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Unit Block Library Of Basic Architectures For Use In Computer-Aided Tissue Engineering Of Bone Replacement Scaffolds, 6/2005

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Information about Unit Block Library Of Basic Architectures For Use In Computer-Aided...
Health & Medicine

Published on September 30, 2008

Author: organprinter

Source: slideshare.net

Description

Presentation given to the American Society of Mechanical Engineers, 6/2005

ABSTRACT
Guided tissue regeneration focuses on the implantation of a scaffold architecture, which acts as a conduit for stimulated tissue growth. Successful scaffolds must fulfill three basic requirements: provide architecture conducive to cell attachment, support adequate fluid perfusion, and provide mechanical stability during healing and degradation. The first two of these concerns have been addressed successfully with standard scaffold fabrication techniques. In instances where load bearing implants are required, such as in treatment of the spine and long bones, application of these normal design criteria is not always feasible. The scaffold may support tissue invasion and fluid perfusion but with insufficient mechanical stability, likely collapsing after implantation as a result of the contradictory nature of the design factors involved.
Addressing mechanical stability of a resorbable implant requires specific control over the scaffold design. With design and manufacturing advancements, such as rapid prototyping and other fabrication methods, research has shifted towards the optimization of scaffolds with both global mechanical properties matching native tissue, and micro-structural dimensions tailored to a site-specific defect.
While previous research has demonstrated the ability to create architectures of repetitious microstructures and characterize them, the ideal implant is one that would readily be assembled in series or parallel, each location corresponding to specific mechanical and perfusion properties. The goal of this study was to design a library of implantable micro-structures (unit blocks) which may be combined piecewise, and seamlessly integrated, according to their mechanical function. Once a library of micro-structures is created, a material may be selected through interpolation to obtain the desired mechanical properties and porosity.
Our study incorporated a linear, isotropic, finite element analysis on a series of various micro-structures to determine their material properties over a wide range of porosities. Furthermore, an analysis of the stress profile throughout the unit blocks was conducted to investigate the effect of the spatial distribution of the building material. Computer Aided Design (CAD) and Finite Element Analysis (FEA) hybridized with manufacturing techniques such as Solid Freeform Fabrication (SFF), is hypothesized to allow for virtual design, characterization, and production of scaffolds optimized for tissue replacement.
This procedure will allow a tissue engineering approach to focus solely on the role of architectural selection by combining symmetric scaffold micro-structures in an anti-symmetric or anisotropic manner as needed. The methodology is discussed in the sphere of bone regeneration, and examples of cataloged shapes are presented. Similar principles may apply for other organs as well.
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Unit Block Library Of Basic Architectures For Use In Computer-Aided Tissue Engineering Of Bone Replacement Scaffolds 1 Wettergreen MA, 1 Bucklen BS, 2 Starly B, 3 Yuksel E, 2 Sun W, 1 Liebschner MAK 1 Department of Bioengineering, Rice University, Houston, TX 2 Department of Mechanical Engineering, Drexel University, Philadelphia, PA 3 Department of Plastic Surgery, Baylor University, Houston, TX

Overview Motivation / Design Goal Biomimetic design theory Generation of unit block library Characterization of library

Motivation / Design Goal

Biomimetic design theory

Generation of unit block library

Characterization of library

Motivation / Design Goal Successful scaffolds must fulfill three basic requirements: Provide mechanical stability during healing and degradation Support adequate fluid perfusion and mass transfer Provide architecture conducive to cell attachment Range of porosities required to fully address scaffold demands Addressing mechanical stability requires architectural control Design goal: Design a library of implantable micro-structures (unit blocks) which may be combined piecewise, and seamlessly integrated, according to their mechanical function.

Successful scaffolds must fulfill three basic requirements:

Provide mechanical stability during healing and degradation

Support adequate fluid perfusion and mass transfer

Provide architecture conducive to cell attachment

Range of porosities required to fully address scaffold demands

Addressing mechanical stability requires architectural control

Design goal: Design a library of implantable micro-structures (unit blocks) which may be combined piecewise, and seamlessly integrated, according to their mechanical function.

Methodology of assigning primitive properties to tissue regions Bone contains complex geometry with mechanical properties that vary spatially and anatomically two continuous phases (bone matrix and interstitial fluid) are responsible for the global mechanical properties Interior properties of bone (or other) can be determined through imaging modalities Computer aided tissue engineering for a defect site utilizes this theory Complete load transfer utilizing an engineered scaffold to mimic the variants with respect to direction. Biomimetic Design Theory

Methodology of assigning primitive properties to tissue regions

Bone contains complex geometry with mechanical properties that vary spatially and anatomically

two continuous phases (bone matrix and interstitial fluid) are responsible for the global mechanical properties

Interior properties of bone (or other) can be determined through imaging modalities

Computer aided tissue engineering for a defect site utilizes this theory

Complete load transfer utilizing an engineered scaffold to mimic the variants with respect to direction.

Generation of Unit Block Library Unit block architectures generated via CAD Solid space filling architectures Wireframe polyhedral approximations Generated within same bounding box, orthotropic, scaled to same porosity Common interface (torus) placed on exterior of every architecture

Unit block architectures generated via CAD

Solid space filling architectures

Wireframe polyhedral approximations

Generated within same bounding box, orthotropic, scaled to same porosity

Common interface (torus) placed on exterior of every architecture

Final Library 12 shapes, 80 % porosity 6 polyhedral derived, 6 space filling 8

12 shapes, 80 % porosity

6 polyhedral derived, 6 space filling

Library Characterization Simulated linear uni-axial displacement to 1.0% strain in confined and unconfined compression Isotropic material properties, E = 2000 GPa, v = 0.3 Convergence study to determine proper seeding density C

Simulated linear uni-axial displacement to 1.0% strain in confined and unconfined compression

Isotropic material properties, E = 2000 GPa, v = 0.3

Convergence study to determine proper seeding density

Analysis of Stress Profiles (1) Elemental principle stress distribution evaluated for all architectures Dissimilar architectures may have similar profiles

Elemental principle stress distribution evaluated for all architectures

Dissimilar architectures may have similar profiles

Analysis of Stress Profiles (2) Decreasing porosity shifts loading profile away from tensile to compressive Decreasing porosity results in higher compressive stress values

Decreasing porosity shifts loading profile away from tensile to compressive

Decreasing porosity results in higher compressive stress values

Analysis of Stress Profiles (3) Similar architectures may have completely dissimilar stress profiles Specific architectural elements may be highlighted through viewing profiles

Similar architectures may have completely dissimilar stress profiles

Specific architectural elements may be highlighted through viewing profiles

Conclusions Highlighted the creation of a unit library of architectures that can be used to assemble a complex scaffold of individual characterized microstructures Allows for tailoring of mechanical stability and connectivity Regularity of architectures promotes mass transfer Mechanical properties vary by a magnitude as a function of architecture Inclusion of common interface promotes union between mechanically dissimilar architectures Stress profiles highlight importance of specific architectural features on modulus Library may be assembled into global scaffold using defect information as input

Highlighted the creation of a unit library of architectures that can be used to assemble a complex scaffold of individual characterized microstructures

Allows for tailoring of mechanical stability and connectivity

Regularity of architectures promotes mass transfer

Mechanical properties vary by a magnitude as a function of architecture

Inclusion of common interface promotes union between mechanically dissimilar architectures

Stress profiles highlight importance of specific architectural features on modulus

Library may be assembled into global scaffold using defect information as input

Future Directions Explore biological relations between implanted regular architecture and tissue repair Create biomaterial implant using library as design input

Explore biological relations between implanted regular architecture and tissue repair

Create biomaterial implant using library as design input

Acknowledgements NSF-ITR as funding source

NSF-ITR as funding source

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