**3.4 Implant post-processing**

After the fabrication has finished, the building platform with implants is taken out of the powder bed (Figure 13). Major part of the unfused powder is directly poured into a sifting system and filtered for reuse. However, in the case of EBM, the working temperature of the chamber reaches high value (650ºC for Ti64). Hence, the unfused powder is semi-sintered which is why, in order to clean the implants thoroughly, they are transferred to *Powder* 

Fig. 13. Implant post-processing. From left to right: hip stem still involved by the powder bed; powder recovery system; knee implant as taken out from the machine and after support removal and hand polishing.

Additive Manufacturing Solutions for Improved Medical Implants 159

porous Ti64 fabricated on EBM are comparable to commercially available materials (titanium foam, tantalum, etc.), even approaching to human bone properties as shown in

> R = 0.1 N ≥ 5.000.000 cycles

**Porosity [wt %]** 57.5 62.5 2 **Compressive modulus [MPa]** 2927 3000 3 **Compressive strength [MPa]** 195 65 3 **Flexural strength [MPa]** 101.98 105 2 **Tensile strength [MPa]** ~78 4 70 2

Table 2. Comparative view of porous Ti64 properties: Electron Beam Melting (EBM) vs

Property H/V **CoCr [EBM] ASTM F75-07** 

**Tensile strength [MPa]** 1171/1188 450 **Yield strength [MPa]** 776/769 655 **Elongation [%]** 5/7 8 **Area reduction [%]** 6/8 8

**[x10-6 1/ºC]** 14-18/13-17 - **Wearing rate [x10-8 mg/cycle]** 3.3/3.44 - Table 3. Comparative view of CoCr F75 properties: Electron Beam Melting (EBM) vs

4 Estimated value upon the results of samples with smaller and bigger pore size

In addition to good mechanical behaviour, material produced by EBM has good biological response as well. Thomsen et al (Thomsen, 2009) have performed a study of surface characterization and early response of porous EBM material in rabbits. According to this

Cobalt Chromium is commonly used in fabrication of implants that are submitted to intensive wearing (knees, shoulders, elbows, etc.). Hence, it is very important for the material processed on AM to show good wearing resistance. For EBM CoCr that corresponds to ASTM F75 is commercially available. Experiments and tests have been made with this material (Petrovic, 2010) and confirm that the main mechanical properties comply

**Alternative foams** 


**(ASTM, 2010)** 

**Property Porous Ti [EBM]** -

**Fatigue properties** Fm = 3820 N

commercially available materials

with the corresponding norm (Table 3).

**Thermal expansion coeficient** 

**5. Biological testing and validation** 

1 Data for EBM porous Ti64 with pore size of 504 μm 2 Data for Ti foam

corresponding norm.

3 Data for tantalum foam

Table 2. 1 <sup>2</sup> <sup>3</sup> <sup>4</sup>

**4.2 CoCr alloy** 

*Recovery System (PRS)*. The rest of powder is wiped away by a jet of compressed air charged with particles of the same material that the implants are made of (so as to prevent implant contamination). After cleaning the implant, the support structure is removed by hand (it is designed to have small contact surface with implant and be easy to eliminate). Also, if necessary, additional machining and/or polishing of certain surface or zone is performed.

As can be seen from this analysis of the supply chain, main advantages of additive technologies are:

	- much bigger geometrical freedom;
	- recycling of major part of material (up to 98%);
	- avoid human errors since the batch of models is stored in electronic way and prepared for "load & play";
	- make hundreds of implants in a week time with very optimized price, depending on size, since the same batch can be built again without additional preparation.
