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CAD based approach for patient specific scaffold for bone tissue engineering.

Introduction

Available solutions for diseased tissue or organ are medical implants and organ transplantation (1). Conversely harms with these are chronic irritation, infection and dislocation from the site which forces us to find new alternatives. Tissue engineering a rising field involves production of new tissues by using the methodology of cell culture and engineering. This technology consists of differentiation of (MSC) mesenchymal stem cell which has capacity to expand when subjected to in-vitro conditions (2). Cell lacks the ability to grow in the preferred direction and rather grow in arbitrary manner. To direct this growth of cells a special medium called as scaffold is essential (3).

Cell propagation on scaffold depends on parameters like its porosity, surface area to volume ratio, cross sectional area, permeability and interconnected porosity. An ideal scaffold should have interconnected pores with minimum pore size in the range of 100 to 350 [micro]m for good release of oxygen and nutrient to facilitate bone cell growth (4). Distribution of cells after perfusion seeding in a tissue engineered scaffolds is influenced by its pore architecture. In a study carried out by Ferry P.W. Melchels in which two scaffold types build by steoreolithography an additive manufacturing technology, one with homogeneous pore size (412 [+ or -] 13 [micro]m) and porosity (62 [+ or -] 1%) and other with pore size (250-500 [micro]m) and porosity of (35%-85%) were modeled. Computational fluid dynamics results showed that there is uniform flow of fluid velocities and wall shear rates of (15-24 [s.sup.-1]) in homogeneous architecture where as for varying pore architecture the fluid velocities and wall shear rates were in a range of (12-38 [s.sup.-1]). Higher cell densities were seen in scaffold with larger pores in gradient architecture since with larger pore size more number of cells are passing through it in a unit time as compared to smaller pores (5).

Traditional approaches for scaffold fabrication involves methods like solvent casting, particulate leaching, gas foaming, fiber meshes/fiber bonding, phase separation, melt moulding, emulsion freeze drying and membrane lamination (6). But the above methods do not ensure internal structure that ensures vascularization and tissue establishment (7). Such intricate structure can be successfully fabricated by (AM) additive manufacturing techniques. Some major techniques involve (3DP) Three Dimensional Printing, (SLA) Stereo lithography, (FDM) Fused Deposition Modeling, (SLS) Selective Laser Sintering, 3D Plotter and Phase-change Jet Printing that let fabricate difficult shape directly from CAD data in .STL (Standard Triangulation Language) file format (8).

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The main aim of the paper is to develop a generalized approach for the direct fabrication of patient specific scaffold with varying porosities for cortical and cancellous bone to enhance its cell fate processes using distinct feature of (CAD & CAM) computer aided design and computer aided manufacturing, imaging technology and (RP) rapid prototyping technology for osseous tissue formation. Scaffold fabricated from FDM technology using PLA a biodegradable polymer for a hypothetical case of tibia bone is discussed.

Materials and Methods

Step 1: Data Acquisition.

CT (Computed tomography) scan at 00 gantry tilt of a 48 year old lady was acquired using GE Prospeed XS Advantage CT scanner to get the external geometry of ankle joint. 128 slices with each slice thickness of 2 mm is used to get the 3D data set. The data obtained was in DICOM format.

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Step 2: Generation of 3D model.

The CT data was then loaded to MIMICS (Materialise, Belgium) software to get the 3D volumetric model as illustrated in Fig.1. Segmentation and editing tools were used to select the region of interest of the tibia bone. Once the area of concern is separated, it is converted to .STL (stereo lithographic) file format.

Step 3: Scaffold design and fabrication

The STL file is then imported to 3-matic 8.0 (Materialise, Belgium) to further visualize in 3D. The cortical part of the bone is obtained by using hollow command and the shell thickness was set to 2 mm for the tibia bone. After getting the cortical bone geometry the cancellous part was modeled by subtracting the tibia model with the shell model to obtain cancellous bone geometry as shown in Fig. 2 & 3.

Step 4: Mechanical Testing

Unit cubic elements of size 20 mm x 20 mm x 20 mm with circular holes having porosity level of 0, 40, 50, 60, 65 and 80% are modeled in Creo Parametric 2.0, USA as illustrated in Fig.4. The models are further saved in .STL file format and are exported to FDM machine (Wanhao, Duplicator 4X--Dual Extrusion 3D Printer). The models were printed layer by layer using PLA material with nozzle diameter of 0.4mm and layer thickness of 0.1mm.

Compression tests were performed on all the six samples (Fig. 5, 6) with loading frame capacity of 50 kN at an axial speed of 0.6 mm/min without preloading. Each division of the dial gauge indicator was of 75.5 N for which time is recorded and stress vs. strain graph is plotted. The compressive modulus is calculated by finding the slope of initial straight line of the graph.

To validate the results the 3D models were again saved in STEP file format and are exported to Ansys Workbench 14.0 for further analysis (Fig.7). The material used is isotropic PLA and hence Elastic modulus of 3 GPa and a poisson's ratio of 0.3 are assigned (11,1). One face of the unit element is fixed while the other face is given a displacement of 0.001mm. The reaction force is calculated on the fixed face and effective compressive modulus is computed by using relation (1) (10).

E = Stress/Strain = ((Reactive force/projected area)/(0.001/20)) (1)

Results and Discussion

It was revealed while doing compression test on PLA material that it continues to bear the load even after getting deformed. Maximum compression strength was noticed when the first hair line crack appeared i.e. when the deformation increases suddenly and the stress value goes down. This is due to inherent ductile property of the material. By fitting a polynomial curve on the stress vs. strain plot the equation of the curve is obtained and is then differentiated to find the slope of the curve and compressive modulus is computed. Elastic modulus goes on decreasing with the increase in the porosity of the samples, since for the same displacement of 0.001 mm, reactive force goes on reducing and so the elastic modulus as shown in (Table 1) (12). Minimum porosity of scaffold for bone cell growth should be 60% with interconnected pore geometry and sufficient mechanical strength for load bearing sites (9,21). Cortical bone is dense and covers 80% of the total mass of the bone. The average value of elastic modulus for human cortical bone is in the range of 0.909 to 1.2 GPa (12) where as cancellous part covers 20% of mass and have a porosity level of 80% with modulus in the range of 88 to 120 MPa (21,26,27,29,31,32,36). PLA unit element with 65% and 80% porosity best fits the condition for cortical bone and cancellous bone scaffold fabrication (10).

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PLA unit element with 65% porosity is then selected as a pore making element for the cortical part of scaffold. The unit element is scaled down to get unit cubic element of (2 x 2 x 2 mm) volume. The scaled model is converted to scaffold model by using the pattern operation. The cortical part is obtained by performing the Boolean intersection operation between cortical model and scaffold model. On the similar lines the cancellous part is acquired first by making the cubical scaffold structure with 80% porous element, and then Boolean intersection is performed between cancellous model and scaffold structure (10). The complete scaffold structure for tibia bone with different porosity level for cortical and cancellous bone is achieve by performing Boolean union operation as shown in Fig (8).

Conclusion

The paper presents rapid prototyping assisted CAD based approach for scaffold fabrication with varying porosity levels using PLA a biodegradable polymer. The study concludes that use of CAD-CAM, RP and imaging techniques in scaffold manufacturing offers more patient specific designed scaffolds with improved strength unlike traditional scaffold fabrication techniques. Patient specific scaffold design having mechanical properties like compressive young modulus, close to that of human cortical bone is achieved. Simulated and experimental results were found to be very close to each other in respect of compression modulus of PLA material as illustrated in Fig: 9. Scaffold for Cortical part is designed at 65% porosity for good mechanical strength whereas cancellous part is kept at 80% porosity for better cell proliferation (19).

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References

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P. S. Sapkal (a), A. M. Kuthe (b)

Department of Mechanical Engineering, V isvesvaraya National Institute of Technology, Nagpur 440 010, Maharashtra

Received 1 January 2016; Accepted 1 January 2016; Published online 1 January 2016

(#) Corespondence: (a) pranav_sapkal@rediffmail.com, (b) amkme2002@yahoo.com
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Title Annotation:Original Article
Author:Sapkal, P.S.; Kuthe, A.M.
Publication:Trends in Biomaterials and Artificial Organs
Article Type:Report
Date:Oct 1, 2015
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