Abstract
Introduction
3D printing (also known as additive manufacturing, AM) is a breakthrough technology that has been developing for more than 30 years, but has attracted more and more attentions in recent years. The American Society for Testing and Materials (ASTM) International defines AM as "A process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies". The seven major additive manufacturing processes as classified per ISO (ASTM F42) are: material jetting, binder jetting, material extrusion, vat polymerization, powder bed fusion, direct energy deposition, sheet lamination. With the development of 3D printing (3DP) from rapid prototyping to the end-of-use product manufacturing process, manufacturing constraints have been greatly relieved and the design freedom has been significantly expanded, including shape complexity, material complexity, hierarchical complexity, and functional complexity
In last few years, 3D printing has been utilized to fabricate electronics and structural electronics. More specifically, electronic/electrical components can be deposited and embedded in a 3D structure to form a multi-functionality product by interrupting the 3D printing process. 3D printing promotes the integrated assemblage and embedded other components as results of layer-by-layer or point-by-point characteristics. Functional elements such as sensors, circuits, and embedded components are now being integrated into 3D-printed products or structures, paving the way for exciting new markets, applications and opportunities. Furthermore, 3D printing can be harnessed to print electronics on stretchable and flexible bio-compatible "skins" with integrated circuitry that can conform to irregularly-shaped mounting surfaces. Therefore, 3D printing electronics can offer great potential and unique capabilities to build complex object with multiple functionalities. Particularly, it has shown the unique ability to produce the embedded electronics, 3D structural electronics, conformal electronics, stretchable electronics, OLED, etc
Recent progresses in 3D printing electronic
Embedded electronics
Many researchers have been conducted to add electronic functionality into the 3D printed structures by embedding electronic/electrical components and fully encapsulating interconnect conductive tracks. The ability of starting or stopping the build at any given layer enables the embedding of electronic components for manufacturing conformal embedded 3D electronic systems. Taking advantage of the layer-based additive manufacturing method and access to individual layers during fabrication, a single object with multiple materials and embedded components can be built now.
Embedded electronics can greatly reduce the mass and assembly complexity due to the elimination of cabled interconnects and redundant electronics packaging. The ability to embed complex functioning components and electronics into 3D printed structures is very crucial for the small-satellite users who are looking to exploit 3DP in a limited space. NASA/GRC (National Aeronautics and Space Administration/Glenn Research Center) and America Makes have performed AM techniques to develop the embedded electronics used in the structures of spacecraft. A manufacturing platform, the multi3D system which integrates two FDM (fused deposition modeling) systems, a CNC (computer numerical control) router for micromachining and a precision dispenser for depositing conductive inks (as shown in
Figure 1.(
Figure 2.(
Figure 3.
A shoe insole with embedded pressure and temperature sensing circuitry, with wireless communications chip for data transmission was fabricated by multi-material 3D printing, shown in
Figure 4.(
Figure 5.
A commercial 3Dn-300 multi-material printer from nScrypt Inc. has been utilized to fabricate a fully embedded low-profile antenna. The 3D printer includes dual deposition heads allowing two different kinds of materials to be dispensed. The thermoplastic stock is dispensed from one head through a filament extrusion process to print the dielectric components. While the other head prints the ink/paste is printed from the other head with feature sizes of as small as 20 μm to build the conductive elements
A hybrid 3D printing process integrating stereolithography (SL) and direct print (DP) was adopted to produce functional, monolithic 3D structures with embedded electronics. The hybrid SL/DP system (as shown in
Figure 6.(
3D structural electronics
A hybrid technology combined with direct-write/cure (DWC) and projection microstereolithography (PμSL) has been utilized to make 3D structural electronics. A PμSL process was applied to build the 3D structures, and the conductive tracks was produced by the combination of DWC with CNT/polymer nanocomposites, which may capacitate a new generation of inexpensive 3D structural electronics in the field of consumer, defense, and medical electronics. The technology of hybrid manufacturing combined with AM technologies will offer capabilities of fully 3D, high-resolution, multimaterial and large-area fabrication as well as requiring only ambient processing conditions (no clean room, vacuum or high temperature environment required).
Figure 7.
Disadvantages in the manufacturing process of printed circuit board include complexity, time consuming, higher cost, and limited product formation as the printed circuit board must be included. In order to get over these disadvantages, Jiang et al. reported a hybrid process using stereolithography and direct writing (DW) to fabricate 3D circuit devices. The insulated structures of circuit boards having high precision were fabricated using SL. Furthermore, the circuits were made on the several layers using DW
The integration of SL in combination with both microdispensing (nozzle deposition) and pick-and-place technology (component insertion) can produce dielectric substrates of intricately-detailed, complex shape where miniature cavities are used for the integration of press-fit electronic components. Printed conductive traces serve as electrical connections deposited by an integrated micro-dispensing system within the SL system and this combination of fabrication technologies stands to revolutionize the integration of electronics within mechanical structures as "3D structural electronics". Wicker and MacDonald demonstrated the development of multiple material and multiple technology SL systems capable of manufacturing multiple material structures with mechanical, electrical, and biochemical functionality. Some functional objects including multi-material tissue engineered implants, multi-material micro-scale parts, 3D structural electronics, have been successfully fabricated. Contamination issues associated with using multiple viscous materials in a single build, throughput, and limited materials as well as conductive inks with low-temperature curing capabilities remain still a challenging.
Figure 8.
Stretchable electronics
With the development of electronics, progresses in manufacturing techniques have promoted the development in the aspects of smaller, faster, more efficient. So far, the main focus has been on rigid electronics. However, recent interest in devices such as wearable electronics and soft robotics has led to an whole new set of electronic devices–stretchable electronics. These new devices require new manufacturing solutions to integrate heterogeneous soft functional materials. 3D printing can be harnessed to print electronics on stretchable and flexible bio-compatible "skins" with integrated circuitry that can conform to irregularly-shaped mounting surfaces.
Muth et al. reported a method of embedded 3D printing (e-3DP) for the fabrication of strain sensors, as shown in
Figure 9.
The ability of printing integrated circuits on the flexible substrate enables the electronic devices with conformity, lightweight structure and shock-resistant construction, which are challenging to be achieved by using rigid substrates such as semiconductor wafers and glass plates. Bijadi et al. have successfully tested the feasibility of a syringe extrusion-based 3D printing process to print stretchable embedded electronics through the use of SS-26S conductive silicone on flexible non-conductive silicone substrates. Instead of merely using the conductive silicone traces as flexible interconnects, this method used the conductive material for creating complete circuitry with SMT components and embedded microcontrollers
Figure 10.
Other electronic/electrical products and related technologies
3D printing possesses the ability of creating complex and conformal electronics integrated within a manufactured product. The Aerosol Jet printing from Optomec has been demonstrated the ability of building the functional antennae on the conformal 3D printing substrates. The whole printing process accurately controls the location, geometry and thickness of the deposit and produces a smooth mirror-like surface finish to insure optimum antenna performance. Some kinds of mobile device antennas, such as the LTE, NFC, GPS, Wifi, WLAN, and BT, have been printed through the Aerosol Jet process. And the performance of such antennas tested by a cell phone component supplier is in the same level with other production methods. For now, the Aerosol Jet technology has been using for the mass production of printed 3D conformal electronics in the application of antenna and sensor. As can been seen that a hybridized DW/AM process presents great potential for creating antennas with 3-dimensional structure.
Figure 11.
Aerosol Jet printing process has the ability to print conformal interconnects on 3D surfaces eliminating the need for wire bonding – for example printing electrical connections on 3D stacked die or for LED chip fabrication. Runge showed Leg prosthesis part produced from PLA (Polylactic Acid) via FDM showing complex non-planar surfaces, with surface-integrated strain gauge sensors produced by Aerosol Jet TM printing and conductive paths deposited via micro-dispensing, both using the modular manufacturing platform. The structural elements of this leg prosthesis shaft were produced via the FDM process from PLA, a thermoplastic polymer. Its functionalization relies on a surface-integrated strain gauge realized via Aerosol Jet printing. The conductive paths that lead across the part as well as the contact pads at their end were deposited through micro-dispensing. The material of the resistive sensor is a silver based ink, while interconnects and contact pads are made from silver particle-filled epoxy, as shown in
Figure 12.Leg prosthesis part produced by FDM and Aerosol Jet printing
The first multi-material 3D electronics printer in the world, named as Voxel8, provides an all-in-one, desktop solution for designing and prototyping next generation 3D electronic devices. Therefore, it has been regarded as a disruptive manufacture platform with the capabilities of printing embedded electronics. It enables prototyping of 3D electronic devices by the method of co-printing both thermoplastics and a highly conductive silver ink, which can be printed and cured at ambient temperature without the need for thermal annealing.
Figure 13.
Prospect, challenges and future trends
3D printing is disrupting the design and manufacture of electronic products. Functionalities of the devices/products fabricated by 3D printing can be significantly expanded by incorporating electronic components, such as sensors and circuits, in predetermined cavities within fabricated structures. 3D printed objects include not only traditional mechanical characteristics, but also embedded optical and electrical functions, such as sensor; all complex structures are difficult to produce with existing manufacturing methods. Many emerging and innovative products, such as embedded electronics, 3D structural electronics, conformal electronics, stretchable electronics, etc., have been fabricated using the technologies. 3D printing electronics has been considered as the next frontier in AM. Optomec has developed a high volume printing solution for the production of 3D antenna and 3D sensors that are tightly integrated with an underlying product ranging from smartphones to industrial components. It can be utilized for high volume printing of conformal sensors and antennas directly onto preformed 3D structures. Complex electronics can be 3D printed at micron resolution which will enable cheaper smartphones and medical gadgets. Aerosol Jet 3D micro-structure printing is capable of ultra-high resolutions with lateral features sizes of 10 μm and aspect ratios of more than 100:1
The conductivity is still one of the major difficulties in both 3D printed electronics and general 2D printed electronics because of the poor conductivity of ink caused by the low curing temperature due to the limitation of substrate material such as cardboard, polymers. More and more challenges in the fields of material types and processing challenges in the process of printing from 2D electronics to 3D integrated objects. Therefore, the compatible material sets should be explored and created to provide the adequate functionality and manufacturability for the product invention by designers. Besides, the adhesion between the materials is also a big issue, because the conductive materials would be stripped from the substrate with a poor adhesion. This is especially important case for traces that are embedded within a print and not on the surface because repair is impossible after a circuit is embedded.
In order to make electronics with 3D printing, new processes should be developed to possess the ability of depositing broader types of materials. To date, there are several solutions which have the ability to fabricate multifunctional 3D structures or products with embedded functional systems. Compared to other methods, the hybrid process combining FDM and direct print/writing shows higher applied potential, more flexibility. Material jetting systems seem currently to be the most successful multi-material 3D printing process among AM technologies. To date, fabrication of true 3D multiple material polymeric components using material jetting processes has been demonstrated. Currently, material jetting of polymers appears to be the nearest approximation to this vision that is currently available: The combination of high resolution, controlled material deposition with the possibility of photo polymerization, which allows immediate solidification of the material after printing and thus facilitates deposition of materials with different functional or structural roles directly besides each other, provides the foundation for effectively printing a structural electronics system directly. Multi-material and multi-scale 3D printing will be the most promising solutions.
More and more 3D printed functional electronics and products with electronics will be fabricated. 3D printing electronic technology provides a powerful tool for innovative product development, and extends 3D printing multiple functionalities. Significant advances in 3D printing electronics have been accomplished in the recent years. However, there is still a long way to go for 3D printed functional electronics and products as well as their industrial-level applications. Further potentials of 3D printing electronics need still to be explored and investigated.
Acknowledgements
We are grateful for financial supports from the National Natural Science Foundation of China (51775288).
Competing interests
The authors declare no competing financial interests.
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