Adjustments for the Tubing Stretching Process

Author: Natalie Smith | Major: Biomedical Engineering

My name is Natalie Smith, and I am a senior Biomedical Engineering major in the College of Engineering working under Dr. Morten Jensen in the Cardiovascular Biomechanics Lab. My research project focuses on the development of a customizable angioplasty balloon forming machine for the treatment of heart disease in coronary artery bifurcations. This past summer I took our prototype machine which produced balloons approximately seven times larger than the average size used for coronary arteries and resized the molds to form smaller balloons. In the fall semester, I continued my project from the summer and addressed the issues that arose from reducing the mold size. In addition to Dr. Jensen and the graduate student working on this project, I also worked with another undergraduate in the research laboratory. Dr. Jensen routinely advised our team on different solutions for the various issues we encountered with the machine.

Coronary artery disease, a type of heart disease, is the leading cause of death in the United States. Angioplasty balloons are a common method of treatment and work by placing a stent at the lesion site to widen the blood vessel. A particularly difficult type of lesion to treat is a bifurcation lesion, due to its complex geometry which varies among patients. To address the issues which hinder the use of standard shaped balloons in bifurcations, our research group developed a balloon forming machine with customizable molds that would allow clinicians to produce angioplasty balloons adapted for each individual patient. The first prototype machine manufactured balloons that were approximately seven times larger than the balloons typically used in coronary arteries to allow for easier preliminary testing. As the initial prototype testing concluded, I endeavored to take this project to the next step and adjust the machine to produce appropriately sized angioplasty balloons for coronary arteries.

Over the summer, I worked to correct several process details which arose from reducing the mold size, including the connection between the luer lock and the tube and the necked tubing process. Coming into the fall, additional issues occurred with the balloon forming process which had to be addressed. One problem was with the necking process. Our previous procedure involved heating up the tubing inside a metal block and then stretching it to decrease its diameter. This was done using two hand-held clamping devices to fix the tubing in place and maintain a hold on the tubing through the stretching process. Two people were required in this procedure as one had to hold the tubing and the other had to assist in the placement of the tubing in the metal block to ensure that the tubing was approximately level. As a result, we noticed that some post-stretched tubing was set at an angle, which hindered the insertion of the parison into the machine. Therefore, we designed an apparatus to stretch the tubing along a level track to make the parisons straight and easy to insert. We constructed a wooden base and mounted a track and slider to run alongside the metal block. Two clamps held the tubing in place through the heating and stretching process. The resulting parisons were significantly straighter than the previous ones.

Another issue which we continued to address was the issue of the tubing diameter. Over the summer, the parisons were adjusted to be formed from larger diameter tubing which was stretched to have smaller ends. However, the resulting balloons had a wall thickness that was much too great for angioplasty balloons. In reviewing the results from the initial prototype machine, the graduate student discovered that the diameter-to-wall thickness ratio was very different between the prototype balloons and the smaller balloons. Therefore, an alternative type of tubing with a similar diameter but smaller wall thickness was purchased. The resulting balloons had a smaller wall thickness and greater flexibility.

Moving into next semester, I will be completing this project and writing my honors thesis. The undergraduate and I will conduct one final set of pressure, temperature, and heat time testing, using the information we collected over the summer and fall. I will also be making slight adjustments to perfect our balloon-forming process and finalizing any last details on this project. I plan to write and present my honors thesis on the project in the spring semester.