3D-printed flexible sensor targets wearables

By Gina Roos, editor-in-chief

Engineers at the University of Waterloo have developed a rugged and flexible sensor using 3D printing and nanotechnology. Researchers said that the new sensor can be used in a variety of wearable devices, ranging from vital sign to athletic performance monitoring, while improving the comfort level. Researchers from the University of California, Los Angeles and the University of British Columbia also collaborated on the project.

Ehsan Toyserkani, research director at the Multi-Scale Additive Manufacturing (MSAM) Lab at Waterloo and a professor of mechanical and mechatronics engineering, said, “The rubber-graphene sensor can be paired with electronic components to make wearable devices that record heart and breathing rates, register the forces exerted when athletes run, allow doctors to remotely monitor patients, and numerous other potential applications.”

The new technology combines silicone rubber with ultra-thin layers of graphene to make a material that is well-suited for making wristbands or insoles in running shoes, said researchers.

The silicone provides the flexibility and durability for biomonitoring, along with it being highly conductive, while the nanoscale embedded graphene makes it an effective sensor.

The rubber-graphene material is flexible, durable, and highly conductive. (Image: University of Waterloo)

Fabricating a silicone rubber structure with complex internal features is only possible using state-of-the-art 3D printing, or additive manufacturing, equipment, and processes, according to the researchers. “When that rubber material bends or moves, electrical signals are created by the highly conductive, nanoscale graphene embedded within its engineered honeycomb structure.”

However, researchers faced some challenges. While dip-coating porous polymers is a low-cost and scalable approach to integrate conductive layers with flexible polymer substrates, products exhibit nanoparticle delamination and, over time, decay. The solution was the development of a fabrication scheme to surface-dope porous silicon sensors with graphene nanoplatelets.

Further explained in a research paper: “The sensors are internally shaped with ordered, interconnected, and tortuous internal geometries (i.e., triply periodic minimal surfaces) using fused deposition modeling (FDM) 3D-printed sacrificial molds. The molds were dip-coated to transfer-embed graphene onto the silicone rubber (SR) surface.”

The result was a stable coating on the porous silicone samples, which delivered long-term electrical resistance durability over about 12 months and high resistance against harsh conditions (exposure to organic solvents). The sensors also retained conductivity — even with over 75% compressive strain — with high-strain recoverability.

Elham Davoodi, an engineering Ph.D. student at Waterloo who led the project, said that the sensor can be used in harsh environments, including extreme temperatures and humidity. “It could even withstand being washed with your laundry,” he said.

The rubber-graphene material, combined with the 3D-printing process, enables custom-made devices to precisely fit the user’s body shape. Researchers said that this will also improve comfort compared to existing wearable devices and reduce manufacturing costs.