Zofia Dziekan, University of Warsaw, Poland
The ability to create physical objects using 3D printers has taken the manufacturing industry by storm and opened up new ways for innovation in a variety of fields (1). But as impressive as it is to print a functional bicycle or a complex medical implant, some researchers have been pushing the limits of this technology in a different direction: down to a nanoscale. With nanoscale printing, we can create structures that are smaller than the width of a human hair, with intricate details and unique properties. In this article, we will explore the history of nanoscale printing, the underlying physics of this process, and the exciting possibilities it offers for the future.
Fig. 1 Size comparison between (A) regular (2) and (B) nano-scale 3D printed object (3).
The Physics of Light-Matter Interactions
In 1930, a young mathematician – Maria Göppert-Mayer attended a Max Born seminar at the University of Göttingen (4). Mesmerized by quantum mechanics, she dedicated herself to the pursuit of theoretical physics, eventually becoming one of four women awarded the Nobel prize in Physics. While today she is best known for her work in the Manhattan Project and her postulation of the nuclear shell model, it is her earlier work that is of interest to our story. Göppert-Mayer’s groundbreaking research into molecular excitation, explored in her doctoral dissertation, demonstrated that molecules can be excited by the simultaneous absorption of two photons with energies smaller than the difference between the excited and ground state (Fig. 2A). Despite the lack of high-intensity light sources to test her theory at the time, Göppert-Mayer’s work laid the foundation for future discoveries. Three decades later, the invention of the laser finally provided the tools necessary to observe two-photon excited fluorescence for the first time in CaF2 crystal doped with europium atoms (5).
Fig. 2 (A) Energy levels involved in one-photon and two-photon absorption (6).
(B) One-photon and two-photon absorption of fluorescent die (7).
The Pulsed Lasers
The two-photon absorption process involves two photons instead of one, making the probability of absorption proportional to the intensity squared (5). As a result, increasing laser power has been crucial in the development of application for two-photon absorption. Pulsed lasers have been a game-changer in this regard, with their ability to release high-intensity bursts of energy that can be precisely controlled in terms of duration and frequency. Unlike their continuous-wave counterparts, pulsed lasers can vaporize materials without causing thermal damage, making them an indispensable tool for surgery and laser material removal (8).
3D Printing Through Direct Laser Writing
In the late 1980s, researchers started investigating the potential of using pulsed laser technology to create nano-scale 3D printers (5). One promising technique that emerged in this process was direct laser writing (DLW), a form of 3D printing in which a focused laser beam scans over the sample in three dimensions until it solidifies the polymer solution into the desired shape. To fabricate structures below the diffraction limit, the intensity, duration and frequency of the laser pulses must be precisely controlled to achieve two-photon absorption that would initiate polymerization. The material is polymerized only in the focal spot of the beam where its intensity is the highest as stated previously, and the probability of the process grows with intensity squared (Fig. 2B). This small volume in the focal spot of the beam is known as a voxel and it serves as a building block of any 3D print in DLW (7).
By moving the laser beam, it is possible to polymerize photosensitive material point-by-point, creating complex structures that are just several microns in size. Just imagine tree-lined avenues, dozens of miniature buildings and little polymer people comfortably sitting on a single strand of hair! It is truly incredible that there is no other method that allows printing on this scale. The resolution of the process is limited mainly by basic properties of an optical setup and material properties. Since its inception, nano-scale printing has come a long way and has found numerous applications in various fields, from micro-robots to drug delivery systems (9 – 10) (Fig. 3). While the technology still faces challenges, including a limited range of building materials and slow printing speed, continued research and development promise more exciting applications in the future. The history of nano-scale printing is a testament to human ingenuity and the power of scientific discovery – and who knows what incredible breakthroughs we’ll see in the years to come!
Fig. 3 SEM images of objects fabricated using DLW. (A) Medical imagining system build by inserting into a needle an optical fiber with 3D printed lenses (9). (B) Light-fueled robot that can walk and jump, placed on a human hair for scale (10). (C) Microfluidic chip designed for the fabrication of drug carrier nanoparticles (11).
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