ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 231

et al., 1994 ). In this technique, the laser beam melts the low sintering temperature metal and uses it
to bind the main metal particles to each other. For instance, in a study where NiTi dental implant was
fabricated using DMLS, the base metal was Ti and the binding agent was Ni ( Shishkovsky et al., 2008 ).
The second technique is called indirect SLS, where a preprocessing of the powder is required to coat the
metal powder particles with some polymer to work as a binding agent of the green sample. This tech-
nique requires a postprocessing of the green sample by debinding the polymer from the green sample
followed by the sintering of the metal particles in a shielded environment at a very high temperature. The
diversity of materials produced by this manufacturing method is one of the most important advantages
of this technique ( Wong and Hernandez, 2012a ).The final product resolution is moderate since heat
transfers to the adjacent area and fuses extra particles to the targeted area, which might lead to a limita-
tion in the accuracy according to the size of particles ( Woesz, 2008a ; Wong and Hernandez, 2012b ).
Economically speaking, fabrication using SLS is costly ( Dabrowski et al., 2010 ). Technically, SLS is
considered a lengthy manufacturing method, since it requires preprocessing of the powder and the laser
sintering process is time-consuming. In addition, the resulting product may require a postprocessing
heat treatment that usually takes hours to complete ( Campanelli et al., 1994; Woesz, 2008b ).
Another complex technique, EBM, works based on the layering process similar to SLS. EBM dif-
fers from SLS in its use of an electron beam rather than a laser to sinter the powders, which is generated
in a tungsten filament and is accelerated and controlled with a magnetic field ( van Noort 2012 ). This
manufacturing technique is relatively fast and the building of samples takes place under vacuum ( Alva-
rez and Nakajima, 2009 ). It offers an attractive opportunity in the fabrication of fully dense or porous
parts due to several controllable parameters such as beam current, scan rate, and sequence variations
( Murr et al., 2009 ). In this technique, an additive or fluxing agent is not required to fulfill the melting
process because the electron beam is powerful enough to raise the temperature of the particles to the
melting point ( van Noort 2012 ). However, there are still some limitations in this method such as the
low dimensional accuracy and surface quality due to shrinkage. Additionally, this technique is costly,
and removing the excess material inside the structure can prove difficult ( Wen et al., 2002 ). Different
metals have been fabricated using EBM, such as Cp-Ti, Ti-6Al-4V, and Co/Cr for orthopedic implant
and maxillofacial surgery ( Alvarez & Nakajima 2009 ; van Noort, 2012 ).
Similarly, SLM utilizes the same mechanism of the EBM, and a wide range of materials in powder
form can be used ( Alvarez and Nakajima, 2009 ). Moreover, SLM is able to produce solid and porous
parts based on the laser energy density ( Mullen et al., 2009 ). Also, this technique is free of binders
and fluxing agents, so there is no need for a postprocessing step ( Pupo et al., 2013; Alvarez and Naka-
jima, 2009; Campanelli et al., 1994 ). Unlike EBM, SLM uses an ytterbium fiber laser 200 W power
and uses Ar or N in the building chamber, which may increase the thermal conductivity and maintain
a consistent rapid cooling of the printed zone more than EBM ( Murr et al., 2012; Mullen et al., 2009 ).
However, high production cost, lengthy fabrication time, and difficulty in removing the trapped pow-
der are the major limitations of this technique ( Dabrowski et al., 2010; Alvarez and Nakajima, 2009;
Campanelli et al., 1994 ). Furthermore, the vaporization phenomenon is one of the drawbacks of this
technique. This phenomenon is generated due to the high temperature of the molten pool caused by the
laser beam evaporating the particles. This leads to an overpressure in the molten pool, which results
in spewing of some molten metals out of the pool ( Campanelli et al., 1994 ). Today, this technique is
utilized in the aerospace industry, orthopedics prostheses, and dental implants ( Pupo et al. 2013 ).
In the same context, laser engineering net shaping (LENS) and direct metal deposition (DMD) are
both AM techniques used in manufacturing bone substitutes. These techniques mainly depend on laser