The composition/structure of a functionally graded material (FGM) changes spatially to achieve a gradient change in its mechanical, thermal expansion, dielectric properties, or other properties. Due to their tunable multi-functionality and high adaptability to specific application requirements, the design, construction and application of functionally graded materials have always been a research hotspot in the field of materials science and materials processing, and they have important application value in the fields of microelectronics, aerospace, soft robotics and biomedical engineering. However, the spatial anisotropy of functionally graded materials is inevitably incompatible with traditional manufacturing techniques (such as casting, machining, injection molding, etc.), and the accurate construction of functionally graded materials in 3D has always been one of the major challenges in the field of material processing.
While 1D/2D gradient materials can be processed using fabrication techniques such as spraying, electrodeposition, and powder metallurgy, 3D, complex gradient materials cannot be fabricated. In response to this problem, the team of Fei Guanghai and Rob Aceloot from the University of Leuven in Belgium has developed a series of new grayscale 3D printing technologies for the manufacture of functionally graded materials based on the stereolithography 3D printing process. (1) The development of halftone grayscale light-curing technology and the construction of mechanically graded materials can produce anisotropic deformation under the stimulation of external conditions (pressure, temperature, light, etc.), and this gradient response characteristic brings new opportunities for soft robotics technology. By using grayscale masks, stereolithography 3D printing technology based on digital micro-galvanometers (DMD) spatially defines the degree of photopolymerization of materials, and the construction of mechanically graded materials can be realized. However, DMD grayscale curing relies on a 256-step grayscale mask (8-bit) to manipulate the gradient distribution of exposure energy, which is not compatible with most projection light-curing equipment such as LCD equipment. In order to solve this limitation, based on digital halftone image processing technology, Fei Guanghai and Rob Ameloot's team developed a new grayscale printing technology that can achieve gradient printing on any projection light-curing equipment (DMD or LCD equipment), breaking through the long-term limitation that grayscale light-curing can only be achieved by DMD printers. The results were published in Cell Reports Physical Science (Cell Press, cover article) under the title of Digital Halftoning for Printer-Independent Stereolithography of Functionally Graded Materials.
Fig.1 Halftone grayscale light-curing technology Original link: https://doi.org/10.1016/j.xcrp.2023.101525(2) The development of digital light-curing and selective dissolution processes and the realization of the dual gradient of "porosity and filler content" The microstructure is complex and has micron pore size ( <50 μm) and gradient porosity porous materials have great potential in areas such as medical implants and microfluidics. However, traditional material processing techniques are not suitable for fabricating such three-dimensional, complex, gradient porous materials. Combining light-curing technology and chemical dissolution strategies, the team of Guanghai Fei and Rob Aceloot developed a digital grayscale light-curing and selective dissolution process to realize the preparation of gradient porous materials. Under the action of digital grayscale exposure strategy, the photosensitive resin undergoes gradient cross-linking reaction to form a polymer network with gradient cross-linking density. After the printed sample is immersed in the cleaning solvent, the degree of swelling of the polymer's cross-linked network is controlled by adjusting the solvent concentration and immersion program, so that the solvent selectively penetrates into the area with lower cross-linking density (faster swelling), dissolves the filler and forms a gradient porous material. The results were published in Cell Reports Physical Science under the title of Stereolithographic 3D Printing of Graded Porous Materials via an Integrated Digital Exposure and Selective Dissolution Strategy.
Fig.2 Original link of digital light curing and selective dissolution technology: https://doi.org/10.1016/j.xcrp.2023.101504(3) Summary and application of various grayscale stereolithography technologies Grayscale stereolithography technology prepares functionally graded materials by accurately controlling the exposure energy of the 3D printing process and the polymerization degree of polymers. This comprehensive work deeply studies various grayscale light-curing 3D printing technologies and their working mechanisms (light energy control principle and polymer polymerization mechanism), analyzes the advantages and disadvantages of various technologies and their application scenarios, and provides methods to improve grayscale light-curing 3D printing. The results were published in Advanced Functional Materials (cover article) under the title From Grayscale Photopolymerization 3D Printing to Functionally Graded Materials.
图3 各类灰度立体光刻技术的对比原文链接：https://doi.org/10.1002/adfm.202314635除了灰度立体光刻技术的开发，费广海团队还开发了电场辅助光固化技术和可逐层调控打印参数的多材料光固化技术，实现了半导体器件的原位梯度封装，扩展了光固化技术在微电子领域的应用。 相关成果分别以Multi-material and parameter-controllable stereolithography 3D printing of graded permittivity composites for high voltage insulators和Electrically assisted stereolithography 3D printing of graded permittivity composites for in-situ encapsulation of insulated gate bipolar transistors (IGBTs)为题发表在材料加工领域权威期刊Virtual and Physical Prototyping和Materials & Design 。 原文链接：https://doi.org/10.1080/17452759.2023.2271447
https://doi.org/10.1016/j.matdes.2023.112220 Source: Frontiers of Polymer Science
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