Nano-Level Additive Manufacturing: Condensed Review of Processes, Materials, and Industrial Applications

: Additive manufacturing, commonly known as 3D printing, represents the forefront of modern manufacturing technology. Its growing popularity spans across research and development, material science, design, processes, and everyday applications. This review paper presents a crucial review of nano-level 3D printing, examining it from the perspectives of processes, materials, industrial applications, and future trends. The authors have synthesized the latest insights from a wide range of archival articles and source books, highlighting the key findings. The primary contribution of this study is a condensed review report that consolidates the newest research on nano-level 3D printing, offering a broad overview of this innovative technology for researchers, inventors, educators, and technologists. It is anticipated that this review study will significantly advance research in nanotechnology, additive manufacturing, and related technological fields.


Introduction
Additive manufacturing (AM), also known as 3D printing (3DP) [1], is a set of technologies that fabricate parts layer by layer from a computer-aided design (CAD)-generated model [2].Historically, Charles ("Chuck") Hull and Scott Crump are recognized as the fathers of AM for their innovative inventions that revolutionized the field.Chuck Hull invented stereolithography in 1984 and received the first patent for it in 1986, laying the groundwork for AM technology.Around the same time, Scott Crump developed Fused Deposition Modeling (FDM), another foundational AM technology, and obtained a patent for it in 1992 [3,4].From its inception to today, AM has evolved from basic prototypes to advanced nanoscale applications, revolutionizing manufacturing and enabling unprecedented precision across various industries.The seven AM technologies identified by ISO/ASTM 52900 [5] are listed below: Material Extrusion (MEX).Vat Photopolymerization (VPP).Material Jetting (MJ).Binder Jetting (BJ).Powder Bed Fusion (PBF).Sheet Lamination (SL).Directed Energy Deposition (DED).
In today's world, there are also several subcategories of these 3DP technologies, such as wire-arc additive manufacturing (WAAM), direct ink writing (DIW), and so on [6].Among all these technologies, MEX is the most widely utilized one [1], since it is easy to use and low-cost in terms of materials [7].The terms Fused Filament Fabrication (FFF) and FDM both refer to a similar process in MEX, but they originate from different contexts and have distinct nuances.FFF is a generic, non-proprietary term commonly used in the open-source community, whereas FDM is a trademarked term associated with Stratasys and commercial applications [8].
Nowadays, 3DP has many applications in research and industry, such as the food industry [9], medical research [10], aerospace [11] et cetera.In the past, 3DP was also studied to fabricate large-scale parts, because of the need to construct large-volume objects like bridges, houses, and shelters with the use of conventional material extrusion technologies.The use of this technology was not popular in any fine-scale production, due to the incapabilities of the available machines and materials in the market.As the development of 3DP technologies has evolved (Figure 1), nano 3DP has been a popular practice in several studies of AM, considering its importance in obtaining high-quality, less resource-intensive, lightweight, and more intricate workpieces.Overall, nano 3DP is a highly precise AM technique producing structures at the nanometer scale.
In today's world, there are also several subcategories of these 3DP technologies, such as wire-arc additive manufacturing (WAAM), direct ink writing (DIW), and so on [6].Among all these technologies, MEX is the most widely utilized one [1], since it is easy to use and low-cost in terms of materials [7].The terms Fused Filament Fabrication (FFF) and FDM both refer to a similar process in MEX, but they originate from different contexts and have distinct nuances.FFF is a generic, non-proprietary term commonly used in the open-source community, whereas FDM is a trademarked term associated with Stratasys and commercial applications [8].
Nowadays, 3DP has many applications in research and industry, such as the food industry [9], medical research [10], aerospace [11] et cetera.In the past, 3DP was also studied to fabricate large-scale parts, because of the need to construct large-volume objects like bridges, houses, and shelters with the use of conventional material extrusion technologies.The use of the technology was not popular in any fine-scale production, due to the capabilities of the available machines and materials on the market.As the development of 3DP technologies has evolved (Figure 1), nano 3DP has been a popular practice in several studies of AM, considering its importance in obtaining high-quality, less resource-intensive, lightweight, and more intricate workpieces.Overall, nano 3DP is a highly precise AM technique producing structures at the nanometer scale.The recent developments in nanotechnologies, which are used to enhance material configuration at the atomic and molecular scale, are very effective in improving the core properties of materials.This technology helps to create a material with a unique combination of enhanced mechanical, electrical, and thermal properties that is not achievable by traditional manufacturing [12].
Hunde et al. are developing highly efficient solar cells with a nanostructured surface that improves the light absorption rate.Traditional solar cells often face challenges due to reflective losses and a suboptimal surface texture; nano 3DP allows the creation of a nonreflective coating at nanoscale which minimizes reflection and thus improves overall efficiency [13].Mohammed et al. are developing a drug delivery system that can target a specific cell or tissue in the body with very high precision.Nano particles enhance the ability to bind to particular cell receptors, to ensure that the drug-loaded nanoparticle will directly reach the targeted cells or tissue [14].Additionally, nano 3DP has been used to produce flexible and stretchable electronics, such as wearable advanced sensors and electronic skin, helping core technological innovations in medical diagnostics and personal health management [15].Integrating nanotechnologies to AM, research studies are pushing the boundaries in material functionality and device performance.The recent developments in nanotechnologies, which are used to enhance material configuration at the atomic and molecular scale, are very effective in improving the core properties of materials.This technology helps to create a material with a unique combination of enhanced mechanical, electrical, and thermal properties that is not achievable by traditional manufacturing [12].
Hunde et al. are developing highly efficient solar cells with a nanostructured surface that improves the light absorption rate.Traditional solar cells often face challenges due to reflective losses and a suboptimal surface texture; nano 3DP allows the creation of a non-reflective coating at nanoscale which minimizes reflection and thus improves overall efficiency [13].Mohammed et al. are developing a drug delivery system that can target a specific cell or tissue in the body with very high precision.Nano particles enhance the ability to bind to particular cell receptors, to ensure that the drug-loaded nanoparticle will directly reach the targeted cells or tissue [14].Additionally, nano 3DP has been used to produce flexible and stretchable electronics, such as wearable advanced sensors and electronic skin, helping core technological innovations in medical diagnostics and personal health management [15].Integrating nanotechnologies with AM, research studies are pushing the boundaries in material functionality and device performance.Today, there are several studies being conducted, measuring the impact of nano-level 3DP in terms of quality, cost, and sustainability.Sikora et al. reviewed the influence of nano additives on the quality of 3DP-fabricated parts [16].Meng et al. performed a 3DP study using a poly-based nano-composite hydrogel as an artificial cartilage replacement and improved the printing accuracy of the entire mechanism.Their results showed that this was a functional method in biomedicine [17].Greer et al. introduced a large-volume nanoscale 3DP process with high speed, improving the quality of nano 3DP-fabricated parts [18].A recent review study explores the extensive use of nanomaterials in AM and showcases recent research findings on their integration into different AM categories, emphasizing their effects on the final product's properties [19].
Recently, numerous researchers have studied the mechanical and physical properties of nanoscale 3DP.Abouzeid et al. reported the in situ mineralization research of nanohydroxyapatite on several nanomaterials (NMs) fabricated by 3DP.The research group has also investigated some physical and mechanical properties, such as the Young's modulus and pH, of these 3D printed parts, and concluded that this manufacturing method would significantly improve these mechanical properties and could be used in bone tissue engineering [20].Ren's group developed a new method to enhance the bioactivity and osteogenesis of nano 3D-printed Ti-6Al-4V implants.Their work is expected to accelerate the application of 3D-printed implants in the healthcare area, especially in orthopedic and dental clinics [21].The article of Chougan's group introduced a nano 3DP method to reinforce the strength of fabricated parts [22].Sezer et al. presented significant improvements in ABS composite parts with enhanced mechanical and electrical properties using nano 3DP technology [23].In direct laser deposition, nano 3DP technology was used to minimize the size of the pores of the directed laser deposition.This is another example of mechanical property improvement in nano 3DP [24].Li et al. used a dual-nozzle nano 3DP method to produce regenerated bones.Nano 3DP offered a good biocompatibility and effectiveness in repairing bone defects [25].The nano 3DP process has already been used in many areas, such as biomedicine and electronic device fabrication.It is projected that its utilization will advance gradually in several high-tech areas such as biomedicine, pharmacy, and aerospace.
Over recent years, because of biomimicry and biocompatibility, nanometric hydroxyapatite has gained interest as a constituent of hybrid systems for bone scaffold fabrication.Cheng et al. summarized many applications and technologies of nano 3DP bone substitutes.Their research concluded that although there were some limitations in biomedicine, it also has a bright future [26].The researchers suggest that nanoscale 3DP was moderately successful simply due to the infancy of the technologies used.Specifically, natural tissues support regenerative growth despite having various cell types.On the other hand, artificial tissues are currently limited when it comes to supporting the collaborative growth of different cell types.Nevertheless, the potential this technology carries can revolutionize tissue and bone surgery due to the customizability of structures using 3DP.
Nano-level 3DP processes have been widely used in several medical research studies for the successful generation of bone tissues [27].Manzoor et al. introduced a biomedicine application by using Polyetheretherketone, which is a biocompatible polymer widely used in the medical field [28].Some researchers used 3DP process to generate bone scaffolds by using nanometric hydroxyapatite.Hassan's group used this technology to open new horizons in tissue engineering by generating customized 3D structures with optimized properties and multifunctionalities. From this work, they also discussed the advantages of harnessing the potential of 3DP to produce unique, multifunctional nanocomposites for tissue engineering [29].In the research of Yeo's group, nano 3DP was applied to culture-engineered skeletal muscle tissue to improve its functionality [30].
Moreover, nano 3DP also has applications in today's industry.In the work of Muldoon et al., the nanoscale 3DP process encourages the innovation of electronic components, because of the ability to control their design, material, and chemical properties at a highly precise level [31].Cao's group developed a new nano 3DP technology to produce carbon nanoelectrodes [32].Ulrich et al. have applied the technology to optics engineering [33].Wei et al. used the technology to fabricate microfluidic sensors [34].
In this paper, the progression of the technology will be presented through its historical transformation from macro to nano-level miniaturization in fabrication processes, materials, and production machines.The latest trends and technologies in processes and material science allow the addition of ultrafine particles to enhance the features (i.e., mechanical, electrical, and physical) of the printed parts.A high number of research studies in material science helped the researchers to develop several knowledge blocks for the needs of practitioners, inventors, and other researchers.It can be observed in Figure 2 that the miniaturization of the material science discoveries in 3DP from macro-to nano-level has been opening new avenues for scientific communities to advance the overall AM realm.This review article presents an original classification of nano-level AM processes and materials.Then, the current standing of the industry is highlighted.Finally, the future perspectives and trends of nano-level 3DP are reported.
In this paper, the progression of the technology will be presented through its h ical transformation from macro to nano-level miniaturization in fabrication processes terials, and production machines.The latest trends and technologies in processes and terial science allow the addition of ultrafine particles to enhance the features (i.e., mec ical, electrical, and physical) of the printed parts.A high number of research studi material science helped the researchers to develop several knowledge blocks for the n of practitioners, inventors, and other researchers.It can be observed in Figure 2 tha miniaturization of the material science discoveries in 3DP from macro-to nano-leve been opening new avenues for scientific communities to advance the overall AM re This review article presents an original classification of nano-level AM processes and terials.Then, the current standing of the industry is highlighted.Finally, the future spectives and trends of nano-level 3DP are reported.

Nano 3D Printing Processes
In this section, traditional and non-traditional 3DP processes are presented in d including their technologies, working parameters, advantages, materials, and app tions.Further, a few of the commonly used conventional and modern 3DP technol that are used for producing nanoscale additively manufactured parts are elaborated at length.

Nano 3D Printing Processes
In this section, traditional and non-traditional 3DP processes are presented in detail, including their technologies, working parameters, advantages, materials, and applications.Further, a few of the commonly used conventional and modern 3DP technologies that are used for producing nanoscale additively manufactured parts are elaborated upon at length.

Material Extrusion
Material Extrusion (MEX) for nano-level 3DP expresses a frontier in AM, offering unprecedented precision at the molecular level.This process involves the deposition of materials, often polymers, through either a tiny nozzle with diameters measured in nanometers, or with feedstock filled with NMs.MEX allows the addition of nanoparticles to polymer matrices to enhance the mechanical and thermal properties of the base matrices.By precisely controlling the extrusion process, complicated structures can be fabricated with resolutions unimaginable with traditional manufacturing methods (i.e., welding, casting, machining) [5].The nano-level 3DP process with the use of MEX holds immense promise in various fields, including biomedical engineering, electronics manufacturing, and nanotechnology, where precise control over material composition and structure is paramount.Its potential to revolutionize fabrication at the ultrafine level indicates a new era of innovation and discovery, since the whole MEX process is based on a simple structure and its operation is the easiest in comparison to other AM processes.One current extrusionbased study presents the creation of a robotic 3D printer tool for a collaborative robot, focusing on enhancing nanofabrication with multi-axis FDM printing and future plans for optical and laser-based improvements [35].
The invention of the MEX process was initiated in the early 1980s as a FDM technique.In this technique, filament material such as thermoplastic polymer and other composites are melted [36].Further, this filler material is deposited in layers and transformed into a final part [37].Further, the categorization of MEX-based AM processes has been divided in several ways.For simplicity of understanding, in this study, a mechanism-based categorization has been deployed.As shown in Figure 3, MEX processes are divided into three categories of mechanisms.These are filament-based, plunger-or syringe-based, and screw-based processes [38].

Material Extrusion
Material Extrusion (MEX) for nano-level 3DP expresses a frontier in AM, offering unprecedented precision at the molecular level.This process involves the deposition of materials, often polymers, through either a tiny nozzle with diameters measured in nanometers, or with feedstock filled with NMs.MEX allows the addition of nanoparticles to polymer matrices to enhance the mechanical and thermal properties of the base matrices.By precisely controlling the extrusion process, complicated structures can be fabricated with resolutions unimaginable with traditional manufacturing methods (i.e., welding, casting, machining) [5].The nano-level 3DP process with the use of MEX holds immense promise in various fields, including biomedical engineering, electronics manufacturing, and nanotechnology, where precise control over material composition and structure is paramount.Its potential to revolutionize fabrication at the ultrafine level indicates a new era of innovation and discovery, since the whole MEX process is based on a simple structure and its operation is the easiest in comparison to other AM processes.One current extrusion-based study presents the creation of a robotic 3D printer tool for a collaborative robot, focusing on enhancing nanofabrication with multi-axis FDM printing and future plans for optical and laser-based improvements [35].
The invention of the MEX process was initiated in the early 1980s as a FDM technique.In this technique, filament material such as thermoplastic polymer and other composites are melted [36].Further, this filler material is deposited in layers and transformed into a final part [37].Further, the categorization of MEX-based AM processes has been divided in several ways.For simplicity of understanding, in this study, a mechanism-based categorization has been deployed.As shown in Figure 3, MEX processes are divided into three categories of mechanisms.These are filament-based, plunger-or syringe-based, and screw-based processes [38].Since the commencement of MEX, some other extrusion processes have been developed with minor modifications.The steps of developing the parts are divided into nine sets (as shown in Figure 4), starting from developing the CAD design to the final developed part or component.Since the commencement of MEX, some other extrusion processes have been developed with minor modifications.The steps of developing the parts are divided into nine sets (as shown in Figure 4), starting from developing the CAD design to the final developed part or component.
In MEX, the major printing parameters that influence the development of the final additively manufactured parts are nozzle temperature ( • C), nozzle diameter (mm), layer thickness (mm), pattern infill, infill density (%), raster angle ( • ), building orientation, and platform temperature ( • C).Amongst all the printing parameters, the nozzle temperature and platform temperature parameters, it has been noticed, should be adaptable to the material used.Since it has direct contact with the final printed part, the nozzle temperature has a significant effect on the strength and quality of the printed part owing to the viscosity of the extruded material, whereas infill density plays a vital role in its mechanical properties.Finally, it can be stated that all the printing parameters have a noteworthy effect on the characteristics of the parts developed by the extrusion process [39].In MEX, the major printing parameters that influence the development of the final additively manufactured parts are nozzle temperature (°C), nozzle diameter (mm), layer thickness (mm), pa ern infill, infill density (%), raster angle (°), building orientation, and platform temperature (°C).Amongst all the printing parameters, the nozzle temperature and platform temperature parameters, it has been noticed, should be adaptable to the material used.Since it has direct contact with the final printed part, the nozzle temperature has a significant effect on the strength and quality of the printed part owing to the viscosity of the extruded material, whereas infill density plays a vital role in its mechanical properties.Finally, it can be stated that all the printing parameters have a noteworthy effect on the characteristics of the parts developed by the extrusion process [39].
Since the development of MEX processes, they have been widely used in several spheres of industry, along with domestic applications, owing to their low equipment and energy costs [40].The health sector is one such major sector, where extrusion processes are currently deployed for biomedical, tissue engineering, bio-composite, and bio-degradability applications.Researchers have progressively developed implants, scaffolds, artificial tissues, cellular structures, sensors, ba eries, and biomedical devices.Along with that, space, marine, automotive, and aviation applications are also explored by industries and research laboratories [41].In recent times, MEX has undergone improvements in the suitability of new materials and processing methods.MEX processes are used for the development of complex-shaped, lightweight constructs in aerospace and automotive engineering, such as engines, as well as exhausts.In the future, energy consumption and life cycle analyses would potentially be investigation factors aimed at Industry 4.0 and sustainable development solutions [38,42].

VAT Photopolymerization
VPP is defined as an AM process by which a liquid polymer resin is selectively cured by a light source in a layer-by-layer manner from a CAD-designed 3D model [5].In this process, the focused source produces polymerization only at the focal point, allowing the formation of intricate and highly detailed nanoscale geometries.Different kinds of VPP are stereolithography, digital light processing, and continuous digital light projection [43,44].The capability to construct complicated polymer parts having ahigh resolution and part strength distinguishes this process from other AM technologies.VPP processes have been differentiated based on their light source, lateral resolution, layer thickness, and printing speed [45].In particular, nanoscale reinforcement materials in VPP resins improve the hardness, tensile strength, impact strength, elongation, and electrical conductivity of the printed products [46].VPP applications include producing nanoscale devices for research and industrial uses, offering tight tolerances and finish qualities.Amongst all the VPP processes, stereolithography (SLA) is the most commonly used technique due to its high accuracy and be er surface finish.A laser beam (UV light) is used as a light source in the SLA technique, while a projector is used in digital light processing and continuous digital light projection [47].Continuous digital light projection Since the development of MEX processes, they have been widely used in several spheres of industry, along with domestic applications, owing to their low equipment and energy costs [40].The health sector is one such major sector, where extrusion processes are currently deployed for biomedical, tissue engineering, bio-composite, and bio-degradability applications.Researchers have progressively developed implants, scaffolds, artificial tissues, cellular structures, sensors, batteries, and biomedical devices.Along with that, space, marine, automotive, and aviation applications are also explored by industries and research laboratories [41].In recent times, MEX has undergone improvements in the suitability of new materials and processing methods.MEX processes are used for the development of complex-shaped, lightweight constructs in aerospace and automotive engineering, such as engines, as well as exhausts.In the future, energy consumption and life cycle analyses would potentially be investigation factors aimed at Industry 4.0 and sustainable development solutions [38,42].

VAT Photopolymerization
VPP is defined as an AM process by which a liquid polymer resin is selectively cured by a light source in a layer-by-layer manner from a CAD-designed 3D model [5].In this process, the focused source produces polymerization only at the focal point, allowing the formation of intricate and highly detailed nanoscale geometries.Different kinds of VPP are stereolithography, digital light processing, and continuous digital light projection [43,44].The capability to construct complicated polymer parts having a high resolution and part strength distinguishes this process from other AM technologies.VPP processes have been differentiated based on their light source, lateral resolution, layer thickness, and printing speed [45].In particular, nanoscale reinforcement materials in VPP resins improve the hardness, tensile strength, impact strength, elongation, and electrical conductivity of the printed products [46].VPP applications include producing nanoscale devices for research and industrial uses, offering tight tolerances and finish qualities.
Amongst all the VPP processes, stereolithography (SLA) is the most commonly used technique due to its high accuracy and better surface finish.A laser beam (UV light) is used as a light source in the SLA technique, while a projector is used in digital light processing and continuous digital light projection [47].Continuous digital light projection has the fastest printing speed (500 to 1000 mm h −1 ) among all the VPP processes.The layer thickness plays an important role in determining the quality of parts and the printing speed of the process.AM parts with a wide layer thickness are printed faster but have less accuracy, resolution, and surface smoothness, while parts with a small layer thickness take more time to print but have a higher resolution, a better-quality surface finish with complicated details, and a finer geometrical accuracy [47].

Inkjet Printing
This MJ method involves using highly controlled jetting mechanisms, often leveraging technologies like inkjet printing with nanoscale nozzles, to accurately place materials such as polymers, metals, or biological substances.The applications of nano-level MJ include creating nano-level electromechanical systems, sensors, high-resolution biomedical devices, and advanced semiconductor devices, allowing for the fabrication of complex and highly detailed structures with an excellent precision, surface finish, and material control.In inkjet printing, a liquid binder is put onto thin layers of powder of the desired shapes as per the command of the printing software [48].One current inkjet printing-based research study presents the creation of high-performance multi-purpose nanostructured thin films which are environmentally friendly and functional [49].
This advanced printing technique highly depends on the specifications of the surface tension and viscosity of the ink.The energy of the droplet gets divided into viscous flow, surface tension, and kinetic energy after the droplet is released [50].The binder liquid used must have a viscosity low enough to fill the channel in about 100 µs.A major challenge known as the "first drop problem" is to avoid clogging the nozzle with dried ink.Liquids having a low volatility and water-mixable properties are added to the ink to prevent drying and clogging [51].To avoid the dripping of the droplet, the surface tension should be high, and the pressure should be low.The viscosity of the generally used inks is up to 2 cP but can have a maximum limit of 100 cP, depending on the design of the printer.The porous film usually dries in 15 s [52].
The droplet deposition technique is similar to traditional inkjet printing but on an ultrafine scale; inkjet printing enables the construction of complicated nanostructures with exceptional details and accuracies.By precisely depositing NMs layer by layer, the inkjet printing process unlocks the potential for creating novel nanoscale instruments, sensors, and devices with applications ranging from semiconductor technology to biomedical engineering.

Non-Traditional Processes
AM is becoming an attractive production technology in several industries such as aerospace [53], construction [54], medicine [55], etc., because of its ability to create complicated prototypes quickly and produce highly customized goods.Academic research environments are also seeing growth in the use of this technology.Non-traditional ultrafine 3DP techniques have also garnered increasing interest since they provide fabrications at extremely smaller scales, and they are much more efficient and flexible than standard microfabrication methods like surface and bulk micromachining.Two nano 3DP techniques are discussed in this section.One is two-photon polymerization (TPP), also known as direct laser writing (DLW), which is a leading method for producing nanoscale products [56] directly from CAD models.The other technique is laser-induced forward transfer (LIFT), which is capable of the precise deposition of materials for various advanced applications.

Two-Photon Polymerization
TPP operates on the principles of the two-photon absorption (TPA) theory which was first proposed by Göppert-Mayer in her doctoral dissertation in 1931 [57].However, at that time, high levels of photon intensity were not available due to the lack of ultrafast lasers, which delayed the actual experimentation and validation of this concept for 30 more years.It was not until 1961, three decades after its initial proposal, that Kaiser and Garrett were able to successfully show the TPA effect in practice [58].
The basic working principle of TPP involves using an ultrafast femtosecond laser, which emits pulses that last on the order of 10-15 s, to focus on a photoresist, usually a liquid monomer (Figure 5).This causes a chemical reaction through TPA at the focal point, which converts the monomer into a solid material, a cross-linked polymer.A laser scanning technique using computer-controlled XYZ positioning is then used to solidify the model.Any excess material is removed using a solvent like acetone or isopropanol, resulting in a detailed printed object.
which emits pulses that last on the order of 10-15 s, to focus on a photoresist, usually a liquid monomer (Figure 5).This causes a chemical reaction through TPA at the focal point, which converts the monomer into a solid material, a cross-linked polymer.A laser scanning technique using computer-controlled XYZ positioning is then used to solidify the model.Any excess material is removed using a solvent like acetone or isopropanol, resulting in a detailed printed object.The TPA process happens when an atom or molecule absorbs two photons at the same time, regardless of their frequencies according to atomic physics.This process boosts the particle from the ground state to an excited state, via a virtual energy level as seen in Figure 6.As the simultaneous absorption of two photons creates a TPA process, the probability of TPA is proportionate to the photon dose (D), and D is directly proportional to the square of the light intensity (D ∝ I 2 ).Therefore, TPA is called a nonlinear optical phenomenon [60].In the context of TPP, FWHM refers to the Full Width at Half-Maximum of the laser focus.In TPP, a focused laser beam is used to polymerize a photosensitive resin by The TPA process happens when an atom or molecule absorbs two photons at the same time, regardless of their frequencies according to atomic physics.This process boosts the particle from the ground state to an excited state, via a virtual energy level as seen in Figure 6.As the simultaneous absorption of two photons creates a TPA process, the probability of TPA is proportionate to the photon dose (D), and D is directly proportional to the square of the light intensity (D ∝ I 2 ).Therefore, TPA is called a nonlinear optical phenomenon [60].liquid monomer (Figure 5).This causes a chemical reaction through TPA at the focal point, which converts the monomer into a solid material, a cross-linked polymer.A laser scanning technique using computer-controlled XYZ positioning is then used to solidify the model.Any excess material is removed using a solvent like acetone or isopropanol, resulting in a detailed printed object.The TPA process happens when an atom or molecule absorbs two photons at the same time, regardless of their frequencies according to atomic physics.This process boosts the particle from the ground state to an excited state, via a virtual energy level as seen in Figure 6.As the simultaneous absorption of two photons creates a TPA process, the probability of TPA is proportionate to the photon dose (D), and D is directly proportional to the square of the light intensity (D ∝ I 2 ).Therefore, TPA is called a nonlinear optical phenomenon [60].In the context of TPP, FWHM refers to the Full Width at Half-Maximum of the laser focus.In TPP, a focused laser beam is used to polymerize a photosensitive resin by In the context of TPP, FWHM refers to the Full Width at Half-Maximum of the laser focus.In TPP, a focused laser beam is used to polymerize a photosensitive resin by inducing a localized chemical reaction.The FWHM of the laser focus determines the resolution and accuracy of the polymerization process.A smaller FWHM typically results in finer features and a higher resolution in the fabricated structures.Therefore, controlling and minimizing the FWHM is important for achieving high-quality results in the TPP process.Based on the threshold model (Figure 7), the minimum feature size depends on the exposure dose received by a material.If the dose is above an FWHM value, the material becomes sufficiently cross-linked, will not be washed away, and a nanoscale egg-shaped voxel is created.Conversely, if the dose is below this threshold, the material is not cross-linked enough and will be removed during washing.and minimizing the FWHM is important for achieving high-quality results in the TPP process.Based on the threshold model (Figure 7), the minimum feature size depends on the exposure dose received by a material.If the dose is above an FWHM value, the material becomes sufficiently cross-linked, will not be washed away, and a nanoscale eggshaped voxel is created.Conversely, if the dose is below this threshold, the material is not cross-linked enough and will be removed during washing.The materials are key components in the TPP process.Photoresists are used in the process, which changes their properties when exposed to light.These photoresists are usually in liquid form or gel substances and contain photo initiators that react to TPA.Photo-initiators are responsible for creating reactive molecules called radicals when they are triggered by the lights to initiate the polymerization process.
Acrylates, methacrylates, and hybrid organic-inorganic resin materials are generally used in TPP because of their diverse properties that range from mechanical strength to biocompatibility.These materials are crucial to achieving the desired results in various 3DP applications [61].
In the last 20 years, joint efforts in physics, chemistry, materials science, and mechanical engineering have greatly progressed in interdisciplinary areas, especially in the advancement of TPP for creating biomedical 3D scaffolds.These scaffolds have accurate designs and customizable shapes and characteristics.However, TPP's extensive use is hindered by restrictions like the limited size of the producible structures, slow production rate, and a limited range of appropriate biomaterials.The 3DP processing of intricate structures can take hours to days, based on the materials employed [62].
Creating complex microstructures is essential to improve the functionality of microfluid devices, and it is possible with the integration of TPP and microfluidics.It has been used to produce microsieves and micromixers, which are vital for mimicking organ functions and drug interactions on chips.In the future, to delve deeper into these physical and chemical processes, more dynamic elements might be involved in TPP due to its potential.Moreover, there is a rising interest in using TPP to explore the photophysics and mechanical properties of microstructures [63].
A high resolution and accuracy are the key advantages of TPP, which are considered the most important features of creating intricate 3D objects directly from CAD files.As a non-contact and mask-free process, TPP might also be considered an efficient process.The materials are key components in the TPP process.Photoresists are used in the process, which changes their properties when exposed to light.These photoresists are usually in liquid form or gel substances and contain photo initiators that react to TPA.Photo-initiators are responsible for creating reactive molecules called radicals when they are triggered by the lights to initiate the polymerization process.
Acrylates, methacrylates, and hybrid organic-inorganic resin materials are generally used in TPP because of their diverse properties that range from mechanical strength to biocompatibility.These materials are crucial to achieving the desired results in various 3DP applications [61].
In the last 20 years, joint efforts in physics, chemistry, materials science, and mechanical engineering have greatly progressed in interdisciplinary areas, especially in the advancement of TPP for creating biomedical 3D scaffolds.These scaffolds have accurate designs and customizable shapes and characteristics.However, TPP's extensive use is hindered by restrictions like the limited size of the producible structures, slow production rate, and a limited range of appropriate biomaterials.The 3DP processing of intricate structures can take hours to days, based on the materials employed [62].
Creating complex microstructures is essential to improve the functionality of microfluid devices, and it is possible with the integration of TPP and microfluidics.It has been used to produce microsieves and micromixers, which are vital for mimicking organ functions and drug interactions on chips.In the future, to delve deeper into these physical and chemical processes, more dynamic elements might be involved in TPP due to its potential.Moreover, there is a rising interest in using TPP to explore the photophysics and mechanical properties of microstructures [63].
A high resolution and accuracy are the key advantages of TPP, which are considered the most important features of creating intricate 3D objects directly from CAD files.As a non-contact and mask-free process, TPP might also be considered an efficient process.However, printing large amounts of material can be difficult due to its slower speed compared to other methods.Additionally, acquiring the necessary tools, like femtosecond lasers, and precise stages is a challenge in terms of cost.

Laser-Induced Forward Transfer
The main principle of LIFT technology is to use the laser to transfer material from a donor substrate to a receiver substrate.The idea was introduced by Bohandy in 1986 [64].It was first applied to bioprinting in 2004, and cell patterns were successfully printed.LIFT technology provides accurate printing and a high resolution and throughput, as well as a high rate of cell survival [65].In this technology, nozzles are not used, and therefore there is no issue of ink clogging during printing.LIFT technology is a promising method for various applications, including biomedical research and tissue engineering [66].
LIFT technology operates by focusing a light beam through a transparent substrate onto a metal or polymer film using lenses.At that point, some of the light is absorbed and then converted into internal energy [67].This process generates heat, which causes the material to expand, deform, and bubble and may even cause liquefaction or vaporization, leading to material transfer [68] to the receiving substrate.The LIFT device normally includes a laser source, lenses, several layers of donor material, and a receiving substrate, as can been seen in Figure 8.The laser source is typically a single-wavelength, single-mode pulsed laser.The donor consists of a transparent substrate, usually a quartz slide with an almost zero absorption of laser light, a laser absorber layer coated with metal or metal oxide, and a biological solution coating layer that contains biological materials such as DNA, proteins, or cells.Finally, the receiving substrate is a buffer-coated slide [66].

Laser-Induced Forward Transfer
The main principle of LIFT technology is to use the laser to transfer material from a donor substrate to a receiver substrate.The idea was introduced by Bohandy in 1986 [64].It was first applied to bioprinting in 2004, and cell pa erns were successfully printed.LIFT technology provides accurate printing and a high resolution and throughput, as well as a high rate of cell survival [65].In this technology, nozzles are not used, and therefore there is no issue of ink clogging during printing.LIFT technology is a promising method for various applications, including biomedical research and tissue engineering [66].
LIFT technology operates by focusing a light beam through a transparent substrate onto a metal or polymer film using lenses.At that point, some of the light is absorbed and then converted into internal energy [67].This process generates heat, which causes the material to expand, deform, and bubble and may even cause liquefaction or vaporization, leading to material transfer [68] to the receiving substrate.The LIFT device normally includes a laser source, lenses, several layers of donor material, and a receiving substrate, as can been seen in Figure 8.The laser source is typically a single-wavelength, single-mode pulsed laser.The donor consists of a transparent substrate, usually a quar slide with an almost zero absorption of laser light, a laser absorber layer coated with metal or metal oxide, and a biological solution coating layer that contains biological materials such as DNA, proteins, or cells.Finally, the receiving substrate is a buffer-coated slide [66].LIFT technology is a precise and biocompatible technique, and it is increasingly used in biomedical applications such as drug testing and tissue engineering.However, some challenges need to be studied for its extensive implementation.Understanding the physics of bubbles is crucial for improving printing behaviors to advance bioprinting.The printing speed is still slow, and adjusting donor properties is vital for creating realistic tissue structures.LIFT technology is applied mainly in lab-scale experiments, but improvements in stability, success rates, and scalability might enable large-scale, efficient commercial production [66].
LIFT has been demonstrated to be viable in printing biomedical products, and it has immense potential for digitally printing any kind of material, whether it is inorganic, LIFT technology is a precise and biocompatible technique, and it is increasingly used in biomedical applications such as drug testing and tissue engineering.However, some challenges need to be studied for its extensive implementation.Understanding the physics of bubbles is crucial for improving printing behaviors to advance bioprinting.The printing speed is still slow, and adjusting donor properties is vital for creating realistic tissue structures.LIFT technology is applied mainly in lab-scale experiments, but improvements in stability, success rates, and scalability might enable large-scale, efficient commercial production [66].
LIFT has been demonstrated to be viable in printing biomedical products, and it has immense potential for digitally printing any kind of material, whether it is inorganic, organic, or living matter.Due to its great versatility, LIFT is expected to be a key technology in other fields, as new functional materials and diverse applications emerge [69].
Overall, these technologies provide meticulous control over material deposition at the nanoscale, facilitating the creation of intricate metal components characterized by a superior resolution and customized mechanical attributes.These capabilities render them indispensable for sophisticated applications spanning semiconductor manufacturing and biomedical engineering.

Materials
In nano 3DP, materials can be divided into several sub-categories based on their properties, applications, and the techniques used for fabrication.The main classification categories are as follows: Polymers.
The current literature presents a number of knowledge bases and publications about the above material categories [70][71][72][73].Nano 3DP is proclaimed as a highly advanced and multidisciplinary manufacturing process, with potential to surpass traditional plastic manufacturing in Industry 5.0 and become the future for advanced manufacturing techniques [74].NM, as defined by the European Commission (EC), is a natural, incidental, or manufactured material containing particles, in an unbound state or as an agglomerate, where for 50% or more of the particles in the number size distribution, one or more external dimension is in the size ranging from 1 nm to 100 nm [75,76].Several research papers have presented the usage of nanoscale composite materials in the AM field [4,77].
This scientific review study has examined various NMs and fillers used in the AM processes from the three different angles highlighted below.The unique features of the materials hold a number of strengths that allow researchers to utilize nano 3DP processes to obtain biocompatible scaffolds for the culture of cell lines or human stem cells in tissue engineering [78].NMs are critical across several emerging fields due to their unique properties, enabling advancements in healthcare, semiconductor manufacturing, and energy through enhanced performance and efficiency [79].Overall, this study highlights the growing research need to discover additively manufactured materials in aerospace, biomedicine, dentistry, and automotive applications.

Types of Nanomaterials
Instead of highlighting the above main material categories, the 3DP NMs review is divided into two main subtypes in this study which are Organic and Inorganic, as shown in Figure 9.In the Organic category, polymers hold a significant impact on AM, considering their ability to enhance part properties and increase the functionality of printed objects.In the Inorganic category, the materials are mostly used for reinforcement and supporting the overall functionality of matrix materials.

Organic Nanoparticles
Organic nanoparticles are biodegradable and nontoxic, and some particles such as micelles and liposomes, which have a hollow core, are known as nanocapsules.These NMs typically have the range of 1 to 100 nm and can be infused into different polymer matrices to improve mechanical strength, flexibility, strain, and other properties [80].
Polymer: At the nanoscale to 100 nm, the high surface-to-volume ratio becomes prominent, and the strengths of NMs are strongly dependent on the increased proportion

Organic Nanoparticles
Organic nanoparticles are biodegradable and nontoxic, and some particles such as micelles and liposomes, which have a hollow core, are known as nanocapsules.These NMs typically have the range of 1 to 100 nm and can be infused into different polymer matrices to improve mechanical strength, flexibility, strain, and other properties [80].
Polymer: At the nanoscale to 100 nm, the high surface-to-volume ratio becomes prominent, and the strengths of NMs are strongly dependent on the increased proportion of atoms or molecules located on the surface [81].This will lead to enhanced reactivity, surface energy, or surface-related phenomena; as the size of the NMs decreases, the confinement of the electrons becomes more pronounced, leading to a change in their electronic structure and an energy bandgap increase [82].Thus, these polymer NMs changes exhibit sizedependent optical, magnetic, and electronic properties; organic nanoparticles are widely used in the biomedical field [83].
Natural fiber: Natural fiber-reinforced degradable composite polymers are a new class of biomaterials that will replace traditional petroleum-based polymers in the near future, because of their eco-friendliness and good performance in an infused matrix [84].The natural fiber serves as a heterogenous crystallization matrix in nano 3DP, which promotes crystallization upon melting as the peak appears and gradually becomes enlarged; thus, the cold crystallization temperature increases gradually.This operation elevates the glass transition temperature of the base material, hence providing vital thermal stability for 3DP.Natural fiber NMs are generally used in optical and automobile applications [85].

Inorganic Nanoparticles
Inorganic nanoparticles are divided into four subtypes, which are carbon fiber, metallic particles, clay, and metal oxides.
Carbon fiber: Carbon-infused NMs in AM offer significant advantages because of their potential for enhancing the electrical properties and mechanical performance of printed materials in various applications [86].Carbon-infused NMs such as carbon nanotubes (CNTs) and graphene have become popular choices for manufacturers and researchers in recent years because of their unique structural, electrical, and mechanical properties, making them an ideal choice for stakeholders who want to improve the strength-to-weight ratio of parts, while increasing the functionality, thermal stability, and hardness of components [87,88].Multi-walled carbon nanotube (MWCNT)-infused NMs exhibit improved temperature and dimensional stability properties, which are used for flexible electrically conductive part manufacturing.A recent case study also showcased the importance of single-walled carbon nanotubes (SWCNTs) in tissue engineering applications because of their biocompatibility [89].Graphene is also a pivotal material for ink formulation in 3DP processes; graphene-based NMs have showcased a good electrical conductivity and have prominent applications in sensors and electronics circuit board manufacturing [90].
Metallic particles: Metallic particle-infused NMs offer a promising research gap to explore changes in functionality and properties in the base material matrix under various loading conditions.Several articles have suggested the inclusion of metallic NMs like gold, silver, titanium, and copper.These NMs have been explored to enhance thermal, mechanical, and electrical properties [91,92].Among these tested materials, gold NMs show promising results in the biomedical field due to their biocompatibility and biodegradable properties.Silver NMs are widely studied in the case of photopolymer resin application; they also deliver positive results because of increasing thermal capabilities [93].Copper NMs have potential applications in photopolymerization resin preparation; it is observed that mechanical properties have been enhanced with infused-copper NMs [94].
Clay: Biodegradable clay composition has been tested in the past decades as alternative ceramics in NMs composition.The research suggests that clay-based NMs show a proven effective application in tissue engineering due to their unique hydroxyapatite (HA) structure, thus exhibiting the biocompatibility and osteoconductive properties needed for bone growth in bone replacement implants [95].
Metal oxides: Metal oxides are an important class of NMs in the electronic industry due to their diverse and infused applications in piezoelectricity, superconductivity, ferromagnetism, and chemical activity in nanoscale.Thus, implementing metal oxides and nanofiller in the base matrix has shown improved results in sensor and electronic applications [96].
Traditional manufacturing concepts in 3D structural metal oxides generally use a complex lithography process, which is very costly and less effective in mass production.NMs with binary and tertiary structures showcase promising results with two-photon lithography and ink-based printing methods, as they have the potential for rapid and high-resolution fabrication for nano/micro-structured metal oxides [97].

Dimensions of Nanomaterials
One of the crucial characteristics of NMs is their dimensions.NMs display different distinct physical and chemical properties to bulk materials because of their size and shape [98].Based on their fine dimensions, these NMs are classified as follows: zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D).

Zero-Dimensional (0D)
In 0D, all the dimensions are measured within the nanoscale, having no dimension larger than 100 nm.Nanoparticles are a great example for this classification because of their 10 −9 m (1 nm) size along the X, Y, and Z directions.Zero-dimensional quantum dots are considered in this category as well [99].They have discrete electronic states and are the main candidates for optical devices in order for them to be finely tuned, with emission properties which are crucial for advanced imaging technologies as well as applications in quantum computing [100].

One-Dimensional (1D)
In 1D, only one dimension is outside the nanoscale, typically greater than 100 nm, while the other two are confined within the nanoscale.Nanowires, nanorods, and nanotubes belong to this class.NMs like nanowires and nanorods, which display properties mostly in one dimension, are used in applications that benefit from their directional properties, such as in solar cells and various sensor devices.Via AM, Guo et al. [101] created 3D-constructed resilient ceramic nanowire aerogels having programmed geometries and engineered mechanical properties.

Two-Dimensional (2D)
In 2D, two dimensions are outside the nanoscale, including plate-like shapes such as graphene, nanofilms, nanolayers, and nanocoating.Graphene is the most notable and well-known two-dimensional NM, with its single layer of carbon atoms arranged in a two-dimensional honeycomb lattice, eminent for its exceptional characteristics across the mechanical [102], electrical [103], and thermal domains [104], performing potential uses in electronics, energy storage, and composite materials.Additionally, 2D graphene is a fundamental building block for a range of well-known carbon materials such as 3D graphite, 1D carbon nanotubes, and 0D fullerene, as given in Figure 10.
well-known two-dimensional NM, with its single layer of carbon atoms arranged in a twodimensional honeycomb la ice, eminent for its exceptional characteristics across the mechanical [102], electrical [103], and thermal domains [104], performing potential uses in electronics, energy storage, and composite materials.Additionally, 2D graphene is a fundamental building block for a range of well-known carbon materials such as 3D graphite, 1D carbon nanotubes, and 0D fullerene, as given in Figure 10.

Three-Dimensional (3D)
Finally, 3D NMs are outside the nanoscale or greater than 100 nm, so not confined to the nanoscale in any direction [105].This class includes bulk powder, multi-layers in which the 0D, 1D, and 2D structural elements are in close contact, the dispersion of nanoparticles, and bundles of nanowires and nanotubes, as well as nanolayers.Moreover, atomically porous thin films, colloidal assemblies, and freely suspended nanoparticles in various forms represent types of 3D NMs [106,107].They have unique application directions.Thus, the 3DP of NMs onto flexible skin-like surfaces innovatively supports the production of sensors [108,109], reusable electronics, communication circuits, and antennas [110].Despite this method being complex, it is adaptable, cost-effective, and efficient.

Geometric Shapes of Nanomaterials
In addition to morphology, the shape of the materials is closely linked to their use and importance.And this feature is a crucial characteristic in the pioneering world of nano 3DP.Understanding and using the advantages that different shapes offer is a characteristic of the development of nano 3DP.Moreover, the creation of nanoparticles, nanofibers, and nanoplates through nano 3DP has sparked new ideas in several industries, using the distinct properties provided by their shapes [111,112].Thus, this specificity of shape, ranging from spherical to fibrous, gives each material a unique set of features that can be used in different technology areas [113][114][115][116].

Spherical Nanomaterials
Spherical Nanomaterials are 3D NMs with a symmetrical round shape, similar to a ball.Spherical or nearly spherical particles, including metals, semiconductors, and oxide nanoparticles, are classified in this group.Mokhtarzadeh et al. [117] have shown how these particles can be used to effectively deliver drugs within biological systems.

Cylindrical Nanomaterials
Cylindrical Nanomaterials are elongated structures with a consistent diameter throughout their length, resembling rods or tubes.This category includes cylindrical tubes

Three-Dimensional (3D)
Finally, 3D NMs are outside the nanoscale or greater than 100 nm, so not confined to the nanoscale in any direction [105].This class includes bulk powder, multi-layers in which the 0D, 1D, and 2D structural elements are in close contact, the dispersion of nanoparticles, and bundles of nanowires and nanotubes, as well as nanolayers.Moreover, atomically porous thin films, colloidal assemblies, and freely suspended nanoparticles in various forms represent types of 3D NMs [106,107].They have unique application directions.Thus, the 3DP of NMs onto flexible skin-like surfaces innovatively supports the production of sensors [108,109], reusable electronics, communication circuits, and antennas [110].Despite this method being complex, it is adaptable, cost-effective, and efficient.

Geometric Shapes of Nanomaterials
In addition to morphology, the shape of the materials is closely linked to their use and importance.And this feature is a crucial characteristic in the pioneering world of nano 3DP.Understanding and using the advantages that different shapes offer is a characteristic of the development of nano 3DP.Moreover, the creation of nanoparticles, nanofibers, and nanoplates through nano 3DP has sparked new ideas in several industries, using the distinct properties provided by their shapes [111,112].Thus, this specificity of shape, ranging from spherical to fibrous, gives each material a unique set of features that can be used in different technology areas [113][114][115][116].

Spherical Nanomaterials
Spherical Nanomaterials are 3D NMs with a symmetrical round shape, similar to a ball.Spherical or nearly spherical particles, including metals, semiconductors, and oxide nanoparticles, are classified in this group.Mokhtarzadeh et al. [117] have shown how these particles can be used to effectively deliver drugs within biological systems.

Cylindrical Nanomaterials
Cylindrical Nanomaterials are elongated structures with a consistent diameter throughout their length, resembling rods or tubes.This category includes cylindrical tubes with hollow and solid forms, such as carbon nanotubes.A Scanning Electron Microscope (SEM) image of CNTs is given in Figure 11.Nanofibers are another variant of cylindrical NMs.In the field of tissue engineering, nanofibers are favored for their ability to facilitate cell growth, acting as scaffolds that support tissue formation.The work of Serafin et.al. for instance, uses nanofibers to create a conducive environment for cell culture and tissue development [118].
with hollow and solid forms, such as carbon nanotubes.A Scanning Electron Microscope (SEM) image of CNTs is given in Figure 11.Nanofibers are another variant of cylindrical NMs.In the field of tissue engineering, nanofibers are favored for their ability to facilitate cell growth, acting as scaffolds that support tissue formation.The work of Serafin et.al for instance, uses nanofibers to create a conducive environment for cell culture and tissue development [118].

Platelet or Disc-Shaped Nanomaterials
Platelet or Disc-Shaped Nanomaterials have a flat and thin shape with a larger surface area relative to their volume, which makes them ideal for enhancing the performance of devices fabricated through nano 3DP techniques.For instance, graphene oxide can be used to strengthen biopolymer structures in 3D-printed biomedical implants [119].

Bricklike or Cuboidal Nanomaterials
Bricklike or Cuboidal Nanomaterials are prism-shaped with rectangular faces, resembling the shape of a brick but on the nanoscale, including materials like cubic zirconia.These are often utilized in 3D-printed ceramics for applications requiring a high thermal stability and toughness, such as aerospace components and high-performance cu ing tools [120].
It is clear that the fine sizes and unique shapes of nanomaterials allow for high-resolution fabrication, enabling the construction of intricate and detailed structures at the nano level.The inherent properties of NMs, such as a high surface area, enhanced reactivity, and superior mechanical strength, translate into the improved performance of the printed objects.Different shapes can be tailored for specific functions, such as conductivity, flexibility, or biocompatibility, allowing for multifunctional components to be printed in a single process.Overall, processing NMs also requires specialized equipment and techniques, to ensure uniform dispersion and prevent aggregation, which can affect the quality and consistency of the printed structures.

Industrial Perspective
With nano-level manufacturing gaining significant traction in various fields, the need for advancements in nano-capable manufacturing equipment is ever-rising.Several companies are leading the advancement in nano-level manufacturing, such as UpNano [121], Nanoss GmbH [122], Nanoscribe GmbH [123], nano3Dprint [124], and Nano Dimension [125].While some have matured expertise in the field of submicron manufacturing and

Platelet or Disc-Shaped Nanomaterials
Platelet or Disc-Shaped Nanomaterials have a flat and thin shape with a larger surface area relative to their volume, which makes them ideal for enhancing the performance of devices fabricated through nano 3DP techniques.For instance, graphene oxide can be used to strengthen biopolymer structures in 3D-printed biomedical implants [119].

Bricklike or Cuboidal Nanomaterials
Bricklike or Cuboidal Nanomaterials are prism-shaped with rectangular faces, resembling the shape of a brick but on the nanoscale, including materials like cubic zirconia.These are often utilized in 3D-printed ceramics for applications requiring a high thermal stability and toughness, such as aerospace components and high-performance cutting tools [120].
It is clear that the fine sizes and unique shapes of nanomaterials allow for highresolution fabrication, enabling the construction of intricate and detailed structures at the nano level.The inherent properties of NMs, such as a high surface area, enhanced reactivity, and superior mechanical strength, translate into the improved performance of the printed objects.Different shapes can be tailored for specific functions, such as conductivity, flexibility, or biocompatibility, allowing for multifunctional components to be printed in a single process.Overall, processing NMs also requires specialized equipment and techniques, to ensure uniform dispersion and prevent aggregation, which can affect the quality and consistency of the printed structures.

Industrial Perspective
With nano-level manufacturing gaining significant traction in various fields, the need for advancements in nano-capable manufacturing equipment is ever-rising.Several companies are leading the advancement in nano-level manufacturing, such as UpNano [121], Nanoss GmbH [122], Nanoscribe GmbH [123], nano3Dprint [124], and Nano Dimension [125].While some have matured expertise in the field of submicron manufacturing and precision, some companies like nano3Dprint are yet to launch a state-of-the-art nanoscale 3D printing machine.

UpNano GmbH
The company UpNano (Vienna, Austria) supplies researchers and advanced manufacturing companies with a high-resolution 3D printer that is claimed to be one of the fastest in its league.The NanoOne (Vienna, Austria) 3D printer uses multiphoton lithography technology with TPP precision.The company provides various modules and materials to meet certain requirements.The NanoOne 3D printer is the first desktop submicron 3D printer to be launched.UpNano's patented process, along with the materials used, are biocompatible, which significantly aids in manufacturing critical components at nanoscale.For example, Dobos et al. [126] explored the use of such technology to 3D print living stem cells using a photosensitive bioink, which they concluded to be successful and impactful.One sample product fabricated with UpNano's NanoOne machine is shown in Figure 12.

UpNano GmbH
The company UpNano (Vienna, Austria) supplies researchers and advanced manufacturing companies with a high-resolution 3D printer that is claimed to be one of the fastest in its league.The NanoOne (Vienna, Austria) 3D printer uses multiphoton lithography technology with TPP precision.The company provides various modules and materials to meet certain requirements.The NanoOne 3D printer is the first desktop submicron 3D printer to be launched.UpNano's patented process, along with the materials used, are biocompatible, which significantly aids in manufacturing critical components at nanoscale.For example, Dobos et al. [126] explored the use of such technology to 3D print living stem cells using a photosensitive bioink, which they concluded to be successful and impactful.One sample product fabricated with UpNano's NanoOne machine is shown in Figure 12.

Nanoss GmbH
Electronics manufacturing has also benefited from micro-and nanoscale precision.Nano Scale Systems (Nanoss) GmbH (Darmstadt, Germany) launched a 3D printer, nano3DSense, that is capable of printing sensors below 10 nanometers.The nano3DSense printer uses an electron beam, similar to a SEM, that is a few nanometers in diameter, to use in nanoscale layer-by-layer production.This 3D printer can be used to manufacture various types of sensors, including force, strain, and pressure sensors.To exploit the capability of those types of sensors, Pfü ner et al. [128] used the nano3DSense 3D printer to create a nanoscale pressure sensor which was successfully used for continuous glucose measurement.One sample product fabricated with Nanoss' nano3DSense machine is shown in Figure 13.

Nanoss GmbH
Electronics manufacturing has also benefited from micro-and nanoscale precision.Nano Scale Systems (Nanoss) GmbH (Darmstadt, Germany) launched a 3D printer, nano3DSense, that is capable of printing sensors below 10 nanometers.The nano3DSense printer uses an electron beam, similar to a SEM, that is a few nanometers in diameter, to use in nanoscale layer-by-layer production.This 3D printer can be used to manufacture various types of sensors, including force, strain, and pressure sensors.To exploit the capability of those types of sensors, Pfützner et al. [128] used the nano3DSense 3D printer to create a nanoscale pressure sensor which was successfully used for continuous glucose measurement.One sample product fabricated with Nanoss' nano3DSense machine is shown in Figure 13.A nanosensor 3D-printed using the nano3DSense machine [129].(Courtesy of Nanoss, nanoss.de).

Nanoscribe
Another pioneering company in nanoscale 3DP is Nanoscribe (Leopoldshafen, Germany), which developed multiple high-resolution 3D printers for different applications, including a bioprinting-capable machine.The highest resolution machine they offer is the Photonic Professional GT2, which is claimed to have nanoscale feature control.Similar to other high-resolution 3D printers, this machine also uses TPP technology.Their machine is developed to be used for a plethora of applications and materials including microfluids, micromechanics, micro-electromechanical systems, micro-optics, nanostructures, and several others.Ko et al. [130] employed this machine to fabricate fused silica microstructures to be used in micro-optics and biomedical applications.Lee et al. [131] used the GT2 Figure 13.A nanosensor 3D-printed using the nano3DSense machine [129].(Courtesy of Nanoss, nanoss.de).

Nanoscribe
Another pioneering company in nanoscale 3DP is Nanoscribe (Leopoldshafen, Germany), which developed multiple high-resolution 3D printers for different applications, including a bioprinting-capable machine.The highest resolution machine they offer is the Photonic Professional GT2, which is claimed to have nanoscale feature control.Similar to other high-resolution 3D printers, this machine also uses TPP technology.Their machine is developed to be used for a plethora of applications and materials including microfluids, micromechanics, micro-electromechanical systems, micro-optics, nanostructures, and several others.Kotz et al. [130] employed this machine to fabricate fused silica microstructures to be used in micro-optics and biomedical applications.Lee et al. [131] used the GT2 machine to fabricate tunable microcubes to deliver catalase drugs.They concluded that this technology is promising for the advancement of biomedical applications, specifically in drug delivery.One sample product fabricated with a Nanoscribe machine is shown in Figure 14. nanoss.de).

Nanoscribe
Another pioneering company in nanoscale 3DP is Nanoscribe (Leopoldshafen, Germany), which developed multiple high-resolution 3D printers for different applications, including a bioprinting-capable machine.The highest resolution machine they offer is the Photonic Professional GT2, which is claimed to have nanoscale feature control.Similar to other high-resolution 3D printers, this machine also uses TPP technology.Their machine is developed to be used for a plethora of applications and materials including microfluids, micromechanics, micro-electromechanical systems, micro-optics, nanostructures, and several others.Ko et al. [130] employed this machine to fabricate fused silica microstructures to be used in micro-optics and biomedical applications.Lee et al. [131] used the GT2 machine to fabricate tunable microcubes to deliver catalase drugs.They concluded that this technology is promising for the advancement of biomedical applications, specifically in drug delivery.One sample product fabricated with a Nanoscribe machine is shown in Figure 14.

Nano3Dprint
Nano3Dprint (Burlingame, CA, USA) is a rising company in the field of nanoscale 3DP.The company offers multiple solutions for multi-material 3D printing, which is what electronics manufacturing needs, and solutions for nanoscale manufacturing.The company sells the MatDep Pro Independent Motion Multimaterials Electronics 3D Printer that combines FDM with Ink/Paste microdispensing, and developed a 3D printing machine, the D4200S, capable of 50 nm resolution manufacturing, which uses extensive scanning probe nanotechnology.The D4200S is compatible with various inks, including polymers, metals, biomolecules, and several others.The D4200S also has a built-in atomic force microscopy capability, which allows easy access to advanced analysis techniques.The MatDep Pro and D4200S are designed to be employed for manufacturing electronics, prototyping, research and development, and bioprinting.This D4200S is claimed to have the highest resolution materials deposition system in 3D printing.At the time of publication, following a successful pre-order campaign, the D4200S is scheduled to be officially launched in August 2024, which explains its limited utilization in research and industry thus far.Nonetheless, this innovation in nanoscale 3D printing is much anticipated for further driving 3D printing to the next level.Figure 15 provides an image showing the capability of the D4200S.
highest resolution materials deposition system in 3D printing.At the time of publication, following a successful pre-order campaign, the D4200S is scheduled to be officially launched in August 2024, which explains its limited utilization in research and industry thus far.Nonetheless, this innovation in nanoscale 3D printing is much anticipated for further driving 3D printing to the next level.

Nano Dimension
Nano Dimension (Ness Ziona, Germany) is a thriving company specializing in advanced manufacturing and automation, including a fully integrated electronics 3D printer.The company's fully integrated electronics 3D printer is called the DragonFly IV, and it is capable of a resolution of 10 microns.Its deposition technology is a piezo dropon-demand inkjet.While this is an ultrafine-quality fabrication technology, the innovative feature of this machine allows for the manufacturing of fully functional electronic devices without the need to outsource.This aids in retaining the confidentiality of designs and ideas, while maintaining the performance of 3D-printed electronics.The company also presents a number of software solutions and materials designed for their AM process.To utilize the capabilities of the DragonFly IV system, Hamid Allah et al. [134] used the system to create a microfluidic sensor used in diabetes diagnosis.Their work was proven to be successful, and these systems show a promising performance for functional microscale components.A fully functional electronic device fabricated by the DragonFly IV is shown in Figure 16.

Nano Dimension
Nano Dimension (Ness Ziona, Germany) is a thriving company specializing in advanced manufacturing and automation, including a fully integrated electronics 3D printer.The company's fully integrated electronics 3D printer is called the DragonFly IV, and it is capable of a resolution of 10 microns.Its deposition technology is a piezo drop-on-demand inkjet.While this is an ultrafine-quality fabrication technology, the innovative feature of this machine allows for the manufacturing of fully functional electronic devices without the need to outsource.This aids in retaining the confidentiality of designs and ideas, while maintaining the performance of 3D-printed electronics.The company also presents a number of software solutions and materials designed for their AM process.To utilize the capabilities of the DragonFly IV system, Hamid Allah et al. [134] used the system to create a microfluidic sensor used in diabetes diagnosis.Their work was proven to be successful, and these systems show a promising performance for functional microscale components.A fully functional electronic device fabricated by the DragonFly IV is shown in Figure 16.

Future Perspectives and Trends
Nanoscale 3DP is growing extensively in various fields and applications.Due to its versatility and the high demand for nanoscale structures and devices, it is one of the growing manufacturing technologies worldwide.This technology is considered as a novel solution to making ultrafine size structures easily, with controlled properties.The alliance of 3DP and nanotechnology is a great opportunity for the production of state-of-the-art, multifunctional, and smart products for nanorobotics, biosensors, electronics, nanooptics, energy storage, etc.Currently, extensive research is being carried out on the printing materials suitable for 3DP, including block copolymers (BCPs), resins, and NMs.Moreover, a number of research studies are also being carried out to improve the process of nano 3DP, including the investigations conducted on the laser type, the nozzle type and size, and the invention of new printing methods [136].In this section, the main goal is to equip readers with information about the opportunities, trends, future perspectives, and challenges of nanoscale 3DP.

Future Perspectives and Trends
Nanoscale 3DP is growing extensively in various fields and applications.Due to its versatility and the high demand for nanoscale structures and devices, it is one of the growing manufacturing technologies worldwide.This technology is considered as a novel solution to making ultrafine size structures easily, with controlled properties.The alliance of 3DP and nanotechnology is a great opportunity for the production of state-of-the-art, multifunctional, and smart products for nanorobotics, biosensors, electronics, nanooptics, energy storage, etc.Currently, extensive research is being carried out on the printing materials suitable for 3DP, including block copolymers (BCPs), resins, and NMs.Moreover, a number of research studies are also being carried out to improve the process of nano 3DP, including the investigations conducted on the laser type, the nozzle type and size, and the invention of new printing methods [136].In this section, the main goal is to equip readers with information about the opportunities, trends, future perspectives, and challenges of nanoscale 3DP.
Electrochemical AM (e-AM) is considered as the most promising technique of nanoscale 3DP [137].This method offers a combination of the capacity to process different materials, namely, its design freedom, and micro-to nanoscale resolution, opening opportunities for a range of applications, including microelectronics, sensors, batteries, etc.
Multiphoton stereolithography is probably the most prevalent technology for nanoscale 3DP that combines nanoscale resolution with complex designs [138].This technique takes advantage of the non-linear light absorption that confines photochemical processes to the nanoscale.The optical AM technique is limited to photocurable materials that have a narrow range of mechanical, electrical, and optical properties.contrast, for electrochemical methods, different material types, including metals, metal oxides, conductive polymers, and biomolecules, in the forms of thin films, single crystals, nanoparticles, and composites can be used.
Electrochemical techniques have not been seriously used so far, mainly because the production of the print nozzles including nanoelectrodes and nanopipettes is difficult, expensive, and hardly reproducible.As the nozzles become more available, e-AM shows tremendous progress.With this method, a variety of features, including pillars, hollow parts, etc., can now be made.Further advancement in resolution, speed/throughput, chemistry, and materials range requires combined multidisciplinary efforts.Thus, the advancement of nanoscale technology is promising for the advancement of micro robotics, sensors, memory devices, and quantum systems.Currently, a handful of metals, including Cu, Ag, Au, and Ni, can be used for e-AM.Expanding the range of materials is needed for the future of nanoscale 3DP [139].
Extensive research is being conducted on material types for nano 3DP.At Stanford University, metal clusters are fused with resin-like mixtures to make stronger and lighter materials.This method was reported for the AM of various materials using metal nanoclusters, which function as initiators, acid generators, and sensitizers, and simultaneously serve as precursors for mechanical reinforcements and nanoscale structure.In the TPP process, it was observed that nanoclusters have the role of accelerator for the printing process and can increase the speed of printing to 100 mm/s.It was also demonstrated that it is possible to print other types of nanocomposites with a different morphology.This shows that photoactive NMs can be used as precursors for the nanoscale 3DP of complex parts with good mechanical properties [140,141].
Dual curing in AM refers to the process of using two distinct mechanisms to harden and set materials [142].Typically, this involves a combination of photo-curing, where light (often UV) initiates polymerization, and thermal curing, where heat is applied to further harden the material.This dual approach allows a precise control over the material's mechanical characteristics, resulting in stronger, more durable, and highly detailed structures.At the nanoscale, these materials enable the creation of complex geometries with exceptional resolution, making them ideal for advanced applications in semiconductor manufacturing and medical devices [143].Several AM technologies like VPP and MJ utilize dual curing to enhance the properties of printed materials.
With advancements in AM, nanoscale technologies such as Electrohydrodynamic Printing and Meniscus-Guided Printing are gaining popularity [144,145].Electrohydrodynamic Printing is a high-precision AM technique that uses an electric field to control the deposition of materials at the nanoscale, while Meniscus-Guided Printing is a high-speed, high-precision AM technique that utilizes the controlled movement of a liquid meniscus to direct material deposition.
BCPs are another class of materials that have been used for nanoscale 3DP [146].BCPs consisting of poly(styrene) (PS) and a polymethacrylate decorated with cross-linkable, printable units were synthesized using controlled radical polymerization.Figure 17 shows a schematic illustration of the steps for the 3D printing of BCPs with TPP.In the first step, to introduce the printable moiety, BCPs consisting of a PS block and a polymethacrylatebased block are synthesized and postmodified.The resultant BCPs are introduced into self-assembled inks by a solvent and photo-initiator.In the second step, the TPP method is employed to make a defined 3D structure.
dynamic Printing is a high-precision AM technique that uses an electric field to control the deposition of materials at the nanoscale, while Meniscus-Guided Printing is a highspeed, high-precision AM technique that utilizes the controlled movement of a liquid meniscus to direct material deposition.
BCPs are another class of materials that have been used for nanoscale 3DP [146].BCPs consisting of poly(styrene) (PS) and a polymethacrylate decorated with cross-linkable, printable units were synthesized using controlled radical polymerization.Figure 17 shows a schematic illustration of the steps for the 3D printing of BCPs with TPP.In the first step, to introduce the printable moiety, BCPs consisting of a PS block and a polymethacrylatebased block are synthesized and postmodified.The resultant BCPs are introduced into self-assembled inks by a solvent and photo-initiator.In the second step, the TPP method is employed to make a defined 3D structure.In the last step, using a SEM, the cross-sections of the 3D-printed microstructures are analyzed.TPP printing is capable of printing complex structures with a controlled nano-order in the entire structure.A library of BCPs with different compositions and functionalizations using controlled radical polymerization has been synthesized for a wide range of applications.The study of the synthesis and rheological properties of 3DP inks has become important, because of the potential future applications of nanoscale 3D-printed structures for such multifunctional 2D nano-inks.Such nano-ink-generated structures have a potential application in biomedicals, solar cells, energy storage, and water purification techniques [147].
In microscale 3DP, SLA and Selective Laser Sintering (SLS) are considered the two main methods of manufacturing, with SLA being considered as the main resin-based printing method and SLS as the main metal-based printing method.Lasers and light perform well at the microscale, but they cannot make complex parts, or they cause rough surfaces and a low accuracy at nanoscale.TPP is a novel method for nanoscale 3DP.TPP uses a resin-like material to make the object layer by layer using a laser.Figure 18a,b demonstrate the different steps of the TPP process.Figure 18a shows the nozzle containing the electrolyte solution and electrode above the substrate, and Figure 18b shows the different steps of printing a voxel.Figure 18c shows a pillar in nanoscale printed by TPP. Figure 18d,e show the side and top view of helical nanostructures printed by the TPP method.Dimitrov et al. used the TPP method to print copper spiral columns with a size of 25 nm [148].
One application for this technology is in batteries, to reduce the ion pathways during the charging process.Using nanoscale 3DP, the distance between anode and cathode can be reduced to a few nanometers, and this allows the interlocking of the anodes and cathodes like fingers at nanoscale.So, it makes it possible to produce batteries that can recharge hundreds of times faster than regular batteries.It means charging EVs faster than filling the gas tank.
demonstrate the different steps of the TPP process.Figure 18a shows the nozzle containing the electrolyte solution and electrode above the substrate, and Figure 18b shows the different steps of printing a voxel.Figure 18c shows a pillar in nanoscale printed by TPP. Figure 18d,e show the side and top view of helical nanostructures printed by the TPP method.Dimitrov et al. used the TPP method to print copper spiral columns with a size of 25 nm [148].One application for this technology is in ba eries, to reduce the ion pathways during the charging process.Using nanoscale 3DP, the distance between anode and cathode can Integrating nanotechnology into AM makes it possible to create nanosized materials with controllable chemical, physical, and biological properties [149].The fabrication of bone substitutes, scaffolds, and implants for various medical applications is one of the most favorable applications of nanoscale production.The huge advancement in 3DP technology in recent years provides immense potential for the fabrication of scaffolds and implants for biomedical applications, especially for bone repair and regeneration.Because surgical methods are not helpful for complete regeneration, it is necessary to find an alternative approach to repairing bone defects.So, the technology can integrate living cells within a construct made of micro or nano biomaterials, to make artificial bone grafts capable of regenerating damaged tissues.Different biomaterials including metal, polymer, and ceramic have been used in clinical applications.This method has the potential to change the way that orthopedic surgeries are conducted [150].
Another promising trend in nano 3DP research is the screening, development, and manufacturing of nano-drug delivery systems [151].Drug carrier nanoparticles can be produced through nanoscale 3DP with high precision.The ability of nanoscale printing to make complex structures makes it possible to develop nanomedicine and to produce functioning tissues and organs.Recent trends in the research on nanoscale 3DP for drug delivery focus on drug delivery systems to improve release kinetics, to enhance therapeutic efficacy, and to minimize side effects.Using AM technologies, making nanoscale drug carriers to deliver medicine to the targeted area becomes more feasible, and this method is very promising for reducing side effects and increasing the therapeutic efficacy in drug delivery [152].
Semiconductors are another promising field for nanoscale AM.Some 3D-printable photoresins based on metal-bound composites were synthesized for semiconductor applications.These photoresins are synthesized by metal-organic framework precursors and available monomers.Metal oxide semiconductors like ZnO and Co 3 O 4 , can be manufactured with AM and with a resolution in the range of 170 nm, a high surface quality, and high shape fidelity.UV photodetectors based on ZnO show potential applications in semiconductor functional devices, especially for optoelectronics devices.A ZnO-based UV photodetector shows a large on-and-off ratio and a high cycling stability, and has potential for further applications [153].
Hybrid AM techniques were used to make piezoelectric wearable sensors without the need for high-voltage poling or high-temperature sintering [154].The electrodes and piezoelectric devices showed excellent stretchability, with no need for a complex design.The fact that the method is free of poling and sintering facilitates the direct printing of the device, and this has great potential for wearable piezoelectric applications.These printed piezoelectric wearable devices can convert mechanical movements of the human body into electrical signals using piezoelectric Tellurium nanowires.The method can be expanded to other materials and structures for new applications, including biomedical wearable sensors.In this method, Tellurium nanowire was used to make a piezoelectric device by a hybrid method of integrating aerosol jet printing and extrusion printing in a single platform.The aerosol jet-printed Tellurium nanowire shows piezoelectric properties and does not need poling.The printed silver nanowire electrodes show great conductivity and stretchability, without the need for sintering.To print the silicon film, an extrusion method is used, which is a stretchable substrate, and also electrical insulation between the silver layer and the Tellurium.The device is attached to the human wrist and neck to detect hand gestures and heartbeat without an external power source.The method opens enormous opportunities for printed electronics and wearable devices [153].
Nanoscale 3DP has also been used for the manufacturing of 3D inorganic glass-ceramic structures with a size of 100 nm using sol-gel precursors [155].The method is based on the laser 3D lithography of inorganic-organic hybrid resin.After printing, heating enables the formation of an inorganic composite.The method made it possible to achieve a resolution of 100 nm for 3D patterns of complex free-form structures.Another advantage of the method is a high throughput of nearly 50 × 103 voxels/s.The method opens a wide range of future applications for the nanoscale printing of inorganic materials in devices that need to function at high temperatures and in harsh physical/chemical conditions [156].
Nanoscale 3DP is considered as one of the advancing technologies for the manufacturing of NMs for advanced applications, including microelectronics, nanorobotics, nanooptics, sensors, batteries, drug delivery, biomaterials, etc. Extensive research is being conducted on novel AM methods to improve the quality and resolution of printed parts.Moreover, many research studies are underway on printing materials suitable for AM, including block copolymers, resins, metals, and inorganic materials, which opens opportunities for new applications.Due to its design freedom, high resolution, and the nanoscale manufacturing of a wide range of materials, overall, nanoscale 3DP has the potential to advance the R&D efforts of today's AM technologies.

Conclusions
Advancements in the field of AM have been growing in every aspect of daily life, from design to manufacturing.R&D organizations and academia continuously create a high number of unique solutions for obtaining high-quality and low-cost additively manufactured products.Researchers have developed several unique knowledge blocks to improve traditional and non-traditional nano 3DP processes, including their working parameters, applications, and quality outcomes.This review paper debriefs the latest nano 3DP R&D knowledge blocks and advancements collected from a high number of archival research papers.These 3DP NMs are uniquely divided into several subtypes, in terms of their dimension, shape, and being organic or inorganic.One of the unique deliverables of

Figure 1 .
Figure 1.Transitioning of 3DP technologies from macro and micro to nano levels.

Figure 1 .
Figure 1.Transitioning of 3DP technologies from macro and micro to nano levels.

Figure 4 .
Figure 4. Steps of MEX from CAD design to final developed part.

Figure 4 .
Figure 4. Steps of MEX from CAD design to final developed part.

Figure 7 .
Figure 7. Intensity profile and threshold of two-photon polymerization.

Figure 7 .
Figure 7. Intensity profile and threshold of two-photon polymerization.

Figure 8 .
Figure 8. Diagram showing key components of standard laser-induced forward transfer (LIFT).

Figure 8 .
Figure 8. Diagram showing key components of standard laser-induced forward transfer (LIFT).

Figure 9 .
Figure 9. Types of nanoparticles by composition.

4. 4 .
Nano3DprintNano3Dprint (Burlingame, CA, USA) is a rising company in the field of nanoscale 3DP.The company offers multiple solutions for multi-material 3D printing, which is what electronics manufacturing needs, and solutions for nanoscale manufacturing.The company sells the MatDep Pro Independent Motion Multimaterials Electronics 3D Printer that combines FDM with Ink/Paste microdispensing, and developed a 3D printing machine, the D4200S, capable of 50 nm resolution manufacturing, which uses extensive scanning
Figure 15 provides an image showing the capability of the D4200S.

Figure 17 .
Figure 17.(a) Pre-assembled functional BCP-based inks exhibiting a lamellar (I) or cylindrical morphology (II) are utilized for (b) the fabrication of defined 3D structures.(c) For characterization, the 3D-printed microstructures are embedded into epoxide resin and sectioned using an ultramicrotome, and the internal nanostructure is imaged via SEM [147].

Figure 17 .
Figure 17.(a) Pre-assembled functional BCP-based inks exhibiting a lamellar (I) or cylindrical morphology (II) are utilized for (b) the fabrication of defined 3D structures.(c) For characterization, the 3D-printed microstructures are embedded into epoxide resin and sectioned using an ultramicrotome, and the internal nanostructure is imaged via SEM [147].

Figure 18 .
Figure 18.(a) TPL printing nozzle.The nozzle, containing the electrolyte solution and the quasireference counter electrode, is placed above a conductive substrate that constitutes the working electrode of the two-electrode electrochemical cell.The substrate and the nozzle are translated with respect to each other by piezoelectric nanopositioners.(b) Schematic of the printing process of a voxel.(c) Sideview of a 10 × 10 array of pillars (656 voxels) forming a Gaussian peak with increasing heights toward the center of the array.Side (d) and top (e) view on four helical structures printed with center-to center spacing of 500 nm [148].

Figure 18 .
Figure 18.(a) TPL printing nozzle.The nozzle, containing the electrolyte solution and the quasireference counter electrode, is placed above a conductive substrate that constitutes the working electrode of the two-electrode electrochemical cell.The substrate and the nozzle are translated with respect to each other by piezoelectric nanopositioners.(b) Schematic of the printing process of a voxel.(c) Sideview of a 10 × 10 array of pillars (656 voxels) forming a Gaussian peak with increasing heights toward the center of the array.Side (d) and top (e) view on four helical structures printed with center-to center spacing of 500 nm [148].