Researchers at Oregon State University’s College of Engineering have taken an important step toward the rapid manufacture of stretchable electronic devices, including soft robots.
What motivated you to undertake this project?
Uranbileg Daalkhaijav: We wanted to do 3D printing with metal that would allow us to quickly and easily make soft sensors. We thought we should be able to use a gallium alloy by altering the material to make it more printable.
How did you alter it?
Daalkhaijav: We used a gallium-indium-tin alloy — Galinstan — that is fairly standard, but we added nickel nanoparticles to it. The pure alloy is a low-viscosity fluid. Adding the nickel particles made it into a paste.
Do the nickel particles affect the conductivity?
Daalkhaijav: To our knowledge, they don’t affect it negatively; it almost appears that they have affected it positively. At high strain, the nickel particles interact with each other and increase the conductivity — at least from preliminary testing, that’s what we’re getting.
What are the benefits of the 3D printing process?
Daalkhaijav: Because we’re trying to use our paste for soft sensors and soft robotics, we don’t want our material to solidify. In traditional printing, there is a polymer melt that comes out in liquid form and then solidifies once it’s in the hot bed. What you end up with is solid plastic. But we wanted our material to be stretchable; therefore, it had to have a somewhat lower viscosity. Once it’s printed, we want it to stay as a soft, conductive material. When it’s enclosed in an elastomer, we’ll be able to stretch the material without losing conductivity.
This is a shear-thinning material. In our printing process, we shear at a high rate, which lowers its viscosity. Once it’s out of the printer it returns to its normal higher viscosity. We do layer-by-layer printing. The layers adhere to each other because of the properties of the paste. Imagine you were pushing out layers of toothpaste on top of each other — each layer will adhere to the next.
How do you see the material being used?
Daalkhaijav: A major goal in this field is to be able to integrate sensors into clothing in order to collect all kinds of data about the wearer. Our material will be the conductive wiring inside the sensor to connect the different parts. The sensor needs a soft, stretchable casing, which could be made from silicone elastomers. Silicones can easily stretch 400 or 500%. We want to use a conductive material that can also stretch 4 or 5 times its original length and not lose conductivity. We have a project in the works now where we’re using our material to print various circuits.
Our next step is to take it down to more practical applications, including making sensors with vertical vias and possibly arching kinds of connections — basically, making soft sensors where the conductor wires are made with our material instead of liquid metal. We also hope to be able to print the elastomer and the liquid metal together in one step.