INNOVATION September-October 2022

F E A T U R E

THE DISCOVERY Ghaffarkhah was reading about spiders and their “slit sensilla” organ, which sits near the leg joints of most spiders and can detect tiny movements. “It was intriguing, because this organ is made of tiny microfractures, and the spiders can sense minute movements because of these microfractures,” Ghaffarkhah said. “Once I read the paper I decided I wanted to try to print a similar 3D sensor that would give us the same effect as the slit organ on the spider,” he said. At the time there was no material available that would achieve the final results needed. To create this first sensor, they created a new substance by using a material called poly (3,4-ethylenedioxythiophene): polystyrene sulfonate, or PEDOT:PSS. “We needed a special polymer that was easy to use in a 3D printer, but it also had to have conductive properties.

PEDOT:PSS has those properties,” Arjmand said. “It’s a special polymer that we outsource. When we get it, the material is 99% water. We couldn’t reduce the amount of water by heating, because that would destroy the polymer, so we did a freeze-dry procedure. First, we place the material into an ultra-low-temperature freezer, and then we put the frozen sample into a freeze dryer, and all the frozen water gets sublimated. The remaining material needs to be dissolved into water. We are able to get it down to 93 percent water and 7 percent polymer, which is perfect for this application.” Then, they mix the PEDOT:PSS with a functional conductive additive called MXene, which, while only a few atoms thick, creates a product that has metallic conductivity. Then, the final mixture—a nanocomposite—is ready for 3D printing. Now they had to create the fractures similar to the slit organs of the spider. It took six months, and they found the answer was to bend the sample material over a rod and stretch it to create fractures.

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