On the other side of the spectrum, micro-scale 3D printing has significant applications in micromechanical devices, optics, and research. However, the current micro-scale limitations include material restrictions, warping and inaccuracy, and speed. Accessible one-photon 3D printing has become a key driver in biological and medical research, including printing tissue scaffolds and microfluidic devices.
Commercial optical 3D printers commonly use stereolithography techniques with z-stage resolutions on the order of 10-100 microns, with x-y minimum feature sizes around 100 microns (for example, Formlabs Form 1, Figure 18-7). These types of printers use one- photon absorption to trigger polymerization of a resin. Positioning of the light source and resin depend on the specifics of the printer and common methods such as galvanometers to steer a laser beam, inkjet deposition of resin, or projection-based systems. Standard one-photon absorption systems usually use UV-curable resins, which require average continuous wave optical power around 100 mW. Typical print times for a 5 x 5 x 5 cm part with these commercial stereolithography printers are around 10 hours, and the current cost can be as low as a few thousand dollars.
Figure 18-7. Scanning electron microscopy image detailing a resolution test print from a Formlabs Form 1 printer
(image: Dr. James Weaver)
Meanwhile, advances in two-photon polymerization have helped realize applications that require high resolution on the nano scale. In contrast to one-photon printers, two-photon polymerization systems function via nonlinear optical absorption to achieve a smaller polymerization voxel unit. Two-photon absorption occurs when two photons are simultaneously absorbed by a molecule to allow an electron to jump to a higher state. This is a third-order process in which higher photon densities are required for two-photon absorption compared to one-photon absorption (linear process). For fabrication, a system typically uses either a pulsed femtosecond or a nanosecond laser operating at double the absorption frequency of the light-curable resin. The latter requires that the laser is tightly focused into a bath of resin, with the focal point being where the two-photon absorption primarily occurs. This generates a small voxel of polymerized resin, typically around 100500 nm in size. Such systems (such as printers made by NanoScribe) are capable of submicron resolutions, but are limited by speed and positioning capability to under 1 mm object size typically. The print time for a 1 x 1 x 1 mm object on a NanoScribe printer is around 50 hours. In addition to the limitations on speed and size, cost is another barrier; commercial two-photon systems such as these start around $500,000.
Work done in collaboration with Will Patrick and Christian Landeros has focused on the limitations of both one-photon and two-photon printers. Our group has taken steps in developing a combination system to take advantage of the two systems’ inherent strengths: fast one-photon polymerization for larger areas and precise two-photon polymerization for small features where needed. We believe the future of 3D printing with this system scales down to the nanometer and will facilitate micromechanical features on product-scale devices, such as structural color, sensing, and actuation mechanisms.
The integrated one- and two-photon polymerization system we designed and built uses an optical setup similar to a fluorescent microscope, as depicted in Figure 18-8. In our configuration, we used two different lasers: a blue diode laser for one-photon polymerization, and a Nd:YAG laser for two-photon polymerization. Early results are promising and show improvements for reliability, measurement data, and the potential to improve resolution based on material monitoring (see Figure 18-9). The work has taken key steps in the direction of coupling the relatively low cost and high speed of one-photon 3D printing with the nano-scale precision of two-photon printing in a combination system. With these advancements, we set the stage for the development of a 3D printing system capable of closing the gap between submicron and centimeter scales. The area of digital fabrication on the small scale continues to push boundaries, allowing for novel structural color fabrication, micromechanical devices, and advances in metamaterials.
Figure 18-8. Schematic diagram of the combination one- and two-photon 3D printing system; note that the mechanical actuation system, comprised of a stepper motor stage with a piezoelectric stage, is not detailed in this
schematic (graphic: Will Patrick)
Figure 18-9. Experimental setup showing the combination one- and two-photon printing system with the 1W blue laser in the process of curing material; designed and built in collaboration with Will Patrick and Christian Landeros