Two-photon lithography creates detailed 3-D prints fast

Ashley N. Paddock, ashley.paddock@photonics.com

A 3-D printing technology that uses two-photon lithography to rapidly create detailed objects on the nanoscale could open the door to new areas of application, such as biomedicine.

Developed at Vienna University of Technology (TU Vienna), the printer uses a liquid resin that is hardened by a focused laser beam guided through the resin by scanning mirrors. The result is a solid polymer line a few hundred nanometers wide.

"Until now, this technique used to be quite slow," Jürgen Stampfl, a professor at TU Vienna's Institute of Materials Science and Technology, said in a university press release. "The printing speed used to be measured in millimeters per second; our device can do five meters in one second."

Several new ideas were combined to reach the higher speed, which now makes it possible to create much larger objects in a given period. The system achieved faster results because of its two-photon active initiators and its fast setup.

"The two-photon active initiator plays an essential role in the polymerization process," Jan Torgersen of TU Vienna told EuroPhotonics. "Originally, conventional ultraviolet-sensitive initiators like Irgacure 369 were used to initiate two-photon polymerization (2PP)."

A miniature version of St. Stephen's Cathedral in Vienna. Images courtesy of TU Vienna.

Unfortunately, these initiators have low-photon absorption cross sections at specific printing wavelengths. High excitation power is required, which results in long exposure times and subsequent damage to polymeric structures, he said.

"Our novel initiators combine much larger two-photon absorption cross sections with high initiation efficiency and thus ensure high writing speeds, a low polymerization threshold and high-quality structures," Torgersen said.

The TU Vienna system also has a galvanoscanner, which consists of two mirrors deflecting the laser beam before a microscope objective can focus it.

"Conventionally, linear axes are used to move the specimen, which creates the movement of the focal point inside the specimen," he said. "Due to inertia and vibrations, these axes have to move slower. The rotating mirrors can accelerate faster and travel at slower angular speed, as slow angular speed creates faster linear movements of the focal point."

This 285-µm race car was printed at the Vienna University of Technology.

The university's project partner, Laser Zentrum Hannover, has used the galvanoscanner to bring the mirrors to speeds of 50 mm/s. "Our system is faster because we synchronized the laser switching and the movement of the mirrors," Torgersen said. "As the galvanoscanner has no feedback control, the right parameters for the triggering of these two devices have to be found."

Chemistry also plays a crucial role.

"The resin contains molecules which are activated by the laser light," Torgersen said in a university press release. "They induce a chain reaction in other components of the resin, so-called monomers, and turn them into a solid."

These novel aromatic ketone-based initiator molecules are activated only if they absorb two photons of the laser beam at once, which happens only in the very center of the beam, where the intensity is the highest. In contrast to conventional 3-D printing techniques, solid material can be created anywhere within the liquid resin, rather than only on top of the previously created layer. Because of this, the working surface does not have to be specially prepared before the next layer is produced, which saves time. Robert Liska and a team of chemists at the university created the suitable ingredients for this special resin.

They fabricated a race car consisting of 100 layers at 300 polymer lines each in 4 min, Torgersen told EuroPhotonics.

"The distance between the lines is one micron," he said. "Two-thirds of the time was used to move to the next layer; thus, total structuring time (the time where the laser is on) was approximately 2.6 minutes."

Two-photon polymerization is attractive for a number of applications, most notably optoelectronics.

"As this technique is the first true 3-D lithography, it is not necessarily limited to a layer-by-layer fabrication process, and it is thus capable of embedding and connecting objects in 3-D," Torgersen said. "2PP is also applicable for fabricating photonic crystals. Some other groups ... focus on the fabrication of 2PP microfluidic devices."

Torgersen and his colleagues see great potential for 2PP in the biomedical field.

"The complexity and the accuracy of the structures that can be produced with this technique are particularly suitable for making scaffolds for tissue engineering," he said. "These scaffolds have to be very accurate to resemble the extracellular environment precisely a demand that 2PP can fulfill."