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1. Introduction
Enormous demands for multi-material three-dimensional (3D) printing technologies have been identified in application areas, such as research, industry, medical, education and entertainment (Bandyopadhyay and Heer, 2018; Gao et al., 2015; Singh et al., 2019), for the benefit of full-color objects (Sitthi-Amorn et al., 2015), meta-material (Chen and Zheng, 2018), soft material (Truby and Lewis, 2016), composite material (Hamidi and Aslani, 2019; Kokkinis et al., 2015; Papon and Haque, 2019; Yang et al., 2018), functionally graded material (Loh et al., 2018) and four-dimensional (4D) printed material (Deng and Chen, 2015; Kim et al., 2018; Tibbits, 2014). Over the years, many multi-material 3D printing methods have been developed. For example, the multi-jetting deposition modeling (MJM) approach (Sitthi-Amorn et al., 2015) was used to fabricate 3D objects with multiple types of polymers and polymer-derived materials. In the MJM process, a piezoelectric material is used in the printhead to generate a pressure pulse in the fluid, which forces a droplet of ink out from the micro-scale nozzles. Such a deposition-on-demand (DOD) method can jet different material droplets from an array of nozzles to fabricate a 3D multi-material object. This ink-jetting method has been successfully adopted in the industry, such as the “Connex3” printer from Stratasys Inc. It uses the 2D ink-jetting printing technology to deposit droplets of different materials from a large number of nozzles. However, the DOD-based methods have two main drawbacks. First, these methods only apply to liquid resins with low viscosity because more viscous materials, such as oil-like materials, cannot be jetted from the micro-nozzles. Another drawback is the limited reliability because of a large number of nozzles and small nozzle sizes. Also, a higher resolution of the DOD method means a smaller nozzle size, which leads to less reliability, higher cost and fewer material choices.
Besides the multi-ink-jetting deposition method, multi-nozzle fusion deposition modeling (FDM) has also been widely used because of its low cost. FDM uses multiple nozzles to extrude different filament materials and fuses them into a component with multiple materials. However, this multi-nozzle FDM process is applicable only to thermosensitive materials, e.g. acrylonitrile butadiene styrene and polylactic acid; it excludes the commonly used photocurable polymers and the polymer-derived materials (Bagheri and Jin,...