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Introduction
3D printing (also known as additive manufacturing, AM) is a breakthrough technology that has been developing for more than 30 years, but has attracted more and more attentions in recent years. The American Society for Testing and Materials (ASTM) International defines AM as "A process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies". The seven major additive manufacturing processes as classified per ISO (ASTM F42) are: material jetting, binder jetting, material extrusion, vat polymerization, powder bed fusion, direct energy deposition, sheet lamination. With the development of 3D printing (3DP) from rapid prototyping to the end-of-use product manufacturing process, manufacturing constraints have been greatly relieved and the design freedom has been significantly expanded, including shape complexity, material complexity, hierarchical complexity, and functional complexity1. In particular, 3D printing has the unique capability to control the point-line-area in geometry and material of each layer for an object at full scale length ranging from micro to macro-scale. The emerging multi-scale and multi-material 3D printing technique possesses great potential to implement the simultaneous and full control of fabricated object which involves the external geometry, internal architecture, functional surface, material composition and ratio as well as gradient distribution, feature size ranging from nano, micro, to macro-scale, embedded components and electro-circuit, etc. Therefore, it is able to construct the heterogeneous and hierarchical structured object with tailored properties and multiple functionalities which cannot be achieved through the existing technologies. Such technology has been considered as a revolutionary technology and next-generation manufacturing tool which can really fulfill the "creating material" and "creating life", especially subvert traditional product design and manufacturing scheme. 3D printing paves the pathway and will result in great breakthrough in various applications for example functional tissue and organ, functionally graded material/structure, lattice material/structure, metamaterial, smart material, functionally embedded electronic component, bio-inspired material/structure, multi-functionality product, soft robot, etc. Furthermore, it may promote the tremendous progress in many subjects involving material, bio-medical, electronics, mechanics, bionics, aerospace, etc2-8.
In last few years, 3D printing has been utilized to fabricate electronics and structural electronics. More specifically, electronic/electrical components can be deposited and embedded in a 3D structure to form a multi-functionality product by interrupting the 3D printing process....