Shape Memory Polymer Stent With Expandable Foam: A New Concept for Endovascular Embolization of Fusiform Aneurysms
Original entry: William Bonificio, AP 225, Fall 2009
Shape Memory Polymer Stent With Expandable Foam: A New Concept for Endovascular Embolization of Fusiform Aneurysms. IEEE Transactions in Biomedical Engineering, vol. 54, no. 6, 2007, pg. 1157
Soft matter keywords
Shape memory Polymer (SMP), aneurysm, embolism, foam, blood rheology.
The purpose of this study was to propose a new device for the therapy of non-necked aneurysms. The researchers proposed a shape memory stent be combined with a shape memory foam in order to produce an embolism that fills the aneurysm with the foam, and a stent which provides blood flow, all in a single device. Current methods are much more invasive, and also take a longer time involving multiple procedures. A further explanation of these aneurysms and the device that was formed is below.
Soft matter discussion
Cerebral aneurysms occur when blood collects in these pockets in the circulatory system, that then grow due to pressure, becoming very weak, and then rupturing, often having fatal results. Many aneurysms are treated by embolizing, or filling the pocket so that the walls of the aneurysm do not feel the pressure of the blood. This can be done with coils, or balloons, or a combination of these things along with stents. These procedures can become very complicated and invasive depending on different factors, such as the size or existence of the neck of the aneurysm, because this often determines what type of embolization can be done. With wide neck aneurysms many current embolizing agents can escape the aneurysm and cause harm down stream.
The researchers on this project proposed a new device which combined a shape memory stent, used to ensure blood flow, and a shape memory foam, used for embolization, for the therapy of the fusiform gyrus nonnecked aneurysm. Shape memory polymers (SMPs) utilize the shape change and deformation that occur during phase changes to take advantage of the glass transition temperature and produce a shape memory effect. First the researchers created a 'memorized' shape of the SMP. They then raised the SMP above its Tg and worked the stent and foam into the desired compacted shape and and cooled it below its Tg and 'froze' it in place. When the SMP is raised above its Tg, the modulus will change and the SMP will retake the original form. in this experiment the SMP used for the stent was MP-5510, and the SMP used for the foam was an in-house blend of hexamethylene diisocyanate (HDI), N, N, N', N'-tetrakis(2-hydroxypropyl)ethylenediamine (HPED), and triethanolamine (TEA) that could have its shape and Tg easily manipulated. In this case the SMPs both had Tg that were slightly below body temperature, so that expansion would occur once inserted into the aneurysm. SMPs obviously has value in this problem because the stent and foam can be inserted laproscopically through blood vessels with minimal invasiveness.
THe investigators prepared the device at low temperature in its compacted form, and then inserted it into a model fusiform aneurysm that had been created. They then raised the temperature above the Tg and the device expanded exactly as anticipated. The results of the study are shown in the figures to the left and right. As shown the foam completely filled the aneurysm and the stent maintained a passageway for blood flow. The device, upon visual inspection, is successful.
The anticipated problems come from tuning the Tg so that it can be inserted into the aneurysm and then to expand, rather then expansion once put into the body. They suggest to raise the Tg above body temperature, and then use a laser heating system that can be focused on the device after it is inserted. Also, the biocompatibility of the SMPs is currently unknown. Overall though, sounds like an interesting device.