A Review on Printed Electronics with Digital 3D Printing: Fabrication Techniques, Materials, Challenges and Future Opportunities

Abstract

The introduction of 3D printing technology has revolutionised the manufacturing and electronic product design in the past few years, where it is used to even produce printed circuit boards. Printed electronics is one of the fastest-growing additive manufacturing technologies and is becoming invaluable to various industries. The evolution of several contact and non-contact types of fabrication techniques have been reported in the recent past. Leveraging these technologies, various types of printed electronic components have been realized. One method is inkjet printing technology, which has been widely accepted for printed electronics manufacturing. As 3D printing uses only those materials which are essential to create the product, it eliminates waste production, with a smaller equipment cost and minimizes the number of process steps, resulting in lower manufacturing costs with reduced turnaround time. Various kinds of conductive and non-conductive materials have emerged in the recent past in conjunction with many manufacturing techniques for printed electronics. Herein, we review the most commonly used substrates, electronic printing materials, and the widespread printing techniques employed at the industrial level, giving an overall vision for a better understanding and evaluation of their different features. The technical challenges of several contact and non-contact techniques with corresponding solutions are also presented. Finally, status on advances in the production of various kinds of materials employed in 3D printed electronics and the methods for producing them, shortcomings, technical challenges, applications, benefits, and the future opportunities pertaining to printed electronics are discussed in detail.

Introduction

Printed electronics (PE) is a comprehensive term for the printing technology used to make electronic devices by printing on a wide range of substrate materials. Generally, it refers to the processes used to fabricate electrical circuits, components, and devices by conventional printing methods. Additive manufacturing (AM) is a technology that enables design and manufacturing of electronic products. In the 3D printing process, various objects are created by adding different materials, layer by layer, quickly and with ease.^1,2,3 Originally, printed electronics began with printing on organic or plastic-based substrates using suitable inks derived from carbon-based compounds. Owing to the expansion of the demand for wearable devices and thinner electronics, printed electronics are used to fabricate a variety of electronic structures, including keyboards, electronic skin patches, thin-film microstrips, sensors, actuators, radio frequency identification (RFID), antennas, biochips, microelectromechanical systems (MEMS), microfluidics, flexible electronics, electronic components, embedded circuits, and many more. Typically, organic electronics are fabricated using conventional procedures such as dip coating, spin coating, and thermal vacuum deposition. Flexibility in design, low customized costs, shorter lead times, flexible digital manufacturing, less waste, lower startup costs, and decentralized manufacturing are just a few of the important benefits of 3D printing technology applied to electronics manufacturing.

Printed electronics technology is emerging at an overwhelming pace adopting various printing techniques for manufacturing electronics. Several decades ago, conductors, resistors, and dielectrics were manufactured on ceramic surfaces using screen printing.^4,5 Hybrid circuits were produced in addition to a few active components to the circuit.⁶ Printed electronics have gained much interest among scientists and engineers in almost every branch of manufacturing owing to numerous advantages such as high throughput, reduction in material wastage, lower processing temperatures, less complexity in processing when compared to that of traditional silicon-based technology, which entails deposition processes at elevated temperatures at higher vacuum levels and advanced lithographic printing techniques.^7,8 Screen printing, flexographic, gravure, and inkjet printing techniques have been successfully employed for manufacturing various electronic structures, such as sensors,⁹ organic thin film transistors,¹⁰ light emitting device (LED),¹¹ displays,¹² RFID tags¹³ and solar cells.¹⁴ Printed electronics technology has also been successfully employed to manufacture printed circuit boards (PCBs) with a wide variety of flexible dielectric materials including different kinds of paper, plastic, and fabrics adopting conventional methods.¹⁵

The present-day electronic products are typically housed in standard plastic mold housings, integrating printed circuit boards with soldered through-hole and surface mount technology (SMT) components. Because of the three-dimensional design of the majority of housings, components may need to be angled, but more importantly, interconnections between components cannot be on a separate piece of FR4 material. In recent years, the printed electronic systems have been successfully accomplished on non-planar surfaces for different applications including biomedical,^16,17 health,¹⁸ robotics,¹⁹ automotive and aerospace.^20,21 Technical feasibility for printing on non-planar surfaces and the compatibility of different polymeric materials combined with new printing techniques working at temperatures matching with various polymeric substrates contributed for the remarkable success of the printed electronics.

The appropriate fabrication technology for printed electronics are selected based on the nature of respective electronic components, considering its size, weight, flexibility, disposability, volume, and the manufacturing cost. The significant factors influencing the printed electronic device are the ease of processing, performance of the resulting product and the long-term consistency of the organic and inorganic materials used in the form of pastes, inks or coatings. Inorganic printable inks are normally metallic-type nanoparticles (eg. aluminum, silver, copper, gold) and are spread in a retaining matrix. Organic printable inks are based on organic materials, such as different polymers, which can act as conductors, semiconductors or dielectrics. The highly conductive polymer inks are used in batteries, electromagnetic shields, and capacitors, while in active devices, inks based on organic semiconductors are used (as active layers) for realizing organic light emitting diodes (OLEDs), organic photodiodes (OPDs), sensors, organic solar cells (OSC) etc. The revolution in next-generation electronics and the growing demand for portable electronic devices requires the development of a variety of new and improved manufacturing procedures for electronics. At present, achieving the significant features and characteristics of printed electronics may not be feasible with conventional electronics, whether it is a single-crystal silicon integrated circuit (IC) or an amorphous silicon thin-film transistor (TFT) display. Electronics printing enables the replacement or support of structures and devices with comparable functionality to conventionally produced electronics, but with the added benefits of increased throughput, rapid turnaround time, minimal material waste, cost effectiveness, and reduced manufacturing complexity.

Conventionally, printed electronics and sensors are realized with the standard approach by using pre-patterned parts (stencils) of a particular component in physical contact with the substrate material and transferring the functional inks to create the desired structure.^22,23 Non-contact and contact printing techniques are the two main approaches generally employed in the development of a system for electronics printing.

The broad classification of printing technologies into two major categories and their further division into different techniques is depicted in Fig. 1. In general, various non-contact printing techniques have been in greater demand owing to several distinct advantages including ease of process adaptability, simplicity, turnaround time, affordability with reduced material wastage, high-resolution printing and ease of process control with adjustable parameters.^24,25 In the recent past, however, miniaturized polymeric stamp-based advanced printing techniques such as micro-contact printing, nanoimprint lithography and transfer printing have evolved particularly for manufacturing flexible electronics with single-crystal inorganic semiconductors.^26,27

Fig. 1

General classification of printed technologies.

Fabrication Technologies for Printed Electronics

Since printed electronics (PE) are ubiquitous at almost every stage of modern society, from individual personal gadgets to the most complex integrated systems, they have gained significance proportionately. While manufacturing most electronic devices with suitable printing technology, the functional materials are sprayed over the substrate to generate thin film and patterns; it can be combined to construct any electronic device for use in electronic circuits.

The printing technologies are broadly classified into two groups. In contact (impact/with master) printing approach, the ink/solution comes in direct contact with the target substrates, whereas in the case of non-contact (maskless) printing, the ink is dispensed with a suitable mechanism and deposited without physically contacting the substrate. The patterned structures with inked surfaces are joined at controlled pressures with physical contact onto the target substrate in contact printing. In non-contact printing, with a pre-programmed pattern design, electronic structures are defined by moving the substrate holder and dispensing the ink (solution) through the nozzles.

Common Printing Techniques Used for Printed Electronics Manufacturing

Contact Printing Techniques for Printed Electronics

Screen Printing

Screen printing (SP) can be used in different methodologies, such as flatbed presses, which are employed for lower throughput (low-volume) production, and rotary systems which are meant for device manufacturing for lower volume, high-speed production roll-to-roll (R2R) with an optimized solution and printing parameters. Screen printing can be repeated with similar results keeping the same process limitations and using optimized solution.^28,29 Screen printing comprises a screen, squeeze, prescribed and suitable substrate fixed on the movable platform. Generally, in flatbed screen printing, a printable solution is applied on the screen, the ink is coated over the structured mesh on the stencil by squeegee, and controlled pressure is applied to dispense the ink through the holes in the mesh. Rotary system screens enable high-speed production, but they are expensive and require frequent cleaning due to material clogging.³⁰ The quality of screen printing, however, is affected by various parameters such as print speed, snap-off between screen and substrate, mesh-size, solution viscosity, and angle geometry of the squeegee.

To avoid undesired flow of ink through the screen mesh, viscosity is maintained higher than other conventional printing methods.^26,31 This printing technique is used for creating patterned structures as well as for large area manufacturing.³² It is feasible to print with high resolution when the substrate's surface energies and the ink's surface tension are compatible.³³ He et al. reported in 2019 that by preparing the conductive ink using graphene nanoparticles (GNPs) and using screen printing technology it is possible to form the conductive patterns on the polyethylene terephthalate (PET) and paper substrates for the flexible electronics applications.³⁴ Figure 2 depicts the screen printing process for the conductive ink application. Nylon, stainless steel and polyester are some of the materials of construction used in the fabrication of screen mesh. Flexible electronic (FE) circuits with many layers and incorporating passive and optical devices have been constructed using an enhanced screen-printing technique. Because screen-printing does not require as much capital as several other manufacturing techniques, it is chosen by various sectors that manufacture printed electronics.

Fig. 2

(a) Screen printing process using graphene ink, (b) Cross-sectional view of the screen-printing process. The figure is reprinted with permission from Ref. 34. Copyright ACS Publications 2019.

There are a few technical challenges encountered with screen printing. Higher risks are involved while repeatedly producing the reliable electronic devices, through printing of multilayered structures with higher wet thickness, and exposure of ink to ambient temperature keeping on the screen bed. When the system is inactive, the rapid evaporation of solution and surfactants from the printing paste causes screen clogging (ink clogging). These technical challenges can, however, be overcome by setting proper process parameters and adopting proper procedure in order to achieve reliable and repeatable results.

Gravure Printing

Gravure printing (GP) is a familiar additive manufacturing techniques, which is capable of producing very high-quality electronic structures at higher speeds. Gravure-printed images are highly stable and reliable due to the robustness of the image.²³ It is capable of depositing film thicknesses that are not possible with flexographic and lithographic techniques. Mizuno and Okazaki showed the offset gravure printing of a black matrix for color filters in active matrix displays in the 1990s.³⁵ Afterwards, a suitable offset gravure technique to print a resist was developed for manufacturing amorphous silicon TFTs. An engraved cylinder, doctor blade, ink fountain, and impression roller are the four basic components to each printing unit in gravure technique. The gravure cylinder is the heart of a gravure process that carries the design to be printed. Electromechanical means or laser technique is used for engraving the microcells in the cylinder for gravure printing.³⁶

The cylinder is built from a thick-walled steel base that has been electroplated with copper and then polished to protect the copper coating from abrasion and scratches generated by the doctor blade during printing. For optimizing the ink transfer, the ratio of width to depth of the microcells in the engraved cylinder plays a significant role.³⁰ Figure 3 illustrates the process of printing pattern using the gravure process. The excess ink from the surface of the engraved cylinder is efficiently sheared and removed by the doctor blade during printing process. The doctor blade mounted at an angle is sufficiently pressurized to ensure uniform contact along the cylinder's length.

Fig. 3

Schematic illustration of the gravure printing process. The figure is reprinted with permission from Ref. 37. Copyright from Wiley Online Library.

It is vital to control the blade's pressure and angle carefully to avoid early cylinder wear caused by the collection of ink particles or blade chips beneath the doctor blade as it oscillates back and forth. Rapid prototyping with increased printing speed is possible with optimized ink viscosities, which allows complete emptying of the ink from the engraved microcells.³⁸ There are many other factors influencing quality of gravure printing, such as impression pressure, doctor blade angle, pressure, speed, substrate properties (smoothness, ink and porosity receptivity, compressibility, wettability, etc.) and ink properties (ink chemistry, rheological behavior, viscosity, and solvent evaporation rate). Gravure offset printing reliability is crucial for supporting printed electronics on rollable substrates in a high-speed manufacturing line.³⁹

Flexographic Printing

In flexographic printing (FP), a polymer-based plate with raised surface patterns is used and is attached to the printing cylinder. Flexographic printing uses a wide variety of solvent-based, UV-curable inks and wafer-based chemically curable (two-part) inks for generating patterns of high-resolution structures on surfaces. The printing takes place when the inked areas of the elevated surfaces come into contract with the moving substrate, after the anilox cylinder draws the ink from the reservoir. The choice of the precise anilox roller is of central importance for an ideal printing result. Thin patterns with sharp edges result with highly efficient transfer of ink in FP. For obtaining better resolution patterns, higher concentrations of solution are essential within the precise range of ink viscosities. FP results in the typical resolutions ranging from 50 µm to 100 µm. With proper tuning of process parameters, however, it is possible to achieve resolutions down to around 20 microns and uniform film quality is achievable. Printing of discontinuous patterns will result due to erosion of any of the cells in flexographic printing. Application of pressure from the impression cylinder on the flexible plate leads to variation in pattern dimensions, and hence, there should be higher tolerance limit for resolution of flexographic printing.^40,41

Figure 4 represents the schematic process of the flexographic printing. Several printing cycles with settings of similar parameters are needed to realize the deposition of thick film, which is one of the challenges necessitating repetition of a similar procedure for printing subsequent layers maintaining proper alignment of the flexographic equipment during individual passes. FP has limitation in realizing highly desirable features with higher switching speed of the signal and reduced supply voltage, which are essential for various electronic applications resulting in degradation of functional parameters including parasitic capacitances, mobility of charge-carriers and registration accurate levels. Generation of fine patterns with FP technique are associated with the challenges of surface irregularities and pores, ragged lines, non-uniform printing and in addition non-availability of appropriate functional materials.⁴³

Fig. 4

Schematic and the machine showing the printing stamp. The figure is reprinted from [42] under the terms of the Creative Commons CC.

Non-Contact Type Printing Techniques for Printed Electronics

Laser Direct Writing

Direct writing (DW) is a type of digital printing technology which is associated with a large number of flexible multi-scale procedures for the deposition of functional materials that may be used to build basic linear and complex conformal electronic structures on a substrate. Direct writing utilizes a range of processes and energy modalities, including laser, inkjet, mechanical pressure, and tips, to facilitate material transfer and generate features ranging in size from nanometers to millimeters. Owing to their conformal writing capabilities, this emerging group of additive on-demand methodologies complement the existing conventional electronics manufacturing approaches particularly in product miniaturization and in the reduction of geometrical footprints. There is a wide range of materials starting from variety of dielectrics, ceramics, metallic, polymers and biomaterials as well. The direct writing approach can produce layers varying in thickness from a single molecule layer to hundreds of micrometers. DW is scalable; it may be utilized for high-volume production, particularly in the microelectronics industry. For the microscopic world, two-photon laser direct writing (LDW) is a 3D printing technique. LDW can typically resolve smaller features from few microns to sub-micron range, thus allowing 3D drawing of complex shapes with fine features (μm to a few nm). The limits of the optical setup and the photochemistry of the materials used determine the fabrication complexity of shapes that can be achieved by LDW.

Laser-Induced Graphene

An infrared laser is used to write directly onto a carbon-based polyimide-like precursor material, generating a multifunctional graphene foam, which is known as laser-induced graphene (LIG). The LIG process is a promising additive 3D printing technology and is a new path towards realizing electronic circuits. Generally, it is achieved using a visible laser that directly converts polyimide into LIG,⁴⁴ enabling the formation of LIG with a spatial micron-level resolution and a thickness of few microns.⁴⁵ This spatial resolution and the minimum feature sizes of LIG is determined by the spot-size of the laser. LIG enables the direct synthesis of graphene for precision electronics applications on a variety of surfaces.⁴⁶ With growing interest from the researchers towards usage of LIG for manufacturing sensors and flexible electronics,⁴⁷ further fine-tuning of this process can result in greater utility for the production of wide range of flexible electronics across many industries.

Inkjet Printing

Inkjet 3D printing is a type of additive manufacturing process used for depositing liquid materials or solid suspensions at lower temperatures and pressures. Inkjet printing (IJP) is a new 3D printing technology which is considered to be in an early stage of advancement.⁴⁸ The materials that can be deposited through IJP include many polymers, nanoparticles of dielectric and conductive inks.⁴⁹ In inkjet printing, a miniaturized nozzle within a print-head extrudes the material in a suitable form of printing. Multiple layers are constructed layer-by-layer with the raster scanning of print-head in inkjet 3D printing process. Individual deposited layers undergo curing instantaneously in between successive depositions and accordingly all inkjet 3D printing systems are supported with suitable equipment for curing every layer within the system. It produces droplets of ink passing through the fluid channel, the dimeters of which range from 10 μm to 150 μm, comparable with the diameter of the printing nozzle. The IJP technology is considered highly advantageous for PE due to the following reasons:

• IJP technology accepts electronic data and prints directly in a droplet-by-droplet method on several kinds of substrates.

• Being a simple and compact setup, inkjet printing needs less investment, space requirements and commissioning time than that of several other printing technologies.

• IJP technology can meet the key requirement of organic electronics by producing patterned thin films.

• Using IJP, it is also possible to add functionalities on a substrate with electronic structures and devices, prefabricated using any other fabrication technology.

In IJP technology, the data in a digital format are transformed into predefined structures without using a mask in non-contact mode by direct deposition (from miniaturized orifices) of droplet fluid, nanoparticles,^50,51 conductive polymers,^52,53 proteins or minerals,^54,55 and a wide variety of different materials including bioactive fluids, which are not compatible for processing through conventional techniques requiring exposure to photolithography and etching chemicals.⁵³ For printing various kinds of fluids, post-processing treatment in the form of thermal annealing or sintering is needed.⁵⁶ IJP requires a smaller quantity of materials, is environmentally friendly producing less waste⁵⁰ and is compatible for patterning on a wide variety of substrates such as glass, plastic,⁵⁷ paper,⁵⁸ and textiles,⁵⁹ when compared to other types of deposition methods. IJP is useful for the production of structural polymers and ceramics, biomedical appliances, transducers,⁵⁰ transistors,⁶⁰ MEMS,⁵¹ sensors,⁵⁸ and 3D electric circuits⁶¹ as well. IJP is a single-step technology with low manufacturing cost, does not require any special processing conditions and makes use of a wide variety of low-cost raw materials.⁶²

Inkjet printers are classified into two categories: continuous and drop-on-demand (DoD).^62,63 The principles of the droplet formation and ejection, however, are extremely different from each other.^64,65 In the case of continuous inkjet (CIJ) printing, a continuous stream of pre-generated ink droplets is transferred between charging plates, allowing them to acquire an electrical charge, and then they are directed towards the substrate by an electric field. Since production of a droplet is a continuous process, there are some unused droplets, which are captured in a gutter and then recirculated back into the ink reservoir. In DoD, ink droplets are generated only when they are required. With this technique, energy is more efficiently utilized; however, nozzle clogging is more likely due to the evaporation of solvent when the print head is idle.

As shown in Fig. 5, the droplets are generated in thermal-type DoD, when a resistor in the ink chamber heats up, vaporizing adjacent liquid and resulting in the formation of a bubble. Piezoelectric DoD ejects ink droplets as a result of a pressure pulse created in the ink chamber by the action of a piezoelectric actuator. Piezo-type DoD inkjet print heads are used by most industrial and research institutes. The actuating voltage is varied for controlling the velocity and the volume of an ejected droplet from a piezo DoD print head. Velocity and volume of the droplets can be increased by increasing the voltage.

Fig. 5

Inkjet printing illustration. The figure is reprinted from [42] under the terms of the Creative Commons CC.

The reliability of the 3D printing system is measured with its resolution as an important process parameter which determines the minimum size of the feature and printing tolerances. The actual size of the print head nozzle determines the droplet size, which is the limiting factor of 3D IJP. The smaller the nozzle, the higher the resolution and results in finer printed features. 3D IJP is extremely advantageous for additive manufacturing of high-resolution PCBs.

Aerosol Jet Printing

Aerosol jet printing (AJP) is a fast expanding contactless direct-write technology for producing tiny features on a range of substrate materials. For printed electronics, it is often referred to as maskless mesoscale materials deposition (M3D).⁶⁶ The latest electronics aerosol jet printer is employed for printing high-resolution transmission lines, sensors and antennas²⁰ on a wide variety of substrate materials including different kinds of polymers, glass, ceramic, IC materials, FR4 (glass-epoxy), and even certain metals. Freeform deposition, a significant feature of AJP enables researchers to generate a variety of devices with augmented geometric complexity compared to conventional electronics manufacturing or generally employed direct-write methods. When AJP proven as a digitally driven approach for integrated electronics manufacturing, comprehensive material compatibility, higher resolutions and independence of orientation have provided uniqueness in several applications. AJP is suitable for printing complex designs including thin film transistors, displays and solar cells. On non-planar surfaces, aerosol jet printing is possible with a variety of materials including metallic conductors, semiconductors, and insulators in viscosities ranging from 1 cps to 1000 cps.⁵⁰ The aerosol printing process is driven by the gas flow, where a stream of micro droplets is generated as a result of atomization or through ultrasonication. AJP is considered most fascinating method among other contactless printing techniques because of its capability to process a wide variety of materials with higher resolution patterns on different substrates.

Based on their operational procedures, AJP is broadly classified into two categories, pneumatic and ultrasonic, as illustrated in Fig. 6. Each of these categories has a distinct set of requirements and objectives, and their applications are tailored to meet those requirements and objectives.⁵¹ Aerosol mist is generated in a pneumatic atomizer by injecting pressured air/gas into a closed chamber containing the ink, resulting in the formation of minute droplets near the ink and air contact. The gas-borne micro-droplets with sizes smaller than 5 µm are accelerated toward the nozzle print head. In the case of the ultrasonic aerosol jet, the ink stored in a container is subjected to ultrasonication, which results in the formation of micro-droplets due to ultrasonic pressure waves. As a result, the created micro-droplets become entrained in the aerosol gas, which is then accelerated towards the print head. Together with the nozzle orifice size, the accompanying sheath gas flow determines the size of the printed pattern on the target substrate. Pneumatic atomizers are often used for large-area printing and use more resources, but ultrasonic aerosol systems can print extremely fine patterns down to 10 µm and require just around 0.5 mL of solution. The annular gas flow, accompanied by the sheath gas at the nozzle print head, reduces the size of the mist contained within the aerosol transport tube. This aerosol mist is converged further by the sheath gas flow and arrives at the surface with a higher effect. Aerosol stream is ejected as an integrated jet, and the step speed is regulated to generate the appropriate pattern structures in a continuous flow.

Fig. 6

Schematics of (a) pneumatic and (b) ultrasonic aerosol jet printing systems. The figure is reprinted from [42] under the terms of the Creative Commons CC.

Materials: Substrates, Dielectric and Conductive Inks for 3D Printing

Materials for Additive Manufacturing

Polymers are the most often utilized materials in additive manufacturing due to their adaptability and ease of application to a variety of 3D printing techniques. Despite the various polymers available, a user must carefully consider the materials to be utilized in the AM process. For fused filament fabrication (FFF), the materials used are in the form of filaments. Because of these physical property necessities, engineers and scientists must assess mechanical properties of materials in order to assure adequate compatibility between the various additive manufacturing techniques.

Fused Filament Fabrication Materials

Among the four AM approaches mentioned in this research, FFF has one of the most diverse material portfolios. All of the materials are thermoplastic, which means they soften at high temperatures and solidify at low temperatures.⁶⁷ The tensile strength of a certain material, such as acrylonitrile butadiene styrene (ABS), are affected by elements such as molecular weight and temperature, and the viscosity of the substance decreases as the temperature rises. Given that FFF makes three-dimensional models by extruding materials through a heated nozzle, the materials utilized must have high melting points and be sufficiently flexible, in order to withstand the highest available heat temperature of the system. The flexibility, stiffness, and viscosity of the materials have a significant impact on their compatibility. For high-quality printers, these characteristics are more precisely regulated.⁶⁸

ABS is the most commonly used FFF material because of its excellent mechanical and printing quality. ABS has thus become increasingly popular for developing functional models for real scale classification.⁶⁹ There have been a number of studies carried out to investigate the performance of ABS in the context of FFF printing. Because of the directed nature of this 3D printing technique, ABS creates items that display anisotropic features. The air gap and raster orientation of an ABS-printed item can have an impact on the tensile strength of the part, but the compressive strength seems to be the parameter with the least direction dependence among the others.⁷⁰ It is possible to find different types of ABS on the market that can be used for FFF printing.

Polylactic acid (PLA) is a linear aliphatic thermoplastic polyester that is made from biodegradable and renewable components. It has higher mechanical properties, processability, thermal stability, and has a minimal environmental effect due to its use of renewable resources.^71,72 The material is well-suited for packaging food and other consumer goods. PLA use aids agriculture because it is derived from agricultural feedstock. However, it is a more expensive substance when compared to other petroleum-based polymers.⁷³ In order to improve the qualities of PLA, researchers are continuing to integrate nanoparticles into the material. The inclusion of montmorillonite-layered silicate is expected to improve barrier characteristics by increasing the modulus and decreasing toughness, while the addition of TiO₂ allows for improved photodegradability under ultraviolet radiation.⁷⁴

Polymer Nanocomposites

Polymer nanocomposites (PNC) are materials that incorporate nanoparticles dispersed throughout the material's matrix. These nano-fillers, which have at least one dimension in the nanoscale range, are utilized in polymer matrixes as reinforcements or as functional additions, depending on the application. Because of the so-called nano effect, polymer nanocomposites have sparked a great deal of interest in research. It is widely acknowledged that modest amounts of nano-fillers can significantly improve the qualities of materials, such as thermal, mechanical, chemical and electrical capabilities.^75,76 In the case of additive manufacturing, PNCs can be treated in the same manner as polymers without the need for machine modification. PNCs for additive manufacturing applications have been produced employing a variety of nano-additives and nano-fillers. In PNCs, the most significant consequence of reinforcements is an upgrade in the mechanical and electrical properties of the composite material. For example, nanoclay-based nanocomposites are commonly utilized to increase the elastic modulus of a variety of materials.⁷⁷ Although nanoparticles are extremely successful at increasing a wide range of material properties, one of the disadvantages of using nanoparticles in many nanocomposites is that their ductility or elongation at the break is reduced.⁷⁸ This phenomenon is generated by two major factors: weak interfacial adhesion and agglomeration of nanoparticles that act as stress concentrator defects, both of which are induced by poor interfacial adhesion. For PNCs to work optimally, adequate dispersion of the nano-fillers is a critical stage in the manufacturing process.

Fiber Nanocomposites and Carbon Nanotubes

Additive manufacturing offers rapid production of a variety of electronic components in an easy process at a lower-cost and with a design freedom.⁷⁹ Material development is a necessary condition for attaining this benefit. In comparison to more conventional conductive fillers such as carbon black,^80,81 only modest amounts of nano-fillers are required. Gnanasekaran stated in 2017 that manufacturing innovative polymer nanocomposites [carbon nanotubes (CNT) and graphene-based polybutylene terephthalate] using a common desktop 3D printer paves the way for the construction of electrically conductive structures. The authors examined the electrical conductivity, printability, and mechanical durability of polymer nanocomposites.⁸² Figure 7 depicts a CNT filament extruded from a 3D printer.

CNT dispersion is crucial not only for optimal polymer effectiveness, but also to avoid poorly dispersed CNT agglomerates that could clog the printer nozzle, considerably delaying or completely stopping the printing process.⁸³ CNT and graphene have extremely high electrical conductivity, which has sparked a great deal of interest in the development of printable CNT nanocomposite materials for electronic applications such as wearable electronics,^84,85 liquid sensors⁸⁶ and flexible circuits.⁸⁷

Fig. 7

(a, c) Extruded composite filament (b, d). 3D-printed layer of composite. The figure is reprinted with permission from Ref. 82. Copyright from Elsevier.

Conductive Inks for Additive Manufacturing

Because of the greater material compatibility of ink-based 3D printing (3DP) over light-based 3D printing, research is focused more on developing devices using graphene-based materials using a variety of promising ink-based 3DP approaches, but additional light-based 3DP techniques are discussed, such as stereolithography (SLA)⁸⁸ and digital light processing (DLP),⁸⁹ or ink-based 3DP techniques, such as powder-bed technology,⁹⁰ have also been reported.

Silver Conductive Ink

Conductive ink is a necessary component of the design in flexible electronics. Ink is used to create patterned objects that are capable of conducting electricity.⁹¹ Various methods, such as drying, curing, and melting, can be used to convert liquid ink into solid print. According to their composition, these types of inks typically contain a variety of distinct ingredients that can grouped into three categories: functional materials, additives, and solvents. Metal nanoparticles, conducting polymers, metal complexes, and inorganic carbon are all examples of functional materials, with the conductivity of metal nanoparticles being the most significant. Solvents can be used to dissolve the ink's ingredients and to thin out the liquid phase of the ink. Additives enhance the functionality of a combination by completing the functions that are required by the mixture.

To date, a variety of materials have been used to create conductive inks, with the most common being polymer. However, because of the material features of these inks, they have a restricted conductivity. When it comes to metallic inks, silver-based inks have seen significant development because of their high conductivity and excellent stability under ambient circumstances. Organic silver inks and nano-silver inks are two types of silver inks.⁹²

In 2021, Lopes et al. used a Voltera V-One printer with silver conductive ink and successfully printed circuits using a versatile method for the fabrication of microchip-integrated ultra-stretchable circuits.⁹³ Figure 8 depicts the inkjet printing process.

Fig. 8

Process of printing using a printer and silver ink. The figure is reprinted with permission from Ref. 93. Copyright from Springer Nature.

Each ink has a number of drawbacks and advantages. Inks containing silver nanoparticles (NPs), organic solvents, and certain surfactants can melt at substantially lower temperatures than silver due to the size impact of nanoparticles. Organic silver inks are made up of an organic silver salt and a volatile organic solvent that transforms to metal when heated to a low temperature. Copper-based inks are being developed as a cost-effective and conductivity-equivalent alternative to silver-based inks. On the other hand, copper nano-inks are not stable and are prone to oxidation during the synthesis process. As a result, they frequently require a reducing environment or a specific sintering technique to produce high conductivity following printing. Compared to the copper NP ink, organic copper decomposition inks are a suitable choice. Thermal decomposition of an ink containing copper (II) formate amino complexes is an excellent instance of this principle.⁹⁴ Although this is the only example of the ink system, it provides clean degradation at low temperatures without leaving any organic residues behind. Graphene ink is made up of graphene, solvents, and stabilizers, among other things. Polycyclic aromatic hydrocarbons (PAHs), surfactants, polymers, and ethyl cellulose are examples of stabilizing agents. Organic solvents such as ethanol, N,N-dimethylformamide (DMF), or water can be used as a solvent. Because of the addition of graphene, graphene ink may address not only the difficulties of polymers, but it can also construct a stable and basic conductive network, which is beneficial in a variety of applications. In a similar vein, graphene ink was developed to take advantage of the properties of these two materials in order to improve conductivity while simultaneously reducing the metal particle concentrations.

Fluid Properties

In order to be compatible with a variety of patterning technologies, the fluid characteristics of conductive ink must be favorable. Although the ink's surface tension and viscosity are important factors in determining its ejection velocity, size, and stability, the shape of the droplets that impinge on a substrate is also influenced by these factors.⁹⁵ The pattern resolution and thickness are determined by the impingement forms, which also have an impact on the mechanical and electrical aspects of the pattern. It is necessary to regulate many ink-rheological characteristics during the formulation process, such as viscosity, surface tension, and wettability, to ensure that the ink has the desired properties.

Viscosity

Flow resistance is a quantity used to measure the resistance to flow shown by a fluid, and it is related to the internal friction of a flowing fluid. A fluid with great viscosity moves gradually because of the high internal friction caused by the molecular interaction in the fluid.⁹⁵ A fluid with low viscosity, on the other hand, flows simply because there is little friction between the liquid and the surrounding environment when the fluid is in motion. To determine the viscosity of the conductive inks, an Ostwald viscometer tube (U-tube) is commonly used.

Wettability

The capability of a liquid to sustain contact with a solid surface is referred to as wetting (the substrate). The formation of a physical attraction due to intermolecular forces between the two phases occurs when sufficient contact is attained between the two phases. This physical attraction causes the liquid to conform to the surface on both a macro- and microscale, dislodging air and minimizing interfacial flaws. In order to provide adequate adhesion between ink and flexible substrates, it is necessary for the surface to be wettable.

Electrical Property

Electrical resistivity is a property of materials that describes how well a substance resists the passage of electricity. An electrically resistive substance has low resistivity, which suggests that it readily facilitates the flow of electric current. To put it another way, the material has excellent conductivity. The conductive ink's electrical resistivity is measured and defined as follows:

$$ {\rm P} \, = {\text{ R}}*{\text{A}}/{\text{L}} $$

(1)

‘Ρ’ is resistivity in Ω m, ‘R’ is the rresistance of the film (Ω), ‘A’ is cross-sectional area, ‘L’ is the length. The inverse of resistivity is conductivity.

$$ \sigma \, = \, 1/ \, {\rm P} \, = {\text{ L}}/\left( {{\text{R}}*{\text{A}}} \right) $$

(2)

Conductivity is measured in siemens per meter (S.m⁻¹)

If the width and length of the measured sample are equal, then ‘R’ is referred to as ‘sheet resistance’, Rs and the electrical resistivity equation simplifies to Ρ = Rs*t, ‘t’ is the thickness of the film.

2D Materials for Inks

The most important characteristics of the 2D materials are discussed in the following section. Graphene, often known as a single layer of graphite, is the most researched member of the family of two-dimensional materials. It is formed from an atomically thin sheet of hexagonally arranged carbon atoms. Additionally, it is the costliest. Graphene's exceptional electrical conductivity is one of its most astounding properties. Additionally, it has a high degree of mechanical strength and flexibility, as well as good optical clarity. Also, it has a high thermal conductivity, an intrinsic high mobility of its charge carriers, and a large surface area.⁹⁶ Other notable characteristics of graphene are its gas imperviousness, chemical resistance (base/acid/salt), antibacterial potential, thermal stability, and environmental friendliness, to name a few examples.^97,98 The most extensively utilized method for scaling up graphene-based product manufacture requires direct delamination from a graphite precursor. Another widely used technique is the oxidative intercalation and exfoliation of graphite to form single- or multiple-layer graphene oxides (GO), followed by partial restoration of thermal/electrical conductivity and structural integrity using thermal or chemical reduction methods.⁹⁹ Reduced graphene oxide, which is generated chemically from graphene sheets, contains functional groups and imperfections, which prevents them from being used in applications such as applied physics and electronics. In comparison to exfoliated graphene, the material can be readily deposited on surfaces by covalent bonding or interfacial adhesion, and it is easily handled as a water-based dispersion, making it suitable for a wide variety of applications.¹⁰⁰

The majority of the time, 2D material inks are processed in three steps that begin with the selection of the optimal 2D material concentration, followed by mechanical dispersion of the materials, and lastly, dilution or let-down with ink varnishes.¹⁰¹ The two-dimensional crystal inks developed in this research provide a diverse supply of two-dimensional crystals with desirable properties for a broad variety of applications, including optoelectronics, sensors,¹⁰² catalysts, and energy storage.¹⁰³

Substrate Materials

The advancement of ink technologies for flexible electronics must be maintained by the development of a various range of substrates. Ideally, such materials would have a unique balance of attributes that make them acceptable for use in a wide range of applications. A flexible substrate, in particular, must be thermally and physically stable while being impermeable, flexible, and, smooth, while remaining transparent, as well as most economically and crucially viable. There are already a variety of materials available that meet the majority of these characteristics and have been successfully employed as substrates for flexible electronics.

Polymer Films

Polymer films are widely employed in the manufacture of flexible electronics. By contrast, when compared to paper substrates, they often have smooth surfaces, homogenous characteristics, and no porosity at all. Despite the fact that this is advantageous in some ways, it is not optimal in others. For example, the non-absorbing surface is not appropriate for the deposition of ink on the surface.

Polyimide (PI) films are frequently employed as substrates for flexible electronics due to their ability to endure prolonged exposure to temperatures of up to 300°C while maintaining flexibility. In contrast, the cost of PI film is quite expensive, and the adhesion between PI and the ink is typically weak due to the lack of functional groups on its surface, which limits the range of applications for which it may be used. Polycarbonate (PC), polyethylene, and polyethylene terephthalate (PET) are all low-cost flexible substrates that can be utilized for flexible electronics as well as other applications. Their softening points are lower than 150°C, and as a result, they are incompatible with the higher temperature sintering process necessary for ink-based metallization applications. Polytetrafluoroethylene (PTFE) is a thermoplastic with chemical resistance and temperature stability. It is, however, prohibitively costly, and its low surface energy might cause problems when printing structures that are not desired.

Materials Properties, Post-Processing, and Challenges for Performance of Printed Electronics

Several materials are used in printed electronics for designated applications. Different printing methods make use of different varieties of substrate and ink materials for attaining optimal properties of the electronic structure, as desired. In this section, commonly used substrates and the functional ink materials, their limitations, and associated challenges with respect to the optimal electrical performance of the resulting printed structures have been discussed in brief.

Substrates for Printed Electronics

The more reliable and normally used substrates for printed electronics are polymeric by nature. Polyethylene derivatives such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are widely used substrate materials for printed electronics,¹⁰⁴ owing to their common advantages of possessing suitable surface roughness (ranging in nanometers), optical transparency, low cost, and are readily available in different thicknesses. Polylactic acid (PLA) is an environmentally friendly biodegradable substrate material commonly employed for printed electronics. These substrates are used in printed electronics applications with a higher degree of optical and electrical performance. However, these substrates cannot withstand high-temperature (>210°C) soldering while bonding with silicon chips for electronic packaging applications. This is a primary limitation of these substrate materials since the soldering temperature is higher than their glass transition temperatures (Tg).¹⁰⁵ Polydimethylsiloxane (PDMS) is a silicone elastomer, possessing suitable elastic properties for stretchable electronics, with optical clarity chemical inertness, and non-toxic properties finding biomedical applications. Its surface wettability and adhesion to metallic inks can be modified with surface treatments using other polymers and used for stretchable electronics.¹⁰⁶ Polyether ether ketone (PEEK) has fir-retardant properties and is another substrate normally used to manufacture wearable devices, but has a limitation of higher surface roughness and is also an expensive material.¹⁰⁷ In order to overcome the limitation of withstanding higher temperature, an alternative and thermally stable substrate material, polyimide (PI), is used. Polyimide substrates are preferred for printed electronics owing to their excellent thermal stability (ranging from 269°C to 400°C), flexibility, and rugged mechanical properties under harsh environmental conditions as evidenced by the usage of polyimide sheets on the International Space Station.¹⁰⁷

The physical properties of the commonly available substrates used in printed electronics ^14,108 are listed in the following Table I.

Table I Properties of various substrate materials used for printed electronics

Depending upon their functional characteristics, the substrates are selected for realizing different printed electronic structures. The print quality is influenced by the surface roughness and porosity of the substrate, whereas the performance of the electronics is determined by the surface energy and absorption capacity of the substrate. The thermal stress and the coefficient of thermal expansion (CTE) mismatch between substrates and the printed materials are very critical for the optimal performance of the printed electronics. Lower CTE (less than 20 ppm/°C) is preferred to match with the corresponding thermal expansion characteristics of the deposited material. Different physical features such as flatness, lightweight conformability, ruggedness, and ease of handling are significant for the selection of substrate materials for different applications.

Functional Inks for Printed Electronics

With the emphasis on active device printing, the development of functional inks for printed electronics is being explored at different levels. For the manufacture of printed electronics, three kinds of inks are required, namely metallic, semiconducting, and insulating (dielectric). Solvents, additives, surfactants, and binders are mixed with original particles of metallic, semiconducting, and dielectric materials in order to improve the printability of the functional inks.¹⁰⁹ Metallic materials create contacts for single electronic elements, and in a circuit, they interconnect multiple electronic components. Because of their solution processability, organic semiconductors and metal oxides are employed as semiconductor materials in printed electronics. Printable metal oxides possess higher carrier mobility (1-100 cm²V⁻¹s⁻¹) than organic semiconductors (1-40 cm²V⁻¹s⁻¹ for p-type and 0.1-5 cm²V⁻¹s⁻¹ for n-type).¹¹⁰ Organic dielectrics are considered to be excellent insulating materials because of their inherent transparency, high dielectric constant (ε ~ 10), and solution processability, which can provide smooth films on plastic substrates and they are compatible with different printing techniques for printed electronics. Polyvinyl pyrrolidone (PVP), polystyrene (PS), polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), and amorphous fluoropolymers are some of the commonly used dielectric materials in printed electronics.

Nanoparticle inks are in abundant usage in the recent past, because of their compatibility for printed electronics applications. They can be easily solubilized in printable solvents and enhanced surface area to volume ratio can be achieved due to their smaller size (particles of less than 10 nm in diameter), resulting in the reduction of their effective melting point, and enabling the fusion of the nanoparticles at lower temperatures of less than 250°C. Nanoparticles are successfully synthesized using wet chemical techniques to produce particles with well-controlled diameters.¹¹¹ The nanoparticles of Au, Cu, ZnO, ZrO2 Al2O3, CuInS, Co, and other similar materials are synthesized encapsulating with suitable organic ligands for obtaining controlled diameters, stabilizing for preventing agglomeration, and allowing them to be solubilized.¹¹² The limitation of organic polymers can be overcome by printing the nanoparticle inks first, and printing the polymer materials that can withstand lower temperature at the last, after the annealing (post-thermal treatment) of nanoparticles. Various properties such as particle size, surface tension, viscosity, and solid loading of nanoparticle inks have a significant effect on printed electronics.

The following Table II specifies various characteristics of nanoparticle inks used in different printing methods.¹¹³

Table II Properties of different nanoparticle inks used in various printing techniques

The decrease in particle size improves the functionality of the nanoparticle metal inks.¹¹⁴ Although it is possible to modify the viscosity of the ink either by adding the solvent or increasing the temperature, maintaining similar electrical properties of the materials is a real challenge.¹¹³ Increasing temperature decreases the viscosity, while solvent evaporation increases the viscosity of the ink. Gravure, flexography, and inkjet printing require inks with low viscosity, whereas high-viscosity inks are used in screen printing. Surface tension is a significant property that determines the interaction between the substrate and the ink.¹¹⁵ Polar liquids normally possess higher surface tension, while non-polar liquids possess lower surface tension. Increasing temperature or the solid content in the ink decreases its surface tension.¹¹⁵ The rheological behavior of the ink can also be modified by varying the solid content in the ink. The electrical conductivity and energy required for annealing/sintering (post-treatment) are influenced by the particle morphologies, as demonstrated by the mixing of nanospheres and nanowires, which results in increased conductivity and reduced sintering energy.¹¹⁶

Substrate-Ink Interactions for Improved Performance of Printed Electronics

To achieve desired electrical performance in printed electronics, it is very important to obtain highly conductive pattern structures. The various interactions of the substrate with the printed ink include ink dropping, spreading, surface wetting, solvent evaporation, drying, and particle absorption onto the substrate. In addition to the environmental conditions, the properties of the substrate and ink particles have a strong impact on the interaction process.¹¹⁵ The equilibrium wetting determines the print quality. The surface wetting in turn can be determined by the contact angle. A substrate with lower surface energy than the surface tension leads to a higher contact angle and decreased spreading of the ink that enables printing of thicker and high-resolution patterns with finer linewidth.¹¹⁵

Post-Treatment for Improved Performance of Printed Electronics

Post-treatment is required to achieve optimal characteristics of printed electronics in order to get rid of the solvents and additives from the ink to improve the morphology and the microstructure.¹¹³ Different kinds of post-treatments can be employed based on the type of substrate and ink, which can be thermal, photonic, microwave, plasma, or chemical sintering.¹¹⁷ Printed pattern thickness, particle size, and shape, temperature, time, radiation energy level are different parameters influencing the sintering process. The degree of sintering increases with temperature and time.¹¹⁵

Nanoparticle inks are thermally sintered, to remove organic stabilizer molecules, in order to create a continuous pattern with higher electrical conductivity at temperatures below their melting point resulting in improved electrical properties. Thermal sintering of metal oxide particle inks is limited with the substrate selection (based on their thermal stability) and metal precipitation.¹¹³ Different nanoparticle inks and their effective sintering temperatures for achieving optimal electrical conductivity^{118,119,120,121} are listed in the Table III below.

Table III Nanoparticle metal inks and their resistivity at different sintering temperatures

Photonic sintering involves heating of printed metallic layer which leads to liquid evaporation.¹²² Selective laser heating is advantageous because of its localized effect, minimizing the impact on the substrate. Microwave sintering is a flash sintering process for metals with lesser penetration depth. Silver, gold, and copper with lower penetrative depth, in the range of 1.3 to 1.6 µm (at 2.54 GHz) can be post-treated with microwave sintering.

With plasma sintering, both metal nanoparticles and metallo-organic complexes (MC) can be post-treated. Printed patterns can be exposed to low-pressure argon plasma sintering to obtain optimal electrical properties in printed electronics.¹²³ Electrical Sintering is a post-treatment process in which voltage is applied over the printed structure with the flow of current, to achieve local heating through dissipation.¹²⁴ The primary advantage of this method is the short duration of sintering and reduced substrate heating. However, the primary requirement for electrical sintering is that the printed pattern needs to possess a certain degree of conductivity before treatment.

Chemical sintering enables the coalescence of metallic nanoparticles at room temperature.¹²⁵ The metal nanoparticles undergo a spontaneous coalescence process when they come into contact with oppositely charged polyelectrolytes resulting in higher conductivity at room temperature. For example, when positively charged polydiallyl dimethyl ammonium chloride (PDAC) is added to negatively charged silver nanoparticles, coalescence of the nanoparticles takes place due to a decrease in their zeta potential at the point of zero charge.⁵⁰

Challenges and Future Opportunities

This article aims to provide a brief overview of commercially available polymers, as well as contemporary breakthroughs in the production of new polymer composites, particularly polymer nanocomposites, for four additive manufacturing technologies, including FFF, SLA, selective laser sintering (SLS), and multi-jet fusion (MJF), among others. In the realm of additive manufacturing, the development of polymer nanocomposites allows for the extension of the material performance spectrum, allowing for the construction of multifunctional parts with higher design flexibility. The additive manufacturing role should not be to fully replace existing manufacturing techniques, but rather to complement and improve their robustness and reliability. Designers must develop new design techniques in order to take full use of additive manufacturing. This can benefit a wide range of enterprises, including, but not limited to, the aircraft industry,¹²⁶ medical, automotive and gas and oil sectors. The following observations concluding offered in order to summarize research areas relevant to the development of polymeric materials for additive manufacturing.

The materials portfolio for additive manufacturing must continue to grow. The majority of current additive manufacturing materials are based on polymers, such as nylon and ABS. It is necessary to investigate the compatibility of undocumented or innovative polymers with various additive manufacturing technologies. New polymer composites and nanocomposites, as well as their processing structure performance relationships in additive manufacturing contexts, will aid in the development of lightweight, stronger, and multifunctional materials, hence significantly expanding the capabilities of additive manufacturing. It is possible to produce strong additive manufacturing components with improved inter-laminar strength by polymer nanocomposites reinforced with continuous fibers.

Despite significant progress in the production of silver inks based on non-toxic solvents and simple synthetic techniques, as well as the achievement of favorable conductivity at low sintering temperatures, printability of the resulting inks remains a challenge that must be overcome in order to obtain high-resolution patterns for practical applications.

As a result, additional study should be conducted in the following areas: (i) The fluidic qualities of silver inks as prepared, such as surface tension, wettability, and viscosity, should be optimized to ensure compatibility with the commercial printer and on a flexible substrate. (ii) For high-resolution and high-quality tracks or patterns, it is required to investigate the printability of the silver inks as they have been made, where the printing conditions are carefully managed to achieve the desired results. (iii) Inkjet printing can be used to construct energy storage or simple electronic devices that can be used to evaluate the performance of the inks. (iv) It is possible to investigate new sintering technologies for the production of flexible electronics that are more energy efficient.

There are various technological hurdles to overcome before printed electronics can be manufactured using traditional printing processes. When it comes to patterning, factors such as resolution, design rules, accuracy, registration, and yield all play a critical role in the manufacturing of printed electronics. While standard printing processes have been refined to be visible to the naked eye, printed electronics require high-resolution continuous lines. The technical challenges associated with the manufacturing of printed electronic devices include difficulty in achieving micron level accuracy required for microelectronics and accomplishment of suitable scaling in feature sizes to enable fabrication of high density, and good performance electronic devices. Moreover, it is required to deposit very thin, homogeneous, defect free layers onto the substrates very accurately to produce different electronic devices for proper functioning. Most suitable printing methodology for manufacturing the high performance printed electronic devices such as various interconnects, passive devices and organic thin film transistors shall be determined only after proper understanding of process modules for metals, dielectrics, and insulators and also the examination of the compatibility between individual technique and the material.

Conclusions

This article provides a comprehensive review on the applications of printed electronics, classification based on contact and non-contact printing technologies, commonly used substrates, different types of conductive and non-conductive materials, and associated technological challenges are briefly covered with a focus on the current trends in industrial and scientific research areas, and fast-growing demand for consumer electronics. In the recent past, there have been concentrated efforts to explore the low-cost electronics manufacturing with commercially feasible techniques and deliver the products at faster turnaround times. However, the technical limitations pertaining to their functional performance and reliability have to be addressed for the printed electronics to replace the conventional manufacturing methods for rapid manufacturing at affordable cost. Though it is improbable that electronics 3D printing will replace all the conventional subtractive processes for the development of high-performance electronic device applications, yet this technology will surely be useful for rapid prototyping with minimized production time and reduced manufacturing costs.

Several research institutes including UC Berkley and Duke University are conducting extensive research on printed electronics for the realization of microelectronics and wireless sensors and conductive thermoplastic materials, respectively. Globally renowned industrial leaders in high-end printed electronics manufacturing, Nano Dimension in collaboration with Harris Corp and Optomec are working towards the realization of high-resolution electronic circuits using inkjet printing for 3D printed circuit boards and aerosol jet technology, respectively. Printed electronics have applications not only in consumer products, but also in various other fields including automobile, biomedical, power and agriculture sectors. With improved performance and reliability of the additively manufactured products, printed electronics are finding significant applications in high-end defense and aerospace sectors.

With the implementation of continuously evolving printing techniques towards electronics manufacturing, new value can be added to the existing technology for printed electronics. This paper mainly presents a comprehensive review of recent advancements in the field of digital 3D printed electronics, focusing on the available printing technologies for the next-generation electronics, substrates, functional conductive and non-conductive materials, and technical challenges with vital observations and future opportunities for further explorations and studies on manufacturability, deposition issues, and common techniques used for examining the product which need to be resolved and improved prior to their usage, with specific reference to a new class of nanomaterials available for digital printing.