Working on the nanoscale, MIT researchers have 3D printed centimeter-long structures that could change the face of electronics and optical sensors. The method relies on the self-assembly principles of colloids – particles between 1 nm and 1 μm wide.
In the method, billions of polystyrene colloids fuse together to build structures programmed by the research team. Alvin Tan, a graduate student in MIT’s Department of Materials Science and Engineering, explains how using different particles such as metal alloys, and quantum dots could unlock a range of possibilities for the method. He says,
“Combining [colloids] into different crystal structures and forming them into different geometries for novel device architectures, I think that would be very effective in fields including sensing, energy storage, and photonics.”
A revolution for 3D printing
Colloids are typically found suspended in a liquid or gas. Milk is one common example of a colloid, as it is made of microscopic butterfat globules suspended in a water-based solution.
Colloidal self-assembly is the process that happens when particles in this liquid or gas come into contact with on another, and fuse together to form a new, larger solid.
“If you blew up each particle to the size of a soccer ball,” says Tan, a “it would be like stacking a whole lot of soccer balls to make something as tall as a skyscraper,”
“That’s what we’re doing at the nanoscale.”
Overall, the process presents scientists with the opportunity to program a material’s structure from the ground up. This means that researchers could make entirely new materials with properties that aren’t possible with traditional chemistry.
Colloidal self assembly has been likened to 3D printing as, essentially, new solid structures are built by layering multiple colloids. In a previous study on the method conducted at New York University Stefano Saccana, assistant professor of chemistry, said, “Colloidal self-assembly has the potential to revolutionize 3D printing.”
3D printing crystals
At MIT, direct-write 3D printing is used to dictate the shape of colloids.
The team uses a custom-made apparatus for the process, arranging a needle above two aluminum plates which form the print bed.
In total, three different water-based colloid inks were tested in the study: one containing polystyrene particles, one with silica and one with gold. These inks were extruded through the needle on to the aluminum plates.
As the plates are heated, the water content of the ink dissipates, leaving only colloids that fuse together. To make a demonstrative helical structure from the ink, the build plate is moved underneath the needle.
By changing the size of the particles, the team also succeeded in creating structures that reflect different colors. MIT graduate student Justin Beroz explians, “By changing the size of these particles, you drastically change the color of the structure. It’s due to the way the particles are assembled, in this periodic, ordered way, and the interference of light as it interacts with particles at this scale,”
“We’re essentially 3D-printing crystals.”
The results of the team’s most recent work, “Direct‐Write Freeform Colloidal Assembly” is published in Advanced Materials journal. It is co-authored by Alvin T. L. Tan, Justin Beroz, Mathias Kolle and A. John Hart.
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Featured image shows crystals 3D printed from colloids at MIT. Photo by Felice Frankel/MIT.