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
Magnetotactic Bacteria, paramagnetism, iron oxide, colloids, debye length.
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
The academic group conducting this research was interested in single domain iron oxide colloids. They noted that the size of the colloids in magnetotactic bacteria (50nm) was much larger than anything that would form spontaneously form ferrofluids (10nm). Because they are so large they are expected to have much stronger dipole forces, and should form different patterns such as chains and rings if removed from the bacteria. The group wants to look at the properties of these colloids when removed from the bacteria, and this is their first study examining the magnetic properties of the bacteria themselves.
The first thing they did was cultivate the bacteria, after this was performed they took some awesome pictures as shown. They also removed the magnetic colloids and let them spontaneously interact. Not surprisingly, they aggregated and formed some interesting structures, such as loops, chains and 'handles' (a combination between a loop and a chain). They noted that for these interactions to occur the colloids must be very magnetic and have large dipole energies. They measured the magnetism and it was in line with there estimates. They also noticed that the ring and handle formation never formed within bacteria. They attributed this to the thought that maybe the bacteria somehow shielded the effects of the magnet so that the colloids could only form the chains they were produced in.
To experiment with this thought they put different magnetostatic bacteria together to see their interaction. They calculated the debye screening length and noted that it should not cause any repulsion between two side by side bacteria. They then calculated the van der waals forces between two bacteria to measure the attraction they should feel towards each other. They then did force measurements to see if the bacteria were at all attracted to each other due to the magnetic dipole force, and the results showed they were not attracted to one another. The magnetic attraction, which is large enough to feel the earth's magnetic field, is also well screened so that it does not see any of its neighbors in other cells. They did some modeling of the magnetic colloids within bacteria to show that the interactions with other colloids does not in any way effect colloidal stability.
FInally, they did sedimentation experiments that showed the bacteria did not aggregate at, again expected based on the previous results. This could have evolutionary advantages basd on competition for nutrients if the bacteria formed large clumps, or their magnetic poles effected one another. This was a neat article!