The uniformity of printed linewidth and edge morphology is critical for circuit performance. The primary challenges at this stage arise from instability in aerosol jet focusing, nonuniform ink droplet size distributions, and variations in droplet trajectories during flight. These issues are influenced by multiple factors, including atomizer parameters, ink properties, airflow conditions, nozzle geometry, printing pitch, and substrate characteristics^[73].

Overspray, which appears as sparse, diffuse, mist-like deposits along the edges of printed traces, is a major process defect. It severely reduces print resolution and edge sharpness and may also cause electrical shorts. Pneumatic focusing has been widely used to reduce excessive spraying^[74], However, the physical mechanisms of overspray and its quantitative prediction are still not fully understood. To address this issue, Chen et al. developed a three-dimensional computational fluid dynamics (CFD) model showing that the interaction between droplet size and sheath-gas velocity governs overspray formation and explains its non-monotonic dependence on flow velocity^[71]. From a dimensionless perspective, the inertial response of droplets is typically characterized by the Stokes number (Stk). Small droplets with Stk << 1 are easily deflected by the airflow and become a major source of overspray^[75]. Building on this work, Feng et al. proposed an image-based quantitative method to evaluate overspray based on area coverage. They showed that overspray generally scales with linewidth and can be suppressed by increasing the ejection velocity and optimizing gas-flow conditions^[76]. The overall airflow regime is characterized by the Reynolds number (Re). Both excessively high and low Reynolds numbers can disrupt jet stability^[55]. Accordingly, precise control of gas dynamics is critical: the sheath and carrier gas flow rates determine jet focusing and print quality^[77], and their ratio, defined as the focusing ratio (FR), is widely used to regulate jet confinement^[78]. However, changing only the FR is not sufficient for comprehensive and stable control of jet characteristics, Ramesh et al. incorporated total gas flow into a systematic analysis, showing that excessive flow broadens line width and must be coordinated with FR, while also revealing that submicron droplets tend to diffuse with the gas flow and contribute to overspray, whereas larger droplets (> 2 μm) are more effectively focused, enabling sharper features^[79]. To address this limitation, Jin et al. developed a high-fidelity CFD model that links in-flight dynamics with post-impact behavior. Their results showed that near-substrate pressure fields can induce droplet splashing and satellite formation, thereby contributing to overspray^[80].

In addition to improving mechanistic understanding through modeling, further improvement in printing precision requires attention to the microscopic phase changes of droplets during flight. Recent studies have shown that in-flight droplet evaporation is a key mechanism for overspray formation. Peripheral droplets can rapidly evaporate and shrink after contacting dry sheath gas. This process reduces their inertia, namely the Stk, and prevents them from effectively reaching the substrate, thereby causing overspray. Based on this mechanism, solvent-vapor presaturation of the sheath gas has been proposed to suppress droplet evaporation^[81]. Experiments showed that this approach reduces overspray by approximately 70% for water-based polyimide (PI) inks and significantly improves line edge morphology [Figure 5A].

To achieve more active and precise control of jet behavior and deposition morphology, researchers have introduced external physical fields to expand the process design space through multi-physics coupling. In the early practices, Park et al. demonstrated that applying an electric field inside a coaxial nozzle can combine aerodynamic and electrostatic forces. This method enables effective focusing and stable deposition of nanoparticles and microparticles, even under low-Stokes-number conditions^[82]. This strategy significantly reduces the feature size of deposited patterns and improves deposition predictability [Figure 5B]. Building on this approach, more multi-physics field-assisted strategies have been explored in recent years. Among them, acoustic focusing has gradually attracted attention as a non-contact regulation method. Annular acoustic field focusing technology applies additional focusing forces to nanoparticles within the jet by introducing acoustic waves at specific frequencies. When combined with an improved nozzle structure, this method achieved ultrafine printing with linewidths below 6 μm and overspray below 0.1 μm. It also increased the line conductivity to 180% of that obtained without focusing^[39], as shown in Figure 5C. In addition, a nozzle design with an internal heating channel has been reported. In this design, constant-temperature hot water circulates through a spiral flow channel inside the nozzle, forming a ring-shaped thermal field around the aerosol flow, as shown in Figure 5D. This thermal field significantly enhances aerodynamic focusing by regulating gas density and pressure distribution. As a result, it improves deposition morphology. The linewidth decreased from 14.78 to 7.02 μm, the thickness increased from 1.14 to 2.06 μm, and the aspect ratio improved to approximately 0.3. This optimization primarily stems from the thermal field’s regulatory effect on the convective flow. This method has been applied to fabricate high-density circuits with a line pitch of 15 μm, high-performance conformal electrodes, and highly conductive heating elements, demonstrating its potential for conformal electronics manufacturing^[40].

Based on the above studies, CFD modeling and parameter optimization are easy to implement because they do not require hardware modification. However, their accuracy in describing complex multi-physics coupling remains limited. The solvent vapor saturation method can reduce over-spraying by approximately 70% and is particularly suitable for water-based inks. However, it requires an additional bubbling device. Electric-, acoustic-, and thermal-field-assisted approaches can achieve higher resolution (line width < 10 μm) and improved deposition morphology, but involve greater hardware integration complexity. Therefore, strategies for overcoming linewidth variation and morphological inconsistency have shifted from early empirical adjustment based on a single FR to more systematic approaches. These approaches include global gas-flow management, full-process simulation, in-flight phase-change control, and multi-physics assisted focusing. These advances have significantly improved the manufacturing accuracy and on-demand printing capability of AJP. However, single-nozzle systems still have low throughput, and scale-up production remains challenging because it is difficult to maintain consistency among multiple nozzles. Moreover, the trade-off between printing speed and resolution has not been sufficiently analyzed, and the long-term operational reliability of AJP still requires further investigation.

Jet optimization and precise deposition ensure accurate initial line formation. However, the spreading, drying, coalescence and final curing processes after droplet landing also directly determine the microstructure and macroscopic properties of the functional layer. In other words, jet formation and deposition control material placement, whereas post-processing determines whether the deposited materials can achieve the desired functional performance. Therefore, after aerosol jet optimization and precise deposition, as discussed in Section “Spraying and focusing: overcoming challenges in line width and morphology consistency”, the final shaping and curing of droplets on the substrate directly determine the core performance of functional devices. Key challenges at this stage include the coffee ring effect^[83], high porosity, and poor microstructural compactness^[84]. These defects can cause nonuniform resistance in conductive patterns, reduce overall conductivity, weaken mechanical strength and adhesion, and decrease pattern resolution and line definition. Consequently, they limit device performance and reliability. In optical applications, they may also cause nonuniform coating thickness or transparency and accelerate material aging. Therefore, active control of post-deposition morphology and effective post-processing are critical for ensuring that printed devices, such as electrodes, sensors, and energy storage components, achieve the expected performance and reliability.

To address these issues, morphological control and post-processing strategies usually focus on two stages: regulating the aerosol state and deposition rate, and optimizing the microstructure and intrinsic properties of the deposited material. The first stage involves regulating the “wet/dry state” during aerosol deposition to influence the diffusion and curing rates. As discussed at the end of Section “Spraying and focusing: overcoming challenges in line width and morphology consistency”, presaturating the sheath gas with solvent vapor during jetting can control droplet evaporation and thereby regulate the physical state of droplets, including viscosity and surface tension, when they contact the substrate^[81,85]. This process directly affects ink spreading, polymerization, and solvent evaporation after deposition, and it serves as an important upstream method for improving deposition morphology. The second stage is to conduct active intervention after droplet deposition, with the focus on precisely controlling the energy input to optimize the physical morphology and functional properties of the deposited ink. The most traditional and direct method is substrate heating, which regulates droplet spreading, wetting, and drying rates. Under appropriate conditions, substrate heating promotes rapid solvent evaporation and suppresses the coffee-ring effect. Winnicki et al. demonstrated that online heating of PI substrates to 90 °C during AJP printing significantly accelerated solvent evaporation in silver nanoparticle (AgNP) ink, facilitating rapid particle aggregation and solidification post-deposition^[84]. This method effectively suppressed the coffee-ring effect and excessive spreading. It reduced the linewidth from 31 to 19 µm, increased the thickness from 1.2 to 5.5 µm, and improved structural density and electrical reliability. In contrast, Zhou et al. employed substrate heating not primarily for linewidth reduction or edge definition, but as a strategy for structural regulation during deposition^[68]. By tuning substrate temperature, they controlled solvent evaporation and particle mobility, thereby governing nanoparticle assembly behavior. This enabled the transition from dense films to loose hierarchical architectures and provided a route for fabricating three-dimensional micro/nanostructures with enhanced functional performance, as shown in Figure 6A. Substrate heating mainly optimizes the macroscopic morphology of the film. However, for functional inks, especially conductive inks containing metallic nanoparticles, better intrinsic electrical and mechanical properties usually require an additional processing step, namely sintering or curing. Furthermore, technologies such as electro-sintering^[86] and microwave sintering^[87] have been extensively studied and verified, offering diverse post-processing options for different material systems and application scenarios.

Nevertheless, conventional thermal sintering often requires high temperatures that exceed the thermal tolerance of flexible polymer substrates, which limits its application in flexible electronics. To address this challenge, alternative sintering strategies compatible with temperature-sensitive substrates have been developed. Among them, photonic sintering uses selective light absorption to induce localized high-temperature sintering while keeping the overall substrate temperature low. Therefore, it is particularly suitable for flexible electronic applications. For instance, laser sintering enables precise control over the spatiotemporal distribution of energy, making it suitable for thermosensitive substrates. Wei et al. used AJP to deposit AgNP ink, Metalon JS-A426, on PI and polyethylene terephthalate (PET) substrates to fabricate a bilayer capacitive strain sensor^[41]. In this work, laser sintering (stable process parameters: 350 mW) was applied to the PI substrate. This process enabled selective melting of AgNPs and the formation of dense electrodes (~10 μm thick) without damaging the substrate, resulting in high conductivity, strong adhesion, and stable electrical performance [Figure 6B]. Beyond laser sintering, other mature photopolymerization/sintering methods include photonic sintering, as well as ultraviolet (UV) lamp sintering, infrared lamp sintering^[88,89], and flash lamp sintering^[90]. Among these methods, real-time UV processing is particularly suitable for polymer inks containing photoinitiators because it enables rapid in situ photopolymerization and curing. Li et al. integrated UV LEDs onto the side of the printhead to enable simultaneous illumination and printing^[42]. This method immediately fixed the printed patterns, effectively prevented excessive spreading of low-viscosity materials, and enabled the printing of microcolumn arrays with linewidths of 30 µm and heights greater than 150 µm, as shown in Figure 6C. In addition, the more mature photonic flash sintering method enables instantaneous and selective sintering of nanoparticle inks through millisecond-scale light pulses, making it suitable for fabricating high-performance electronic devices on low-melting-point flexible substrates. For example, AJP was used to deposit gold nanoparticle and indium tin oxide (ITO) nanoparticle inks onto PI substrates^[67]. By optimizing the intense pulsed light sintering parameters, the sintering process was completed in less than two seconds, resulting in dense and conductive networks of gold and ITO nanoparticles, as shown in Figure 6D. After flash sintering, the sensors exhibit excellent stability at temperatures up to 350 °C, while maintaining nearly unchanged performance even after 500 bending and torsion cycles. This approach provides a new pathway for the rapid and reliable fabrication of high-temperature-resistant flexible multimodal sensors.

In addition, plasma sintering has been preliminarily studied to minimize substrate damage. Du et al. coupled AJP with an atmospheric-pressure non-thermal plasma jet, enabling room-temperature in situ sintering of metal nanoparticle inks during the deposition process^[91], as shown in Figure 6E. By synchronizing deposition and sintering, this approach eliminates the need for conventional post-processing and significantly reduces the overall fabrication time. Moreover, the coupled printing strategy demonstrates excellent substrate compatibility, allowing for the direct fabrication of functional devices on a wide range of temperature-sensitive materials, including biological substrates. Finally, reactive precursor inks suitable for AJP have recently been developed to avoid the effect of additional energy fields on flexible or curved substrates. These inks undergo chemical reactions during or after the printing process^[92,93]. Because this approach does not rely on nanoparticle-based inks, it can significantly reduce or completely eliminate the sintering requirements after deposition, further minimizing the damage to the flexible substrate.

Compared with the above post-processing methods, the substrate heating is simple but has a limited temperature range (< 150 °C). Laser sintering offers good spatial selectivity but has low efficiency for large-area processing. Intense pulsed light sintering is extremely fast, with processing times at the millisecond scale, and is suitable for roll-to-roll manufacturing. However, it strongly depends on the optical absorption properties of the ink. Plasma sintering can achieve in situ processing at room temperature and is compatible with biological substrates, but the equipment is complex. Reactive precursor inks can avoid thermal damage, but their conductivity is usually lower than that of sintered nanoparticle films. Therefore, in flexible electronics manufacturing, the choice of post-processing method should be determined based on the thermal tolerance of the substrate, conductivity requirements, and production efficiency. Overall, post-deposition processing has evolved from simple substrate heating to more precise energy-management strategies. These strategies include controlling heat transfer during deposition, achieving rapid and selective sintering or curing with tailored photon energy, using low-temperature plasma sintering for thermally or energetically sensitive substrates such as biological materials, and developing reactive precursor inks that reduce or eliminate post-treatment. The core principle is precise spatial and temporal control of energy input. This strategy enables defect mitigation, morphological optimization, functional performance enhancement, and preservation of flexible substrate integrity.

As flexible electronics evolve toward wearable, stretchable, and conformal integrated systems, especially for devices deployed on complex three-dimensional surfaces, the related printing challenges have become increasingly important. For flexible and stretchable substrates, the main challenges include maintaining electrical stability under repeated mechanical deformation, suppressing crack formation caused by stress concentration, and achieving uniform deposition on dynamically changing geometries. Flexible conductive lines fabricated by AJP can maintain stable electrical performance under repeated bending, showing clear advantages for addressing these challenges. Luo et al. fabricated MXene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) flexible micro-capacitors on both planar and curved fiber surfaces, thereby improving the stability of flexible electronic devices^[94]. In addition, the high resolution and mask-less nature of AJP enable the rapid fabrication of complex patterns. Xu et al. printed various strain-adaptive structures, such as serpentine and wavy geometries, for flexible electronic devices^[95] [Figure 7A]. The resulting strain sensors showed a wide sensing range of up to 660%, providing a systematic strategy for structural optimization in flexible and stretchable electronic devices. These structural designs effectively redistribute mechanical strain and reduce the risk of failure, thereby improving the durability and performance of stretchable electronic systems on complex curved surfaces.

When AJP is applied to complex three-dimensional surfaces, the nozzle often operates at a larger stand-off distance from the substrate and is difficult to keep perpendicular to the local surface. This condition can cause tilted deposition and disrupt jet symmetry, resulting in asymmetric line cross-sections, increased linewidth, blurred edges, and reduced conductivity^[38]. Under these conditions, printing quality is governed not only by material properties and process parameters but also, more critically, by the dynamic geometric relationship between the nozzle and the substrate. To address this issue, Mosa et al. developed a laser-scattering-based jet visualization method for real-time observation and analysis of spray morphology and dynamic behavior during AJP^[73], as shown in Figure 7B. By measuring the spray diameter and penetration length, this method clarifies how parameters such as spray velocity and sheath-gas flow rate affect jet focusing and stability. This approach extends the working distance to more than 10 mm and enables high-quality printing on complex three-dimensional surfaces. It also provides an effective visualization tool and theoretical basis for real-time monitoring, parameter optimization, and precise remote printing in AJP. For path planning, the core challenge is to accurately, continuously, and adaptively map two-dimensional printing paths onto three-dimensional surfaces. Traditional approaches usually triangulate CAD models before generating printing paths, which often introduces geometric approximation errors and path discontinuities. To address this problem, Jignasu et al. proposed a direct path planning method based on non-uniform rational B-splines (NURBS) parametrized surfaces^[43]. This method bypasses the triangulation step and directly uses the high-precision mathematical representation of CAD models. As a result, it generates smooth and continuous printing trajectories that strictly conform to the surface geometry, as shown in Figure 7C. Compared with previous mesh-based approaches^[96], this strategy significantly enhances geometric fidelity and trajectory continuity. It also maintains a constant nozzle-to-surface distance and avoids collisions. These advantages represent important progress toward higher digitalization and automation in conformal path generation.

Precise trajectory planning alone is insufficient. The hardware motion system must also be able to implement the planned trajectory. For specific surface types, such as rotationally symmetric bodies, customized motion platforms can simplify the printing process and improve print quality. One study integrated a lathe-type rotating attachment into a three-axis AJP system and extended it into a lathe-type aerosol jet printing (LAJ) platform with cylindrical-coordinate motion capability^[44], as shown in Figure 7D. Through coordinated substrate rotation and nozzle translation, the system maintains the nozzle perpendicular to the printing surface, thereby reducing defects induced by tilted deposition. This enables uniform, high-resolution conformal circuit printing on flexible films, medical catheter balloons, conical surfaces, and other stretchable curved substrates. The study demonstrates the feasibility of extending AJP from planar printing to three-dimensional conformal integration in flexible electronics, although further experimental validation is still required. It also highlights the importance of application-specific hardware optimization for the reliable fabrication of high-performance devices.

In summary, extending AJP from planar substrates to flexible, stretchable, and complex three-dimensional surfaces introduces new challenges, including mechanical reliability, strain adaptability, and geometry-dependent deposition behavior. At present, AJP still has inherent limitations in printing on complex curved surfaces. Path planning mainly relies on offline programming and lacks real-time deformation feedback, which limits its applicability to dynamically deforming substrates. Addressing these challenges requires a shift from isolated process optimization to an integrated strategy that combines structural design, adaptive path planning, and advanced motion control. This strategy can enable high-fidelity and mechanically robust conformal printing on diverse substrates. It also supports the reliable fabrication of flexible and stretchable electronic devices and provides a basis for large-scale conformal manufacturing on complex three-dimensional surfaces. This section establishes the physical constraints and process guidelines for subsequent material design and ink-formulation optimization, thereby providing a basis for the discussion of material printability and performance control in Section “AJP MATERIALS”.

In AJP, processability depends on the interactions among ink formulation, aerosol generation, transport, focusing, and deposition. The ink serves as the aerosol source, and its key physicochemical properties, including viscosity, surface tension, solvent volatility, particle size, and solid loading, directly affect aerosol atomization efficiency, aerosol stability, and printing accuracy^[97,98]. Although AJP is generally considered to have broad material compatibility, its practical processing window is still limited by ink-related properties. Therefore, precise control of ink formulation is essential for achieving repeatable and high-quality printing. This section summarizes how the physicochemical properties of AJP inks affect key process steps, including atomization, transport, focusing, and deposition. It also summarizes the recommended parameter ranges for two different atomization mechanisms, providing guidance for the preparation of functional inks suitable for high-quality AJP.

AJP is widely regarded as one of the most versatile direct-write additive manufacturing techniques because it can deposit a broad range of functional materials with micrometer-scale precision. In principle, any material that can be formulated into a stable and jettable ink that meets the atomization requirements discussed in Section “AJP processability” can be processed by AJP. This capability has established AJP as an important manufacturing platform for printed and flexible electronics, sensors, energy storage devices, and bio-integrated devices. In this section, the state of the art of AJP is reviewed according to the functional roles of the printed materials, with a focus on conductive materials, semiconductors, dielectrics and biological materials. For each material class, representative ink formulations, processing strategies, and device-level demonstrations are discussed. The discussion highlights both the opportunities enabled by AJP and the material-specific challenges that still limit its wider technological adoption.

Printed and flexible electronics constitute one of the most mature and industrially relevant domains for AJP, largely due to the demand for lightweight, thin, and mechanically compliant electronic systems. Unlike conventional microfabrication, which is often limited to planar substrates and subtractive patterning, printed electronics relies on the additive deposition of functional materials and supports a broader range of substrate chemistries and geometries. As a result, direct-write printing techniques can fabricate functional structures on substrates such as PET, PI, thermoplastic polyurethane, and other elastomers, allowing electronic systems to maintain performance under bending, twisting, or stretching^[138]. Within this field, AJP occupies a distinctive position. It enables mask-less, high-resolution patterning while maintaining compatibility with conductive, semiconducting, and dielectric inks over a wide viscosity range. A key strength of AJP is its ability to form high-resolution conductive traces and interconnects, typically in the 10-50 µm linewidth range, with precise placement enabled by the sheath-gas focusing mechanism. This capability allows AJP to bridge the gap between inkjet printing, which is limited in resolution and surface conformity, and photolithography, which requires rigid substrates and multistep processing. As a result, AJP has been widely used in applications such as antennas, sensors, transparent electrodes, and conformal circuit routing. With appropriate sintering strategies, AJP-printed metallic traces can achieve conductivities close to bulk values. Combined with the ability to deposit materials on nonplanar or irregular surfaces, this feature makes AJP particularly suitable for heterogeneous integration and advanced packaging, where interconnects must reliably link components with different geometries or orientations^[139,140].

Given the promising results achieved in conductive trace fabrication, the increasing use of AJP-printed electronics, especially on flexible substrates, is expected^[141-143]. In emerging applications such as wearable sensors, epidermal electronic devices, soft robots, smart packaging, and foldable consumer devices, AJP can maintain high spatial resolution even when the substrate is deformed. This provides an important advantage over traditional microfabrication processes, which are mainly optimized for rigid silicon wafers. The broad material compatibility of AJP further supports this advantage because it enables the printing of dielectric materials and facilitates the integration of complete electronic architectures^[144,145]. A major area in which the capabilities of AJP have been extensively leveraged is the fabrication of flexible antennas and radio-frequency identification (RFID) tags. In these devices, electrical performance depends not only on the conductivity of the printed traces but also on the geometric precision, surface uniformity, and substrate conformity of the patterned conductors. These requirements align well with the intrinsic strengths of AJP, and recent developments have expanded its relevance to higher-frequency operation. Flexible RF components are particularly attractive for conformal wireless systems, where antennas must operate reliably even when bent or wrapped around curved surfaces. The ability of AJP to maintain geometric precision on flexible substrates helps ensure stable impedance matching and predictable electromagnetic responses under mechanical deformation. Several studies have demonstrated that AJP can reliably produce fine conductive patterns suitable for ultrahigh-frequency, microwave, and emerging millimeter-wave applications^[146-150]. AJP has also been used to fabricate terahertz (THz) metasurfaces on flexible substrates. As shown in Figure 11A, Ghosh et al. demonstrated aerosol-jet patterning combined with a parylene lift-off process to fabricate high-resolution, conformal metasurfaces on a flexible substrate^[45]. Compared with traditional screen printing^[151], AJP can achieve digital rapid prototyping without the need for masks, significantly shortening the design iteration cycle. Compared with transfer printing technology^[152], AJP simplifies the process flow through one-step direct writing and enables truly conformal printing on complex curved surfaces, which is difficult to achieve using traditional planar transfer methods.

In addition to high-resolution interconnects and RF components, AJP has been increasingly used to fabricate transparent conductive electrodes. These electrodes are essential for flexible optoelectronic systems, such as transparent heaters, touch panels, displays, light-emitting devices, and photovoltaic devices^[153-156]. AJP can deposit fine conductive features on diverse substrates and is compatible with both metal and metal oxide inks. These advantages enable the additive patterning of electrodes with tailored optical and electrical properties that meet the requirements for transparency, conductivity, and mechanical compliance. As shown in Figure 11B, a recent breakthrough in this area is the direct patterning of ITO nanoparticle electrodes using a co-solvent-assisted AJP process, which achieves high optical transmittance (~90.6%) and relatively low sheet resistance (~23.2 Ω/sq)^[137]. In this work, optimization of ink composition and post-annealing enabled smooth, uniform ITO films suitable for transparent thin film heaters, demonstrating both functional performance and reliable thermal stability across varied operating conditions. Beyond metal oxides, AJP has been used to create metallic grid and mesh architectures that balance transparency and conductivity. Studies on silver nanosheet grids shows that high-resolution printed grids can reach sheet resistances in the low single-digit to tens of ohms per square while maintaining significant optical transmittance^[157-159]. Because the optical transparency scales with the ratio of aperture size to linewidth, these grid structures allow designers to tailor electrodes toward specific device requirements while exploiting AJP’s superior resolution relative to conventional inkjet approaches. In addition to passive layers, AJP has been used to fabricate optically active devices and light-emitting films. For example, polymer dispersed liquid crystal (PDLC) devices with directly printed electrodes and LC material have been demonstrated on curved optical surfaces, showing the feasibility of fully printed electro-optical device structures^[160]. Metal-organic framework (MOF) nanosheets with tunable luminescence have also been patterned by AJP, yielding RGB and white-light emission films that can serve as micrometer-scale light-emitting elements in optoelectronic architectures^[161]. Finally, 3D AJP of lead halide perovskites has been leveraged to obtain micrometer-sized X-ray photodetectors with groundbreaking sensitivity^[130], and its sensitivity was improved by 4 times compared to the best-in-class devices.

Sensors and actuators represent a broad class of functional devices in which AJP has been increasingly used to enable high-resolution patterning of sensing materials and electrodes for environmental, physiological, and industrial monitoring. The versatility of AJP makes it suitable for integrating sensing elements directly onto flexible, wearable, and miniaturized platforms. The same direct-write capability that enables high-performance conductive traces also allows sensor elements, microelectrode arrays, and actuators to be fabricated with fine geometric control, multi-material integration, and scalability. These advantages support emerging applications in flexible wearable devices, healthcare, environmental safety, and Internet of Things (IoT) systems.

For physical sensors, AJP-fabricated temperature sensors most commonly rely on resistive transduction mechanisms, in which the electrical resistance of a printed material varies with temperature. Metallic traces based on silver^[162] or gold^[163] nanoparticle inks are frequently used, as their temperature coefficients of resistance is well characterized and reproducible. AJP enables the fabrication of fine serpentine resistor geometries with controlled linewidth and thickness, allowing accurate tuning of sensitivity while minimizing thermal mass and improving response time. Moreover, the high resolution of AJP traces enables the fabrication of compact sensor arrays that meet device-level integration requirements. In recent studies, the temperature coefficient of resistance was increased by approximately 10%, allowing precise temperature sensing over a wide range from 0 to 240 °C, as shown in Figure 11C^[164]. Carbon-based materials and conductive polymers have also been explored for temperature sensing, particularly in flexible or stretchable formats^[46,165]. Humidity sensors fabricated by AJP typically use capacitive or resistive mechanisms associated with moisture absorption in hygroscopic materials. In capacitive humidity sensors, water uptake by a polymer or composite dielectric layer changes its effective dielectric constant, leading to a measurable capacitance variation between printed electrodes^[65]. In this regard, AJP is well suited for printing interdigitated electrode geometries with high aspect ratios, which enhance sensitivity by increasing the interaction volume between the electric field and the sensing material. In resistive humidity sensors, the absorption of water molecules changes ionic or electronic conduction pathways within the sensing layer, resulting in resistance variations^[66,166,167]. Although most recent studies have focused on sensors based on resistive or capacitive transduction mechanisms, as summarized in Table 4, alternative sensing approaches have also been reported. McKibben et al. demonstrated an aerosol jet-printed thermometer based on surface acoustic wave interrogation^[168], whereas Basnayaka et al. reported a passive RFID-based humidity sensor. In the latter device, a carboxymethyl cellulose (CMC) sensing layer modulated both the magnitude of the S₁₁ reflection coefficient and the resonant frequency in response to changes in ambient moisture content^[169].

Stress and strain sensors produced using AJP are also predominantly based on piezoresistive or capacitive transduction mechanisms [Table 5]. In piezoresistive strain sensors, mechanical deformation of the substrate induces a change in the electrical resistance of a printed conductive or semiconducting element. This effect can arise from geometric deformation, microcrack formation, or changes in the tunneling distance between conductive domains^{[178,183,185]}. Pressure sensors fabricated by AJP frequently rely on capacitive transduction, in which applied pressure changes the separation between electrodes or the effective dielectric constant of a compressible layer^[41,186]. AJP allows precise alignment of electrodes and localized deposition on flexible substrates, facilitating compact pressure sensor arrays and tactile sensing systems^[179,191]. Beyond sensing, AJP-printed physical sensor architectures can also be operated as actuators. For example, conductive traces printed on elastomeric substrates can serve as electrothermal actuators. In these devices, Joule heating induces thermal expansion or softening of the substrate, resulting in controlled mechanical motion, as shown in Figure 11D^[189]. In addition to passive mechanical sensing, AJP has been used to fabricate fully printed electrochemical actuators. In these actuators, electrically driven ion transport within multilayer polymer structures produces controlled mechanical deformation. Zhang et al. demonstrated ultrathin, micro-patterned electrochemical actuators fabricated entirely by AJP, exhibiting programmable bending and multimodal actuation under low applied voltages^[180,190].

Chemical sensors are widely used in environmental monitoring and disease diagnosis. Following the success of other printing techniques, such as screen printing and inkjet printing, AJP has also been used to fabricate various chemical and electrochemical sensors [Table 6] These studies demonstrate the advantages of AJP for manufacturing miniaturized, portable, and highly sensitive chemical sensors. In gas sensors, AJP is used either to deposit the active sensing layer or to fabricate the electrodes required for sensor operation. Most AJP-based gas sensors rely on chemiresistive mechanisms, in which adsorption of gas molecules on the surface of a sensing material changes its electrical resistance. Electrochemical sensors, by contrast, use redox reactions, ion transport, or interfacial charge-transfer processes at electrode–electrolyte interfaces. AJP is commonly used to print working, counter, and reference electrodes with well-defined geometries, enabling miniaturized and multiplexed electrochemical sensor arrays. In amperometric and voltammetric sensors, analyte oxidation or reduction generates a current proportional to concentration. In impedimetric sensors, analyte binding changes the interfacial impedance. By carefully selecting materials for the AJP process, electrochemical sensors can be designed to detect specific molecules or biological compounds. A representative example is the detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [Figure 11E], the virus responsible for the COVID-19 pandemic^{[119,193,204]}. In addition, the ability of AJP to fabricate three-dimensional structures can be used to increase the surface area of sensors and thereby improve their sensitivity^[206,208]. Similarly, AJP has been used to fabricate organic electrochemical transistors, in which the sensing material modulates electronic conduction between the source and drain in response to an analyte. Compared with conventional electrochemical sensors, organic electrochemical transistors provide intrinsic signal amplification, as the source-drain currents are typically much larger than the currents directly involved in the electrochemical sensing process^[210]. Compared with traditional manufacturing processes, such as melt extrusion^[211], lithography^[212], the non-contact nature of AJP can avoid mechanical pressure-induced damage to flexible substrates. Its one-step direct-write process also simplifies fabrication and is more suitable for wearable electronics. Moreover, its capability for fabricating three-dimensional structures can significantly increase the specific surface area of active materials and enhance chemical sensing sensitivity.

AJP has emerged as a promising enabling technology for energy harvesting and storage systems, especially when device miniaturization, mechanical flexibility, and integration with printed electronics are required. In photovoltaic devices, AJP has mainly been investigated for patterning high-resolution conductive features rather than replacing full-area deposition techniques. Although AJP has been demonstrated for depositing photoactive materials^{[105,213-216]} or for cell encapsulation^[217], these layers usually do not require stringent patterning. Therefore, conventional methods, such as spin coating and hydrothermal deposition, are often adequate. In contrast, fine-grid current collectors and interconnects can be printed with narrow linewidths and controlled thicknesses, enabling efficient charge extraction while reducing optical shading and series resistance^[218-220]. This capability is particularly relevant for thin-film and emerging photovoltaic technologies, including organic, perovskite, and other solution-processed solar cells, in which the photoactive layers are often mechanically fragile and incompatible with aggressive processing. Beyond front-side metallization, AJP has also been explored for back-contact architectures and module-level interconnections, supporting flexible and lightweight photovoltaic systems^[221]. As shown in Figure 12A, Vlnieska et al. fabricated interconnected perovskite photovoltaic modules using AJP. This method reduced the line area by 33%, which helped simplify the module layout. It also reduced processing steps such as laser etching, thereby improving the stability of the overall photovoltaic module^[47].

Thermoelectric energy harvesting represents another area in which AJP offers distinct advantages. Thermoelectric harvesters convert temperature gradients directly into electrical energy through the Seebeck effect. When two dissimilar conductors or semiconductors are electrically connected and their junctions are kept at different temperatures, charge carriers migrate from the hot region to the cold region, generating a measurable voltage. For thermoelectric harvesters, the primary advantage of AJP lies in its ability to deposit conductive features and contacts on flexible, curved, or otherwise irregular substrates, where traditional planar fabrication methods are impractical^[222]. Although AJP can provide fine geometric control, high-resolution patterning is generally a secondary consideration because thermoelectric performance is more strongly governed by material properties and thermoelectric coupling than by lateral feature size^[223]. Another strength of AJP in this context is its broad material compatibility, which enables the direct deposition of effective thermoelectric materials, such as chalcogenide compounds, including Bi₂Te₃^[224,225], Sb₂Te₃^[226,227], and their combinations^[228,229]. Beyond the use of preformulated inks, combinatorial AJP has emerged as an interesting strategy for synthesizing thermoelectric alloys through precursor co-deposition, which facilitates rapid exploration of composition–property relationships^[230]. Such approaches generally require post-deposition sintering to achieve adequate electrical connectivity, which can be incompatible with flexible substrates. However, these constraints can be mitigated by nonthermal or low-temperature sintering strategies that preserve substrate integrity^[224,231]. From a system-level perspective, thermoelectric harvesters and other passive harvesters^[232] are well suited for powering AJP-printed electronic devices with very low power consumption. They support the development of self-sufficient, battery-free systems that operate by harvesting ambient or waste heat and reduce the need for external power wiring.

In electrochemical batteries, most AJP-based studies have focused on the direct printing of active electrode materials^[233-236] or on the deposition of solid-state electrolytes^[237,238], rather than the fabrication of fully integrated microscale battery architectures. In practice, AJP is more commonly applied to produce electrodes and components at practical, device-relevant dimensions, where its material versatility and three-dimensional deposition capability can be fully exploited. One widely recognized feature of AJP-printed battery electrodes is their inherent micro- and nanoscale porosity, which increases the electrochemically active surface area and improves interfacial charge-transfer kinetics and ion transport^[239,240]. This porous morphology, together with the ability of AJP to deposit materials in a controlled layer-by-layer manner, improves electrode utilization compared with dense films produced by conventional coating methods. Furthermore, three-dimensional AJP has been employed to fabricate structured Ag current collectors with tailored architectures, providing improved current distribution within the electrode^[241]. Finally, Morzy et al. used AJP to fabricate samples for advanced operando transmission electron microscopy characterization of battery active materials, thereby expanding the range of battery materials and structures compatible with AJP-based characterization^[242].

Supercapacitors constitute a complementary energy storage technology that benefits strongly from the geometric freedom afforded by AJP. Their performance is highly dependent on electrode surface area, porosity, and interfacial quality, all of which can be engineered through additive patterning. In contrast to batteries, most AJP-related studies on electrochemical capacitors focus on microscale supercapacitors, which are well suited for powering the low-power flexible devices discussed in the preceding sections because they provide rapid charge-discharge capability and long cycle life^[94,243,244]. Using AJP, flexible and stretchable microsupercapacitors have been demonstrated on polymeric and elastomeric substrates, enabling energy storage components that can mechanically conform to flexible electronic systems^[85,245,246]. In this regard, Zhou et al. recently demonstrated the fabrication of stretchable microsupercapacitors by AJP electrode materials onto pre-stretched elastomeric substrates, which were then relaxed to induce controlled crumpling of the printed layers^[247]. This approach produced devices with nearly unchanged performance even under biaxial 200% × 200% deformation. Moreover, three-dimensional AJP can be used to fabricate structured or vertically integrated electrodes, further increasing the accessible surface area and volumetric capacitance beyond what can be achieved by traditional planar coating or scraping processes, as shown in Figure 12B^[48].

Bioelectronics and biomedical devices are often constrained by anatomical structures, mechanical compliance requirements, and sensitivity to processing conditions, which limits the application scope of traditional planar microfabrication techniques. In this context, AJP offers clear advantages because it can deposit functional materials with micrometer-scale precision on soft, flexible, and irregular substrates. These features enable compact system-level integration in wearable and implantable biomedical devices and make AJP a promising manufacturing technology for biomedical engineering.

In addition to the previously mentioned applications, one of the most established biomedical applications of AJP is the fabrication of electrodes and conductive features for neural and electrophysiological interfaces^[248,249]. High-resolution traces printed by AJP have been used to define microelectrodes^[250], microcoils^[251,252], and fine-pitch routing structures for neural stimulation and recording^[253]. In particular, electrophysiological interfaces benefit from the ability of AJP to build vertically structured features. In this regard, Saleh et al. demonstrated an AJP-fabricated microneedle array with high probe density and successfully recorded action potentials from a mouse brain [Figure 12C]^[49]. The ability to process materials such as AgNP inks and conductive polymers on flexible substrates enables mechanically compliant interfaces that better match the elastic modulus of biological tissues. This mechanical compliance is critical for reducing micromotion-induced damage and improving long-term device stability. Moreover, AJP enables direct patterning on curved or irregular implant surfaces, allowing electrode placement and interconnect routing in anatomically relevant geometries that are difficult to address using traditional lithographic techniques.

Tissue engineering and organ-on-a-chip systems are particularly important applications for AJP because they require precise spatial control of materials to regulate cell behavior and mimic native tissue organization. Beyond electrophysiological recording, AJP-fabricated microelectrode arrays can also serve as physical and electrical guidance cues, promoting selective cell adhesion and alignment for advanced tissue engineering, as shown in Figure 12D^[50]. More generally, microscale patterning has become a key design parameter for guiding cell adhesion, alignment, migration, and differentiation. Patterned substrates have been shown to promote anisotropic cell organization, such as directed neuronal alignment and neurite outgrowth^[254]. AJP can support this control through the direct deposition of conductive polymers, hydrogels, extracellular matrix components, and protein-based inks with high positional accuracy^[255]. In scaffold-related applications, AJP has been used to deposit extracellular-matrix-like materials, such as collagen^[256,257], in dense and spatially defined architectures with microstructural fidelity relevant to connective tissue environments. Compared to extrusion-based bioprinting, AJP offers finer feature control and compatibility with relatively low-viscosity biomaterials, making it particularly suited for localized scaffold functionalization rather than bulk tissue fabrication. These capabilities also translate naturally to organ-on-a-chip systems, in which multiple material functions must be integrated within confined microfluidic architectures. Such as Rich et al. reduced the time required to manufacture one interdigital transducer using AJP from ~40 h to ~5 min, significantly shortening the development cycle^[51]. This approach enabled rapid prototyping of acoustofluidic systems capable of acoustic streaming and particle manipulation for lab-on-chip applications.

Beyond neural interfaces and tissue engineering, AJP has also been widely explored for wearable and implantable medical devices. Wearable health-monitoring systems require electronic components that maintain electrical performance under repeated bending, twisting, or stretching while remaining lightweight and unobtrusive^[30,258]. AJP enables direct deposition of conductive traces and circuit elements on elastomers, polymer films, and textile substrates, facilitating the integration of electronics into garments or skin-mounted platforms^[259,260]. In implantable devices, the ability to route electrical connections over three-dimensional housings, sharp edges, or stepped topographies supports miniaturization and heterogeneous integration^[261]. Herbert et al. reported a fully implantable, battery-free vascular platform in which AJP-printed strain sensors were integrated onto an inductive stent, as shown in Figure 12E. This platform enabled wireless, real-time monitoring of arterial stiffness and restenosis in both arterial models and ex vivo coronary arteries^[184]. Moreover, proteins and other biomolecules have been deposited by AJP without significant loss of activity, enabling the patterned presentation of adhesion ligands, signaling molecules, and biochemical gradients^[136]. This capability expands the design space for engineered microenvironments in which biological responses can be modulated through spatially defined chemical cues, supporting applications in in vitro disease modeling, drug screening, and tissue development studies^[262].

Overall, the biomedical relevance of AJP lies in its versatility as an enabling fabrication platform rather than in a single application area. Its ability to combine high-resolution patterning, conformal deposition, and multi-material compatibility addresses key challenges in neural interfaces, wearable and implantable electronics, tissue-engineered substrates, and organ-on-a-chip systems. As printable biomaterials and low-temperature processing strategies continue to advance, AJP is expected to play an increasingly important role in niche and high-value biomedical applications, where geometric complexity and multi-material integration are more important than the advantages of conventional mass-production technologies.

Across the four application domains discussed in this section, AJP provides a common advantage through its conformal deposition capability, material versatility, and mask-less direct-write process. These features allow functional components to be integrated onto substrates and geometries that are difficult to process using conventional microfabrication. In flexible and printed electronics, this advantage enables high-resolution patterning on mechanically compliant substrates. In sensors and actuators, it supports compact multi-material architectures with tailored transduction properties. In energy devices, it enables structured electrodes and conformal harvesters on irregular surfaces. In biomedical systems, it supports mechanically compliant interfaces that match the complexity of biological environments. Across these fields, the same characteristics also make AJP suitable for rapid prototyping and small-batch production. However, challenges related to print consistency, long-term reliability, and scalability still need to be addressed. These issues are discussed in the following section in the broader context of the development trajectory of AJP.

This review systematically summarizes the recent progress of AJP in the fabrication of micro-nano scale functional structures and the manufacturing of complex surfaces, with particular emphasis on the demands of flexible electronics. Starting from the fundamental working principles of AJP, this review discusses the key processes of aerosol generation, aerodynamic focusing, material deposition, and post-processing. Particular attention is paid to the physical mechanisms that govern printing resolution, morphological uniformity, and functional performance. These mechanisms directly affect jet stability, droplet evaporation, and final deposition morphology. Representative process optimization strategies reported in recent years are also reviewed, including jet stabilization, multi-physics assisted control, deposition morphology regulation, and conformal printing on complex geometries and flexible substrates.

The reviewed studies indicate that AJP achieves a unique balance among printing resolution, material compatibility, and three-dimensional conformal manufacturability. By rationally tuning the coupling between the carrier gas and sheath gas and introducing auxiliary physical fields, such as thermal, acoustic, or solvent-vapor fields, the stability and focusing quality of the aerosol jet can be effectively improved. These strategies suppress overspray and edge blurring, thereby improving the geometric accuracy and edge sharpness of printed lines. At the deposition and post-processing stages, this review summarizes various low-temperature and substrate-compatible processing strategies for flexible substrates, including thermal sintering, photonic sintering, plasma sintering, and related methods. These techniques enable precise control of structural densification, microstructural evolution, and functional properties. As a result, they improve the electrical conductivity, flexibility, mechanical robustness, and long-term stability of printed features. In other words, process parameters affect the microstructure of the deposited layer, which in turn determines the electrical and mechanical properties of the device.

From a materials perspective, AJP has been extended to a broad range of ink systems, including metallic nanoparticles, MOD inks, carbon-based materials, selected semiconductor and dielectric formulations, and bio-inks. The rheological properties of the ink strongly influence atomization efficiency and jet focusing quality, thereby determining whether high-precision printing can be achieved across different material systems. In particular, this review highlights stretchable and low-temperature-curable conductive inks developed for flexible electronics, which show strong versatility and scalability. In terms of applications, AJP has shown strong potential in flexible electronics, wearable devices, patch antennas, physical and chemical sensors, micro-energy systems, and bioelectronics. These applications are closely related to flexible and wearable scenarios and demonstrate the ability of AJP to achieve functional integration on complex three-dimensional substrates. Overall, this review focuses on the intersection between flexible electronics and additive manufacturing.

At present, AJP is evolving from a laboratory-scale high-resolution patterning technique into a more general additive manufacturing platform for flexible substrates and complex three-dimensional structures. Accordingly, the research focus is shifting from isolated process-parameter optimization to integrated design strategies involving multiscale, multi-physics, and multisystem coordination. However, AJP still faces several key challenges for practical engineering applications. These challenges include the long-term reliability of flexible components, such as performance degradation under thermal cycling, mechanical bending, humid environments, and interfacial delamination. They also include limitations in scalable manufacturing, such as low single-nozzle throughput, poor consistency among multiple nozzles, and insufficient long-term printing stability. In addition, material and process constraints, including ink storage stability, nozzle clogging, and defects during curved-surface deposition, remain unresolved. Systematic studies on these engineering challenges are still limited. Therefore, future research should place greater emphasis on reliability, scalability, material stability, and process robustness to accelerate the transition of AJP from laboratory demonstrations to practical manufacturing.

Despite the substantial progress achieved to date, AJP still faces several challenges printing consistency, reliability, and large-scale manufacturing. Future research directions can be broadly summarized as follows.

(1) Systematic modeling and unified theory of multi-physics couplingCurrent theoretical studies on AJP are mostly limited to individual process stages, and a unified multi-physics framework covering the entire workflow, from aerosol formation and transport to jet focusing, deposition, and solidification, is still lacking. Future efforts should integrate CFD, multiphase flow, heat and mass transfer, and phase-transformation kinetics to establish high-fidelity predictive models. These models should correlate process parameters with printing morphology and functional performance, thereby enabling rational process design and improved reproducibility.

(2) Intelligent conformal printing systems for complex surfacesAs AJP applications expand from planar substrates to complex freeform geometries, printing quality increasingly depends on the dynamic geometric relationship between the nozzle and the substrate. The development of next-generation AJP systems is expected to focus on intelligent integration of perception, planning, and printing. By incorporating three-dimensional vision or laser-based surface sensing, adaptive path-planning algorithms, and multi-axis motion platforms, these systems could enable real-time adjustment of nozzle orientation, stand-off distance, and deposition angle. Specifically, reinforcement-learning-based online path correction should be explored to address the dynamic deformation of flexible substrates, which cannot be effectively handled by traditional offline programming.

(3) Synergistic development of high-performance inksAlthough AJP exhibits strong material compatibility, long-term ink stability, deposition uniformity, and interfacial reliability of printed structures remain critical bottlenecks for practical applications. Future work should emphasize the coordinated optimization of ink formulations, solvent systems, additives, and substrate surface engineering. A closed-loop database linking ink formulation, process parameters, and device performance should also be established. In parallel, integrated screening approaches that combine high-throughput experimentation and machine learning should be developed to accelerate the discovery and optimization of new functional inks.

(4) Application-oriented reliability assessment and scalable manufacturingMany existing studies primarily focus on proof-of-concept demonstrations, whereas systematic investigations of device reliability under thermal cycling, mechanical deformation, and long-term operation remain limited. Future work should clarify the failure mechanisms of AJP-printed structures and explore scalable manufacturing pathways, such as integration with roll-to-roll processing, parallel printing, and multi-nozzle architectures. These efforts are essential for facilitating the transition of AJP from prototyping to practical engineering deployment.

(5) AI- and data-driven process intelligenceTraditional process optimization relies on sequential single-parameter scanning, which is inefficient and cannot fully capture multiparameter coupling effects. In future work, machine learning methods, such as Gaussian process regression and neural networks, should be used to establish nonlinear mapping models between input parameters, including gas flow rate, FR, temperature, and ink viscosity, and output metrics, including linewidth, conductivity, and edge roughness. These models could enable rapid exploration of the parameter space and inverse process design. Furthermore, in situ monitoring techniques, such as light scattering and machine vision, can be integrated with closed-loop feedback control to establish an intelligent print-measure-learn-adjust loop. This strategy could fundamentally transform empirical trial-and-error-based parameter tuning into data-driven process optimization.

In summary, AJP is at a critical stage of transition from a process-driven paradigm to a system- and data-driven paradigm. With the continued integration of multi-physics modeling, intelligent hardware, advanced materials, and AI methodologies, AJP is expected to play an increasingly important role in flexible and wearable systems, three-dimensional conformal electronics, biomedical devices, and heterogeneous integration technologies.