A joint team of researchers from the Lawrence Livermore National Laboratory (LLNL) and the Chinese University of Hong Kong have unlocked dramatically faster nanoscale 3D printing rates thanks to a new time-based technique for controlling the light from an ultrafast laser. The new approach can reportedly achieve fabrication speeds up to a thousand times faster than existing two-photon lithography (TPL) processes with superior resolution.
The innovative nanoscale 3D printing approach, called femtosecond projection TPL (FP-TPL), offers a number of improvements compared to existing TPL technologies. In addition to its high throughput, the process is also able to produce parts with a depth resolution of 175 nanometers (higher than current methods) and can create geometries with 90 degree overhangs, which up until now have been challenging or impossible to realize.
In terms of applications, the novel process could be used for producing bioscaffolds, flexible electronics, electrochemical interfaces, micro-optics, mechanical and optical meta-materials and other functional micro- or nanostructures.
TPL vs. FP-TPL
How exactly does the FP-TPL process differ from more conventional TPL technologies? As the researchers explain in a recent study published in the journal Science, the process’ high throughput is achieved by using a million light points at once, rather than a single spot of high-intensity light.
Conventional TPL technologies rely on a single spot of high-intensity light (of about 700 to 800 nanometers in diameter), which is exposed to liquid photopolymers to harden layers of the material. Though effective, this approach requires the point to scan through the entire printed structure, which leads to long print times, especially for complex structures. Because it can take several hours to produce a complex 3D structure, traditional TPL has been limited in terms of scalability.
“Instead of using a single point of light, we project a million points simultaneously,” explained Sourabh Saha, an assistant professor at Georgia Tech’s George W. Woodruff’s School of Mechanical Engineering and the study’s lead and corresponding author. “This scales up the process dramatically because instead of working with a single point that has to be scanned to create the structure, we can use an entire plane of projected light. Instead of focusing a single point, we have an entire focused plane that can be patterned into arbitrary structures.”
A million light points
In order to create a million light points, the researchers devised a digital mask similar to those used in projectors. Rather than create images or videos, however, the digital mask in the nanoscale printing technology controls a femtosecond laser to create specific light patterns in the liquid photopolymer to solidify a 3D structure.
According to the researchers, each layers of the printed object is solidified by a 35-femtosecond burst of high-intensity light. This is repeated, layer by layer, until the 3D structure is completed. The process enables the creation of 3D structures in about 8 minutes—a drastic difference compared to several hours using TPL.
Chris Spadaccini, the Director of LLNL’s Center for Engineered Materials and Manufacturing, said of the new process: “The parallel two-photon system that has been developed is a breakthrough in nanoscale printing that will enable the remarkable performance in materials and structures at this size scale to be realized in usable components.”
The technology’s ability to project the laser into any depth of the photopolymer material also enables the creation of previously impossible geometries, like 90-degree overhangs or 1,000:1 aspect ratios of length to feature size.
As part of the research, the team 3D printed suspended structures measuring a millimeter in length between bases smaller than 100 x 100 microns. The structure showed no signs of collapse during the fabrication process thanks to the similar densities of the liquid and solidified material. The speed of the process also reduced the risk of disturbing the liquid.
The researchers also experimented with other structures, including micro-pillars, cuboids, log-piles, wires and spirals.
While the joint research team worked with conventional polymer precursors, it believes that the FP-TPL process could be adapted for metals and ceramics that can be generated from precursor polymers. This, says Saha, would create numerous industrial uses for the nanoscale 3D printing technology.
“The real application for this would be in industrial-scale production of small devices that may be integrated into larger products, such as components in smartphones,” he said. “The next step is to demonstrate that we can print with other materials to expand the material palette.”
Maintaining nanoscale resolutions
“Traditionally, there are tradeoffs between speed and resolution,” Saha said of the process’ high resolution capabilities. “If you want a faster process, you would lose resolution. We have broken this engineering tradeoff, allowing us to print a thousand times faster with the smallest of features.
“So far, we have shown that we can do pretty well on speed and resolution. The next questions will be how well we can predict the features and how well we can control the quality over large scales. That will require more work to understand the process itself.”