[Image above] Aarhus University assistant professor Shweta Agarwala holds up an example of a printed electronic. Credit:Shweta Agarwala，奥尔胡斯大学
When talking about additive manufacturing, we often imagine using these techniques to create large-scale 3D objects such asceramic implants,涡轮零件， 甚至homes. But by its nature, additive manufacturing excels at creating much less bulky devices as well, such as printed electronics.
“Printed electronics (PE) is an emerging technology under the umbrella of additive manufacturing that uses functional inks to print circuits and electrical components on various substrates,” researchers explain in arecent paper. “So far, PE has enabled fabrication of devices including but not limited to thin film solar cells, antennas, memories, transistors, displays, batteries, capacitors, and sensors.”
研究人员由Shweta Agarwala, assistant professor of electrical and computer engineering at Aarhus University (Denmark). She is joined by professorPreben Kidmoseand Ph.D. studentHamed Abdolmaleki.
In an Aarhuspress release, Abdolmaleki explains that compared to传统的基于光刻技术对于制造半导体，使用光将预定义的图案转移到刚性基板上，打印的电子技术打开了“多种新的可能性，可以将柔性电路直接打印到诸如塑料，纸张，衣服和相当大的基板等广泛的基板上任何其他平面和非平面表面。”
关于印刷电子产品的研究正在迅速发展，因此保持最新发现是具有挑战性的。Agarwala在新闻稿中说：“通过本文，我们试图通过讨论技术，材料油墨，墨水性能，后处理，基板和应用程序来填补文献中现有的空白。”
Below is a brief summary of the main findings from the29-page review paper.
Types of printing processes
打印过程可以广泛归类为“联系”和“非接触”。在接触打印中，将墨水直接从图案介质（例如雕刻辊或模板）转移到基板上。在非接触打印中，将墨水从一系列喷嘴弹出到预期的表面上。
The authors focus this review on the latter category of noncontact printing, which consists mainly of three droplet-based printing techniques: inkjet, aerosol‐jet, and electrohydrodynamic‐jet printing.
Inkjet technology是个most widely used noncontact methodfor printing electronics.
In this technique, ink droplets are ejected onto a substrate via either continuous inkjet or drop‐on‐demand ejection modes. In the former method, a continuous column of ink ejects from a nozzle and spontaneously splits into droplets due to an acoustic wave generated by a piezoelectric ceramic. In the latter method, ink droplets are ejected in a controlled manner by passing trigger signals through actuators (either thermal or piezoelectric) located within the printhead.
Compared to the other noncontact printing techniques, inkjet printers have fewer operating parameters and so can reach optimal printing conditions more easily and in less time.
Aerosol jet technology与喷墨技术相比，相对较新，但是研究人员已经成功地使用了这种技术来制造transistors,锂离子电池， 和solar cells, among other devices.
This technique involves formation and deposition of an aerosol on a substrate. “Aerosol” is defined as a suspension of liquid or solid particles in a gas carrier. While techniques to produce and deposit aerosols have been known since the mid‐20thCentury，直到最近才开发了通过数字信号来控制气溶胶喷射沉积的技术。
Compared to the other noncontact printing techniques, aerosol jet printers have enhanced capabilities for printing complex designs on 3D substrates (rather than just planar substrates) and have relaxed requirements for ink rheology and fluid mechanic properties.
Electrohydrodynamic jet printingwas1998年获得专利by Choi and Smith, but it wasn’t until 2007 that researchers at the University of Illinois at Urbana-Champaignsignificantly enhanced the resolutionfor printed electronics.
This technique uses an electric field to create fluid flow from the nozzle to the substrate. Flow rate and electrical field are two important parameters that determine the jetting behavior of inks. When these parameter values are low, droplets are ejected from the nozzle by gravitational forces in a “dripping” mode. As the values of these parameters increase, droplets are ejected in a “pulsating” mode and, at even higher values, a “cone-jet” mode.
Compared to the other noncontact printing techniques, electrohydrodynamic jet printers cater particularly to printing resolutions smaller than 100 nm, which is promising for printing cells and tissues for biotechnology and biomedical applications.
Types of functional inks
To date, researchers have explored several kinds of organic and inorganic materials for printed electronics. While organic materials show better flexibility and processibility, inorganic materials often possess better electrical properties.
A functional ink typically has four constituents:
- Solvent- 溶剂溶解或分散其他三个成分，并调整墨水的最终粘度。
- Functional material—The functional material endows the ink with electrical properties.
- Polymer binder- 聚合物粘合剂可最大程度地减少纳米颗粒之间的分子间相互作用，从而提高了墨水的稳定性和保质期。
- Additives- 添加剂可以增强稳定性和性能。使用最广泛的添加剂是分散剂（表面活性剂），流变学修饰剂，散装剂，增塑剂和杀菌剂。
就功能材料而言，墨水可以分为四个主要类别：金属油墨，基于碳的油墨，聚合物油墨和陶瓷油墨。
Formetal inks、银是应用最广泛的功能性马特里al due to its outstanding electrical conductivity and high chemical resistance. Gold, though expensive, is used as well due to its high functionality and superior chemical stability, which makes it a preferred choice for applications involving corrosive and extreme environments. Copper is considered a low-cost alternative to silver and gold, but it can undergo rapid oxidation when exposed to ambient oxygen and thus is difficult to handle.
Forcarbon-based inks, graphene and carbon nanotubes are widely used. Graphene inks are prepared using four main approaches: oxidative exfoliation, sonication‐induced exfoliation, shear exfoliation, and electrochemical exfoliation. Preparing carbon nanotube-based inks is difficult because of the material’s poor dispersion in water and organic solvents, but researchers have devised several physical and chemical strategies for overcoming this challenge, such as using different dispersants or surface modification techniques.
聚合物油墨由于出色的机械合规性和通过基于溶液的方法的易于加工性，因此引起了人们的兴趣。功能聚合物有两种主要类别：共轭聚合物和电活性聚合物。通常，共轭聚合物由于其半结构性质而显示出较差的设备性能和低电荷载体迁移率。相比之下，由于电场的响应能力和形状，电活性聚合物受到了高度追捧，这是制造传感器和执行器的有用特征。
Ceramic inkscan show interesting electrical properties, such as superconductivity, semiconductivity, piezoelectricity, and magnetic features. For ceramic inks to yield a uniform pattern when printed, they should fulfill three main requirements: 1) adequate particle size and stability of the suspension, 2) proper fluid mechanics, and 3) proper drying behavior.
限制因素和挑战
在审查了后处理方法和底物的类型之后，您可以阅读更多信息纸, the authors conclude by detailing some areas in which research and innovation are needed to push this emerging technology toward commercialization. For example,
- Printability—Printability is the ability of a final product to follow the initial computer designed pattern. However, most of the software used for printed electronics have constraints on curved edges or multilayer designs, which restricts the design possibilities. In addition, due to fluid mechanic requirements, the viscosity and solid content of inks cannot exceed a certain value, meaning many printing passes may be required to achieve a thicker printed pattern.
- 设备制造- 封装层通常用于保护印刷的电子产品，由于其薄而柔韧性，该电子设备容易受到破坏。但是，选择封装层的挑战包括弹性恒定不匹配，热膨胀不匹配以及从层到印刷电路的应力浓度。
The paper, published in高级材料, is “Droplet‐based techniques for printing of functional inks for flexible physical sensors” (DOI: 10.1002/adma.202006792).