Ink-jetting-based conformal additive manufacturing: advantages, opportunities, and challenges

Ink-jetting printing stands out in conformal manufacturing for its ability to deposit multiple materials with different functions or states continuously. This enables the creation of composite structures with gradients in visual and mechanical properties, including active elements like semiconductors and circuits on conformal surfaces. These benefits come from ink-jetting printing's strengths in multi-material deposition [48, 49]. The main advantages include continuous deposition of various functional materials, continuous deposition of materials in different physical states (liquid and solid), and in-situ mixed deposition of multiple materials.

This technology can continuously deposit different functional materials [50]. Traditional methods usually involve single materials or complex steps to combine multiple materials. Ink-jetting printing simplifies this by precisely controlling nozzle movements and ink deposition, allowing continuous placement of functional materials directly onto conformal substrates. This gives ink-jetting-based conformal additive manufacturing a unique edge in creating multifunctional devices and complex structures.

Ink-jetting printing can also continuously depose materials in different physical states, such as liquid and solid. As shown in figure 3(a), a new liquid-solid co-printing method for multi-material 3D fluid equipment utilizes ink-jetting printing's adaptability, enabling the co-deposition of cured and uncured materials during the printing process [48]. Cured materials harden quickly after deposition, showing a sharp increase in shear modulus, while non-cured materials remain liquid. This allows easy removal of liquid supports and efficient manufacturing of complex 3D conformal structures. This capability provides ink-jetting printing with greater flexibility in creating structures with specific mechanical properties and stability. It broadens the application of ink-jetting-based conformal additive manufacturing to areas like fluid logic circuits, electrochemical sensors and amplifiers, microfluidics, lab-on-chip devices, and robotics. These complex structures often require extensive 3D data processing before manufacturing.

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Ink-jetting printing's multi-material capability also includes in-situ mixing of materials. By precisely controlling the injection ratios and mixing methods of different inks, microscale spatial resolution can be achieved, enabling the fabrication of complex structures with gradient compositions. For example, AJP allows in-situ mixing of multiple components during the gas carrier stage and offers real-time adjustments of their mixing ratios during printing. This key feature, not achievable by traditional multi-material printing, enables the creation of functional gradients within conformal structures, as shown in figure 3(b) [51]. High-throughput combinatorial printing can produce functional gradient materials with unique compositional and structural arrangements, outperforming uniform-component materials. This advantage is particularly useful for printing multifunctional structures or thin films in conformal manufacturing. Recently, Duan et al introduced a programmable hybrid electrohydrodynamic printing (M-ePrinting) technology for multi-material EJP [52]. As illustrated in figure 3(c), this technology uses programmable control of the driving voltage applied to a multi-channel nozzle, allowing in-situ mixing and spraying of various material solutions. These advancements in in-situ multi-material hybrid ink-jetting technologies have laid a robust foundation for ink-jetting-based multi-material conformal additive manufacturing. This in-situ mixing capability enhances manufacturing precision and efficiency, enabling the production of materials with specialized properties and functions.

The advantages of multi-material printing are anticipated to expand its applications in manufacturing integrated multifunctional gradient structures and complex shapes in the future. As shown in figure 3(d), multi-material printing technology demonstrates significant advantages across various fields, including flexible electronics and wearable devices, energy storage, aerospace and automotive lightweight structures, and personalized consumer products and art design. This technology not only enhances device integration and functional diversity but also drives advancements in efficient energy storage, lightweight and high-strength structures, and custom product design, showcasing its innovative potential across these applications.

However, achieving optimal interfacial compatibility between different materials is essential to realize these benefits. Key factors such as surface wettability, surface roughness, and ink chemical compatibility play a significant role in influencing interfacial bonding. For instance, surface treatments like plasma or UV irradiation can enhance wettability and adhesion at interfaces, enabling more uniform layer deposition. Additionally, selecting materials with compatible thermal and chemical properties is crucial to prevent degradation during post-processing stages, such as sintering or curing. Advanced techniques like selective laser sintering can further localize thermal effects, minimizing potential damage to thermally sensitive layers. These considerations are vital for maintaining the structural and functional integrity of multilayered and multi-material printed electronics [46].

A key advantage of ink-jetting-based conformal additive manufacturing is its ability to interface with digital driving technologies such as ML and artificial intelligence. This integration enables advanced digital manufacturing capabilities, allowing for intelligent control of droplet spraying, precise deposition on complex substrates, and effective error management. These capabilities facilitate intelligent conformal manufacturing on intricate substrates and provide a solid foundation for tackling future manufacturing challenges that involve multi-dimensional and high-difficulty requirements. Figure 4 highlights the benefits of advanced digital control and path planning, showcasing how ink-jetting printing can achieve conformal additive manufacturing on diverse topological surfaces [43].

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The benefits of multi-dimensional digital driving include a deeper understanding of the relationship between droplet ejection behavior and printing outcomes and the optimization of process parameters. Ink-jetting-based conformal additive manufacturing involves designing jetting inks to address non-spraying, interference, satellite droplets, multiple droplets, droplet breakage, and nozzle blockage. Researchers often address these challenges through material consumption tests and error experiments. However, ML offers a predictive approach that can reduce both costs and workload associated with these configurations. Various inks are used to print conductive, insulating, or semiconductor materials in conformal manufacturing, each with distinct electrical and material properties such as mass load, concentration, particle size, viscosity, density, and surface tension. These properties can significantly influence pattern formation on substrates. Understanding the effects of these diverse physical properties on conformal manufacturing outcomes is complex. ML excels in capturing the intricate dependencies between input parameters and desired results, thereby optimizing printing conditions. For example, PJP can analyze the relationship between parameters (e.g. frequency, dwell voltage, return voltage, dwell time, return time, rise time, fall time, and nozzle diameter) and ink properties. This approach streamlines experimental processes by deriving spray results from signal and material features, predicting material spray behavior, and providing insights for optimizing printing conditions and ink design [53, 54]. Additionally, leveraging digital driving advantages, closed-loop ML algorithms can be employed to design optimal piezoelectric driving waveforms [55].

The adjustable workspace between the AJP nozzle and the substrate makes it ideal for conformal manufacturing [45, 56, 57]. However, precise aerosol jet printing on complex surfaces requires understanding the relationship between substrate features and process parameters. Key factors such as sheath gas flow rate and carrier gas flow rate significantly influence AJP quality. Traditional methods often use linear models to link printing objectives with process parameters, but these are inadequate for the nonlinear nature of conformal printing on curved surfaces. ML offers a data-driven approach to establish linear and nonlinear relationships between process parameters and printing quality, enabling effective process optimization and precise deposition on complex substrates. It helps reveal the link between droplet morphology and printed line characteristics under various conditions [58]. This method uses the average droplet edge as a reference and defines edge roughness (Er) and overspray (Od) to assess droplet quality. ML allows for a systematic analysis of the formation of printing features and optimizes quality based on droplet morphology. This approach is more efficient than traditional methods and supports the broader application of AJP in conformal manufacturing.

Ink-jetting-based conformal additive manufacturing stands out for its high flexibility and integration capabilities, making it a state-of-the-art technology. It integrates seamlessly with advanced digital manufacturing techniques, including ML and artificial intelligence, providing robust support for digital-driven innovations. This integration enables unprecedented precision in droplet ejection, accurate deposition on complex surfaces, and effective error management, pushing the boundaries of manufacturing to meet higher challenges. By leveraging digital technologies, this system can autonomously optimize jetting parameters, enhancing efficiency and quality. It offers precise control over droplet behavior, meeting the demands of intricate and complex printing tasks. This intelligent approach provides a distinct advantage in complex surface manufacturing, allowing it to adapt to various substrate geometries and materials, achieving superior precision and performance.

The flexibility of manufacturing technology is a crucial attribute of conformal manufacturing. As a non-contact technique, ink-jetting printing is unrestricted by substrate shape, enabling printing on complex surfaces with high flexibility and adaptability.

This flexibility is multi-dimensional. Complex curved substrate shapes exhibit continuously varying Gaussian curvatures in the vertical manufacturing dimension. Direct-contact conformal manufacturing techniques often face challenges in maintaining continuous, precise contact with complex surfaces, complicating the process. As conformal manufacturing advances, soft substrates have also emerged as critical targets for this technology. However, direct contact with these substrates can induce deformation, negatively impacting accuracy and resolution. Meanwhile, manufacturing non-thin film structures on these conformal substrates often requires dynamic masking or complex transfer-deformation processes [17, 59, 60]. However, ink-jetting printing technology does not require masking or active deformation. It can deposit materials directly onto conformal surfaces in a non-contact manner, simplifying the vertical manufacturing process. This method allows for a controlled distance between the nozzle and the substrate, reducing friction and deformation caused by contact. It offers high flexibility for high-precision and high-resolution conformal manufacturing on any surface.

In the horizontal manufacturing dimension, ink-jetting-based conformal additive manufacturing can leverage hundreds or thousands of nozzles to deposit materials simultaneously, allowing for the concurrent printing of multiple conformal structures or simultaneous deposition at various points on a single structure. This multi-nozzle capability offers significant potential for high-throughput conformal manufacturing, a concept usually associated with fields like drug screening, parallel reactors, and gene sequencing [61–63]. In conformal manufacturing, high-throughput is rarely emphasized due to the stringent precision and resolution requirements. Multi-nozzle ink-jetting printing can significantly enhance the speed of material deposition, as demonstrated in DNA synthesis where coupling agents are precisely deposited using a multi-nozzle mode, ensuring isolation of each coupling reaction and improving efficiency [62]. This flexibility provides a robust technical foundation for efficiently producing complex three-dimensional conformal structures.

However, achieving accurate high-throughput manufacturing requires stable material deposition on substrates. During droplet drying, solvent evaporation at the contact line is faster than that in other regions, leading to solute accumulation, supersaturation, and the Marangoni effect, which can cause uneven nucleation and zoning, disrupting the conformal manufacturing process. Recent advances, such as topologically mediated molecular nucleation anchoring combined with ink-jetting printing, address these challenges. By introducing tear-shaped microstructures on the substrate surface, the three-phase contact line of droplets can be precisely controlled [64], as shown in figure 5(a). This approach stabilizes droplet nucleation, supporting stable, high-throughput conformal manufacturing.

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For material development, non-contact ink-jetting printing easily integrates with other manufacturing processes, enhancing its flexibility in conformal material exploration. Identifying suitable materials for conformal manufacturing requires analyzing the physical properties of inks and their binding characteristics with conformal surfaces [68], often demanding significant resources. With its broad material adaptability, ink-jetting printing enables high-throughput screening of conformal materials, expediting the search for new materials with exceptional properties that are often difficult to predict theoretically [69]. In renewable energy storage, ink-jetting printing has facilitated rapid screening of solid electrocatalysts [65], and it has accelerated the discovery of novel tribromide peroxides for high-voltage solar cells through efficient screening schemes for peroxide thin films [66, 67], as illustrated in figures 5(b)–(d). When combined with selective laser melting, this approach quickly identifies superior thermoelectric (TE) materials, broadening their applications [70]. The ability of ink-jetting printing to screen unknown materials based on specific application requirements is particularly advantageous for conformal manufacturing, where substrates demand distinct rheological properties. Thus, ink-jetting printing offers a unique capability for high-throughput material screening tailored to various conformal substrates, strengthening its position in advanced manufacturing.

The scalability and sustainability of conformal electronic devices are hindered primarily by ink formulations and printing processes. Some inks require complex formulations, while others pose challenges due to properties like insufficient conductivity. Additionally, certain inks necessitate extensive post-processing, increasing production time and costs. Balancing these factors is crucial, underscoring the need for printing techniques with broad ink compatibility.

As the structural complexity of conformal electronic products grows, particularly in wireless multifunctional systems, the demand for high-precision and integrated manufacturing intensifies. Figure 6(a) illustrates curved electronic products created using current ink-jetting printing technology [117]. Currently, most conformal electronic manufacturing involves printing on flexible substrates and transferring them to complex surfaces [118]. This method is limited when using non-stretchable or low-tensile materials. Ink-jetting-based conformal additive manufacturing eliminates the need for additional masks and accessories, expanding material and substrate options and enabling scalable printing from coplanar to three-dimensional forms. The key advancement lies in avoiding time-consuming transfer and assembly processes, facilitating direct conformal manufacturing on curved surfaces. This section will explore conformal electronic manufacturing and its potential applications in electronics.

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4.1.1. Conformal antennas and sensors.

4.1.2. Wearable devices.

With the advancement of technology, energy devices are also constantly evolving. Global advocacy for a low-carbon culture and spirit is driving energy devices towards higher energy utilization efficiency and higher energy conversion rates. The emergence of conformal structures has brought new inspiration to energy devices. Taking TEGs and ion batteries as examples, this section discusses the potential application in energy equipment manufacturing.

4.2.1. Thermoelectric generator (TEG).

The global energy consumption is about 1.63 × 10⁸ kilowatt hours, of which about two-thirds of the energy is wasted in thermal energy, and most is released into the atmosphere. A TEG is considered one of the most promising devices for autonomous energy collection as it can convert waste heat into usable electrical energy [86, 87]. According to the Seebeck effect, to achieve high energy conversion efficiency [88–90], minimizing the parasitic heat losses generated from heat source to heat source is necessary. With the continuous improvement of TE material performance, achieving higher energy conversion efficiency is necessary to ensure excellent mechanical reliability through appropriate TEG design [8].

To achieve efficient heat collection, TEGs must construct a conformal interface that closely fits any shape or shape-changing 3D heat source (such as the human body). The purpose of establishing this interface is to reduce heat loss caused by unexpected gaps and improve heat transfer efficiency to the TE foot. In addition, TEGs must maintain stable operation under deformation conditions such as bending or stretching to ensure that their TE performance does not significantly decrease due to deformation [85, 127]. Usually, to achieve this conformal contact on a three-dimensional heat source, TE materials with inherent flexibility can be deposited on a soft platform. This lightweight material can be processed at low temperatures, providing a massive opportunity for low-cost and large-area energy collection systems [128]. Most of the printable TEGs reported so far are mainly prepared by depositing TE materials on a planar substrate to form planar supports parallel to the substrate, which limits the device from collecting heat in the vertical direction. However, in practical applications, temperature gradients mainly exist in the direction perpendicular to the heat pipe plane. To achieve effective heat collection in the out-of-plane direction, it is essential to construct a TE support foot array perpendicular to the heat pipe support feet, a potential application opportunity for ink-jetting-based conformal additive manufacturing.

Ink-jetting-based conformal additive manufacturing enables the direct printing of TEGs in arbitrary three-dimensional shapes and structures. Figures 7(a) and (b) illustrate the design concept, manufacturing process, and temperature distribution of TEGs, as well as the compatibility of 35 pairs of pn junctions with 3D TEGs. The conformal integration of 72 pairs of hH TE modules directly on cylindrical heat pipes was demonstrated [129]. A concept diagram of a flexible TEG for self-powered circuit applications using soft electrodes and soft thermal conductors (s-hc) was also shown [130]. This design can effectively absorb strain energy, allowing the TEG to adhere to surfaces perfectly. Adjusting the rheological properties of ink enables the structural manufacturing of both inorganic TE materials and conductive polymer materials. Compared to traditional rigid TEG devices, conformal designs offer variable filling factors and greater flexibility, enhancing device performance. The materials used in these new TEGs can all be deposited via ink-jetting. The shape retention capability of ink-jetting-based conformal additive manufacturing effectively meets these structures' manufacturing requirements. In the future, this technology will support the conformal manufacturing of TEGs.

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4.2.2. Ionic batteries.

In recent years, conformal bioelectronic devices have become a research hotspot. They have close contact with the skin, can accurately monitor biological signals, provide efficient treatment plans, improve patient comfort, and effectively reduce surgical risks [98, 99]. Especially with the rapid development of implantable devices, careful design, treatment, or data recording can be directly performed at the lesion site, ensuring accurate diagnosis and effective treatment of the disease [100–102]. Whether implantable devices can provide accurate diagnosis largely depends on whether stable and firm interface adhesion can be established between the device and the lesion site. However, due to the presence of skin secretions and mechanical mismatches at the interface, the adhesion between bioelectronic devices and target points is severely hindered, which requires the adhesive at the interface to achieve adequate conformal bonding without causing a human immune response.

The human body is a vast conformal structure, from the highly aspect-ratio brain cortex and complex organ surfaces to the body skin, where different physiological phenomena exist. Finding adhesives that meet the requirements of a wide range of human conformal applications is challenging. Although recent analyses by Park et al have provided in-depth insights into bio-adhesive technologies and their practical applications in healthcare [133], the reliance on external devices for biological research disrupts the integrity of the device-biological system. It limits the subjects' freedom of movement. Research has shown that soft electronic devices with mechanical properties matched to the brain can achieve stable and reliable contact with neurons. Reducing the mechanical mismatch between electronic devices and the brain can significantly reduce the immune response induced by chronic implants [134]. This principle is similar to ink-jetting-based conformal additive manufacturing, providing theoretical support for establishing interfaces between electronic devices and living tissue using this technology. At the same time, combined with the superior adaptability of TJP to the deposition of biological materials, it has the potential for application in biological conformal manufacturing [44].

Many researchers have conducted related research regarding applying ink-jetting-based conformal additive manufacturing in biomanufacturing: Jung et al demonstrated a soft neural probe for recording neuronal activity in the brain and its overall integration through a neural interface system. Using ink-jetting to directly print liquid metal-based electrical interconnections and accessory electronic components on the surface of a living skull can be implanted into the brain with minimal invasiveness, significantly improving the integrity of electronic devices and biological systems. Figure 8(a) shows the schematic diagram of ink-jetting-based conformal additive manufacturing of bioelectronics and its enlarged image [135]. Bachman et al used full ink-jetting technology to manufacture disposable microelectrode arrays, demonstrating the feasibility of this technology for cellular bioelectronics [136]. As shown in figure 8(b), the rise of the concept of sensory digitization also provides an opportunity for ink-jetting printing to deposit functional materials on bio-substrates.

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In addition to the application of functional materials on biological surfaces mentioned above, scholars have recently proposed the concept of bioprinting pixels. It breaks away from traditional non-biological functional materials and regards biological materials as a pixel type, treating various organisms as traditional manufacturing substrates [20]. Combined with the extensive compatibility of ink-jetting printing with biological materials, it is possible to achieve conformal manufacturing of bio-pixels on complex substrates closer to nature [138]. This new concept can help further expand the application of ink-jetting conformal manufacturing technology in biomaterials implants, in-situ organ repair, and more in the future. As shown in figure 8(c), ink-jetting printing has a future opportunity to achieve conformal deposition of biomaterials on biological substrates.

All of these indicate that it can achieve effective deposition of biological materials, and compared to traditional silicon devices, ink-jetting-based conformal additive manufacturing biological devices can conduct electricity, be transparent, bend or stretch, and have biocompatibility with living organisms [139, 140]. In addition, their surfaces can be freely designed to form optimal contact with biological samples, including adjusting wettability, controlling surface roughness, adjusting mechanical properties, and deploying chemical patterns, which typically require additional post-processing steps [141]. The advantages of this technology in bioprinting and its creativity in combining with conformal manufacturing will provide new insights for the future of bioprinting.

4.4.1. Metasurfaces manufacturing.

Metasurfaces are 2D metamaterials that flexibly adjust physical parameters such as amplitude, phase, and polarization [103–105]. Compared with traditional materials, they not only have significant applications in the field of optics [142] but also extend to multiple fields, such as elasticity [143] and acoustics [144]. These advantages include reducing energy loss, simplifying design processes, applicability of nanomanufacturing technology, and convenience in practical applications. To overcome the limitations of planar and rigid structures in metasurfaces, a growing demand exists for conformal metasurfaces capable of operating with arbitrary bending shapes. Among them, the manufacturing of conformal metasurfaces is one of the recent hotspots in the field of metasurfaces. Figure 9(b) shows the process of effectively deploying conformal metasurface antennas directly on actual geometric shapes using aerosol printing [106]. The current manufacturing solutions for conformal metasurfaces can be divided into four categories: (i) flexible substrates, (ii) multi-plane approximation, (iii) kirigami and origami design, and (iv) nonplanar 3D manufacturing [107]. These methods rely on soft materials or are limited to simple deployable surfaces. For ink-jetting-based conformal additive manufacturing, these limitations can be effectively avoided for conformal manufacturing of metasurfaces.

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Conformal additive manufacturing of metasurfaces mainly faces challenges in structural design and positioning algorithms. In terms of structural design, in addition to meeting the requirements of conformal deposition and shaping on complex surfaces, they also need specific electromagnetic, optical, and acoustic characteristics. These characteristics are crucial for achieving special effects, such as high transmittance and reflectivity, to effectively regulate electromagnetic and light waves in various application scenarios. In terms of positioning algorithms, the metasurfaces are planar arrays composed of sub-wavelength-sized super atoms, and the geometric structure and spatial arrangement of these super atoms need to be accurately designed based on the target phase distribution to achieve flexible control of parameters such as light and electromagnetic fields. This is reflected in the manufacturing industry's demand for precise positioning capabilities. Ink-jetting-based conformal additive manufacturing technology can provide corresponding manufacturing foundations for these two aspects.

A relatively common manufacturing method is conformal frequency selective metasurfaces based on five-axis 3D printing. As shown in figure 9(a), this manufacturing mode can be perfectly integrated into ink-jetting-based conformal additive manufacturing, achieving more accurate and high-throughput metasurfaces manufacturing. In addition, Liu et al successfully manufactured zero refractive index metasurfaces for microwave power transfer harvesters using liquid metal eutectic gallium indium, polyimide combined with ink-jetting technology [148]. Donnie et al introduced a universal and industrially scalable deposition method based on polymer mixture ink-jetting to address the advantages of phase-separated nanostructures (PSNs) in forming quasi-periodic or disordered light scattering components at high throughput and low cost. The feature size of PSNs was adjusted from a few micrometers to below 100 nm to improve light management in photonic devices [149]. By 2016, Kim et al had already applied ink-jetting printing technology in manufacturing metasurfaces devices. They used silver nanoparticle ink jetting printing to create periodic split ring cross resonators patterns [116]. However, it had not yet introduced conformal manufacturing. This is consistent with the strict requirements for more advanced trajectory planning algorithms and real-time in-situ process error control in ink-jetting-based conformal additive manufacturing, discussed earlier. It is a powerful means to ensure the precise deposition of metasurfaces conformal materials. This technology can effectively meet the challenges of material selection and positioning algorithms in metasurfaces manufacturing processes.

4.4.2. Area-selective atomic layer conformal mask.

4.4.3. Conformal information encryption.

Although ink-jetting-based conformal additive manufacturing has a promising future, most existing printing systems operate on an open-loop paradigm. These systems use pre-designed 3D models or nozzle paths for offline manufacturing on calibrated flat substrates. However, many natural surfaces do not always remain static and undergo continuous motion and deformation. This time-varying geometric structure fundamentally limits the application of existing open-loop paradigm-based 3D printing systems, making it difficult to achieve offline manufacturing with models and paths prepared. For example, in biological manufacturing involving human bodies with variable skin and living organs or when working with deformable objects such as robots with dynamic shapes, this preset manufacturing method faces significant challenges due to continuously growing or deforming target substrates. Overall, when interacting with these flexibly changing target objects, existing methods still require further improvement to adapt to their constantly changing characteristics.

To achieve conformal additive manufacturing on a continuously deformed and moving substrate, ink materials must be flexible to adapt to these dynamic changes. However, determining how to effectively and accurately deposit functional materials on irregularly changing substrates is more important than discussing material properties. This is a test of the combination of this technology and advanced multi-dimensional digital drive technology, requiring establishing a dynamic surface printing closed-loop control system. Current ink-jetting-based conformal additive manufacturing methods involve establishing a robust visual positioning system and employing advanced perception technology for precise motion tracking [156]. Alternatively, artificial intelligence can predict subsequent movements through localized positioning methods and ML, thus achieving precise, comprehensive positioning [157]. For this dynamic surface capture method, Albanna et al initially developed a real-time closed-loop system that can track the movement of non-deformable human hands and directly perform in-situ 3D printing of electronic tattoos on the skin. However, simple closed-loop systems are no longer applicable when complex surface deformations such as soft tissue expansion and contraction occur [158]. Later algorithm iterations enabled more robust and accurate tracking of high-dimensional deformation data. Using this advanced technology, as shown in figure 11(a), the hydrogel-based sensor successfully printed the lung deformation caused by pig breathing [159]. This method is the most direct way to achieve dynamic surface conformal manufacturing, but it relies on iterative development of relevant high-precision equipment and usually requires expensive costs. Technological breakthroughs require extensive mathematical analysis and calculations by relevant personnel.

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Compared to establishing a highly accurate and intelligent closed-loop conformal printing system, which demands substantial investment in manpower and resources, using CAD masks to print dynamic conformal structures in real-time on moderately complex three-dimensional surfaces is more practical. The process is shown in figure 11(b), a schematic diagram of printing dynamic, flexible liquid alloy circuits on deformed objects using shape-preserving, adhesive, and dynamic (CAD) masks. The manufacturing process includes UV laser planar mask patterns, mask transfer, atomized liquid alloy printing, and conformal printing. Ultimately, the connection of the dynamic flexible liquid metal alloy circuit to the deformed three-dimensional surface was finalized through the removal of the masking layer. This method transforms the originally complex surface deposition localization problem into a dynamic adaptability problem between the mask and the substrate without considering the dynamic printing of the pattern. By setting patterns on the mask beforehand, various materials with three-dimensional complex static and even dynamic shapes/shapes can be indirectly and seamlessly covered through the mask. It provides an effective method for real-time manufacturing of dynamic, flexible liquid alloy circuits on deformable objects such as flexible robots and living organs/organisms. It endows them with interesting characteristics/functions. However, this method also poses some challenges: it reduces the requirement for the size of the pattern or related structure, and when the base surface relaxes, the corresponding pattern will also undergo size changes during the printing process.

This technology is expected to be applied to dynamic and deformable substrates such as biological tissues and flexible robots. However, the dependence on open-loop systems limits their adaptability to real-time changes in surface geometry. The advancement of ink material flexibility and digital control technology is crucial for overcoming these obstacles. Integrating comprehensive positioning systems and artificial intelligence-driven positioning methods is expected to improve the accuracy of dynamic surface printing. Meanwhile, CAD masks are a practical method for real-time manufacturing of complex structures on complex surfaces. Future research should focus on improving these technologies to ensure consistent and reliable performance in various applications, thereby enhancing the ability of ink-jetting-based conformal additive manufacturing.

Traditional multifunctional electronics integrate multiple single-function devices to achieve multimodal sensing, including pressure, strain, temperature, and humidity detection [162]. This strategy has limitations, such as high energy consumption, insufficient integration, and relatively complex manufacturing processes [163]. Ink-jetting-based conformal additive manufacturing, leveraging its capability for multi-material deposition, enables continuous deposition of various functional sensing materials on the same structure. This facilitates the creation of an integrated multifunctional conformal structure during the manufacturing process. It is an important means to solve the multimodal integration problem of complex substrate surfaces, but many challenges must be addressed.

To continuously deposit different functional materials on complex substrates with changing surface aspect ratios, the first challenge is to match the rheological properties of the ink to the substrate. Different functional materials exhibit varying ink rheological properties, and the rheological requirements vary with changes in aspect ratio across different deposition areas. In essence, the goal is to achieve real-time control of rheological properties; for functional materials whose rheological properties vary with time and temperature, more precise and controllable rheological methods are required to assess the printing suitability of inks [164]. It is necessary to deeply understand the rheological properties of various inks (not just single materials, but also research the relationship between multi-component inks and substrates) [165]. However, this demand goes far beyond manufacturing points and lines in 2D structures. In fact, with the advancement of technology and the increasing industry demand, achieving multifunctional integrated manufacturing with three-dimensional shape preservation has become an urgent and important goal. When the manufacturing dimension shifts from 2D to 3D, during the 3D printing process, each layer needs to be quickly solidified after deposition to prevent deformation or mixing with the next layer. At the same time, the solidification process itself may also lead to shape changes and performance losses. To achieve gradient changes in interlayer shape and function, it is necessary to introduce real-time control of curing behavior based on the challenge of ink rheological properties.

To achieve the industrial application of ink-jetting-based conformal additive manufacturing technology in the manufacturing of integrated multifunctional conformal structures, it is necessary to invest a lot of time and resources in technical research (developing new curing technologies) and process optimization (deeply understanding the rheological properties of ink). Although the future application prospects of conformal manufacturing based on the advantages of multi-material deposition are worth looking forward to, new ideas and methods are still needed to solve the problem of multifunctional integrated manufacturing on complex substrates.

Additionally, real-time error control is fundamental for integrating multifunctional conformal structures via ink-jetting manufacturing. Errors in the ink-jetting process primarily originate from the printing process itself and the deviation of ink deposition from the ideal position. The error due to ink deposition deviation is significantly magnified during the manufacturing of multifunctional conformal structures. Because the process requires continuous deposition of various materials, any deviation will impact the independent functionality of each material. Ink deposition errors can arise from the accuracy and freedom of the motion platform, surface complexity, and ink flight mode. These errors can be mitigated using advanced algorithms and control methods. Figure 12(c) illustrates schematic diagrams of error control strategies [166].

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These inevitable manufacturing errors, including geometric, positioning, and ink droplet flight errors [170], are typically addressed through error compensation or open-loop control methods [171–173]. For example, high-precision probes can measure errors on curved substrates for feedback compensation [166]. To address droplet uniformity, volume uniformity within pixels is achieved by ensuring uniform ink droplet volumes across all pixels [174]. These methods are examples of open-loop calibration, which is ineffective at compensating for uncertainties in the printing process, such as nozzle size variations, pressure and temperature fluctuations, and inconsistent ink characteristics. These uncertainties negatively impact the consistency of droplet volume and injection speed. Thus, the superiority of closed-loop error control algorithms becomes evident. Jie et al proposed a one-step predictive control algorithm based on droplet images to adjust droplet volume and injection speed. They demonstrated the boundedness and convergence of parameter estimation errors, as well as the stability of the closed-loop system. This control algorithm significantly reduced relative errors in droplet volume and injection velocity to within 1% [175].

Moving beyond error compensation and weakening, ink-jetting-based conformal additive manufacturing, being a non-contact process, eliminates deformation errors from direct contact with the substrate and tolerates significant geometric errors along the spraying direction. Combined with the unique characteristics of this technology, such as high surface accuracy and low spraying direction accuracy, each axis's motion and error situation can be digitized. By sacrificing spraying direction accuracy, the error transfer algorithm can enhance in-plane geometric accuracy and reduce geometric errors in conformal printing. As shown in figure 12(d), this calculation process includes printer modeling, geometric error representation, geometric error optimization, and selecting the optimal feeding value [169]. The method proposed by this shift in thinking has the advantage of addressing specific characteristics of non-contact and conformal printing processes. It can altogether avoid the overall deviation or deformation of printed patterns. In addition, compared to other primary precision enhancement methods, this method has no intervention or impact on manufacturing in terms of equipment and printing process. However, limitations remain, such as the requirements for unique print heads and limited compression rates. As complexity increases, error control algorithms require more robust adaptability and robustness. This means the algorithm needs to dynamically adjust according to the system's real-time status and environmental changes to achieve integrated and precise manufacturing of multifunctional conformal structures.

Ink-jetting-based conformal additive manufacturing has high resolution, rapid prototyping and customization, and comprehensive material compatibility, making it highly selective when depositing on conformal surfaces. However, to apply it directly to manufacturing high aspect ratios and large surfaces, especially in industries such as automotive and aerospace, it is necessary to maximize the deposition rate while maintaining high resolution. The trade-off between resolution and deposition rate challenges process mode and path planning algorithms for it.

In terms of process mode, although innovative approaches have been adopted, modes such as EJP and AJP are still limited by various inherent trade-offs, including limitations on printing indicators, low material throughput, insufficient stability, and the complexity of material and process optimization processes [46, 176–179]. Excessive ink deposition on the surface may lead to liquid phase diffusion. At high deposition rates, this diffusion becomes more pronounced. Solvent-rich inks may exhibit increased diffusion, leading to more comprehensive lines, decreased resolution, and uncontrollable wetting. This results in irregular shapes at the edges of the lines. These issues limit manufacturing throughput and make resolution particularly sensitive to deposition rate [18]. Many laboratories intentionally or unintentionally reduce the sedimentation rate during demonstrations to achieve better resolution based on experience and avoid these potential issues. However, this approach can lead to decreased productivity, which may be acceptable in research laboratories but is not applicable for large-area conformal surfaces that pursue high efficiency. As shown in figures 11(c) and (d), two types of researchers conducted exploratory studies on high-flux conformal deposition without sacrificing resolution: the former used an online heater to regulate the evaporation of droplets in the aerosol phase, reducing the sensitivity of resolution to deposition rate, and thus achieving higher throughput. The latter studied a coaxial-focused current dynamic jet printing technology, which can achieve reliable micro and nano batch production with further improvements in the equipment's automation, assembly, and intelligence [161]. However, the relationship between resolution and sedimentation rate cannot be completely decoupled. Collaborative optimization of resolution and deposition rate in ink-jetting-based conformal additive manufacturing remains challenging.

Path planning plays a crucial role in high-throughput printing. Advanced path-planning algorithms are required for precise material deposition on complex surfaces [180]. These algorithms can adapt to the topological complexity of the printed surface, optimize the motion path of the print head, improve printing efficiency, and ensure precise placement of each layer of material, thereby maintaining high resolution. The development of path-planning algorithms is a powerful guarantee for achieving collaborative optimization of resolution and printing speed. Currently, the development of algorithms mainly includes three aspects: slicing layered paths, continuous paths, and predicted paths.

In conformal additive manufacturing, due to the complexity and topological structure of the sliced surface, the Z-axis coordinates will constantly change within the same layer [181]. If the surface is converted into an array of data points on a grid of the same size as the surface in the x–y plane, and the surface equation is used to calculate the z-value, there will still be roughness issues between layers [182]. Researchers have attempted to use mathematical methods to represent surface layers, change the layer thickness within each layer, and achieve smooth transitions between non-planar layers, thereby reducing the surface roughness of surface layering algorithms [183]. Some have also proposed a new method that combines 3D scanning, multi-axis 3D printing, and conformal printing to generate paths on unknown surfaces, but this problem has not been completely solved. Although these algorithms can effectively generate printing paths for surface conformal structures, the key is to more effectively and naturally generate non-planar layers to reduce weak connections and step effects between layers.

Regarding continuous path development, the thermal guidance algorithm using isothermal surfaces to generate non-planar layers can quickly and effectively generate unsupported high-quality structures [167]. Based on the similarity between heat conduction and ink-jetting processes (heat always transfers from higher temperature areas to lower temperature areas, and conformal printing structures 'grow' by continuously adding new materials on the previous layer), researchers simulated the printing process as a particular form of the heat transfer process. The 'virtual' isothermal surface can naturally divide the surface conformal structure into multiple curved layers in this model. Adjusting the boundary conditions and thermal conductivity of the 'virtual' heat transfer problem can create a surface layer with specific functions. As shown in figure 12(a), it dramatically improves printing efficiency and quality, balancing resolution and throughput.

The difference in predicting paths lies in whether a CAD model of the part needs to be created. Facing printing models with highly complex structures, such as wearable devices, electronic textiles, shaping devices, and anatomical models, this step is time-consuming or challenging. With the help of 3D scanning technology, researchers can print functional materials on polymer films with random non-planar shapes based on point cloud data of object geometry. This method is precious for printing scenes lacking CAD models. More importantly, this data-driven trajectory planning algorithm has iterability and can be further improved based on the rheological properties of sedimentary materials and real-time object perception research [168]. Figure 12(b) is a schematic diagram of the algorithm. The most prominent feature of this closed-loop path planning method is that it does not require describing the global geometric shape of the object, significantly improving printing efficiency. However, regardless of which path planning method is used, much effort is still needed to achieve high flux and resolution in ink-jetting-based conformal additive manufacturing.