Technical Analysis

Material Requirement

Before starting the technical analysis, the first step is to find the proper material for the guidewire. After completing literature reviews, major material requirements were determined to be:

The maximum transition (critical) temperature of the selected polymer cannot be higher than 45 degrees Celsius.
It contains no toxic elements for human bodies.
Good thermal performance.
Good shape-memory effect.
Available to using the additive manufacturing process.

The team chose to use shape memory polymers (SMPs), which are also known as smart polymers. SMPs have the ability to generate shape transformation due to phase transitions in response to different stimuli. In the case of our project, the outside stimuli will be temperature and the way for the team to manipulate the degree of deformation of the guidewire will be controlling the temperature of the guidewire. The figure on the right side shows a brief diagram of the shape-memory effect mechanism when the temperature is the stimuli from the outside environment.

CAD Model

According to the actual guidewire in the market, the group searched online and decided to choose a diameter of 0.018 in (0.45mm). Also, after measuring the parameters of the passive guidewire obtained in the current market, the group then picked 70 mm as the length of the tip of guidewire, with a 20 mm polymer as the actuator. However, in order to print the guidewire by 3D printers, the team decided to scale up the guidewire to 6 times larger than the designed size. The final design of the guidewire is shown below.

In the first version (shown below) there were 4 main components: base (gray), tip (yellow), actuator (light blue), and wire (red). The mechanism of the design is described here: The tip is made of a relatively soft polymer with a bullet-shaped head, this characteristic could avoid unintentional scratches as the guidewire is being pushed forward. The base is made of a relatively hard polymer with low thermal expansion rate, to hold up other components and give support. The actuator is another polymer with high thermal expansion rate, such that when this part is being heated it would expand more than the base, to cause a shape deformation (bending). And finally the wire (though called wire, it’s still a conductive polymer) with windings over the actuator is to provide heat to make the actuator bend.

When there’s a situation that needs the wire to bend, operator needs to close the circuit. As electricity passes through the wire, heat is generated and absorbed by the actuator. The actuator then expand, but the base doesn’t. The difference in expansion rate then cause the guidewire to bend as expected. After the bending process, operator stops the current and heat is dissipated through blood flowing through the wire, and the whole wire restores its original shape. The red wire is attached to the surface of the guidewire. There’s also fin-shaped structures to help dissipate heat from the actuator.

After demonstrating the version 1 design, some professor queried if the attached wire and fin shapes would cause turbulence in blood. The group seriously considered that and redesigned the guidewire. In the second version (shown below) the wire is embedded into the wire and those fin shapes are removed to avoid turbulence. More winding wires are added to increase the contact area between wire and actuator. The group also reduced the length of the tip and the actuator, in order to improve the efficiency of bending movement. With shorter tip and actuator the guidewire could bend with a smaller radius and more easily maneuver through vessels.

During the process of trying to manufacture the alpha prototype, several critical problems were encountered. The team found that because the dimensions of version 2 were still too tiny (thinnest part is around 0.025mm), none of the 3D printers in Stevens PROOF lab could handle the printing. Also, the circular cross-section shape made the manufacturing process even more difficult. After carefully discussed with the advisor, the team decided to make two major adjustments in the design: change the circular cross-section into square one, and instead of using the real-scaled version, a scaled-up version will be manufactured and tested. Also, the overall length is shortened again. Because the team decided to fabricate a scaled-up model, dimensional analysis for the guidewire was utilized to prove that the scaled-up prototype will work the same as the real size model. The final version of guidewire was shown below:

Version 1

Version 2

Version 3

Alpha-Prototype

Alpha-prototype was created in phase 4, and it was printed in PLA by the FDM 3D printer based on the version 3 CAD model. For this prototype, the team only printed the base part, tip part, and the actuator part because the wire part was too small to be printed by the FDM printer. The team utilized the prototype for further technical analysis with the properties of PLA. The picture of alpha-prototype was shown below:

Mechanical Analysis For Alpha-Prototype

Reynold's Number

The example of analyzing Reynold’s number for the real size model and the scaled-up prototype showed that if the team wanted to ensure that the prototype would work under the same scenario as the design goal within the real size model. When the real size model and the scaled-up prototype worked in environments with the same Renold’s number, it can be said that the real size model would have identical performances as the scaled-up prototype. The team got a result of 9.46 for the ratio between the flow velocity of the model and the velocity of the scaled-up model, which means that the team can control this ratio to obtain the same Renold’s number under both circumstances.

Drag Force Analysis

As for the drag force, the team got a ratio of 0.315 for the relationship between drag coefficients of the real size model and the scaled-up prototype. This number indicates that if the team needs to ensure that the drag force would have equivalent effects on the guidewire, the model should have a drag coefficient of 0.315 times of the drag coefficient of the prototype. There are several ways to manage the value of the drag coefficient, including changing shapes of the tip on the guidewire and modifying the length of the guidewire.

Thermal Analysis For Alpha-Prototype

Conductive Polymer

The team will utilize two commercial conductive polymer products to fabricate the conductive wire on the guidewire. Both selected products are available for purchase and easily to be printed out using additive manufacturing. The goal of our design is to find a proper material that has fast response time under temperature stimuli since shorter time in stroke surgery means more chances to live for patients. To find out how much heat does the polymer need to increase temperature by 1 Celsius degree, the team has done analysis on 3 different conductive polymers with different resistance. As it shown in the chart below, commercial products have much better performance than SiC, which is a self-made polymer on response time. Considering working efficiency and manufactuability, the team will choose one or both of these two products to future fabrication.

Electrical Subsystem

In order to apply an electrical signal to the guidewire by the conductive polymer, the team had to design an electrical subsystem that could provide electricity to generate heat. Arduino board was used to build the electrical subsystem. The overall design was to use coding to produce 3 different levels of output voltages to let the guidewire bend in desired angles. This design requires an analog output voltage higher than 5V so that together with the UNO board, the team also installed a motor shield to the system. The picture of the actual electrical subsystem showed below:

The coding utilized what is taught in Design VI, including digital in/output, analog output, motor control, and also uses an LCD screen.

There were three main parts in the code: buttons, motor, and the display. Buttons were used as the switches of the output. The motor here acts as the load. In the real project, the motor would be replaced by the joule heating system. And the screen showed what level of output the system was in. Lines of codes were shown here:

A live demonstration of how it actually works is uploaded below:

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