Mar 24 2017
A rendering of the 3D printed device. (Credit: Subramanian Sundaram)
With the popularity of tablet computers and smartphones, touch-sensitive surfaces are everywhere. They are, however, brittle, as people with cracked phone screens can confirm.
Using sensors to cover a robot - or a bridge or an airplane - will require a technology that is not only flexible but also economical to mass produce. Researchers at MIT’s Computer Science and Artificial Intelligence Laboratory believe that the answer lies in 3D printing.
In an effort to exhibit the viability of flexible, printable electronics that integrate processing circuitry and sensors, and can act on their environments, the MIT researchers have engineered and constructed a device that reacts to mechanical stresses by changing the color of a spot on its surface.
The device drew inspiration from the golden tortoise beetle, or “goldbug,” an insect whose exterior typically looks golden but changes to reddish orange if the insect is prodded or poked - that is, mechanically stressed.
In nature, networks of sensors and interconnects are called sensorimotor pathways. We were trying to see whether we could replicate sensorimotor pathways inside a 3D-printed object. So we considered the simplest organism we could find.
Subramanian Sundaram, Graduate, MIT
The researchers describe their new design in the current issue of the Advanced Materials Technologies journal. Sundaram is the paper’s first author. The senior authors of the paper are Sundaram’s advisor, Wojciech Matusik, an associate professor of EECS; and Marc Baldo, a professor of EECS and director of the Research Laboratory of Electronics.
Others who contributed to the paper are Pitchaya Sitthi-Amorn, a former postdoc in Matusik’s lab; Ziwen Jiang, an undergraduate EECS student; and David Kim, a technical assistant in Matusik’s Computational Fabrication Group.
Bottom Up
For decades, a major area of research has been printable electronics in which flexible circuitry is deposited on a specific type of plastic substrate.
But Sundaram says that if the substrate itself can be printed, it would significantly increase the range of devices the method can produce.
The choice of substrate restricts the types of materials that can be deposited on it. Since a printed substrate could comprise of Different materials, interlocked in complex but regular patterns, it widens the range of functional materials that printable electronics can make use of.
Printed substrates can also make way for the possibility of devices that, though printed as flat sheets, can fold themselves up into more intricate, 3D shapes.
Printable robots that naturally self-assemble when heated, for example, are an area of constant research at the CSAIL Distributed Robotics Laboratory, led by Daniela Rus, the Andrew and Erna Viterbi Professor of Electrical Engineering and Computer Science at MIT.
We believe that only if you’re able to print the underlying substrate can you begin to think about printing a more complex shape.
Subramanian Sundaram, Graduate, MIT
Selective Signaling
The MIT team’s new device is roughly T-shaped, but with a wide, squat base and an extended crossbar. The crossbar is built from an elastic plastic, with a strip of silver covering its length; in the researchers’ experiments, electrodes were linked to the crossbar’s ends.
The base of the T is built using a more rigid plastic. It comprises of two printed transistors and what the researchers term as a “pixel,” a circle of semiconducting polymer whose color turns when the crossbars stretch, altering the silver strip’s electrical resistance.
Actually, the transistors and the pixel are composed of the same material; the transistors also turn color slightly when the crossbars expand. The effect is more intense in the pixel, however, because the transistors intensify the electrical signal from the crossbar. Exhibiting working transistors was necessary, Sundaram says, because large, dense sensor arrays need some capacity for onboard signal processing.
“You wouldn’t want to connect all the sensors to your main computer, because then you would have tons of data coming in,” he says. “You want to be able to make clever connections and to select just the relevant signals.”
The MIT team used the MultiFab, a custom 3D printer built by Matusik’s group to build the device. The MultiFab already had two diverse “print heads,” one for emitting cool materials and one for hot materials, and an array of UV light-emitting diodes. Applying UV radiation to “cure” fluids deposited by the print heads forms the device’s substrate.
Sundaram incorporated a copper-and-ceramic heater, which was required to deposit the semiconducting plastic: The plastic is suspended in a fluid that is sprayed on the surface of the device, and the heater evaporates the fluid, leaving behind a layer of plastic measuring just 200 nm in thickness.
Fluid Boundaries
A transistor comprises of semiconductor channel above which is placed a “gate,” a metal wire that, when charged, produces an electric field that shifts the semiconductor between its electrically conductive and nonconductive states. In a regular transistor, there is an insulator between the semiconductor and the gate, to stop the gate current from seeping into the semiconductor channel.
The transistors placed in the device instead separate the semiconductor and the gate with a layer of water having a potassium salt. By charging the gate, potassium ions are driven into the semiconductor, altering its conductivity.
The saltwater layer decreases the operational voltage of the device, so that it can be powered with a basic 1.5-volt battery. But it does make the device less durable.
I think we can probably get it to work stably for two months, maybe. One option is to replace that liquid with something between a solid and a liquid, like a hydrogel, perhaps. But that’s something we would work on later. This is an initial demonstration.
Subramanian Sundaram, Graduate, MIT
“I am very impressed with both the concept and the realization of the system,” says Hagen Klauk, who heads the Organic Electronic Research Group at the Max Planck Institute for Solid State Research, in Stuttgart, Germany. “The approach of printing an entire optoelectronic system — including the substrate and all the components — by depositing all the materials, including solids and liquids, by 3D printing is certainly novel, interesting, and useful, and the demonstration of the functional system confirms that the approach is also doable. By fabricating the substrate on the fly, the approach is particularly useful for improvised manufacturing environments where dedicated substrate materials may not be available.”