A 3D topographic image of a single voxel of polymerized resin surrounded by liquid resin; the image was produced by NIST’s new sample-coupled-resonance photo-rheology (SCRPR) technique. Image: NIST.
A 3D topographic image of a single voxel of polymerized resin surrounded by liquid resin; the image was produced by NIST’s new sample-coupled-resonance photo-rheology (SCRPR) technique. Image: NIST.

Resins and other materials that react under light to form polymers, or long chains of molecules, are attractive for the 3D printing of parts ranging from architectural models to functioning human organs. But it's been a mystery what happens to the materials' mechanical and flow properties during the curing process at the scale of a single voxel – a 3D unit of volume, the equivalent of a pixel in a photo.

Now, researchers at the US National Institute of Standards and Technology (NIST) have demonstrated a novel light-based atomic force microscopy (AFM) technique – sample-coupled-resonance photorheology (SCRPR) – that can measure how and where a material's properties change in real time at the smallest scales during the curing process.

"We have had a ton of interest in the method from industry, just as a result of a few conference talks," said Jason Killgore, NIST materials research engineer. He and his colleagues report the technique in a paper in Small Methods.

Lauded for the flexible, efficient production of complex parts, 3D printing, or additive manufacturing, has the disadvantage of introducing microscopic variations in a material's properties. Because software renders the parts as thin layers and then reconstructs them in 3D before printing, the physical material's bulk properties no longer match those of the printed parts. Instead, the performance of fabricated parts depends on printing conditions.

NIST's new method measures how materials evolve with submicrometer spatial resolution and submillisecond time resolution – thousands of times smaller-scale and faster than bulk measurement techniques. Researchers can use SCRPR to measure changes throughout a cure, collecting critical data for optimizing the processing of materials that can range from biological gels to stiff resins.

The new method combines AFM with a form of 3D printing known as stereolithography, which uses light to pattern photo-reactive materials such as hydrogels and reinforced acrylics. A printed voxel may turn out uneven due to variations in light intensity or the diffusion of reactive molecules.

AFM can sense rapid, minute changes in surfaces. In the NIST SCRPR method, the AFM probe is continuously in contact with the sample. The researchers adapted a commercial AFM so that it could use an ultraviolet laser to start the formation of the polymer (‘polymerization’) at or near the point where the AFM probe contacts the sample.

The method is able to measure two values at one location in space during a finite timespan. Specifically, it measures the resonance frequency (the frequency of maximum vibration) and quality factor (an indicator of energy dissipation) of the AFM probe, tracking changes in these values throughout the polymerization process. These data can then be analyzed with mathematical models to determine material properties such as stiffness and damping.

The researchers demonstrated the method on two materials. One was a polymer film that is transformed by light from a rubber into a glass. They found that the curing process and material properties depended on exposure power and time, and were spatially complex, confirming the need for fast, high-resolution measurements. The second material was a commercial 3D printing resin that changed from liquid to solid in 12 milliseconds. A rise in resonance frequency seemed to signal polymerization and increased elasticity of the curing resin, and so the researchers used the AFM to make topographic images of a single polymerized voxel.

Surprising the researchers, interest in the NIST technique has extended well beyond the initial 3D printing applications. Companies in the coatings, optics and additive manufacturing fields have reached out, and some are pursuing formal collaborations, the NIST researchers say.

This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.