle of 18°. Subsequently, a hydrophobic coating was applied to the micro-structured substrate using a dust-free cloth. This resulted in a micro-structured substrate that was hydrophilic inside the grooves and hydrophobic outside the grooves on the upper surface, with a static contact angle of 111°. The impact of oxygen plasma etching process parameters on the static contact angle was investigated (Figure S1).

thumbnail Figure 2

Numerical simulations and experimental verifications of self-confined printing of droplets on microgroove surfaces. (A) Schematic representation of the local hydrophilic and hydrophobic treatment; (B), (C) numerical simulation and experimental phenomenon when the droplet diameter exceeds the groove width; (D), (E) numerical simulation and experimental phenomenon when the droplet deposition position deviates from the groove. The yellow dotted line in the figure represents the groove edge line.

This study employs numerical simulations and experiments to investigate the deposition and flow pattern of droplets in microgrooves. Details of the numerical simulation based on OpenFOAM are available in the Supplementary information. OpenFOAM numerical simulations revealed that droplets on a substrate with localized hydrophilic and hydrophobic treatment gradually flowed entirely into the groove under the influence of surface tension without leaving any residue when the ratio of droplet diameter to the width of the structured groove was 1.4, as depicted in Figure 2B. Ink droplets with diameters significantly larger than the width of the grooves were generated by electrohydrodynamic printing. Upon turning off the voltage, the ink droplet volume gradually decreased, and all ink droplets flowed back into the grooves eventually, as shown in Figure 2C. When droplet deposition deviates from the groove, the contact angle of the droplets on the upper surface of the groove increases after the localized hydrophilic treatment. Instead of spreading out in all directions, the droplets tend to aggregate in the interior. The droplets slowly flow into the groove under the combined effects of gravity and surface tension, as shown in Figure 2D. In the experiment depicted in Figure 2E, droplets initially sprayed by electrohydrodynamic printing landed entirely on the upper surface of the groove. The difference in surface tension between the left and right sides of the droplet caused them to move towards the right side, leading to flattened droplet profiles and a reduction in volume, leaving no residue on the upper surface. The numerical simulation and experimental results of electrohydrodynamic printing on a micro-structured substrate with hydrophilic treatment only are presented in Figure S2. Localized hydrophilic and hydrophobic treatment of the substrate enables complete deposition of droplets in the grooves, even when the droplet diameter exceeds the groove width, and the deposition position is not perfectly aligned with the groove center, which is called self-attractive filling. This method prevents substrate staining outside the groove, reduces the need for precise electrohydrodynamic printing motion platform movement and positioning accuracy, and streamlines the process operation. Moreover, it holds promise for industrial applications.

To systematically characterize the effect of localized hydrophilic and hydrophobic treatment of the substrate on the self-attractive filling of droplets, we gradually increased the offset distance between the centerline of the control nozzle and the symmetry axis in the direction of the width of the groove (referred to as the offset distance) from 0 to 70 μm for grooves of the same depth (8 μm) and different widths. Based on the ink deposition, it was categorized into three groups: self-attractive filling in the grooves, ink partially remaining outside the groove, and ink completely remaining outside the groove. Figure 3A illustrates the established process phase diagram. It is evident that increasing the groove width from 8 to 24 μm results in a linear and significant increase in the offset distance required for self-attractive filling. However, when the groove width is further increased from 24 to 32 μm, the increase in the offset distance for self-attractive filling decreases. When the groove width is increased from 32 to 40 μm, the offset distance required for self-attractive filling remains almost constant. This indicates that the increase of groove width at the same depth helps to realize the self-attractive filling of ink, but there is a saturation value, and when the groove width increases to a certain value, the offset distance that can be self-attractively filled remains unchanged.

thumbnail Figure 3

Process rules and effect characterization of self-confined electrohydrodynamic printing. (A) Filling phase diagrams for grooves with different line widths; (B) ink flow distance in grooves at different voltages; (C) resistance and resistivity of electrodes with varying widths; (D)–(H) micro-structured substrate filling effect with various patterns.

This paper presents the results of an experimental investigation into the effects of groove width, groove shape, and voltage on ink flow, as shown in Figure 3B (groove depth 8 μm, nozzle diameter 25 μm, fill time 1 min, nozzle to substrate distance 30–35 μm, reservoir diameter uniform 60 μm). This study reveals that ink flows a significantly greater distance directly from the groove filling compared to the reservoir filling. Here, we utilize the “flow distance” to compare the groove-filling ability in quantity. Flow distance represents the distance between the ink injection position and the front surface of the flowing ink. Specifically, under a voltage of 500 V, the average flow distance of ink from an 8 μm wide groove is 1570.57 μm, while the average flow distance of ink from the reservoir is only 371.78 μm. This gap decreases as the groove width increases. Inks with high dynamic viscosity may benefit from an increase in the voltage of the electrohydrodynamic printing process. This increase in voltage results in a higher initial velocity of ink droplets and a longer flow distance of ink in the groove. During the initial printing stage, the voltage is set to 450 V and then rapidly increased to 700 V after the Taylor cone jet is generated from the nozzle. In these operational conditions, the average flow distance of ink in the 8 μm wide groove is 4151.14 μm, which is 2.64 times higher than that of the 500 V voltage condition. This substantially improves the filling efficiency.

The resistivity of pure copper is 1.6 μΩ cm. The resistivity of the transparent electrode produced in this paper ranges from 2.987 to 3.697 μΩ cm, which is only 1.9 to 2.3 times that of pure copper, as shown in Figure 3C. This method enables the preparation of not only linear metal meshes (Figure 3D) but also serpentine curves (Figure 3E). Furthermore, it is capable of accommodating variations in wire thickness (Figure 3F and G). The method described can produce metal conductor lines with a width of 1 μm and an aspect ratio of 1 (Figure 3H). Figure S3 displays the optical and 3D structural information for micro-structured substrates, including unfilled, silver ink-filled, and electroplated copper.

Stretchable and flexible transparent electrodes play a crucial role in the development of stretchable optoelectronic devices, as they can withstand large deformations and conform to irregular surfaces [12,57–59]. This study proposes the use of a serpentine wire electrode grid to enhance the tensile resistance of FTEs. The results of the bending and stretching tests for the transparent electrodes are presented in Figure 4A–D (see Figure S5, Supplementary information for more details). The degree of electrode deformation increased as the bending radius decreased (5, 4, 3, and 2 mm), with inward and outward bending as the two bending directions were analyzed. Following the completion of 1000 bends with a 5 mm radius, the resistance increase rate was found to be 0.22% for inward bending and 0.41% for outward bending. In the case of bending with a 2 mm radius, the resistance increase rate was 1.92% for inward bending and 4.45% for outward bending. It has been observed that when the transparent electrode is bent outward, the conductive lines elongate, resulting in a higher rate of resistance change compared to inward bending. However, even in this case, the rate of resistance increase remains small and does not significantly affect conductivity. This exceptional bending resistance is attributed to the conductive lines being embedded in the flexible substrate. The resistance values of various types of transparent electrodes were compared with the stretching rate, as shown in Figure 4D. The resistance of the square metal grid flexible transparent electrode increased from 2.6 to 10 Ω at a stretching rate of 2.4%. The initial resistance of the commercial ITO film was 18.8 Ω, which increased to 35.6 Ω at a stretching rate of 3.3%. Such a large rate of increase in resistance does not ensure the functional stability of flexible electronic devices. In contrast, the resistance of the serpentine wire metal mesh remained unchanged in the stretch rate range of 0–24%, with a resistance change rate of 9.8% at a stretch rate of 45%. Although the resistance value of the embedded serpentine wire metal mesh transparent electrodes initially increased and then decreased at a certain stretching rate during the stretching process, the rate of change of the resistance value remained low. This characteristic makes it suitable for applications on the human body surface and on complicated objects that are subject to tensile deformation.

thumbnail Figure 4

Comprehensive performance testing of FTEs. (A) Physical drawing of various bending radii; (B) inward bend test; (C) outward bend test; (D) comparison of tensile properties of various types of FTEs; (E) transmittance of FTEs with different spacings of metal grids; (F) square resistance and FoM of FTEs with different spacings of metal grids.

Transmittance and sheet resistance are crucial parameters for evaluating the optical and electrical properties of flexible transparent electrodes. This study investigates the impact of varying the side length of ortho-hexagonal metal grid transparent electrodes with a line width of 4 μm and a depth of 8 μm on their optical and electrical properties as shown in Figure 4E and F (Figure S6). It is observed that increasing the side length of the ortho-hexagonal shape has a slight positive effect on the transmittance of the transparent electrode. However, the effect is minimal, with only a 1% increase in transmittance observed when the side length is increased by 30 μm as shown in Figure 4E. A similar result is observed when the side length is increased from 90 to 180 μm, with a slight increase in the square resistance of the transparent electrode, from 0.1475 to 0.175 Ω/sq. The FoMs of the transparent electrodes with varying spacings exceeded 12,000 as shown in Figure 4F, indicating that the transparent electrodes have a superior combination of optical and electrical performance.

This study evaluates the practical application value of FTEs with embedded metal mesh as transparent flexible heaters and electromagnetic shielding membranes, as depicted in Figure 5A–H. The prepared flexible transparent heater has an electrode width and depth of 12 μm, with dimensions of 30 mm × 25 mm (Figure 5A–C). When different voltages were applied to the heater, a rapid temperature increase was observed within 0–75 s. The rate of temperature rise was found to be higher with higher applied voltage, and the temperature after stabilization also increased as shown in Figure 5D and E. For instance, increasing the voltage from 1 to 4 V resulted in a temperature increase from 30 to 130°C after stabilization. Furthermore, the heating cycle characteristics of the flexible transparent heater were examined, as illustrated in Figure 5E. The durability of the heater during multiple heating and cooling cycles was assessed. The transparent film heater was subjected to a voltage of 2 V for 120 s, followed by a 120-s disconnection, and then another 2 V voltage application for 120 s. This cycle was repeated 15 times as shown in Figure 5F. It was observed that the maximum temperature of the heater remained within the range of 65–72°C during each cycle and that the heating and cooling rates were stable, indicating that the heater is suitable for practical applications.

thumbnail Figure 5

Application validation of FTEs. (A)–(C) Optical images of the transparent flexible heater; (D) schematic diagram of the transparent flexible heater; (E) temperature versus time at different voltages; (F) cycle curve for heating of the flexible transparent heater; (G) schematic diagram of electromagnetic shielding test device; (H) electromagnetic shielding performance of FTEs with different square resistances.

The frequency range of the X-band, which encompasses the range of 8.2 to 12.4 GHz, is utilized in several applications, including weather detection, defense tracking, air traffic control, and satellite communications. This paper examines the EMI performance of EMI shielding film in the X-band. A vector network analyzer (VNA) was employed to measure the scattering parameters, and the EMI shielding efficiency of the sample was calculated as shown in Figure 5G. FTEs with different square resistances were obtained by modulating the wire width and spacing of the electrodes. The results demonstrated that lower square resistance led to higher EMI shielding efficiency. This is attributed to the increased electrical conductivity of the FTEs, which improves both the electromagnetic reflecting and absorbing efficiency of electromagnetic waves [60,61]. The average sheet resistance of the transparent electrode was measured to be 0.1475 Ω/sq, with an average EMI SE of 29.414 dB as shown in Figure 5H. The highest recorded SE value was 35.277 dB. This paper compares the performance of flexible EMI shielding films obtained by different manufacturing methods, as shown in Table 1. The findings indicate that the EMI shielding film prepared in this study demonstrates a superior balance between optical transmittance and EMI shielding efficiency, positioning it as a promising candidate for strategic material development in the field of EMI shielding.

Table 1

Comparison of the performance of different types of flexible EMI shielding films

CONCLUSION

In conclusion, this study has successfully realized a high aspect ratio, high electrical conductivity, and high-resolution flexible transparent electrodes with embedded metal mesh through self-confined electrohydrodynamic printing on a micro-structured substrate, which has the advantages of non-contact filling and high ink utilization. This method enables the complete filling of imprinted microgrooves with a high aspect ratio of 2 (4 μm width and 8 μm depth) with Ag/Cu metal. In addition, the FTE exhibits outstanding mechanical flexibility with a resistance change of less than 5% after 1000 bending cycles with a curvature radius of 2 mm and high resolution with a minimum line width of 1 μm. When employed as a film heater, the fabricated transparent electrode exhibited a high heating temperature (130°C) at a low input voltage (4 V) with a fast thermal response, and its performance remained consistent even after several heating cycles. As an electromagnetic shielding film, it demonstrated an electromagnetic interference shielding efficiency of greater than 29 dB in the X-band. The proposed method provides another potential excellent option to fabricate high-quality FTE and flexible optical electronics. For future advancements, it is recommended to employ roll-to-roll nanoimprinting technology to produce large-area micro-structured substrates. Furthermore, using arrayed electrohydrodynamic printing nozzles is expected to increase the filling rate significantly.

METHODS

Materials

The PDMS (Dow Corning SYLGARD 184) and curing agent were purchased from Dow Corning (USA). The silver nanoparticle ink (EHD50, dynamic viscosity of 100 cP, silver particle diameter of 30–50 nm) was purchased from the Beijing BroadTeko Intelligent Technology Co., Ltd. (China). The superhydrophobic coatings were obtained from the Yantai Zixilai New Material Co., Ltd. (China). The copper electroplating solution and device were purchased from Shenzhen Gaon Technology Co., Ltd. (China).

Fabrication of the FTEs with embedded metal mesh

The micro-structured substrate with patterned grooves was obtained by spin-coating the PDMS mixture solution (ratio of the PDMS elastomer and curing agent is 10:1) onto the PDMS template and peeling it off with tweezers after complete curing. The PDMS substrate with a structured surface underwent oxygen plasma etching. The etching parameters were as follows: oxygen flow rate of 50 sccm, reaction power of 150 W, and reaction time of 150 s. Thereafter, a hydrophobic coating was applied to the micro-structured substrate using a dust-free cloth. The cloth was left on the substrate for 5 min and then allowed to dry naturally for 5 min under room temperature conditions. The micro-grooves were filled with silver ink using electrohydrodynamic printing equipment. Following this, the transparent electrodes were maintained in a controlled environment at 30°C and 80% relative humidity for 5 min, after which they were subjected to a further 40 min on a 150°C hot plate or oven. Finally, electroplating of copper was performed at a current density of 1.5 A/dm2.

Characterization method

Oxygen plasma etching was conducted in a reactive ion etching machine (Etchlab200, SENTECH Instruments GmbH, Germany). The process of filling the microgrooves with silver ink by electrohydrodynamic printing was documented by a high-resolution camera and lens (acA2440-75 μm, Basler, Germany; MML4-HR65VI-5M, Moritex, Japan). Optical images of the metal mesh were captured using a 3D microscope with ultra-depth capabilities (DSX 510, OLYMPUS, Japan). The microstructure and cross-section of the metal mesh are characterized using a field emission scanning electron microscope (JSM-7600F, Japan Electronics Co., Ltd, Japan). The 3D structure information of the micro-structured substrate and embedded metal mesh flexible transparent electrode was captured by a white light interferometer (NewView 8300, Zygo, USA). The optical transmittance of the FTEs with embedded metal mesh was measured with an ultraviolet spectrophotometer (UH4150, Hitachi, Japan). The square resistance of the FTEs was measured using a four-probe square resistance tester (ST2258C, Lattice Electronics Co., Ltd., China). The tensile and bending tests of the transparent electrode were conducted using a single-column bench universal material test system (5944, Instron, USA). Resistance changes of transparent electrodes were recorded during the bending and stretching tests using a digital multimeter (34465A, Keysight, USA). The scattering parameters were measured using a vector network analyzer (N5234B PNA-L, Keysight, USA).

Data availability

All data generated during this study are included in this published article.

Acknowledgments

The general characterization facilities are provided by the Flexible Electronics Manufacturing Laboratory in the Comprehensive Experiment Center for Advanced Manufacturing Equipment and Technology at Huazhong University of Science and Technology.

Funding

This work was supported by the National Key Research and Development Program of China (2021YFB3200700) and the National Natural Science Foundation of China (51925503, 52175537).

Author contributions

B.W did the investigation, performed experiments, analyzed the data, and wrote the manuscript. R.H. and D.Y. provided suggestions on the groove design and data analyses. Y.P. and Y.A.H. optimized the experimental design, and reviewed and revised the paper.

Conflict of interest

The authors declare no conflict of interest.

Supplementary information

Supplementary file provided by the authors. Access here

The supporting information is available online at https://doi.org/10.1360/nso/20240016. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.

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All Tables

Table 1

Comparison of the performance of different types of flexible EMI shielding films

In the text

All Figures

thumbnail Figure 1

Principle and process effect of self-confined electrohydrodynamic printing on micro-structured substrates. (A) Schematics for the fabrication process of Ag/Cu mesh film; (B) FTEs with an embedded metal mesh; (C), (D) the optical and SEM images of FTEs with an embedded Ag/Cu mesh; (E) comparison of copper electroplating on the PDMS plane and in the PDMS micro-grooves; (F) comparison of FTEs made in other works [15–36].
In the text
thumbnail Figure 2

Numerical simulations and experimental verifications of self-confined printing of droplets on microgroove surfaces. (A) Schematic representation of the local hydrophilic and hydrophobic treatment; (B), (C) numerical simulation and experimental phenomenon when the droplet diameter exceeds the groove width; (D), (E) numerical simulation and experimental phenomenon when the droplet deposition position deviates from the groove. The yellow dotted line in the figure represents the groove edge line.
In the text
thumbnail Figure 3

Process rules and effect characterization of self-confined electrohydrodynamic printing. (A) Filling phase diagrams for grooves with different line widths; (B) ink flow distance in grooves at different voltages; (C) resistance and resistivity of electrodes with varying widths; (D)–(H) micro-structured substrate filling effect with various patterns.
In the text
thumbnail Figure 4

Comprehensive performance testing of FTEs. (A) Physical drawing of various bending radii; (B) inward bend test; (C) outward bend test; (D) comparison of tensile properties of various types of FTEs; (E) transmittance of FTEs with different spacings of metal grids; (F) square resistance and FoM of FTEs with different spacings of metal grids.
In the text
thumbnail Figure 5

Application validation of FTEs. (A)–(C) Optical images of the transparent flexible heater; (D) schematic diagram of the transparent flexible heater; (E) temperature versus time at different voltages; (F) cycle curve for heating of the flexible transparent heater; (G) schematic diagram of electromagnetic shielding test device; (H) electromagnetic shielding performance of FTEs with different square resistances.
In the text

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