Aerosol Printing of 3D Conductive Microstructures via Precision Dot Modulation

Abstract

The advancement of electronic devices necessitates the fabrication of high‐precision, 3D conductive microstructures using functional materials. This study introduces an improved pneumatic shuttering method for aerosol printing (AP), enabling the fabrication of 3D microstructures. The approach overcomes the limitations of conventional AP techniques, which struggle to print dot‐based structures essential for constructing intricate 3D geometries layer by layer. To address this challenge, a pneumatic shuttering mechanism based on flow‐path control is developed, enabling rapid on–off jet for both line and dot printing. This technique allows precise dot modulation (ranging from 20 to 144 µm), facilitating high‐resolution and scalable patterning. Leveraging this capability, an analog halftoning technique is implemented, enabling precise control of the deposition of functional materials. Additionally, the method supports the fabrication of complex 3D microstructures, including conductive pillars with customizable angles relative to the substrate. These pillars serve as interconnects for chips with uneven surfaces, effectively addressing challenges associated with large height variations. This advancement in AP technology significantly enhances deposition precision and patterning flexibility, broadening its potential for advanced material applications in next‐generation electronics and additive manufacturing.

A fully integrated aerosol printing system is developed featuring an enhanced printhead, high‐performance pneumatic shuttering, precise dot control, advanced software, and optimized material processes for accurate deposition of functional materials with tunable dot sizes. This system enables the fabrication of 3D structures with minimal overspray, significantly advancing aerosol printing for complex microstructures in next‐generation electronic applications.

1. Introduction

3D printing has revolutionized microstructure fabrication, enabling precise control over geometry and material properties for applications in electronics,^{[ 1 ]} biomedical engineering,^{[ 2} , ^{3 ]} sensing technologies,^{[ 4 ]} and microfluidics.^{[ 5} , ^{6 ]} Due to the unique advantages of 3D microstructures, such as increased sensing surface area, enhanced stretchability, and higher device integration density, 3D microstructures, including micro/nanopillars, nanowires (NWs), and scaffolds, are highly attractive for advanced applications.^{[ 7} , ⁸ , ^{9 ]} Conventional microfabrication methods, such as lithography, high‐vacuum deposition, selective laser melting, fused deposition modeling, and chemical etching, have also been widely used to build 3D micro‐architectures on planar substrates.^{[ 10} , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ^{18 ]} These techniques differ in terms of cost, spatial resolution, and compatible materials.^{[ 18} , ^{19 ]} While these approaches can achieve high‐resolution microstructures at the micrometer scale, they typically require specialized equipment, complex processing, and highly trained personnel.^{[ 13} , ²⁰ , ^{21 ]} Some of these methods are time‐consuming, material‐intensive, and generate significant chemical waste due to their reliance on vacuum‐based processes.^{[ 22} , ²³ , ^{24 ]}

To overcome these limitations, additive manufacturing techniques have emerged as promising alternatives for the direct deposition of 3D microstructures. Methods such as electrohydrodynamic (EHD) printing,^{[ 9 ]} liquid metal printing (LMP),^{[ 25 ]} and aerosol printing (AP)^{[ 26} , ^{27 ]} have been explored for high‐resolution microfabrication. However, both EHD and LMP involve direct contact or near‐contact deposition, making them less suitable for printing on complex or non‐planar surfaces. In contrast, AP, a non‐contact approach, offers significant advantages in terms of material versatility, scalability, and the ability to print onto delicate or non‐planar substrates.^{[ 28 ]} This technique utilizes a continuous and focused stream of aerosol to create line patterns with typical line widths ranging from 10 to 100 µm.^{[ 28} , ^{29 ]} It enables precise material deposition through a nozzle at a variable stand‐off distance (typically 1–5 mm) from the substrate. One of the key advantages of AP is its ability to accommodate a wide range of ink viscosities, from 1 to 1000 cp.^{[ 28 ]} This capability has led to its successful application in various fields, including interconnects, sensors, circuits, energy conversion, medical imaging devices, and wireless communication.^{[ 30} , ³¹ , ³² , ³³ , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ^{40 ]}

Research in AP has expanded to cover various aspects, including ink formulation development,^{[ 41} , ⁴² , ^{43 ]} optimization of process parameters,^{[ 44} , ⁴⁵ , ⁴⁶ , ⁴⁷ , ^{48 ]} quality prediction methods,^{[ 49} , ⁵⁰ , ^{51 ]} nozzle design,^{[ 52} , ⁵³ , ^{54 ]} head design,^{[ 55} , ^{56 ]} and shutter design.^{[ 57} , ^{58 ]} Among these areas, the development of precise on‐off jet control has emerged as a crucial focus, driven by the need to manage the continuous aerosol jet streams characteristic of the AP system.^{[ 57 ]}

To date, the application of AP has been predominantly limited to line‐based printing^{[ 28 ]}, which requires minimal on‐off control. However, dot‐based printing is essential for 3D microstructure fabrication, as it serves as the fundamental building block for constructing intricate geometries layer by layer with high precision. Despite this potential, the advanced drop‐on‐demand method for AP and its applications remain underexplored. Note that most modern printers require digital bitmap images as printing data, which can be generated by photos, camera images, or even CAD information.^{[ 59 ]} As a result, most printing technologies rely on dot‐based drop‐on‐demand printing. To fully harness the benefits of drop‐on‐demand functionality, this study explores new aspects of AP by developing an efficient shuttering method, thereby expanding its potential applications.

The original design of the system includes an external mechanical shutter positioned outside the nozzle.^{[ 60 ]} Its purpose is to block or redirect the aerosol jet emitted from the nozzle when in standby mode. However, this setup requires a significant distance between the nozzle and the substrate to accommodate the operation of the mechanical shutter. Unfortunately, such a large stand‐off distance often leads to non‐optimal deposition because it deviates from the ideal nozzle‐to‐substrate distance (1–5 mm), where the aerosol jet is most focused.^{[ 57 ]} Researchers have observed that the stand‐off distance tends to be greater than optimal when utilizing an external shutter.^{[ 58 ]} Moreover, the external shutter can create complications when dealing with uneven surfaces such as concave areas or protruding objects like chips mounted on printed circuit boards (PCBs).^{[ 58 ]} Additional drawbacks of external shutter include mist scattering and material buildup during the blocking process.^{[ 61 ]}

In contrast, internal shuttering offers advantages as it occurs within the print head, upstream of the deposition nozzle orifice, enabling optimal stand‐off distance printing.^{[ 57} , ⁵⁸ , ^{61 ]} In our previous report,^{[ 57 ]} we proposed an AP system having a rotary valve as an internal shutter as an alternative to external shutters. However, directly diverting the aerosol internally induces a fluctuation in the pressure of the flow cell, resulting in additional transition time for consistent on‐off printing. To mitigate the pressure fluctuation issue, a method for compensating the flow rate of the virtual impactor during shuttering was proposed to restore the internal pressure within a short time and reduce the jetting delay.^{[ 57 ]} Moreover, this flow rate compensation method applies only to AP systems using pneumatic atomization and is limited when applied to systems with ultrasonic atomization.

Keicher et al. introduced a pneumatic‐based external shuttering system that effectively prevents pressure drops during the shuttering process, with reported transition times as short as a few milliseconds.^{[ 62 ]} Their approach utilizes a perpendicular gas flow, known as the bypass flow, by mounting the shuttering mechanism externally to the collimated aerosol stream. During shuttering, the aerosol stream is redirected through a secondary flow channel, allowing the bypass flow to efficiently divert the particle stream away from the flow axis toward an exhaust port. However, the use of external pneumatic shuttering requires the installation of the shuttering component below the nozzle, which can introduce complexities in the printing process.

Recently, Optomec Inc.^{[ 63 ]} introduced an internal pneumatic shuttering method based on boost gas, where an additional boost gas is added into the sheath gas.^{[ 58 ]} This boost gas flows in the opposite direction to the aerosol stream, effectively blocking mist flow during the non‐printing phase. However, this research has primarily been published in patent form, with a focus on legal claims rather than scientific analysis or experimental validation. As a result, there is limited insight into the actual performance, efficiency, and potential constraints of pneumatic shuttering systems in practical applications. Furthermore, there has been little exploration of dot‐based printing methods using these systems, despite their significance for many advanced applications.

This study presents an improved pneumatic‐based internal shuttering mechanism that provides simplified yet high‐speed on–off control. The approach offers advantages over conventional mechanical shutters, particularly in fast‐switching applications. Mechanical shutters often cause flow disturbances and momentum‐driven mist dispersion during short‐duration operations (e.g., ≈20 ms), which can result in significant overspray. In contrast, the proposed pneumatic design maintains a constant nozzle flow while internally redirecting the mist, thereby avoiding these issues and enabling smoother transitions, as demonstrated experimentally. The effects of various parameters on the on–off performance were thoroughly investigated, resulting in the development of an unconventional dot‐based printing approach. This method enables continuous (analog) grayscale modulation by tuning the deposition time (valve on‐time), allowing precise control of dot size and height. Such analog grayscale printing significantly enhances image quality compared to conventional inkjet printing, which typically depends on digital halftoning algorithms.

Furthermore, the dot‐based AP method enables the printing of diverse 3D structures using functional materials, including conductive inks, without the need for support structures. Although previous reports have demonstrated the ability to print vertical pillar‐like shapes^{[ 64} , ^{65 ]}, these methods suffered from significant overspray due to limited control over the shuttering process. In contrast, precise on–off jetting control enables cleaner drop‐on‐demand printing with tunable drop volumes, allowing the fabrication of 3D structures with diameters ranging from 15 to 100 µm. Additionally, this method facilitates the fabrication of complex geometries, including pillars with precisely designed angles and shapes. These capabilities are essential for microelectronics applications, where well‐controlled, conductive 3D pillars can serve as interconnects.

2. Results and Discussion

2.1. Dot‐Based AP

To date, AP has primarily been applied for line printing, with limited demonstrations of dot‐based, drop‐on‐demand applications. This section focuses on the drop‐on‐demand capabilities of AP, utilizing the pneumatic shuttering method described in the “Experimental Section”. Unlike conventional inkjet printing, the proposed AP method offers unique dot modulation capabilities for drop‐on‐demand printing. The dot size (or height) can be smoothly and proportionally controlled by adjusting the duration of deposition on a single spot. Here, the deposition time (T_d) is regulated by a three‐way solenoid valve that controls the mist flow path. During the “on” phase, the mist flow is directed toward the nozzle; in the “off” phase, the boost gas blocks the mist flow, redirecting it to the vent. This precise on‐off jet control, in conjunction with stage motion, allows for accurate deposition on targeted substrate locations. Note that the T_d refers to the solenoid “valve on‐time” which defines the duration for which the aerosol droplets are deposited to form a single dot.

To print consistent dots, precise control of the T_d is crucial. The dot size can be easily varied by adjusting T_d, as shown in Figure  1a. (For better understanding, refer to Movie S1, Supporting Information). It is important to note that, due to the switching speed limitation of the solenoid valve and the travel time of the aerosol mist from the shutter to the nozzle, a minimum T_d of 15 ms is required. At this minimum T_d (15 ms), variations in drop size may occur, likely caused by slight inconsistencies (a few milliseconds) in the valve response time. However, the dot size becomes significantly more uniform when T_d exceeds 20 ms. Given practical constraints, the operating shutter frequency for dot modulation is limited to ≈10 Hz, enabling effective control of dot sizes while maintaining jet uniformity (Text S1 and Movie S2, Supporting Information). To increase jet frequency, a high‐speed switching valve can be employed. Additionally, a sophisticated head design could be used to reduce the aerosol mist's travel time between the nozzle and the vent.

Figure 1.

a) Printed dot images with respect to T_d, [scale bar 100 µm], b‐d) effects of T_d on b) diameter, c) height, and d) volume.

Figure 1b,c show measured dot diameter and height as a function of T_d, respectively. Each diameter was determined by averaging 12 different dots. When T_d is set to 15 ms, the printed dot diameter is ≈20 µm, with a corresponding height of about 1.41 µm. In contrast, when T_d is increased to 120 ms, the dot diameter expands to ≈144 µm, with a corresponding height of about 6.25 µm. It is worth noting from Figure 1c that the maximum height remains below 7 µm, primarily due to droplet spreading rather than stacking. One strategy to address this issue is to promote rapid evaporation of atomized droplets, either by adjusting the substrate temperature or using solvents with lower boiling points. The dot volume in Figure 1d, calculated using Equation (S1) in Supporting Information, Section S2 (Supporting Information), shows proportionality with respect to T_d. This indicates that the deposition amount can be easily controlled by adjusting T_d, which is an important parameter for process optimization.

The dots generated by AP exhibit unique characteristics, including a higher aspect ratio than those produced by conventional inkjet printing. Comparative 3D shapes and corresponding height profiles of dots printed by AP and inkjet printing are shown in Figure  2 . The height of the dots produced by AP with a T_d of 50 ms reaches 6.1 µm (Figure 2a), while the height of the dots from inkjet printing is ≈0.15 µm (Figure 2b). The height difference can be attributed to two main factors. First, AP enables the use of higher viscosity inks. Second, unlike inkjet printing, which relies on individual droplets, AP employs a focused atomized jet stream consisting of smaller, multiple droplets that promote faster evaporation. As a result, even with a very short T_d, dots printed with AP are ≈40 times taller than those produced by inkjet printing. Additionally, inkjet‐printed dots often exhibit the coffee‐ring effect, where particles accumulate along the droplet edges during drying, leading to non‐uniform particle distribution. In contrast, AP‐generated dots show no evidence of the coffee‐ring effect.

Figure 2.

3D shape and thickness profile of Ag NPs dots printed using: a) AP and b) inkjet printing.

2.2. Dot Modulation

In conventional inkjet printing, the size of printed dots is typically controlled by producing multiple dots using a series of driving waveforms (a method known as grayscale printing)^{[ 59 ]}; however, this approach offers only limited control over droplet size modulation. In contrast, AP enables continuous dot modulation, allowing for highly accurate control over dot size. Figure  3a illustrates AP's dot modulation capability (refer to Movie S3, Supporting Information), where an array of dots was printed with T_d incrementally increased from 20 ms to 65 ms in 5 ms steps. Notably, the time increment was arbitrarily chosen, and the T_d can be adjusted to any value, as long as stable on–off performance is maintained.

Figure 3.

Dot modulation printing. a–c) AP results: (a) microscopic image of the dot patterns, (b) 3D image of the printed dots, (c) height profile. d–f) Inkjet printing results: (d) microscopic image of dot patterns, (e) 3D image of printed dots, (f) height profile. g–i) Gradient circle image and printed results using continuous dot modulation with AP: (g) sample gradient circle image with a pixel size of 30 × 30, (h) 3D profile of printed results, and (i) height profile along the radial direction.

For comparison, an array of dots was printed using inkjet printing, where the dot size was modulated (increased) by increasing the number of drops, as shown in Figure 3d. Interestingly, the 3D surface morphology of the printed dot arrays produced by AP and inkjet printing differ significantly, as shown in Figure 3b,e, respectively. In inkjet printing, increasing the number of droplets from 1 to 10 results in dot diameters ranging from 70 to 130 µm (Figure 3f), but achieving a substantial increase in dot height remains challenging. Inkjet‐printed dots exhibit heights ranging from only 0.15 µm to 0.4 µm and are prone to the coffee‐ring effect.

In contrast, AP achieves dot diameters ranging from 30 to 100 µm by simply adjusting T_d, eliminating the need for multiple droplets. Moreover, although the dot diameters produced by both methods may appear similar, AP significantly outperforms inkjet printing in dot height, producing heights ranging from 3 to 5.40 µm (Figure 3c). This demonstrates AP's superior capability to fabricate taller dots with precise control while avoiding the coffee‐ring effect.

To further demonstrate AP's drop modulation capability, an 8‐bit grayscale image (Figure 3g) of a radial gradient circle with a pixel size of 30 × 30 was printed. The grayscale bitmap image contains values from 0 to 255, representing brightness levels. In the image example shown in Figure 3g, the center has the highest grayscale value (255), gradually decreasing toward the edges. These grayscale values were converted into deposition amounts, with T_d adjusted proportionally to the image value at each corresponding location. For this demonstration, T_d was set to range from 20 to 60 ms, depending on the grayscale level. Within this range, the minimum T_d (20 ms) corresponds to the smallest dot size, representing the lowest grayscale value in the circle, while the maximum T_d (60 ms) produces the largest dot size, corresponding to the highest grayscale value. Figure 3h shows the 3D surface profile of the printed gradient circle. As shown in the cross‐sectional height profile in Figure 3i, the approach ensures a smooth gradient of dot heights, with dot sizes (or drop amounts) modulated according to grayscale values. This precise control over droplet deposition highlights AP's ability to create complex patterns with finely tuned dot sizes, tailored to target deposition amounts at specific locations, as demonstrated with grayscale images.

2.3. Analog Halftone Printing

This section further delves into digital image printing using dot‐modulated AP. In conventional digital photo printing, grayscale images are converted into binary images, where each pixel is represented by a single on/off value.^{[ 66 ]} The simplest method for generating binary printing data is image thresholding, a basic form of image segmentation that divides the image into printing and non‐printing locations. For demonstration, a 32‐bit color image (Figure  4a) was converted to an 8‐bit grayscale image (Figure 4b) by extracting luminance information.^{[ 67 ]} The grayscale image can then be converted to a binary image using a threshold value (e.g., 164)^{[ 67 ]}, which can be directly used as printing data. Figure 4c shows the printed image resulting from the converted binary image. However, compared to the original photo, significant information is lost through this direct thresholding approach.

Figure 4.

Photographs and microscopic images of the aerosol printed samples using Ag NPs ink: a) original color image b) extracted grayscale image, c) aerosol printed result of binary image obtained by simple thresholding (pixel threshold = 164, T_d = 40 ms), and d) aerosol printed image of binary pixels obtained by error diffusion halftone algorithm (pixel threshold = 127, T_d = 50 ms). The color photo in (a) is the corresponding author's own image and is used with permission; no copyright issues apply.

To overcome the limitations of simple thresholding, halftoning algorithms generate the illusion of continuous‐tone images on binary displays by utilizing the human eye's ability to blend halftone patterns into a smooth image.^{[ 68 ]} Among these methods, digital halftoning based on the error diffusion algorithm is commonly used in digital printing. Figure 4d shows the printed result using a binary image generated through the error diffusion halftoning method with a threshold value of 127.^{[ 69} , ^{70 ]} Although this method enhances the printed image quality, the result still lacks smooth transitions and contains missing details, particularly when magnified. These limitations arise from the constraints of binary on‐off pixel printing, which cannot be fully addressed without adopting an analog printing approach.

The proposed dot‐based AP method is capable of printing analog information, as T_d can be adjusted continuously. Unlike conventional methods, this approach allows for variation in both dot size and height (deposition amount) at each pixel, directly corresponding to the image's intensity values. This approach produces images with enhanced precision, smoothness, and accuracy, as shown in Figure  5 . The proposed technique bypasses the binary image conversion typically required in other digital printing methods. Instead, it maps intensity levels from 0 to 255 (8‐bit) directly to T_d values, which control the deposition amount. The T_d values range from 20 to 60 ms across 255 gradations, with locations at zero intensity values excluded from the printing process to represent non‐printing areas. Furthermore, this method is versatile and can be applied to any scenario that requires variable material intensities or quantities at specified locations. To the best of the authors' knowledge, no commercially available digital on‐demand printing technique currently exists that can fully represent images with continuous tones (analog halftone). It is also worth noting that the printed results may not be perfectly analog, as the original photo is digital, with values from 0 to 255.

Figure 5.

Aerosol printed results using proposed analog halftoning method (T_d = 20–60 ms).

2.4. 3D Microstructure Printing Using Dots

Efficient shuttering performance is essential for fabricating 3D microstructures of functional materials through dot‐based printing. Without precise and rapid on‐off control, achieving complex and intricate patterns becomes challenging. By implementing a fast and accurate jet shuttering method, the spreading effect can be minimized, allowing the formation of pillar‐like structures with less reliance on ink properties.

T_d control alone can print pillar‐like 3D microstructures, as shown in Figure 3. However, this approach has limitations on effectiveness and precision, particularly when creating diverse micro 3D structures. To address these limitations, we propose a multiple‐dot printing method that enhances the evaporation rate and increases the vertical growth of printed features. By combining T_d control with multiple dot stacking, this approach offers unique advantages for AP, enabling the fabrication of structures that are difficult to achieve with other methods.

Figure  6a illustrates the fabrication of pillars using a multiple‐dot printing technique, emphasizing the direct correlation between the number of deposited dots and the resulting pillar height. For example, a pillar printed with 10 dots reaches a height of 130 µm, whereas a pillar with 50 dots attains a height of 613 µm, with minimal overspray on the substrate (see Movie S4, Supporting Information). To ensure the successful formation of high‐aspect‐ratio structures, the 3D microstructures were printed on a platen maintained at 80 °C, which promoted rapid droplet evaporation and helped prevent the spreading or deformation of successive dots. While previous continuous aerosol jet methods^{[ 65 ]} have achieved upright pillar printing, they often exhibit significant overspray around structures, presenting challenges, particularly in fabricating interconnection lines for electronic devices. Furthermore, continuous jet methods also face limitations in creating complex structures, such as pillars at arbitrary angles.

Figure 6.

a) 3D pillar height variation with different numbers of stacked dots, b) diameter control of the 3D pillars by adjusting the T_d (20–100 ms), c) 3D pillars at various angles relative to the substrate by lateral shifting of dots and multi‐directional pillars at different angle (inset) d) inverted V‐like 3D structures and non‐connected crossline printing achieved through 3D bridge structure formation (inset), e) complex 3D spiral structure, f) 3D bridges to connect electrodes on surfaces with different height levels, and g) demonstration of electrical connection of printed 3D bridges using LED lights.

This method offers a significant advantage in precisely controlling the pillar diameter, a capability lacking in most previous AP techniques. A notable feature of this approach is the ability to adjust pillar diameter through drop‐on‐demand jetting by varying the T_d. By adjusting the T_d from 20 to 100 ms, the pillar diameter can range from 15 to 100 µm, as shown in Figure 6b. Another key feature is the ability to print pillars at various angles relative to the substrate, which is essential for constructing complex structures. This versatility is demonstrated in Figure 6c, where pillars at varying angles were achieved by laterally shifting the position of each successive dot by 0 to 10 µm. The minimum achievable angle of a pillar relative substrate depends on the height of each dot and the magnitude of the lateral shift. Furthermore, the maximum lateral shift is influenced by factors such as ink properties (e.g., viscosity and evaporation rate of solvents), platen heating temperature, and dot size (pillar diameter). A 20 µm diameter pillar with a 42‐degree angle was fabricated successfully by applying a 10 µm lateral shift during each sequential dot deposition. Attempts to print pillars with tilt angles below 40 degrees or above 140 degrees may result in practical challenges, such as excessive overspray around the deposition area, which compromises the functionality and precision of the printed structures (inset of Figure 6c; Movie S5, Supporting Information).

The angle (θ) of a pillar relative to substrate can be estimated from the lateral shift (d_s) (the distance of a dot from the previous dot) and dot height (h_dot) using the following relation:

$$\theta =\mathrm{co}{\mathrm{s}}^{-1}\left(\frac{d_s}{\sqrt{\left({h_{dot}}^2+{d_s}^2\right)}}\right)$$ (1)

Based on Equation (1), custom printing software was developed to fabricate user‐defined 3D structures. Figure 6d shows inverted V‐like structures printed at designed angles, suitable for creating 3D bridges (see inset) that enhance circuit design flexibility (see Section S3 and Movie S6, Supporting Information). Additionally, more intricate 3D structures, such as 3D spiral patterns can be printed without requiring support structures, as demonstrated in Figure 6e (see Movie S7, Supporting Information). Notably, this approach significantly reduces overspray compared to earlier studies^{[ 65 ]}, as further highlighted in Figure S4 (Supporting Information) (SEM images).

Figure 6f demonstrates 3D interconnection lines bridging two surfaces of different height levels (see Movie S8, Supporting Information). To validate this functionality, 3D electrical lines were printed to connect a bottom glass substrate to two LEDs and two resistors mounted on a top substrate (0.5 mm thick), as shown in Figure 6g (for details explanations, refer to Section S5, Supporting Information). Following the printing process, the entire structure was sintered on a hotplate at 240 °C for 15 min to ensure electrical connectivity, including across the 3D bridge lines spanning the pads. Successful LED illumination upon applying voltage confirms the effectiveness of these 3D‐printed interconnects (see Movie S9, Supporting Information).

3. Conclusion

In summary, this research introduces a significant advancement in AP technology through the introduction of a novel dot‐based printing technique enabled by an efficient internal pneumatic shuttering system. This shuttering mechanism enhances the precision and versatility of AP's by allowing precise modulation of dot sizes and shapes, marking a major improvement in both control and application scope.

Key achievements include:

• Development of a pneumatic shuttering system that enables fast and efficient control of aerosol jet, minimizing the transition delays between printing and non‐printing states by reducing pressure fluctuations.

• Implementation of dot‐based printing, which achieves Ag NPs ink dots with diameters ranging from ≈20 to 144 µm and thicknesses from 1 to 6.25 µm by adjusting T_d from 15 to 120 ms. This provides a high degree of flexibility in dot modulation.

• Realization of grayscale bitmap printing by directly mapping image intensity values to T_d without converting to binary images. This enables precise material deposition at the target location, offering great potential for applications requiring complex and highly accurate on‐demand deposition.

• Demonstration of 3D structure printing, including vertical and angled pillars ranging from 40° to 140° relative to the substrate, using functional materials. These structures were printed with high aspect ratios and without the need for support structures. In addition, 3D interconnects and bridge structures were printed with minimal overspray, an essential requirement for microstructures in electronic applications.

Finally, this research not only improves resolution and feature definition in AP but also extends its capability from conventional 2D patterning to high‐precision 3D printing. Unlike the previous works, which rely solely on off‐the‐shelf systems, often with clear limitations, this work represents a comprehensive innovation across the entire platform, including hardware, methodology, and software. Ultimately, the innovations presented here offer a scalable and versatile technique for AP, paving the way for broader adoption across industries that require high‐resolution, high‐accuracy additive manufacturing solutions.

4. Experimental Section

AP Process

To demonstrate the proposed internal pneumatic shuttering method, an AP system was developed. Figure  7a shows a schematic of the system incorporating the pneumatic shuttering unit, while Figure 7b presents an experimental photograph taken during operation. The AP system consists of four main components: an atomizer, a virtual impactor, a shuttering unit, and a focusing unit.^{[ 57 ]} To precisely regulate the flow rates of the atomizer, virtual impactor, and sheath gases, three mass flow controllers (MFCs) (SFC5500‐2 slm, SENSIRION, South Korea) were employed. Aerosol droplets were generated using a nitrogen‐based pneumatic atomization method. These droplets, along with the carrier gas, were directed into the virtual impactor for refinement. The virtual impactor served a dual function: removing excess carrier gas and filtering out smaller droplets that contribute to overspray during deposition.

Figure 7.

a) Schematic of the AP technique, b) experimental photograph.

The refined aerosol mist was then enveloped by a co‐flowing sheath gas (dry nitrogen), forming a collimated stream. This stream (aerosol mist) was passed through a 0.2 mm diameter converging nozzle to produce a high‐speed focused aerosol jet. The pneumatic shuttering system was composed of a three‐port external air pilot solenoid valve (SYJ514R‐5LZ‐01, SMC Pneumatics, South Korea) and a vent MFC, as shown in Figure 7a. Additional details of the shuttering process are discussed in the “Pneumatic Shuttering Process” section.

Two cameras (acA1300‐60gc, BASLER, Germany) were used to monitor the printing process: one for a side view and another for a top view, as shown in Figure 7b. The side‐view camera enabled real‐time observation of jetting status, while the top‐view camera was used to inspect printed patterns. Both cameras were equipped with 6× magnification lenses (MML‐HR65, MORITEX, Japan) for high‐resolution imaging. A custom LabVIEW‐based software was developed to control the entire system, including motion control and the on/off operation of the aerosol jet.

Pneumatic Shuttering Process

Figure  8 illustrates the proposed internal pneumatic shuttering method. During the shuttering phase (off condition), the entire aerosol stream is redirected from its original flow path to a vent outlet located in the middle of the flow cell. A vent MFC is used to extract the aerosol mist through this outlet, as shown in Figure 8b. A three‐port solenoid valve controls the flow direction, while two MFCs, a sheath‐boost MFC and a vent MFC, regulate the flow rates during the shuttering process.

Figure 8.

Schematic of internal pneumatic shuttering for AP: a) printing and b) shuttering.

The sheath‐boost MFC performs dual functions: it supplies sheath gas to collimate the aerosol during printing (Figure 8a) and provides boost gas for the shuttering (Figure 8b). These functions are alternated using a three‐way solenoid valve. The vent MFC performs suction of the aerosol mist and the boost gas during shuttering and printing, respectively. Here, a vacuum pump is used for extracting the mist through the vent MFC. A filter was installed between the solenoid valve and the vent port to prevent aerosol mist from entering in solenoid valve, thereby avoiding contamination and potential damage.

One advantage of the proposed shuttering method is its simplified design compared to the Optomec system, which requires an additional MFC for boost gas input. In contrast, the approach eliminates the need for extra hardware by utilizing the sheath gas MFC with a newly developed flow control algorithm. This internal pneumatic shuttering mechanism also supports precision dot control techniques, with detailed performance data provided. The precise on/off control method is effective for both dot and line‐based printing. A comprehensive comparison shuttering mechanisms, including the Optomec pneumatic shutter and the proposed method, is provided in the Supporting Information (Tables S1–S3, Supporting Information).

The ratio between the mist flow rate (Q_mist) and sheath flow rate (Q_Sh), also known as the focusing ratio, is a key parameter for achieving fine features in AP.^{[ 28 ]} To illustrate the flow dynamics in the pneumatic shuttering system, a focusing ratio of 2.5 is considered, with typical flow rates of 20 sccm (standard cubic centimeters per minute) for Q_mist and 50 sccm for Q_Sh. To completely block aerosol particles from exiting the nozzle, the boost gas flow rate (Q_B) must exceed Q_mist, typically by a factor of 1.2 to 2. In this demonstration, Q_B is set to 30 sccm (1.5 times the Q_mist). These flow rates, as shown in Figure 8, are used to explain the directional flow of Q_B during both printing and shuttering, though they may vary depending on the specific purposes and conditions.

During printing, Q_B (30 sccm), considered as excess gas, bypasses the sheath inlet and is vented directly through the MFC control, as shown in Figure 8a. Meanwhile, the Q_Sh (50 sccm) is directed to the sheath inlet to collimate the Q_mist (20 sccm) through the nozzle. The vent outlet remains closed during this phase, ensuring that the aerosol mist reaches the nozzle without diversion.

During shuttering, the combined sheath and boost gas flow (Q_Sh + Q_B = 50 sccm + 30 sccm) is directed into the sheath inlet (Figure 8b). At the same time, the vent at the flow cell opens and extracts the mist at a rate equal to Q_B (Q_V = Q_B = 30 sccm). The Q_B remains fixed at 30 sccm during both the printing and shuttering phases, maintaining flow stability and ensuring reliable MFC operation. Because the Q_B exceeds the Q_mist, all Q_mist (20 sccm) and a portion of Q_B (10 sccm) are diverted through the vent. This diversion occurs in the transition region of the flow cell (indicated in Figure 8b) and prevents Q_mist from reaching the nozzle. At this transition region, the sheath‐boost gas splits into two parts. A portion of the flow (10 sccm) moves in the opposite direction of the original aerosol flow and is exhausted through the vent port along with the Q_mist. The remaining portion (70 sccm) flows to the nozzle, which maintains a constant flow through the nozzle both during printing and shuttering. Importantly, the total flow rate at the nozzle should be constant regardless of the printing on/off condition to ensure a constant pressure inside the flow cell, unlike the mechanical valve used in the previous work.^{[ 57 ]} This can reduce the transition time between on and off switching, significantly enhancing on‐off performance and enabling precise dot‐based printing. For a more detailed explanation of the pressure behavior, please refer to the Section S6 and Movie S10 (Supporting Information). To minimize on/off delays in aerosol jet shuttering, it is critical to reduce the distance between the vent port and the transition region (Figure 8b). A shorter distance leads to a faster mist flow response time, since it shortens the path that aerosol must travel from the switching point to the nozzle.

Materials

For the experimental demonstrations, glass microscope slides (Marienfeld Superior, Germany) were used as substrates. The printing experiments of AP employed a commercially available Ag NPs ink (PARU‐PG‐007‐AP, PARU Printed Electronics, South Korea) with a solid content of 60% and a viscosity of 70 cp. To compare the drop morphology of AP and most used inkjet printing, inkjet printing was performed using Ag ink (ANP‐Silverjet DGP 40LT‐15C, ANP, South Korea), which has a solid content of 30% and a viscosity of 10 cP. It should be noted that different inks were used for AP and inkjet printing due to the viscosity constraints of inkjet technology, which typically requires low‐viscosity inks (1–20 cP). In contrast, low‐viscosity inks are not ideal for 3D microprinting using AP, as their low solid content limits structural integrity. To more effectively demonstrate the system's capabilities, a higher‐viscosity ink was selected for AP. The inkjet printing was executed with a commercial inkjet printhead (Dimatix, DMC‐11610, FUJIFILM, USA). A trapezoidal waveform with a jetting voltage of 30 V, equal rising and falling times of 2 µs, and a dwell time of 3 µs was used for dot printing.

Characterization

The surface morphology of the printed features was examined using an optical microscopy system (MF‐UA1020THD, Mitutoyo, Japan). 3D surface profiles, including feature width and thickness, were characterized using a coherence scanning interferometer (CSI) (ZeGage Pro, Zygo Corp., USA). The 3D micropillar structures fabricated via dot‐based AP were further analyzed using a field emission scanning electron microscope (FE‐SEM) (MIRA LMH, TESCAN, Czech Republic).

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

Md. A.M.: Data curation, Writing – original draft, Methodology, Investigation, etc. J.J.Y.: Software, Data curation. S.‐H.P.: Software, Data curation K.K.‐S.: Writing – review & editing, Supervision, Project administration, Investigation, Funding acquisition.

Supporting information

Supporting Information

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Supporting Information

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Supporting Information

Mosa M. A., Jo J. Y., Park S.‐H., Kwon K.‐S., Aerosol Printing of 3D Conductive Microstructures via Precision Dot Modulation. Small 2025, 21, 2504037. 10.1002/smll.202504037

Data Availability Statement

The data that supports the findings of this study are available from the corresponding author upon reasonable request.