3D Printed Liquid Metal and Stretchable Electronics

by Abdul Rehman on September 8, 2013

Terminator 2 Style Flexible MetalShape-shifting robots and machines; self-healing wires, wearable electronics and artificial skin with artificial nerves.

All these concepts were made famous by the “Terminator” series.

However, few expected that they would become possible in the real world so soon.

3D liquid metal printing, which has recently been invented by North California State University (NCSU) researchers, has given life to these and many other concepts from science fiction.

3D Liquid Metal Printing

Michael Dickey and Collin Ladd, along with some other researchers at NCSU, have developed a technique to make coherent 3D structures using a liquid metal alloy of indium and gallium. Using this technique, they have made several beautiful structures, such as antennae of an insect, which can be viewed on their YouTube Channel.

The thing that’s surprising about their invention is that while ordinary liquids don’t maintain specific shapes, their liquid structures remain intact and can even be made to stand upright without any support. Michael Dickey explained this, in his post at Reddit, in the following words:

“The part that’s so cool is that liquids don’t behave this way normally. My wires should break up into a rain of droplets, and my structures should just melt into a puddle.”

So, why don’t the structures break up into drops? The reason is that as the liquid mixture of indium and gallium is exposed to oxygen, a very thin layer of oxide forms on its surface, which, despite being a few nanometers thick, is strong enough to keep the liquid structure from falling apart.

Taking advantage of this property, Michael Dickey and Collin Ladd have developed two methods for forming solid, coherent structures. In the first method, drops of liquid metal are extruded at a high pressure from a syringe and then stacked over one another to form a complete structure. In the second method, liquid metal is extruded out into a template of polymer.

The template is later dissolved, leaving an intact structure of desired shape. By simply replacing the syringe with a 3D printer, this method can easily be extended to 3D printing. Their paper “3-D Printing of Free Standing Liquid Metal Microstructures”, published online at Advanced Material, describes the details of their methods in the following words:

“Here, we show it is possible to direct-write a low viscosity liquid metal at room temperature into a variety of stable free-standing 3-D microstructures….. A thin (~ 1 nm thick), passivating oxide skin forms rapidly on the surface of the liquid metal and stabilizes the microstructures despite the low viscosity and large surface energy of the liquid.”

3D Liquid Metal Printing vs Ordinary 3D Metal Printing

“But I’ve been hearing about 3D metal printing for a long time.” You may object here. “What’s so special about this?”

Although, it is true that 3D Metal Printing isn’t a new concept, the thing that makes NCSU researchers’ invention so special is that they use liquid metal for 3D printing. How is this better than ordinary metal printing?

  • No Binding Needed: Firstly, ordinary metal printing requires several complex techniques to bind together particles of metal powder, which are used as raw materials. These include melting the metal powder and cooling it later, using binding materials to hold the metal particles together and using laser to bind the particles together. But no such technique is required in 3D printing liquid metal. The droplets of liquid metals are liquid at room temperature, eliminating the need for heating. Furthermore, they fuse automatically, requiring no binding material to hold them together.
  • Flexibility: The second major advantage that liquid metal 3D printing provides is that ordinary metal structures are rigid and fixed. Their shapes can’t be changed once the structures are completely formed. In contrast, because the major part of the 3D liquid metal printed structure remains liquid (the oxide layer, though very thin, is solid), its shape can be changed i.e. it can be bent and stretched to any desired shape.

One fact, however, that hinders 3D liquid metal printing is the extremely high cost of the liquid metal. An analyst at NewScientist.com has estimated that a 3D printed liquid metal structure could cost a 100 times more than a 3D printed plastic structure of similar size.

Answering a comment on reddit that raised this problem, Michael Dickey said that, for wires and other electronic applications, very small amount of liquid metal will be needed, which won’t cost a lot.

Future Prospects – Machines of the Future

3D printed liquid metal retains the properties of both liquid and metal. This means that it is flexible like liquids and at the same time it can conduct electricity, like metals. This fact makes liquid metal perfect for use in future electronics. In their paper, Michael Dickey and Collin Ladd mention the following potential applications of their invention:

“The ability to directly print metals with liquid-like properties is important for soft, stretchable, and shape reconfigurable analogs to wires, electrical interconnects, electrodes, antennas, meta-materials, and optical materials.”

Some of the most important of these prospects are discussed below:

  • Self-Healing Wires and Electronics: A major problem encountered with wires that we use today, is that they can break easily. This leads to short circuits and accidents. Because liquid metal parts fuse together spontaneously, wires and electric appliances made of liquid metals won’t break at all. Any damage occurring over time, would be automatically repaired as damaged parts would fuse together without any external assistance.
  • Stretchable, Wearable Electronics: Another interesting prospect about 3D liquid metal printing is that because of their flexibility, they could be used to incorporate entire circuits and electronic boards in everyday Wearable Items Like Clothes, Glasses, Shoes etc. For example, in the future, entire computers may be incorporated into our shirts to help us with our daily tasks. Such computers on clothes of patients could monitor their status continuously without the need for hospitalization. In the distant future, stretchable electronics may even be used to make Artificial Skin and Artificial Nerves which will revolutionize treatment of several diseases.
  • Shape-shifting Machines: 3D printed liquid metal is the perfect material for shape-shifting objects and machines. A pen could transform into a key for opening your door, then a knife for cutting vegetables, then into a spoon for eating, then into a brush for brushing teeth and finally, into a pen again. A car, at one moment, could transform instantly into a boat or an airplane, the next moment.

Conclusion – Straight from Science Fiction

In conclusion, it is obvious that 3D liquid metal printing will revolutionize every aspect of our machines from the wires that connect them to the computers that control them. As this technology progresses, science fiction will gradually become a reality. “Transformers” and “Terminators” will soon come out from comics and movies into our real world.

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