MIT researchers 3D print colour changing, sensor-laden device
The device can be used to develop robot "skins" that change colours when mechanically stressed.
Inspired by the golden tortoise beetle, a team of researchers at MIT’s Computer Science and Artificial Intelligence Laboratory have designed and built a device that responds to mechanical stresses by changing the colour of a spot on its surface.
Touch-sensitive surfaces are everywhere, but they tend to be brittle and break easily. The researchers believe this sensor-laden skin has significant implications for developing 3D printed objects.
“In nature, networks of sensors and interconnects are called sensorimotor pathways,” says Subramanian Sundaram, an MIT graduate student in electrical engineering and computer science (EECS), who led the project. “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.”
One of the challenges researchers faced was determining the best possible substrate for printable electronics. According to the team, being able to print the substrate itself gave them a wider range of devices the technique can yield. Certain substrates limit the type of material that can be deposited. Whereas a printed substrate can consist of many materials, interlocked in intricate, regular patterns, therefore broadening the range of functional materials printable electronics can use.
This also opens up the ability to use flexible materials that can be printed flat and then folded into 3D shapes.
“We believe that only if you’re able to print the underlying substrate can you begin to think about printing a more complex shape,” Sundaram says.
The new device is a t-shape, but with a wide, squat base and an elongated crossbar. The crossbar is made from an elastic plastic, with a strip of silver running its length. In experiments, electrodes were connected to the crossbar’s ends. The base of the T is made from a more rigid plastic. It includes two printed transistors and what the researchers call a “pixel,” a circle of semiconducting polymer that changes color when the crossbars stretch, modifying the electrical resistance of the silver strip. The transistors also change color slightly when the crossbars stretch. The effect is more dramatic in the pixel, however, because the transistors amplify the electrical signal from the crossbar.
Demonstrating working transistors was essential, Sundaram says, because large, dense sensor arrays require 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.”
A transistor consists of semiconductor channel on top of which sits a “gate,” a metal wire that, when charged, generates an electric field that switches the semiconductor between its electrically conductive and nonconductive states.
The transistors in the MIT researchers’ device differs from traditional transistors by separating the gate and the semiconductor with a layer of water containing a potassium salt. Charging the gate drives potassium ions into the semiconductor, changing its conductivity. This saltwater lowers the device’s operational voltage so that it can be powered by a 1.5 volt battery. However, this does render the device less durable.
The team used a custom-built 3D printer developed by Wojciech Matusik, an associate professor of EECS, and his team. The “MultiFab” printer already included two different “print heads,” one for emitting hot materials and one for cool, and an array of ultraviolet light-emitting diodes. Using ultraviolet radiation to “cure” fluids deposited by the print heads produces the device’s substrate.
A copper-and-ceramic heater was added in order to deposit the semiconducting plastic. The plastic is suspended in a fluid that’s sprayed onto the device surface, and the heater evaporates the fluid, leaving behind a layer of plastic only 200 nanometers thick.
“I am very impressed with both the concept and the realization of the system,” says Hagen Klauk, who leads 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.”
The research is presented in the journal Advanced Materials Technologies. The team believes the applications for such a device are vast. The technique could be used to make sensor-laden skins for robots or even covering bridges or aircraft to determine stress points.
Sundaram is the first author on the paper, and the senior authors are Sundaram’s advisor, Matusik and Marc Baldo, a professor of EECS and director of the Research Laboratory of Electronics. Joining them on 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.