Purdue research could add electronic sensors through 3D printed parts

Purdue University researchers have developed a method to embed electrically-conductive particles, or sensors, in 3D printed filament polymers and disperse them evenly.

The patent-pending wet-mixing method could be used for a "large variety" of products and uses, including assessing materials and parts themselves, Brittany Newell, associate professor at Purdue's School of Engineering Technology, told Plastics News in an interview. "By embedding a sensor within the 3D printed material itself, we can determine if the part is doing what it's supposed to."

Traditional foil-type strain gauges are adhered to the surface of a printed part with an epoxy resin, Newell said.

Sensor particles used in the new mixing method are too small to be seen without a microscope, allowing a printed part to maintain strength it would have sacrificed for large, built-in sensors, she added.

"A limitation of application of 3D printed parts has been in their durability," Newell said. "Generally, we apply [a] strain gauge [on the surface of the] part or apply it to the top and bottom of the part to get information on overall strain across the part. However, the middle and internal structures are never monitored since the gauges are glued to the surface.

"You're assuming constant values throughout, where here you can put it in the center of the object," she said. "With this development, we can continually monitor the structural health of the part with the sensor embedded in the print."

Manufacturers and researchers can use the method to create "complex 3D structures with embedded strain gauges, rapidly moving traditional prototype pieces into fully functional and structurally assessable parts," she added.

"By adding this conductive material directly to the 3D prints, we can monitor things like deformation of the actuator itself, so you know where in space the material is," Newell said. "You can look at things like grip.

"Since we have enhanced the electrical properties of the material, there's always a tradeoff between the mechanical and electrical properties," she said. "We can actually optimize for specific sensing ranges, which allows us to better balance the mechanical and electrical needs and … vary the amount [of conductive components] to tune our sensors."

Users can also "plan the path for the signal to come through, almost as if it was like a wire coming out," Newell said.

"When we adjust the filament as it is being manufactured, and we can control the electrical mechanical properties, that allows us to check for certain sensing ranges," she added. That "enhances the sensor's ability to detect problems or changes."

The wet-mixing method could expand sensor development in general, Newell said.

"Usually people think of sensors [as] discrete things," Tyler Tallman, assistant professor Purdue's School of Aeronautics and Astronautics said. "We're actually printing the material as a sensor, so it's possible to make the entire component or structure, or whatever you're printing out of the sensing material. In that sense, the entire thing is a sensor, rather than a single point

"We can also print whatever component entirely out of the material so it's all self-sensing," Tallman said. "Then you're no longer constrained by point-base sensors."

"This work can be further expanded to add other particle types," Newell said. "This could include magnetic particles for electromagnetic fields, fluorescent particles and other functionalities."

"We need to increase the batch size to an industrial scale and integrate the customizable aspect of this work with industrial 3D printers," Newell said. "Testing should be done to expand to new prototypes … the sky is the limit as far as what end products it could be used in."

The researchers primarily used two base materials with the wet-mix method, including thermoplastic polyurethane and used polylactic acid (PLA) in a similar method, she said.

"I would expect us to be able to expand further into some of the other common 3D printer materials. We haven't even begun to look at things like ABS … nylons," Newell said.

The researchers are seeking out industry partners to create a process to scale up and further test the method, Newell said.

The mixing method "expands the electrical applications of 3D printed parts and sensor designs," Tallman said. "Current [3D printed] products are inconsistent and not of great electrical or functional properties and because of that it keeps demand down. But by making something much better, like we have done, that could drive demand up even further because there's a viable material for printing these things."

The researchers' initial sponsor, the Naval Engineering Education Consortium, a program from the NAVSEA warfare centers, drove their focus on conductive particles for strain sensing, Tallman said.

"Usually strain sensors are pre-packaged, you get what you get when you buy one," he added."[We can] print whatever shape for whatever application we want."

"We can mix in whatever functionalities we want based on the particles we select," Tallman said, including electromagnetic shielding or other connective or magnetic uses.

"I don't think we can anticipate all the things they could do yet because we're completely opening that design space in this application," he said. "We have ideas … but because of the flexibility of the manufacturing process there's going to be many more than we could think of."

The potential uses for the new method are "sky high, at this point," Jose Garcia-Bravo associate professor Purdue's School of Engineering Technology told PN. "Everybody's wearing smart watches and things for measuring different physiological thing in your body like heart rate and blood pressure … You could 3D print an entire circuit board … use sensors and actuators together that are fully 3D printed to create things like … micro robots."

"We're not there yet but [the ideas] … provide for us a good direction for the research going forward," Garcia-Bravo said.

"We're just opening the door for something that could mean so many things," he said. "It's a fourth dimension into materials or parts printing that is just starting to become a reality."

The research was published in the July 2022 edition of the peer-reviewed journal Advanced Engineering Materials and in the 2020, 2021 and 2022 editions of the journal American Society of Mechanical Engineers Smart Materials, Adaptive Structures and Intelligent Systems.