**2. Additive manufacturing process**

Additive fabrication is performed directly from a 3D CAD file in which a geometrical model of part is stored. The part model can be designed in many different commercial 3D modelers but it is exported in STL file format. The STL file is imported into a specific software (such as Magics® by Materialise, Viscam® by Marcam, Netfabb® by FIT, etc.), where it is pre-

*interconnected porosity* which, combined with solid material, provides better bone ingrowth into implants. Also, AM implants are characterized by *rough surface quality* per se. Undesirable in other sectors, in medical implants rough surface is an advantage because it enhances bone-implant fixing. Furthermore, AM technologies perform fabrication of metal implants in a highly controlled atmosphere with restricted presence of oxygen, which results in especially *high purity*. Finally, layer-by-layer principle allows the fabrication of *customized net-shape implants* that fully fit patient's data. In addition, high power and

processing velocity open the possibility of serial production of *standardized implants*.

Fig. 1. Acetabular cup with designed porous surface (Courtesy of ARCAM AB).

with one of the most powerful active principles: Electron Beam Melting (EBM).

**2. Additive manufacturing process** 

The aim of this chapter is to illustrate the capabilities of AM on the example of a technology

Additive fabrication is performed directly from a 3D CAD file in which a geometrical model of part is stored. The part model can be designed in many different commercial 3D modelers but it is exported in STL file format. The STL file is imported into a specific software (such as Magics® by Materialise, Viscam® by Marcam, Netfabb® by FIT, etc.), where it is preprocessed. The part is oriented for building and a support structure is made for the downfacing surfaces of the part. Afterwards, cross-sections of a given thickness, known as 'slices', are generated virtually from 3D CAD descriptions of the part and support structures. The slices are saved in a 'slice file' (ABF, SLI, SLC or any other format that may depend on patented technology). After pre-processing, the slice file is sent to the machine to be 'printed' slice-by-slice.

Fig. 2. Steps in AM process (left) and fused layer (right)

The fabrication process consists of two basic steps: *coating* and *selective melting*. The coating step is the process in which material is laid over the working surface in a very thin layer. The thickness of the layer depends on the chosen technology and it ranges between 0.03 mm to 0.20 mm. The selective melting step refers to the process of printing the part slice by the action of a source of energy. The active principle can be a *light source*, *laser beam*, *electron beam*, etc. It acts over the layer of material and produces fusion of slice's footprint to the layer below. The power of the energy source depends on the chosen technology: in the low end, we have Stereolithography that uses ~100 mW, and in the high end, we have Electron Beam Melting that uses up to 3000 W. Only the contour of part slice and its interior are fused (Figure 2, right); the rest of the material is left untouched and may be recycled (more or less, depending on the process and material). The actions of coating and slice printing follow each other until all the slices are correctly printed, generating the final threedimensional part. Once fabrication has finished, the part is taken out to be cleaned of

Additive Manufacturing Solutions for Improved Medical Implants 151

6. *Fabrication of free form enclosed structures.* Additive technologies are capable of fabricating free form channels as well as different forms of latticed structures.

However, from the point of view of their application in biomedical field, additive

1. *Remove the 'stigma' of its original name 'Rapid Prototyping'.* Although AM has evolved to deliver ready-to-use products made from a wide variety of metals and polymers, it is

2. *Validation of mechanical properties of existing materials and AM technologies.* Unlike conventionally produced parts, AM parts may not behave identically in all directions. Depending on particular additive technology, processed material has better behaviour when the load is applied along the direction of the layer as compared to the build-up

3. *Development and characterization of new materials for AM.* Alloys like stainless steel, titanium alloys, cobalt chromium, etc. are already being processed. Nevertheless, there are many interesting materials that are under research or considered for further

EBM is one of free-form fabrication technologies capable of processing ferrous and non ferrous metallic powders to fully-dense material, using layer-by-layer principle. In the case of EBM, the energy is delivered through an electric circuit of 60kV that is created between a

tungsten filament placed inside of the electron gun and the building plate (Figure 4).

Fig. 4. Inside the EBM machine (left); scheme of an additive machine (middle); electron

The filament is heated by electric current and emits a beam of electrons which is conducted by a set of different coils until it impacts the powder surface. During the impact, electric energy is transformed to heat energy which fully melts the powder. The working chamber is kept under deep vacuum (order of magnitude 10-4 mbar). Hence, powder is released from containers and distributed over the build platform in fine 70-100 μm layers. The beam melts powder to a solid slice, following the cross-section of the part at that layer and merging it with previous slices (Figure 4). The build platform descends for the value of layer thickness and a new powder layer is dispatched. The process repeats until the part is completed.

still wrongly considered that AM is valid only for prototype fabrication.

technologies are facing some challenges:

research (see Future development section).

beam making the contour of tibia prosthesis.

direction.

*Electron Beam Melting* 

material that is not fused and, if necessary, to be given further post-processing. The most common post-processing techniques are sanding, polishing, homogenization, thermal treatment, etc.

Fig. 3. Support structure for downfacing surfaces

It is important to highlight that, in general, material that is not processed (be it powder or liquid) is not capable of supporting fused material. Hence, if a part has overhanging zones, they may need to be supported by a *support structure* (Figure 3). Additionally, the support structure acts as a conductor of excess heat created in the process of selective melting. Finally, in some technologies, the support structure prevents warping of the part due to the thermal stress created during the process. Hence, the first step of pre-processing actually consists in generating an efficient support structure, which is then sliced and fabricated together with the part. The support structure is eliminated in the post-processing.

The most important advantages of Additive Manufacturing are:


6. *Fabrication of free form enclosed structures.* Additive technologies are capable of fabricating free form channels as well as different forms of latticed structures.

However, from the point of view of their application in biomedical field, additive technologies are facing some challenges:

