## 4 Discussion  
The spherical occlusion device is a new treatment option for liver cancers. We believe that such occlusion devices could also be extended to other types of cancers elsewhere in the body. The spherical shape was chosen over other geometries for better fit inside an artery. The typical shape of the commercial plug devices is usually double-cone (dog-bone), cylindrical, or umbrella which has a longer contacting zone with the artery. Since most of the arteries are tapered from the proximal to the distal end, this may create some size mismatch issue where the distal section of the artery is over-expanded by the plug yet the proximal section is mal-apposed (not fully contacted). Our proposed spherical occlusion device's contact zone with the artery is merely a circumferential ring, potentially minimizing the size mismatch issue. Nitinol alloy was chosen as our device's material due to its excellent super-elasticity and biocompatibility. Biocompatibility of an alloy is directly related to the corrosion behavior of the material in a specified solution and the tendency for the alloy to release its potential toxic ions. To quantify toxicity, inhibition of mitosis in human fibroblasts in tissue cultures exposed to test materials is an accepted screening method. Results showed that no significant toxicity effects were found in nitinol [28]. Literature reviews generally indicate that nitinol has extremely good biocompatibility due to the formation of a passive titanium-oxide layer (TiO\({}_{2}\)). This oxide layer not only increases the stability of the surface layers by protecting the bulk material from corrosion, but also creates a physical and chemical barrier against Ni oxidation and modifies the oxidation pathways of Ni [13]. However, nitinol has its limitations when applied as an implant in some cases. Peripheral stents made of nitinol suffer stent fracture issues which may in turn cause restenosis. [PERSON] et al. compared the rates of restenosis, and their results showed that restenosis remains a major problem even with current nitinol stent technology [29]. Another potential indication for the spherical occlusion device is the intracranial aneurysm. An intracranial aneurysm is a cerebrovascular disorder in which weakness in the cerebral arterial wall causes localized ballooning of the blood vessel. As the aneurysm grows, it puts pressure on surrounding brain tissues and may eventually rupture, leading to life-threatening hemorrhagic stroke. Our spherical occlusion device is able to fit the aneurysm geometry nicely and provide more gentle and uniform radial support to an intracranial aneurysm, thereby reducing the risk of rupture. In addition, it can be directly deployed inside the aneurysm and block the blood flow in a single step, which significantly shortens the surgery time. When the spherical occlusion device is used to treat intracranial aneurysm, the occlusion rate has to be as high as possible. As mentioned in this paper, the occlusion rate is mainly controlled by the device's metal density. However, since the device has to be crimped inside a catheter for delivery, the device's metal density has an upper limit, beyond which the device becomes too dense to pack. Figure 12: Digital particle image velocimetry (DPIV) image showing successful occlusion performance. To overcome this issue, we are currently pursuing several alternative ideas. For example, a covered stent made of a metallic frame covered by an extremely durable fabric, such as polyester, is likely to provide almost total occlusion. Similar effects could also be achieved via electro-spinning to coat a thin layer of film on our spherical occlusion device. Another novel concept is to place a smaller sphere inside a larger sphere to effectively increase the overall metal density without having to change the metal density of an individual sphere. These two spheres could be crimped inside a catheter end-to-end and delivered sequentially in a single deployment.