## Materials and Methods  
### Design of Spherical Occlusion Device  
Catheres are thin hollow tubes that can be inserted in the body to treat diseases or perform surgical procedures. Our spherical occlusion device is designed to be crimped cylindrically inside a 1.6 mm catheter for delivery. Upon arriving at the lesion site, it is released from the catheter and expands from its original cylindrical shape to the final spherical diameter of 5.2 mm. To achieve such a dramatic shape and size transformation via self-expansion, nitinol alloy (NiTi) was chosen as the device's material due to its exceptional super-elasticity and shape-memory properties. Nitinol is the primary material for self-expanding stents and many other advanced medical devices today. With appropriate tooling and heat treatments, nitinol devices can be molded into various shapes and sizes depending on their final clinical applications [13; 14]. Nitinol has excellent biocompatibility due to the formation of a passive titanium-oxide (TiO\({}_{2}\)) layer on the surface to protect the device's body from corrosion.  
Stent design is an art of balance. A stent designer is required to strike the perfect balance among many conflicting design requirements. For example, interventional cardiologists in recent years have observed a new clinical issue called longitudinal stent compression (LSC), a failure mode not previously reported in coronary stents [15]. This new clinical issue emerges as a consequence of the bold attempts made by the industry to dramatically improve the stent deliverability in the human body. Our spherical occlusion device is another good example; the higher mesh density (for better occlusion) and smaller device profile (for easier delivery) have to be balanced carefully. In order to reach a compromise among conflicting design requirements, countless design iterations, along with fast prototyping, have become common practice in the industry before product launch.  
In this paper, a parametric design scheme, integrated with the developed FEA and CFD models, was implemented in our design process to reduce the design iteration time [16; 17]. It is a method of linking device dimensions to geometry in such a way that when the defined design parameters change, the design drawing changes as well. Important device geometries such as tubing diameter, strut geometry, and crown radius and number are parameterized by mathematical equations. Once the drawings are generated, computational analysis is used to screen out inadequate designs before lengthy prototyping and bench testing procedures begin. This integrated CAD (Computer Aided Design)/FEA/CFD scheme is able to cut the design iteration time significantly; used in the medical device industry, it can save precious time and reduce a single-product development cycle from months to weeks. Figure 1 shows the initial cylindrical configuration of the spherical occlusion device at 2.0 mm. Here, a 2-D drawing of the spherical occlusion device was first sketched and then wrapped onto a 3-D cylinder to create the 3-D cylindrical design pattern in SolidWorks (Dassault Systemes SolidWorks Corp., Waltham, MA, USA). This device has to withstand large deformation without fracture during the dramatic shape and size transformations from crimping to expansion and from cylinder to sphere. Computer simulations including FEA and CFD were implemented next to predict the device integrity and find the optimized design.