Design Selection

 

When considering which design would best help us meet our goal of developing a working active needle for use in brachytherapy we weighed the following design factors: Cost, overall diameter, safety (encompassing durability, thermally safe, etc.), flexibility, ease of operation, repeatability, and environmentally friendly (during manufacturing, and disposal). Safety was given the highest weight, as is expected for surgical tools, followed by ease of use and flexibility. Factors that fall under safety include durability and tissue safety (from elevated temperatures and sharp geometries, although, because of the needle’s small scale, the latter is less of an issue). Unlike many design projects, cost was initially weighed relatively low as medical instruments usually purchased at high prices. Although, the lack of sufficient project funding may have an unfortunate impact on the materials and manufacturing processes available. That being said, we are proceeding with our initial evaluation of a low weight on cost. Other design factors considered, based on performance, were angle of deflection, bend smoothness, stability on entering tissue, and force required to bend the needle (more force potentially means more higher wire temperature).

 

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Prototype 1

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Based on these factors we initially selected design one (Figure 2 on Generating Design Concepts page) to prototype. This design is composed of few parts, lowering the risk of malfunction during use. It also would allow for stable entry into tissue, as there is no flexible joint. Additionally, this design allows the wires to be installed within the needle tubes, lowering the risk of tissue damage due to heat, and potentially erasing the need for a cooling system (further examination and testing will have to be done to verify this).

 

The design was modeled in SolidWorks and 3D printed. Upon assembly, it was discovered a few adjustments needed to be made to successfully test the prototype and so a second prototype was printed (see Figures 7 and 8 below).

 

 

 

 

 

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Figure 1. Prototype 1 SolidWorks model and schematic

 

 

Based on the schematic for prototype 1, the model uses the same ball and socket mechanism to allow for multiple degrees of freedom. In both of the tubes (tip and body), three hooks are placed inside to allow for the nitinol wires to be attached. The three nitinol wires allow for a larger degree of freedom when compared to having only one or two wires. In order to reset the wires, a ram rod is still incorporated into this model. With further research and experimentation, it would be more ideal to have the prototype revert without any external forces.

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Prototype 2

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The benefit of printing multiple design prototypes was later realized, as there is no cost to us and this increases the chances of success in meeting our goals.  Therefore, a second design (design six from the preliminary design concepts; Figure 7 on Generating Design Concepts page)  was selected to prototype. This design was chosen as it allows a seed to be passed through the needle and is expected to have high stability when inserting the needle into tissue. This is a factor that design three lacked.

 

 

 

 

 

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Figure 10. Prototype 2 SolidWorks model

 

 

Unlike prototype 1, prototype 2 incorporates multiple segments to increase the number of actuation joints in order to increase the amount of deflection. With the addition of multiple segments, this allows for a smoother curvature during actuation. This would potentially decreases the amount of discomfort the patient felt following surgery and would allow for smoother maneuvering of the needle inside the patient. It also increases the chances of passing a seed through the joint while it is bent. While prototype 2 has better maneuverability compared to prototype 1, this causes complications in the reset mechanism. This prototype relies on the spring stiffness and force provided by the other Nitinol wires to reset the needle. Testing will need to be conducted to evaluate whether this form of reset will be effective or not.