34th INTERNATIONAL CONFERENCE ON PRODUCTION ENGINEERING28. - 30. September 2011, Niš, Serbia

University of Niš, Faculty of Mechanical Engineering

APPROACHES TO AUTOMATED CREATION OF TISSUE ENGINEERING SCAFFOLDS

Milan TRIFUNOVIĆ, Jelena MILOVANOVIĆ, Miroslav TRAJANOVIĆ, Nikola KORUNOVIĆ,Miloš STOJKOVIĆ

Faculty of Mechanical Engineering, University of Niš, Aleksandra Medvedeva 14, Niš, Serbiamilant@masfak.ni.ac.rs, jeki@masfak.ni.ac.rs, traja@masfak.ni.ac.rs, nikola.korunovic@masfak.ni.ac.rs,

miloss@masfak.ni.ac.rs

Abstract: Tissue engineering (TE) is a relatively new field in biomedical engineering that is receiving much attention among the scientific community and the general public. One approach to promote the regeneration of new tissue involves using scaffolds as a template to guide the proliferation, growth and development of cells appropriately. For this purpose, the scaffolds internal structure plays an important role. The process of creating customized scaffolds includes defining the internal architecture suitable for the development of the desired tissue and creating external geometry to match the geometry of fragment being replaced. This paper analyzes the approaches which aim to simplify and automate the process of creating customized scaffolds.

Key words: tissue, engineering, scaffold

1. INTRODUCTION

Tissue engineering is an interdisciplinary field that applies the principles of engineering and the life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function [1]. One approach to tissue engineering involves the use of scaffold as a structure which allows cells attachment, proliferation, differentiation and migration. After seeding the cells into a scaffold they are cultured in vitro until the scaffold is ready to be implanted. Along with the process of tissue regeneration scaffold gradually degrades leaving newly formed tissue in place. Three primary requirements for scaffolds are [2]: (1) to define anatomic shape and volume, (2) to provide temporary mechanical support, and (3) to enhance tissue regeneration through delivery of cells, genes, and/or proteins.Initial research has shown that the internal microstructure of scaffolds in terms of shape, pore sizes and pore networks influences the proliferation, differentiation as well as the angiogenesis of the forming tissue [3]. Scaffolds made with conventional fabrication techniques have a number of shortcomings. They are primarily reflected in lack of mechanical strength, nonuniformity in terms of size and distribution of pores and arbitrary pore shape. This is because conventional fabrication techniques offer minimal control over the internal architecture of scaffold. With the advent of rapid prototyping technology it has become possible to create complex scaffolds and control of the internal architecture of the scaffolds. Fabrication of scaffolds using rapid prototyping technologies implies the existence of scaffold CAD model.Problem of creating customized scaffolds to meet patients requirements is very complex. On one hand it is necessary

to generate the appropriate internal geometry that will be suitable for the development of different types of tissues, while on the other hand it is necessary to ensure that the external geometry corresponds to the outer geometry of the organ or organ part that is to be replaced. There are several approaches to solving this problem which simplify and automate the process of creating scaffolds. These approaches offer to the user possibility to choose internal geometry that provides the desired scaffold properties and then perform automated scaffold creation. This paper provides an overview and comparison of existing approaches to automated scaffold creation. It also aims to define possibilities for their improvement.

2. ANALYSES OF APPROACHES

Naing and his collaborators developed system called Computer aided system for tissue scaffolds (CASTS) which provides the users with a database of designs to choose from, generates scaffolds with desired parameters and properties, and customises scaffolds according to patients requirements [4]. System operates on a graphics workstation which runs ProENGINEER. CASTS has three separate modules, namely, an input module, a designer’s toolbox and an output module.Input module is used to convert raw patient data obtained through magnetic resonance imaging (MRI) and computed tomography (CT) scan to surface files in IGES format. The conversion is carried out using image processing software MIMICS.Designers toolbar has several functions. Firstly the appropriate unit cell is selected depending on porosity, surface area to volume ratio and strength requirements from the parametric library developed by the creators of the system [5]. Parametric library contains 11 unit cells

(Fig.1.) – polyhedral shapes – that can be divided into two categories: cells that can fill space (without leaving gaps) and cells that can span space (leaving gaps). After selecting the unit cell designer can change its size to conform to pore size requirements and defines the overall dimensions of the scaffold using sizing routines. The next step is the automated generation of scaffold of given size using a specially developed algorithm [6]. Output values, such as pore sizes, surface area to volume ratio and porosity of the scaffold are calculated and displayed also.

Fig.1. The 11 unit cells

To ensure better ingrowth of cells the authors have came up with an approach where created scaffold is cut by using developed routines into layers that can be seeded separately and then assembled prior to implantation (Fig.2.).

Fig.2. (a) Rectangular scaffold block model (b) Sliced scaffold model with orientation features

Output module is used to create customized scaffold through applying Boolean operation on previously generated scaffold and solid model of a part of patients bone which was created based on input module model. The final output is scaffold structure in near net shape of the patients defect represented by model in STL format.For validation purposes authors generated and fabricated patient specific scaffold using the system. Octahedron-tetrahedron space filling system was used for scaffold generation (Fig.3.). The length of strut in each unit of the scaffold was set to 2.5mm which gave a porosity of 93 per cent with a macro pore size of 1.193mm. SLS fabricated femur scaffold with the scaffold model and the surface profile are shown in Fig.4.. Under the light microscope, the scaffold was found to exhibit regular predesigned microarchitecture, proving the effectiveness of the system.

Fig.3. Femur scaffold model (a) before and (b) after slicing

Fig.4. A femur segment (left) and the fabricated customised scaffold (right)

Another approach to automated creation of tissue engineering scaffolds is the result of the work of Ramin and Harris [7]. It involves the use of library of routines that interact with the CAD software and enable automated creation of geometric elements. The authors have chosen multi section solid (curved rod) as a geometric element because it is the most suitable form for the creation of trabecular bone (Fig.5.) like structure that was their area of interest. The resulting geometry is represented by a network of irregular and interconnected rods and corresponds to geometry of the regenerated tissue. Scaffold is obtained by representing the multi section solid in reverse form using the appropriate routines as a channel in the reference volume.

Fig.5. Example of an STL representation of human trabecular bone structure from a μCT scan of a femoral

neck

Multi section solid is defined with one or two cross sections while the path is defined by a straight or curved line in space (Fig.6.). Curved paths are defined by three control points. The authors have chosen pore size, pore shape, number and orientation of pores as design variables.

Fig.6. Example of a multi section solid

Pore size is controlled by planar curve that is defined by a number of control points (Fig.7.), while the shape of the pore is controlled by the values of the tangents at the control points. Authors used five control points to define a pore section, with the first and the fifth vertex coincide and each control point was positioned as a vertex of an ideal square. Pore interconnectivity and porosity are calculated as output from the system. Pore location was defined by the absolute coordinates of the centre of the enclosed square on the surface. All design parameters can be set as either constant or variable.

Fig.7. Definition of pore size

In order to prove validity of this approach five cubic scaffolds were created. They had interconnecting pore channels that range from 200 to 800 μm in diameter, and each was with an increasing complexity of the internal geometrical arrangement. One of them is presented in Fig.8.

Fig.8. Details of some curved paths and their random connections

Authors also presented a clinical case study where they integrated one of the geometries generated for cubic scaffolds with the cranoficial implant (Fig.9.) [8].

Fig.9. (up) Global network of rods enclosed by the surfaces surrounding the craniofacial implant (middle)

Simulated trabecular and compact bone structures (down) Scaffold geometry for the craniofacial implant

analysed

3. DISCUSSION

One of the advantages of the CASTS system compared to Ramins approach is that the size and shape of the pores are defined more precisely. CASTS system has also undergone validation in terms of fabricating customized scaffolds. In one case [9] scaffold fabricated using this system was used for culturing C2C12 myoblast cells in vitro for 21 days in order to determine its suitability for cardiac tissue engineering. High density of cells was recorded after 4 days of culture. Fusion and differentiation of C2C12 were observed as early as 6 days in vitro. A steady population of cells was then maintained throughout 21 days of culturing.Ramins approach provides creating irregularities in the scaffold structure which is a feature that is found in the structure of trabecular bone. The advantage of this approach compared to the CASTS system is that it does not imply the use of Boolean operations when creating customized scaffolds. The main reason is that the application of these operations makes the process of generating scaffold ineffective in cases when the geometries to be merged are very complex and irregular.

1 Name of the author, title, company, address and e-mail.2 Name of the author, title, company, address and e-mail.

Time savings when creating scaffolds using this approach ranges from 75 to 90% compared to the manual creation [7]. With this approach it is also possible to define that input variables are changed by a mathematical function in order to mimic the different gradients of density that exist in bone structure. On the other hand Ramins approach does not involve the analysis of mechanical properties of the scaffold, and the porosity is not defined as an input parameter but is calculated as output value after the creation of geometry.Scaffolds are aimed to temporarily replace and promote growth of biological structures which are usually inhomogeneous. Having this in mind it could be useful to further improve existing systems or develop a completely new one with the possibility to create heterogeneous scaffold both structurally and in terms of materials. Another direction in which the research in this field can be directed is attempt of further automation of the system that would ensure that it automatically selects unit cell and generates scaffold structure with appropriate parameters, all based on specifications defined by the tissue engineer.

4. CONCLUSION

Although different approaches were analysed, their main advantage is that they do not rely on the users skill for creation of customized scaffold. Automation of processes in these approaches is the key factor that ensures the design of a large number of geometric elements in a short time, while maintaining a high level of flexibility. These systems represent significant scientific contribution on which further investigations in this field can be based on.

ACKNOWLEDGEMENT

This paper reports on the research which is undertaken in the scope of project "Virtual human osteoarticular system and its application in preclinical and clinical practice" (project id III 41017) funded by the Ministry of Science and Technological Development of Republic of Serbia for the period 2011-2014.

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