Researchers from the Karlsruhe Institute of Technology (KIT) and Heidelberg University in Germany have developed photoresists for two-photon microprinting. Now, it has been used for the first time to produce three-dimensional polymer microstructures with nano-scale cavities. The research results were published in the journal Advanced Materials through the article "3D TwoPhoton Microprinting of Nanoporous Architectures".
Photoresist is an ink used to print the smallest three-dimensional microstructure through two-photon lithography. Photoresist is one of the key materials for micro-pattern processing in microelectronics technology. Especially in recent years, the development of large-scale and very large-scale integrated circuits has greatly promoted the research, development and application of photoresist. The printing industry is an important field of photoresist applications. In 1954, the polyvinyl alcohol cinnamate, which was first successfully studied by Minsk and others, was used in the printing industry and later in the electronics industry (photoresist explanation comes from Baidu Encyclopedia). During the printing process, the laser beam moves through the initial liquid photoresist in all spatial directions. The photoresist hardens only at the focal point of the laser beam. This complex microstructure can be built little by little. In the second step, a solvent is used to remove those areas that are not exposed to radiation, while the micro- and nano-scale complex polymer structures still exist. After the photoresist is exposed to ultraviolet light, its solubility in the developing solution changes. The photoresist used in silicon wafer manufacturing is coated on the surface of the silicon wafer in a liquid state, and then dried into a glue film.
Two-photon polymerization (or two-photon micro-imprinting based on this process) has been extensively studied for several years, especially with regard to the production of micro-optics, metamaterials, and micro-scaffolds for single biological cell experiments.
The neuron cell culture microstructure made by Nanoscribe's two-photon micro-nano 3D printing device is used for detailed study of neuron networks.
In order to expand the range of applications, new printable materials are needed. This is the starting point for the scientists involved in KIT and Heidelberg University's Outstanding 3D Customized Object Group (3DMM2O). With the help of traditional photoresist, it is possible to print only transparent glass-like polymers. Frederik Mayer, a physicist at KIT and the lead author of this study, said that the new photoresist they studied made it possible for the first time to print 3D microstructures from porous nanofoam. This polymer foam has a cavity of 30 to 100 nanometers in size, which is filled with air.
The researchers introduced a photoresist formulation for 3D laser microprinting based on two-photon absorption, which allows the combination of self-assembly and additive manufacturing methods to achieve 3D nanometers with a pore size of about 50nm porous structure. Researchers foresee the application of this structure in the control of diffuse light scattering, such as 3D printed miniature Ulbricht condenser spheres, nanoparticle filters in microfluidics, superhydrophobic surfaces or scaffolds for cell and tissue culture.
In 3D laser microprinting, one of the most important requirements for photoresist is that the photoresist must neither absorb nor scatter at the excitation wavelength used (usually about 800nm). In particular, if a template is used to introduce porosity into the photoresist, the refractive index matching between the monomer and the template must be avoided to avoid light scattering during laser writing. Another well-known method for generating intrinsic nanoporous polymers in bulk is polymerization-induced phase separation. Applications include chromatographic separation media and superhydrophobic surfaces. Polymerization-induced phase separation has been used to 3D print glass structures using stereolithography. Following the method of polymerization-induced phase separation, the researchers developed photoresists for 3D laser microprinting. In order to prove the difference in light scattering between the structure printed by "traditional" photoresist (Nanoscribe IPS) and the phase-separated photoresist presented here.
Color change: As light is scattered in a sponge-like structure, the right micro-cylinder printed with the new photoresist appears white, while the cylinder printed with the conventional photoresist appears transparent.
Frederik Mayer pointed out that there has never been a photoresist used for 3D laser microprinting, which can be used to print'white' materials. Just like in porous eggshells, the many small pores in the porous nanostructure make them look white. Mixing white particles into conventional photoresist will not produce this effect, because the photoresist must be transparent to the (red) laser beam during the printing process. Frederik Mayer said: "Our photoresist is transparent before printing, but the printed object is white and has high reflectivity." Researchers from Karlsruhe Institute of Technology and Heidelberg University printed as thin as hair Ulbricht sphere to prove this.
An oblique view of the miniature 3D printed Ulbricht integrating sphere, using the high diffuse reflectance of the optically thick light scattering porous wall. d) Demonstration of functional micro 3D printing optical integrating sphere. The top-down optical microscope image under white light illumination (left), the green light (λ = 532 nm) emitted from the top hole when the laser illuminates the side hole (right) and the composite image. Note that very little (if any) green light leaks from the wall of the light integrating sphere.
In short, scientists have developed a photoresist for 3D two-photon microprinting for the first time, which can be used to fabricate inherent nanoporous polymer microstructures with an average pore diameter of about 50 nanometers.