Rapid prototyping technology used for manufacturing of an adapted medical implant prototype.

Druga, Corneliu ; Barbu, Daniela Mariana ; Serban, Ionel 等

1. INTRODUCTION

Rapid Prototyping (RP) is a relatively new technique that was invented over a decade ago to rapidly produce solid 3-D objects of complex shapes directly from CAD files (Surana, 1997). RP constructs solid physical models from 3D computer data by the addition of layers of material. These techniques provide ways for making a variety of complex shaped parts which are difficult, costly or sometimes impossible to make by conventional methods of material removal.

The selection of a particular process will depend on the medical model application. Some of the most commonly available systems are: Fused Deposition Modeling (FDM), Stereolithography, Selective Laser Sintering (SLS), Sanders Prototyping Technology and Z Corporation Fabrication Machine (Berce et al., 2000). All these systems employ the same basic five-step process. The steps are to:

* create a CAD model of the design,

* convert the CAD model to STL format,

* slice the STL file into thin cross-sectional layers,

* construct the model one layer atop another,

* clean and finish the model.

2. THE EXPERIMENTAL SYSTEM

2.1 Working principle of 3D Printing Elitte

Principle of operation of the 3D Printing Elitte is based on heating the material near its melting point and then deposit the molten material where needed, to build the desired model (Figure 1). The success of the process lies in the rigorous control of temperature at which the material is heated and maintained during the deposition (Steidle et al., 1999).

The material used may be a wire (filament) made of special wax, nylon, polyamide or plastic ABS Plus (Acrilonitril Butadian Styrene). ABS Plastic heating is performed at a temperature of 270[degrees] C, at which the material is found in semi-liquid state, it may continue to be extruded through a nozzle of small diameter (0.178 mm or 0.254 mm) and is deposited where the configuration part of the layer requires. Nozzle, through which plastic material is extruded, in a semi-liquid state, can be moved with the heating head, to which it is attached. This movement is in xOy plane, the movement is numerically controlled by computer. Track construction is mounted on a platform that moves vertically, along the axis Oz, the movement is also controlled by the command panel.

[FIGURE 1 OMITTED]

The software generates automatically the support for the physical model, without the need of the operator. Support is not part of the model, but it is necessary to support layers of material during manufacture where the model has a complex configuration and present interior goals. Material support is all ABS plastic, but the mechanical properties differ from those of the material work.

In this way a model can be obtained by material deposition, where the configuration requires it. Important is the materialization time from a virtual model, made in a CAD program, into a physical model which can be used further on for preparation of manufacturing or direct, as a functional piece, is very short in comparison with classical methods of manufacture. Dimensional accuracy of models produced by this process is about. 0.125 mm axes Ox, Oy, and Oz in a volume of size 205 x 205 x 305 mm.

3. EXPERIMENT'S PROCEDURE

In order to obtain anatomical model and osteosynthesis implant prototype there are three consecutive stages that need to be followed, i.e: preprocessing stage, processing stage and postprocessing stage.

3.1 Preprocessing stage

At this stage, the first phase consists in importing of STL models of fibula and osteosynthesis implant in the specialized program, Catalyst, (program which generates command code for 3D Printing machine Elitte). STL files of both models to be built have been generated in the SolidWorks 2007 software (Radu& Rosca, 2009). Thus, using this program, the models were divided into triangular facets resulting a number of facets of 21,920 to the osteosynthesis implant and 17,060 facets to the fibula (Figure 2. a, b), (Stanciulescu, 2003).

After the STL models are read, the next step is to shift the models on the working platform of the machine, so that the construction of models is optimal in terms of working time and consumption of materials. This targeting is done with specialized functions (rotation, translation) within the Catalyst software.

Guideline STL models is preceded by cutting with the horizontal plane, the operation result in sets of level curves (the so-called perimeter). Depending on the orientation of normal forces to STL model's surface, Catalyst generate routes for the extrusion head in order to create a section through the piece.

[FIGURE 2 OMITTED]

In case of fibula it's preferable a thickness of 0.2540 mm, due to model's size and geometry. The thickness of the osteosynthesis implant is of 0.1778 mm, due to the small size of the model and its complex geometrical configuration.

Generating the curves, is followed by a global analysis of virtual models, and for suspended parts, Catalyst generates supports.

3.2 Processing stage

In the construction stage it's made both physical model of fibula and osteosynthesis implant, layer by layer, as follows:

* Extrusion-heating head of the apparatus deposits a thin material wire along the curves that define the perimeter of the section;

* after perimeter's materialization takes place deposition of material in the full areas of model;

* support material is deposited through the second nozzle;

* after materialization of entire current section, platform descend with an equal distance to the step section of the virtual model;

* for the next section follows the same steps mentioned above.

Processing stage ends after constructing the last section of the virtual model of fibula, respectively osteosynthesis implant. Melting temperature of both materials, controlled by electronic command block of 3D Printing Elitte machine, is set between 265-270[degrees] C. Regarding machine room's temperature, it's set to the value of 70[degrees]C.

3.3 Postprocessing stage

Postprocesing stage of both obtained physical models consists in their separation from the working platform and support, by mechanical or chemical action. In the latter sense is to use the bathroom with substance pickle.

4. DISCUSSION AND CONCLUSION

Physical models obtained are shown in Figure 3.a, and b, in two phases: the final phase of the technological process of rapid prototyping, where it can be seen in both models the appearance of harsh surfaces.

Areas with harsh appearance are due to the scale effect produced by model's high inclination curvature (Figure 3.a., b). By applying the polishing process, a large proportion of asperity was removed.

To better highlight the osteosynthesis implant, it was painted, spray with a thin layer of chromium paint.

[FIGURE 3 OMITTED]

Major disadvantage of the models made by Rapid Prototyping, irrespective of the type of process used, it refers to area's roughness. This important parameter of the mechanical parts, produced using the FDM process, is directly influenced by several factors as: STL model accuracy, model targeting on the working platform, material's extrusion nozzle diameter, the step section of the virtual model, prototype's scale dimension, complexity geometric shape of the model and type of material used.

For most osteosynthesis implants, the roughness is a key factor in implant performance, but on the other hand, for the mechanical elements used in fasteners, classical postprocessing operations are required.

In conclusion, one can say that the technique of Rapid Prototyping, revolutionary in the technology field can be used with success in achieving any prosthetic element or anatomical model, out of different materials as plastics, metals, nemet.

Elements obtained using the FDM Rapid Prototyping, are part of the rigors imposed by the shape and size, achievement time (from CAD models to physical plastic models) and structure.

5. ACKNOWLEDGEMENTS

This research was made with the help of our collaborator, George Muntean, which is an orthopaedist doctor in the University County Hospital from Brasov. Also, the research represents part of PhD studies of Ciprian Radu and was supported by one grant from Romanian Educational Minister, C.N.C.S.I.S type, ID no. 147/2009.

6. REFERENCES

Berce, P. et al. (2000). Fabricarea Rapida a Prototipurilor, Tehnica, ISBN 973-31-1503-7, Bucuresti

Radu, C. & Rosca, I.C. (2009). Some contribution to the design of osteosysnthesis implant. Estonian Journal of Engineering, Vol. 15, No. 2, (March 2009) pp. (121-130), ISSN 1736-7522

Stanciulescu, V. (2003). Modelarea computerizata a structurii osoase faciale in vederea protezarii si interventiilor chirurgicale reparatorii, PhD thesis, Politechnical University of Timisoara, Timisoara

Surana, R. (1997). An Integrated Rapid Prototyping and Vacuum Casting System for Medical Applications, Master's Thesis, Thesis submitted to the Faculty of the Graduate School of the University of Maryland at College Park

Steidle, C. Et al. (1999). Automated Fabrication of Custom Bone Implants Using Rapid Prototyping, 44th International SAMPE Symposium and Exhibition, pp. 23-27, University of Daytona, May 1999, Rapid Prototyping Development Laboratory, Long Beach, CA