Introduction: additive manufacturing is considered as "a technology that will change the world". UV curing 3D printing is one of the important directions. Based on the digital model, the structure is formed by the reaction of light and materials (mostly resin, ceramic slurry, nano metal particle slurry, etc.), and by local photopolymerization reaction, relatively high optical resolution and printing accuracy can be achieved.
At present, from the perspective of the development of UV curing 3D printing technology, it focuses on two dimensions: one is the macro dimension, that is, to achieve large format, large size and high speed 3D printing; the other is the micro dimension, that is, to achieve micro and nano size fine 3D printing.
In the fields of micro nano mechanical and electrical systems, biomedicine, new materials (super materials, composite materials, photonic crystals, functional gradient materials, etc.), new energy (solar cells, micro fuel cells, etc.), micro nano sensors, micro nano optical devices, microelectronics, biomedicine, printing electronics, etc., the complex three-dimensional micro nano structure has a huge industrial demand [1].
Micro / nano scale UV curable 3D printing has high potential and outstanding advantages in the manufacturing of complex 3D micro / nano structure, high aspect ratio micro / nano structure and composite (multi-material) micro / nano structure. It also has the advantages of simple equipment, low cost, high efficiency, wide variety of materials to be used, no need of mask or mold, direct forming, etc. Therefore, micro / nano UV curable 3D printing technology in the near future In recent years, it is favored by more and more scientific research institutions, enterprises and end users. Two-photon polymerization based direct laser writing (TPP) and Projection Micro Stereolithography (P μ SL) are the most mature and commercialized 3D printing technologies in the world.
TPP is a process for curing photosensitive material (resin, gel, etc.) in focus area by ultrafast pulse laser. P μ SL is a process of curing resin by using ultraviolet light to expose the whole picture on the dynamic mask. These two technologies are commonly used in micro nano scale 3D printing technology, among which the precision of TPP printing can be less than 100 nm. At present, Germany, Lithuania and other countries have commercialized equipment products. At present, P μ SL can achieve hundreds of nanometer precision in the laboratory stage, and the products that have been commercialized can reach several micrometers of printing precision. Most of them are found in nanoarch series micro nano 3D printing equipment of Shenzhen morfang Materials Co., Ltd., which is the first commercial P μ SL micro scale 3D printing equipment product in the world. This paper will systematically introduce the above two technologies from several aspects.
Technical principle
Photocuring refers to the curing process of monomer, oligomer or polymer matrix under the light induction. Light curing 3D printing refers to that the liquid resin in the exposed area is polymerized into solid substance by controlling the pattern of light spot or scanning path of galvanometer, while the resin in the unexposed area is not involved in polymerization reaction, and the rapid prototyping samples are stacked layer by layer by precisely controlling the z-axis movement. At present, there are two kinds of resin polymerization methods: single photon absorption polymerization and two photon absorption polymerization. Single photon absorption (SPA) refers to the process in which the excited state electrons absorb a energy level difference from the low energy level to the high energy level. The light absorption efficiency is linearly related to the incident light intensity.
P μ SL is a printing technology formed by single photon absorption polymerization. After the incident light enters the liquid resin, the light intensity decreases gradually under the action of the absorbent. Therefore, the effective polymerization only takes place in a very thin layer on the resin surface, as shown in Figure 1. Two photon absorption (TPA) is the process of stimulated electrons absorbing two photon energy at the same time to realize the transition, which is a nonlinear effect, that is, with the increase of optical energy density, the effect will rapidly strengthen. Therefore, the incident light can pass through the liquid resin and solidify the body pixels in a very small area of its space. As shown in Figure 1, two-photon absorption mainly occurs at a certain point, usually the focus of the beam. This is also because the light intensity here is high enough to promote the two-photon absorption effect of polymer.
Figure 1. Single photon absorption and two-photon absorption [2]. Among them, 3D printing equipment based on single photon absorption can use point light source or face light source (such as P μ SL), while TPP uses point light source.
It can also be seen from Fig. 1 that the two-photon absorption has high localization, which can not be realized by single light. With this high local property, 3D printing with the scale less than 100 nm has become a reality. By focusing the laser, the light intensity at the laser focus exceeds the two-photon absorption threshold, controlling the reaction area in a very small area near the focus, and changing the relative position of the laser focus in the sample, 3D micro nanostructures can be printed with high printing accuracy. Single photon absorption, with a large exposure area, achieves high printing accuracy and high printing speed.
Preparation process and equipment
In the process of two-photon polymerization TPP micro nano 3D printing, figure 2 is taken as an example: femtosecond laser is focused on the photosensitive material through an ultra-high power focusing system, and the polymerization takes place by the two-photon absorption of the photosensitive material. Among them, photosensitive materials are usually coated on glass or silicon wafer, which is placed on piezoelectric ceramic platform. By moving the precision piezoelectric ceramic platform or galvanometer scanning and controlling the laser focus position, the micro nano 3D structure can be formed. After forming, the samples are washed (soaked) with organic solvent to remove the remaining unpolymerized materials, and finally the 3D structure samples are obtained. The printing process generally does not need to peel the print from the bottom of the resin tank, or install a scraper to coat the photosensitive resin level.
Figure 2 typical TPP printing system diagram [3]
The operation process of P μ SL (as shown in Figure 3) is to reflect the ultraviolet band light emitted by the LED on a digital micro mirror device (DMD), and then let the ultraviolet light carry out a thin layer exposure of the liquid resin according to the set figure. After the surface resin is cured, the printing platform will be lowered, more liquid resin will flow onto the cured layer, and a new layer of liquid material will continue to be exposed by ultraviolet radiation. The finished printed article can be used as a device, sample or mold only by removing the residual liquid resin.
The usual TPP printing uses the infrared femtosecond pulse laser as the light source. The femtosecond pulse laser is expensive and has the attenuation problem with the use time accumulation. P μ SL can choose industrial UV-LED as light source, which has long life (10000 hours), low cost (usually less than 100000), and relatively low replacement cost. In terms of environmental requirements for equipment use, most TPP printing equipment is recommended to use yellow light dust-free room, and P μ SL 3D printing system only needs normal and clean space to be placed, without the requirements of yellow light dust-free room.
Fig. 3 equipment diagram of typical P μ SL printing system
3D printing performance
In terms of printing resolution, P μ SL technology can achieve printing resolution in the range of hundreds of nanometers to tens of microns through DMD chip selection and projection objective miniaturization. However, TPP two-photon polymerization can achieve ultra-high printing resolution of about 100 nm because of its highly localized polymerization and breaking through the optical diffraction limit.
In terms of printing speed, because P μ SL technology uses full face projection exposure and TPP technology uses point by point scanning processing, there is also a big difference in printing speed. Taking the bionic model of Sophora japonica with the overall size of 2 m m (L) × 2 mm (W) × 70 μ m (H) and the minimum characteristic size of 5 μ m as an example, the P μ SL printing device can print in 15 minutes, while the TPP printing device needs 16 hours [4].
In terms of printing format, TPP technology is usually provided by Precision Piezoelectric ceramic platform or scanning galvanometer due to the precise movement of laser focus position. The range of movement is limited, supplemented by scanning galvanometer technology or mechanical splicing. The typical printing format is about 3mm × 3mm. P μ SL technology determines the single projection exposure size by DMD chip size and projection objective magnification, and it can also achieve larger size through mechanical splicing. As shown in Figure 2, the sample of high-precision large-scale cross-scale printing prepared by the equipment of Shenzhen Mofang Material Technology Co., Ltd. has an overall size of 88 × 44 × 11 mm3 and a rod diameter of 160 μ M. The maximum printing size of the equipment of morfang material company can reach 100mm × 100mm.
Figure 4 high precision cross scale printing
In terms of printing materials, the particularity of two-photon absorption also makes TPP printing more demanding in material selection. For example, it is required that the resin must be transparent to the laser of working wavelength to ensure that the laser energy can be focused in the resin, and has a high two-photon absorption conversion rate, because the materials used are relatively limited (such as SCR resin, IP series resin, SU8 resin, PETA, etc.) 。 P and SL printing materials are mostly photosensitive resins, which can print transparent resin materials and opaque composite resin materials. They are widely and commercially available (such as rigid resin, toughness resin, high temperature resistant resin, biocompatible resin, flexible resin, transparent resin, hydrogel, ceramic resin, etc.).
Application level
TPP technology is currently a common processing technology for three-dimensional nano scale processing, which is widely used in many scientific research fields, including nano optics (such as photonic crystal, metamaterials, etc.), Life Science (cell culture tissue, vascular stent, etc.), bionics, microfluidic devices (valves, pumps, sensors, etc.), biochip, etc., as shown in Figure 5. But on the other hand, due to the limitation of its processing size and speed, the industrial application of TPP printing is less, and it is still in urgent need of breakthrough.
Figure 5 case of TPP micro nano 3D printing [5]
The application of P μ SL in scientific research includes bionics (Sophora japonica structure [4]), biomedicine (scaffold structure, micro needle), microfluidic pipe, mechanics, 3D micro nano manufacturing, micro machinery, acoustics, etc., as shown in Fig. 6.
Figure 6 case of P μ SL micro nano 3D printing [4]
Compared with TPP, P μ SL has the advantages of fast processing speed, large print size, low processing cost and loose environmental requirements, which makes its industrial application field realize the batch processing and application of endoscope, guide pin, connector, packaging test material, etc. For example, in the eye hospital, the guide pin used for glaucoma treatment (as shown in Figure 7), the diameter of the micro spring in the guide pin can reach 200 μ m, and the printing material has excellent biocompatibility. The guide pin can effectively improve the intraocular pressure and flow rate in the treatment. In addition, there are socket sockets used by communication companies for chip testing, as shown in Figure 8, which can achieve a compact structure with a radius of up to 100 microns and an interval of 50 microns. The well-known endoscope manufacturers in the medical field have also used P μ SL to manufacture the endoscope base with high aspect ratio and thin aperture, with the minimum thin wall thickness of 70 μ m and the height of 13.8 mm. In addition, in addition to printing resin materials, P μ SL process can also print ceramics (Fig. 9 is ceramic printing sample).
Figure 7 guide screw (drainage tube, short process, wing collar) for glaucoma treatment in ophthalmic hospital
Figure 8 endoscope lens end and socket socket
Figure 9 ceramic print sample
In a word, TPP and P μ SL have their own printing characteristics and related application fields. TPP has been widely used in the fields of optics, metamaterials, biology and other scientific research because of its high printing accuracy of about 100 nm and relatively limited processing size and materials. In the aspect of large format micro scale 3D printing technology, P μ SL projection stereolithography has the advantages of long processing time, low cost and high efficiency