I could go on about the golden tortoise beetle, which is a fascinating insect in many respects (Go look up what it does in its larval stage. I dare you), but its sensitivity to touch is what a group of researchers at MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) are particularly interested in. A lot of things that we rely on every day – smartphones, tablets, etc. – operate by touch sensitivity, and these scientists, among many others, envision a future in which much larger objects, like robots or even vehicles, are made from touch-sensitive material. That will require a flexible, cost-effective technology, and the CSAIL team thinks that technology could be 3D printing – with some help from the goldbug.
“In nature, networks of sensors and interconnects are called sensorimotor pathways,” said project leader Subramanian Sundaram, a graduate student in electrical engineering and computer science (EECS). “We were trying to see whether we could replicate sensorimotor pathways inside a 3-D-printed object. So we considered the simplest organism we could find.”
While 3D printed electronics are progressing, the types of devices that can be printed are still limited. According to Sundaram, the ability to print the substrate as well as the circuitry could greatly expand what can be done with electronics. A 3D printed substrate could, for example, be made from multiple materials in interlocking patterns, resulting in a much more complex device. In addition, a substrate could be printed from a material that reacts to external stimulants, like heat, to self-assemble or otherwise move.
To demonstrate the feasibility of flexible, 3D printable electronics that react to their environments, the CSAIL team 3D printed a T-shaped device with a base made from a rigid plastic and a crossbar printed from a flexible plastic with a strip of silver running along its length. The base includes two 3D printed transistors and a “pixel,” or a semi-conductive polymer circle that changes color when the crossbars are stretched, modifying the electrical resistance of the silver strip.
The transistors were actually made from the same material as the pixel, but they change color much more subtly when the crossbars are stretched, because they amplify the electrical signal from the silver. It was essential that they be able to demonstrate working transistors, said Sundaram, because large, dense sensor arrays need to have some capacity for onboard signal processing.
The device was printed on the MIT-created mulitmaterial MultiFab 3D printer, which can print up to 10 different materials in one build. The MultiFab has two print heads, one for hot materials and one for cool, and Sundaram added a copper-and-ceramic heater to print the semiconducting plastic, which was suspended in a liquid that was sprayed onto the surface of the device. Once deposited, the liquid was evaporated by the heater, leaving behind a layer of plastic only 200 nanometers thick.
“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 is a semiconductor channel atop which sits a device known as a gate: a metal wire that, when charged, generates an electric field that flips the semiconductor from a conductive to a nonconductive state and back again. A standard transistor has an insulator between the gate and the semiconductor to keep the gate current from leaking into the semiconductor channel. In the CSAIL team’s device, that insulator consists of a layer of water containing a potassium salt. When the gate is charged, potassium ions are driven into the semiconductor, changing its conductivity.
The saltwater lowers the device’s operational voltage so that it can be powered by a standard 1.5-volt battery. It does make it less durable, though.
“I think we can probably get it to work stably for two months, maybe,” Sundaram said. “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.”
The research has been published in a paper entitled ” 3-D Printed Autonomous Sensory Composites,” which you can access here. Additional authors include Ziwen Jiang, Pitchaya Sitthi-Amorn, David S. Kim, Marc A. Baldo, and Wojciech Matusik. Discuss in the Goldbug forum at 3DPB.com.MIT]