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Geometric Characterization Of Scaffold Building Blocks For Tissue Engineering, Orthopaedic Research Society, 10/2005

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Information about Geometric Characterization Of Scaffold Building Blocks For Tissue...
Health & Medicine

Published on September 30, 2008

Author: organprinter

Source: slideshare.net

Description

Poster presented to the Orthopaedic Research Society, 10/2005

ABSTRACT
GEOMETRIC CHARACTERIZATION OF SCAFFOLD BUILDING BLOCKS FOR TISSUE ENGINEERING

+Wettergreen, MA; Sun, W; Mikos, AG; Liebschner, MAK
Rice University, Houston, TX
Mattheww@rice.edu


INTRODUCTION
Minimization schema in nature affects the arrangement of material in all objects regardless of the scale of magnitude. Be it minimal energy expenditure (soap bubbles) or structural integrity (honeycombs, bone), these rules give rise to highly ordered systems with defined, repeated architecture. The field of cellular solids has focused heavily on the elastic and plastic properties of two dimensional architectures, such as the honeycomb. Few studies, however, have investigated the effects of these minimization schemes in the formation of complex, three- dimensional architectures, such as bone. Even fewer studies have attempted to characterize the architectural properties of these shapes for the determination of the effect of material arrangement on structural integrity.
The goal of this study was to characterize the difference in mechanical properties of a group of architectures as the result of material arrangement between dissimilar shapes. Through geometric characterization of regular polyhedra, we determined quantitative differences in grossly different architectures. This research presents the first step towards developing rules which govern the effect material arrangements exert upon the specific properties of a material. This research is immediately applicable for the design of tissue engineering scaffolds which require either a specific loading conditions or are needed in load bearing applications.
METHODS
The polyhedra subset was chosen from the Platonic and Archimedean solids, which are the simplest three dimensional shapes in nature exhibiting symmetry. Four shapes were chosen for this characterization schema, displayed here in their single wireframe approximations for illustrative purposes. A hexahedron (Fig 1A) and a truncated hexahedron (Fig 1B) were chosen to represent the simplest approximations of beam structures. A rhombitruncated cuboctahedron (Fig 1C) and a truncated octahedron (Fig 1D) were chosen as more complex shapes to compare to the hexahedron and octahedron. Additionally, the truncated hexahedron and the truncated octahedron contain the same number of struts and vertices, with varied material arrangement. Table 1 illustrates the differences in the polyhedra in terms of struts and vertices.
The chosen polyhedra were generated as wireframe approximations using computer aided design (CAD) (Fig 1). Each polyhedra was created in four volumetric porosities (50, 60, 70, and 80) and a constant material envelope (bounding box), allowing for a direct comparison of only the specific arrangement of material in each shape in comparison with each other. Geometric characterization of the architectures was completed following the generation of the architectures. Surface area, strut length, strut diameter, and porosity were used to quantitatively compare the architectures.
The shapes were then evaluated with a prescribed displacement finite element simulation for cases of both confined and unconfined compression. Stiffness and elastic modulus were calculated at each porosity. Material property definitions used were the same for each shape, further aiding in the calculation of effects due solely to arrangement. Elastic modulus was averaged from the confined and unconfined compression results. Elemental principle stress distribution was evaluated for each polyhedra as a method to characterize the loading on each architecture as a result of the spatial arrangement of the building
material. Von Mises stress was also calculated as a method to localize any stress concentrations and stress-free elements.
RESULTS
For all polyhedra, strut length and strut diameter were linearly related, but only between each polyhedra. Additionally, the sur
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INTRODUCTION The major determinant of mechanical properties of a structure is the apparent density, ρ. Density cannot describe architectures with directionally dependent properties. Previous studies have demonstrated a three-fold difference in the apparent property variation as a result of geometric arrangement (1,2). A systematic exploration into the effects of geometric arrangement on apparent properties utilizing the Platonic and Archimedean polyhedra is presented. CONCLUSIONS AND DISCUSSION Morphological differences were the most dramatic for the hexahedron Modulus of solids illustrates power law relationship discussed in previous studies (1,4) The range of Moduli between architectures was 25% and 50% of total value at 50 and 80% porosity, respectively Modulus values for the hexahedron agrees with previous studies of cellular solids (4) The stress distribution profile shifts with porosity for each architecture Significantly different internal stress profiles are observed for each architecture regardless of similar material property assignment Variation of microarchitecture and porosity may be used to tailor the microenvironment of scaffold building blocks for tissue engineering REFERENCES Gibson, LJ; Ashby, MF. Cellular Solids. 1988, Pergamon Press, New York. Wettergreen, M et al. Creation of a Unit Block Library of Architectures for Use in Assembled Scaffold Engineering, In Press , Computer Aided Design, 2005. Mattheck, C. Mat.-wiss. U. Werkstofftech, 1990. 21: 143-168. Woesz, A et al. Cellular Solids Beyond The Apparent Density – An Experimental Assessment of Mechanical Properties. Adv Eng Mat, 2004. 6(3): p. 134-138. GEOMETRIC CHARACTERIZATION OF SCAFFOLD BUILDING BLOCKS FOR TISSUE ENGINEERING *Wettergreen, M A; **Sun, W; *Mikos, A G; *Liebschner, M A K *Department of Bioengineering, Rice University, Houston, TX **Drexel University, Philadelphia, PA STRESS DISTRIBUTION SELECTION OF POLYHEDRA Figure 1. Left to right: Hexahedron (H), Truncated Hexahedron (TH), Rhombitruncated Cuboctahedron, Truncated Octahedron (TO). Meshed using ABAQUS/CAE (Abaqus, Inc., Pawtuckett, RI) based upon convergence study (Figure 2) Single step, linear elastic finite element analysis performed using ABAQUS Standard Boundary conditions were unconfined uniaxial compression with a prescribed displacement of 1% between virtual platens All elements were assigned isotropic material properties with a Young’s modulus (E) of 2GPa and a Poisson’s Ratio (v) of 0.3 Apparent (structural) Modulus was calculated in each case MORPHOLOGICAL CHARACTERIZATION Figure 2. Finite element analysis procedure for architectures. Single architectures were created using computer aided design with the same volumetric bounding box and ranged porosity (50-80%). Virtual platens were created on the top and bottom faces of the architectures as boundary conditions for finite element analysis to simulate unconfined compression. x y z Figure 3. Convergence study to determine adequate mesh density vs. computational expenditure. Figure 6. Modulus vs. Porosity for evaluated architectures. Modulus was normalized to a solid block of material, highlighting the sole effect of architecture on apparent properties. The hexahedron demonstrated the highest modulus at all porosities. The Rhombitruncated Cuboctahedron has twice the struts of the Truncated Hexahedron but both architectures exhibit similar architectural properties through the porosities. The Truncated Octahedron is the weakest architecture at 50% porosity but eclipses the TH and RC at porosity values higher than 60%. Figure 4. Strut length vs. strut diameter for varying architectures with increasing porosity. The TO demonstrates the largest range of strut diameters across porosities. This is as a result of the obscuring of beams at lower porosities in the TO. The Hexahedron has the highest volume per strut based on low beam number. Figure 5. Surface area vs. porosity for the architectures. The surface area is normalized to the surface area of the bounding box at zero percent porosity. The simplified shapes (H & TH) demonstrate a decrease in surface area over the reduction in porosity. Shapes which are more complex or poorly utilize the strut arrangement of the polyhedra (TO specifically, but also RC) exhibit a peak where the surface area is maximized for the volumetric porosity. Figure 7. Elemental stress distribution for the hexahedron with increasing porosity. At 50% porosity, a near equal percentage of elements are loaded in between 0 and -10. With increasing porosity, a peak develops near 0 and below -10. Also with increasing porosity, elements loaded in compression shift to tension (bending). Tension Compression Figure 8. Elemental stress distribution for varied architectures at 80 percent porosity. All architectures have a similar loading profile in tension. The TH has similar profiles in tension and compression. The Hexahedron exhibits the highest stress peak in compression. The RC and TO have dual peaks in the range of zero to 10 MPa compression. Figure 9. Rapid prototyped approximation of human vertebral body matching the properties of microarchitectures (as selected from a unit block library) to calculated regional stiffness as derived from apparent density (QCT). 3 3 3 3 Connectivity Index 24 48 24 8 # Vertices 36 72 36 12 # Struts Truncated Octahedron Rhombitruncated Cubocatahedron Truncated Hexahedron Hexahedron

CONCLUSIONS AND DISCUSSION

Morphological differences were the most dramatic for the hexahedron

Modulus of solids illustrates power law relationship discussed in previous studies (1,4)

The range of Moduli between architectures was 25% and 50% of total value at 50 and 80% porosity, respectively

Modulus values for the hexahedron agrees with previous studies of cellular solids (4)

The stress distribution profile shifts with porosity for each architecture

Significantly different internal stress profiles are observed for each architecture regardless of similar material property assignment

Variation of microarchitecture and porosity may be used to tailor the microenvironment of scaffold building blocks for tissue engineering

REFERENCES

Gibson, LJ; Ashby, MF. Cellular Solids. 1988, Pergamon Press, New York.

Wettergreen, M et al. Creation of a Unit Block Library of Architectures for Use in Assembled Scaffold Engineering, In Press , Computer Aided Design, 2005.

Mattheck, C. Mat.-wiss. U. Werkstofftech, 1990. 21: 143-168.

Woesz, A et al. Cellular Solids Beyond The Apparent Density – An Experimental Assessment of Mechanical Properties. Adv Eng Mat, 2004. 6(3): p. 134-138.

Meshed using ABAQUS/CAE (Abaqus, Inc., Pawtuckett, RI) based upon convergence study (Figure 2)

Single step, linear elastic finite element analysis performed using ABAQUS Standard

Boundary conditions were unconfined uniaxial compression with a prescribed displacement of 1% between virtual platens

All elements were assigned isotropic material properties with a Young’s modulus (E) of 2GPa and a Poisson’s Ratio (v) of 0.3

Apparent (structural) Modulus was calculated in each case

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