permeability and form factor values. Because we could control the pore size of the foams, the samples of SMP foams that we used for testing had different pore sizes, which leads to alternative material properties. The alternative material properties arise because a relationship exists between material geometry and the factors we were trying to calculate. Identifying material properties of different-sized pores allowed future optimization of the foam geometry to offer the most favorable possible fluid dynamic conditions needed in any new application of SMP foam technology. Therefore, we used a large pore and a small pore sample size. Also, because different-sized aneurysms can be packed with embolic coils to various degrees depending on the volume of the aneurysm and ease of access, we tested different packing densities for the mock embolic coils. This approach offered us insight into how densely the coils would need to be packed to have a certain effect on fluid flow in a treated aneurysm. We used four different packing densities to give a range of effective data. Results and Conclusions For our SMP foams to reduce more flow and cause more blood clot formation, the permeability and form factor values for the SMP foams in relation to the mock embolic coils needed to be lower and higher, respectively. Our testing revealed that the SMP foams had permeability values 10 times lower than the mock
References 1. Burns JD, Huston J, Layton KF, et al. Intracranial aneurysm enlargement on serial magnetic resonance angiography frequency and risk factors. Stroke 2009;40:406–411. 2. Juvela S, Poussa K, Porras M. Factors affecting formation and growth of intracranial
Explorations | Fall 2013
embolic coils for all clinically relevant cases. Form factors were 1,000 times higher for the SMP foams than for the mock embolic coils in all cases. These findings suggest that, compared with embolic coils, the SMP foam geometry produces a better environment to form a blood clot. Furthermore, this environment could offer substantially better filling of an aneurysm body, giving an SMP foam device the edge in successful treatment. The larger-pore-sized foams had higher permeability values and lower form factors than foams with smaller pores. From these data, we concluded that the smaller-pore-sized foams have material properties that would produce the most flow stagnation in aneurysm treatment. Therefore, smaller-pore-sized foams will probably induce the most clot formation and provide the best filling of an aneurysm. The permeability values of the mock embolic coils also decreased as packing density increased, and the form factors of the mock embolic coils increased as packing density increased. This finding suggests that in large aneurysms, where high packing densities cannot be achieved, the amount of residual flow is high. With a high residual flow, not as much clot formation is facilitated. A lack of clot formation could then mean increased future risk of aneurysm rupture, indicating that in such cases, SMP foam treatment may succeed where embolic coils would fail.
aneurysms: A long-term followup study. Stroke 2001;32:485–491. 3. Brilstra EH, Rinkel GJ, van der Graaf Y, et al. Treatment of intracranial aneurysms by embolization with coils. Stroke 1999;30:470–476. 4. Maitland DJ, Small W 4th, Ortega JM, et al. Prototype laser-activated shape memory polymer foam device for
Applications Though the implications of these data for using SMP foams as an aneurysm treatment option are important, the scope of our project can be considered on a wider basis. By using data obtained for permeability and form factor for different-pore-sized SMP foams, we will be able to customize the material properties of an SMP foam for a specific application. For example, in applications where blood flow needs to be reestablished in a blocked artery, the blood flow reduction properties of our aneurysm treatment foam are less desirable. Using SMP foam on a device meant to treat such a condition would then mean creating foam with a particularly high permeability and low form factor, which, from the results of this study, could be accomplished with a very-large-pore-sized foam. The potential uses of these alterable material properties are nearly endless. In fact, the ability to modify the material properties of SMP foams, in conjunction with the ability to deliver them noninvasively by catheter, would make them powerful tools for inclusion in many medical devices. Acknowledgments I thank Andrea Muschenborn for mentorship. I also thank Dr. Duncan Maitland for assistance with editing and general support. The National Institutes of Health/National Institute of Biomedical Imaging and Bioengineering funded this work (grant no. R01EB000462).
embolic treatment of aneurysms. Journal of Biomedical Optics 2007;12(3):030504. doi:10.1117/1.2743983. 5. Hwang W, Volk BL, Akberali F, et al. Estimation of aneurysm wall stresses created by treatment with a shape memory polymer foam device. Biomechanics and Modeling in Mechanobiology 2012;11:715–729.
Published on Nov 22, 2013