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Computer-aided design or more commonly knon as CAD uses computer systems in assisting creation and modification of design. CAD is also used in analysis and optimization of design. The use of CAD software increases the productivity if the designer and at the same time improve the quality of the design. In addition, the use of CAD software in design has the ability to improve communications through standardized documentation where a database for manufacturing can be created. The output of computer-aided design is mainly in the form of electronic files. These electronic files are used for printing or machining operations. CAD may be used to design curves and figures in two-dimensional (2D) space; or curves, surfaces and solids in three-dimensional (3D) space (Farin, 2002). CAD is an important tool in many applications such as automotive, aerospace industries, architectural and many more. The use of CAD is more obvious to the public in its use in producing computer animation which is use in movies, advertisements and many more. Due to its importance, CAD has been major driving force for research in computational geometry, computer graphics and discrete differential geometry (Pottmann, 2007).
CAD, nowadays, is also used together with other tools such as finite element analysis (FEA), computer numerical control (CNC) machines, product data management (PDM) and more. In the preparation of Environmental Impact Reports, CADs of intended buildings are often inserted into photographs of existing environments for simulation and analysis purposes. For engineers, the CAD's construction history can be reviewed on the single area rather than the whole model and CAD's features allow measurement of stress, strain and many other parameters.
Traditionally, computer-aided design assisted engineers in design and modeling for analysis and manufacturing purposes. The advancement in information technology and biomedicine has extended the uses of CAD. One in many new applications is in tissue engineering, where important information of tissue properties can be modeled and analyzed using Bio-CAD modeling. In Bio-CAD modeling, images are usually obtained from computed tomography (CT), magnetic resonance imaging (MRI) and more. Computer models of human joints for stress analysis, dynamic force analysis and simulation can be produced using these images with CAD software. Computer-Aided Tissue Engineering (CATE) embraces 3 major applications in tissue engineering: (1) computer-aided tissue modeling; (2) computer-aided tissue scaffold informatics and biomimetic design; and (3) bio-manufacturing for tissue and organ regeneration. An overview of CATE is outline in Figure 1 (Sun, 2005).
Figure 1. Overview of computer-aided tissue engineering (Sun, 2005).
IMAGE BASED ON BIO-CAD MODELING TECHNIQUE
Bio-CAD modeling involves these steps: (1) non-invasive image acquisition; (2) imaging process and 3-D reconstruction and (3) construction of CAD-based model.
Non-invasive imaging data acquisition
The primary methods of obtaining imaging data are CT, MRI and optical microscopy. By using CT, the sample is exposed to a small quantity of ionizing radiation. The absorption of this radiation is then detected and imaged. Stacking the 2-D images which display density map of the sample creates 3-D representation of the sample. The use of CT has an advantage of reasonably high resolution. MRI provides images for soft tissues as well as for hard tissues and as such is vastly superior in differentiating soft tissue types and recognizing border regions of tissues of similar density (Sun, 2005). On the other hand, optical microscopy has limited applications in this case as it requires an intensive data manipulation. One may have to stack thousands of 2-D images to for a 3-D representation. CT is at disadvantage compared to MRI and optical microscopy in differentiating soft tissues of similar density. MRI has high tissue differentiation capacity, but its resolution is no better than CT and optical microscopy. Also, MRI has been great use in assembling anatomic atlases of increasingly fine resolution as the technology matures, and find more clinical applications because it does not expose patient to ionizing radiation (Sun, 2005). A more appropriate way for a more precise 3-D reconstruction is by combining modality to obtain images.
Reconstruction for 3-D image representation
By using 2-D segmentation and 3-D region growth, the images are integrated. The 3-D images will then to generate anatomic modeling. The anatomic modeling generated is then used for contour based generation and 3D shaded surface extraction. These will then create the CAD-based medical modeling. Prototype modeling is done through additive/constructive processes as opposed to subtractive processes (Sun, 2005). It starts with model slicing, then model processing which will lead to model assisted application. Segmentation or volumetric representation is usually used to represent 3-D anatomical image. 2-D segmentation is extraction of the geometry of the scan data set. Each slice is processed independently leading to the detection of the inner and outer contours of the living tissue (Sun, 2005). By using skinning operations, these contours are stacked in 3-D to make a solid model. 3-D segmentation of the CT data set are able to identify, within the CT data set, voxels bounding the bone and extract a 'tiled surface' from them(Sun, 2005). By manipulating CT scans or MR images, a more constructive way of viewing 3-D anatomy of patient is provided.
Construction of CAD based biomodelling
There are 3 different process paths to produce a CAD model from imaging data: (1) MedCAD, (2) reverse engineering, and (3) STL-triangulated model.
This interface is usually used to fill the gap between CAD design software and medical imaging. Using this software, data can be imported and exported between CAD platform and imaging tools through International Graphics Exchange Standard (IGES), STL format or Standard for Exchange of Product (STEP). The limitation of this interface is where it is unable to capture detail and complex tissue anatomical features, especially those with complex geometry.
Reverse engineering interface
The example of reverse engineering software is Geomagic Studios. 3-D voxel model generated by segmentation in converted into data points and then imported into the software. With these points, a faceted model is then formed. Model formed using this interface has much more aesthetic value and stability in configuration. Also, the errors in data transfer formats are lesser.
STL-triangulated model converting approach
This method is more or less similar with the reverse engineering interface. However, it differs in its input. This method uses STL-triangulated surface instead of point clouds data.
APPLICATION TO BIOMIMETIC AND TISSUE SCAFFOLD DESIGN
3-D scaffolds play important roles as extra-cellular matrices onto which cells can attach, grow and form new tissues (Sun, 2005). In order to fabricate complex structural architectures, solid freedom fabrication (SFF), of which is an advanced manufacturing technique. The integration of biomimetic and non-biomimetic features is used to form high fidelity and smart scaffolds. Biomimetic features can be based upon real anatomical data regenerated from CT/MRI images, or can be created purely within a CAD environment such as channels and porous structures (Sun, 2005). Non-biomimetic features do not imitate nature but can be designed as drug storage chambers, mechanical elements, and attachment interfaces for tubes, sensors, electronics and other devices (Sun, 2005).
Biomimetic design for load bearing tissue scaffolds
The characteristics for load bearing tissue scaffolds to possess in order to function like a true substitute are (1) biological requirement, (2) mechanical requirement, and (3) anatomical requirement. The CATE based design is illustrated on the flow chart below:
Non invasive image acquisition
2D segmentation and 3D region growing of bone structure
Heterogeneous bone structural characterization using homogenization approach
Reverse engineering approaches to generate CAD model
Design of unit cells and its mechanical property evaluation using FEA
Unit cells used to fill up the bone structure using heterogeneous Boolean operations
Fabrication of biomimetic structure supported by appropriate slicing and control algorithm
CAD in modeling and representing customized tissue scaffold primitives
To biomimic the natural morphologies of bone through the use of CT and μCT imagery, 3-D reconstruction and modeling techniques, one can further design customized feature primitives for specific tissue structures, morphologies and functional requirements (Sun, 2005). The process of image-based 3-D reconstruction from CT and MRI, reverse engineering to develop Non-unifom Rational B-Spline (NURBS) based bio-CAD model, and reasoning Boolean algebra for heterogeneous primitive operations defined in the CATE paradigm has laid a critically important foundation for integrating both biological tissue and non-biological artificial elements, such as syringes, drugs, tubes, sensors, electronics, and nano- or micro-scale bio-devices for next generation 'smart' and 'functional' scaffolds (Sun, 2005).
CAD-BASED BIO-BLUEPRINT MODEL FOR 3D CELL AND ORGAN PRINTING
The most important requirement for 3-D cell and organ printing is its ability to continuously deliver scaffolding materials, nutrients, living cells, therapeutic drugs and growth factors and other components at the precise time, position, amount and environment for in vivo growth. The requirements for cell and organ printing are (1) blueprint model, (2) process model, (3) process machine and (4) tissue/organ culture system. The functions of the bio-blueprint model will:
Describe anatomy, geometry and internal architecture of a organ of interest;
Define a vascular network and the 3-D topology in a organ of interest;
Provide a needed database on organ /tissue geometry, heterogeneity and the associate vascular network that can be used for toolpath generation of 3-D cell and organ printing (Sun, 2005).
The bio-blueprint model is developed by firstly developing a computer modeling representation of a 3-D organ. It is followed by developing a 3-D vascularization network. Lastly, a CAD based organ bio-blueprint model is developed. The bio-blueprint model has many uses such as, providing biological data for organ anatomy. Also, it can be used to generate process planning for entire organ printing.
The use of bio-CAD had made modeling, design and fabrication of tissue scaffolds improved over the years. These models can be used by biologists and tissue engineers to communicate and thus advances the CATE technology.