ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 232

technology to fuse the metal powder. LENS systems use a neodymium-yttrium-aluminum-garnet
(Nd:YAG) laser, while DMD uses the CO 2 laser beam. The powder feeding systems are completely dif-
ferent, since the powder feeding in the DMD comes through a concentric ring at the tip of the laser
nozzle, while in the LENS the powder comes through different powder feeders to the melting zone. Both
techniques use an inert gas during the manufacturing process to avoid oxidation ( Dinda et al., 2008 ). With
these techniques, it is possible to use a wide range of metal powders such as stainless steel, nickel-based
alloys, Ti and its alloys, tooling steel, copper alloys, alumina, or a combination of these ( Lin et al., 2009;
Wong and Hernandez, 2012b ; Woesz, 2008b ). Thus, LENS can be useful for the repairing or re-manufac-
turing process that cannot be implemented by other AM techniques ( Wong & Hernandez, 2012b ). LENS
and DMD work through the process of depositing the metal layer-over-layer. Both are similar in terms of
processing; however, the high cost of DMD has boosted LENS's popularity. The need for support struc-
tures for overhanging features is one of the limitations of the use of these techniques in the fabrication of
orthopedic prostheses ( Woesz, 2008b ). Another drawback is the residual stress caused by the rapid heat-
ing and solidifying process ( Wong and Hernandez, 2012b ).
Many studies have been conducted using two techniques: fiber deposition (FD) and 3DP. The two
have been chosen mainly due to their manufacturing simplicity compared with SLS or EBM and cost
efficiency. The base material of FD is metal powder. In the FD method, the powders are mixed with a
solution to form the slurry which can be deposited from the machine onto a substrate. The scaffold is
built by the process of layering from bottom to top. The FD technique requires postprocessing of the
product by the sintering of the produced scaffold in a very high temperature furnace. When using FD
in the fabrication of a scaffold, several parameters can influence the strength and the porosity of the
structure, such as the gap between fibers, the fiber's lay down angle, and the nozzle's diameter ( P. J. Li
et al., 2007 ; Li et al., 2005 ). The dimensional accuracy of the final product is poor compared to other
techniques such as 3DP due to the wet nature of the fiber slurry, which leaves a high level of shrinkage
after drying and sintering ( Basalah et al., 2012 ). In fact, this technique is still in the experimental stage.
One of the simplest rapid prototyping techniques is the ink-jet-based 3DP, as shown in Figure 11.2 .
In this technique, the printer functionality is similar to an ink-jet printer, where instead of ink, a binder
FIGURE 11.2
Simple sketch showing the working principle of the 3DP machine.