Digital Fabrication of Multi-Material Objects for Biomedical Applications

Recent developments in medical and dental fields have warranted biomedical objects or implants with desirable properties for biomedical applications. For example, artificial hip joints, tissue scaffolds, and bone and jaw structures are now commonly used in hospitals to assist complex surgical operations, and as specimens for experiments in pharmaceutical manufacturing enterprises. But most biomedical objects are not economical to fabricate by the traditional manufacturing processes because of their complex shapes and internal structures with delicate material variations. Layered manufacturing (LM) has been widely recognized as a potential technology for fabrication of such biomedical objects. Wang et al. (2004) developed a precision extruding deposition (PED) system to fabricate interconnected 3D scaffolds. Zeng et al. (2008) used fused deposition modelling (FDM) technology to build an artificial human bone based on computed tomography (CT) images. However, most commercial LM systems can only fabricate single-material objects, which cannot meet the needs for biomedical applications. A typical example of dental implantation requires a dental implant with functionally graded multi-material (FGM) structures to be composed of titanium (Ti) and hydroxyapatite (HAP) in order to satisfy both mechanical and biocompatible property requirements (Watari et al., 1997). Therefore, it is desirable to develop multi-material layered manufacturing (MMLM) technology for fabrication of biomedical objects. Multi-material (or heterogeneous) objects may be classified into two major types, namely (i) discrete multi-material (DMM) objects with a collection of distinct materials, and (ii) functionally graded multi-material (FGM) objects with materials that change gradually from one type to another. In comparison with single-material objects, a DMM object can differentiate clearly one part from others, or tissues from blood vessels of a human organ, while an FGM object can perform better in rigorous environments. In particular, suitably graded composition transitions across multi-material interfaces can create an object of very different properties to suit various applications (Kumar, 1999; Shin & Dutta, 2001). Multi-material layered manufacturing (MMLM) refers to a process of fabricating an object or an assembly of objects consisting of more than one material layer by layer from a CAD model with sufficient material information. Some researchers have explored different techniques to fabricate multi-material objects. A few experimental MMLM machines, such as a discrete multiple material selective laser sintering (M2SLS) machine (Jepson et al., 1997;