Stroke is the third leading cause of death in the United States and will occur in 700,000 Americans this year, of these 700,000, over 163,000 of them will die. Ischemic stroke is when a blood clot, called a thrombus, blocks blood in a cerebral artery from entering the brain causing brain cells to die. Ischemic stroke is the most common type of stroke and accounts for 85% of all strokes. Time is very important when treating a stroke since every second that passes, 32,000 brain cells die. Treatments of ischemic stroke include pharmacologic thrombolysis (intravenous drugs that dissolve blood clots) and mechanical thrombectomy (removing the clot mechanically).

The objective of this project is to design a MEMS (microelectromechanical system) device to mechanically remove or break apart a blood clot in the cerebral artery in stroke patients. The device will need to completely relieve the effects of the stroke and replenish blood to the brain in a small amount of time. A macro scale prototype of the device will be built and demonstrated in an in vitro model.

Considering the medical arena we have entered with the commencement of this project, the impact of it directly depends on the efficiency of our process. The removal of blood clots from a patient’s artery could be a life or death situation. With the current method of injecting tissue plasminogen activator (tPA) to dissolve blood clots, the probability of recurrence makes it a short term solution with negative long term effects. Our device aims to provide a more complete treatment and reduce the recurrence rate while limiting vessel damage for our patients. The fact that our device can be used in a proactive process (rather than waiting for a removing agent to do the work) makes it all the more significant.

The MERCI (Mechanical Embolus Removal in Cerebral Ischemia) retrieval system is the only device approved by the FDA (Food and Drug Administration) as a treatment for ischemic stroke. The device is introduced into the body using a catheter and navigated though blood vessels to the brain. The end of the device is fitted with a corkscrew where it can be driven though the clot and mechanically remove it. A recent study with the device showed success in 48% of patients where just the device alone was used and the success rate increased to 60% when the device was used in combination with drug therapy.

Pros:
− The device was approved by the Food & Drug Administration
− 48% recanalization rate from the device alone
− 60% recanalization rate from the device with adjuvant therapy
− 28% mortality rate with a Rankin score of 2
Cons:
− Overall mortality rate of 44%


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With previous research on existing designs and their pros/cons, certain criteria were necessary in developing conceptual designs. One significant consideration was the nature of a blood clot. A blood clot is composed of platelets that exist within blood and has a viscous consistency. Although it blocks the passage way of blood, it does not have a solid form that is easily grasped. Putting this into consideration, various designs were constructed. All the devices would be accompanied by a guide wire which would be directed by a surgeon through a patient via external controls. The location of the device would be monitored via CAT scan.


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One design was an “umbrella” like device designed to capture the clot and remove it from the body. The device would initially be collapsed and enclosed by a tip. The surgeon would guide the tip past the clot. Once the device was properly placed, the “umbrella” would inflate, creating a scoop which could then be redirected out the body.
Another design designated for removal is the “jaw” device. Its design is comprised of a clamp-like mechanism which encloses the clot within itself. The device would prevent the mass from escape. The surgeon could then pull out the clot via the path entered with the aid of the guide wire.


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One more consideration made for removal was a “tree” design. The concept behind the design was to have multiple levels of wire orientated in different directions. Ideally the clot would be engrossed within the matrix of the tree and thereby able to be removed from the body.


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Another form of treatment is to break up the blood clot into particles minute enough to reenter the bloodstream and no longer prove harmful. A viable option is to use a vacuum strainer; the concept behind this would be to strain the blood clot through a micro-mesh or push the clot through a small opening. To do this, there would need to be some type of encapsulated tip that would be able to push and/or pull the clot through the strain/hole using some type of vacuum.

Applying the same principle of breakdown, a method that would use an active-tip is also a plausible option. This method would involve using minimal vibration induced from an active-tip. At proper frequencies, the tip would be able to induce breakdown of the clot.

This idea was further developed to use multiple tips. At certain frequencies the fingers are enticed to move and at times will collide. This collision will provide for a “chomping” motion which would in turn destroy the clot.

A similar method would be using a bristle device. This device would encompass closely to the walls of the blood vessel rotating within the clot and thereby breaking it down. This concept is closely related to a street cleaning brush.

Several factors need to be considered in the selection of the final design. The most important area of concern is in minimizing further damage. This provides to major concerns. Does the product have potential to affect the blood flow and/or does the product have the potential of damaging the vessel wall? The effect on blood flow is seen in devices designed to remove clots to outside the body (i.e. The MERCI Device). The potential damage to vessel walls is present in devices that have moving parts that are very near to the vessel walls. Other criteria include ease of use, cost to manufacture, durability, and the ability to be used in various sized vessels. The ease of use is concerned with how easy the device is to use and if any additional training is needed to operate. Cost is of vital importance to ensure that as many patients as possible could benefit from the design’s advantages. Durability is also an important factor because any type of damage to the device has the potential to harm the patient. Also, there is the consideration of using the device in various sized vessels. This would reduce costs to both manufacturing and consumer.

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According to the concept matrix, the vacuum strainer design and the multiple finger vibrating tips had the most positive outcome. We decided that minimizing further damage meant having the least effect on blood flow and the vacuum strainer and vibrating fingers will have the least effect because they would work from within the thrombus, leaving the flow of blood unobstructed. The conceptual design leaves room for the device to work in vessels of any size, unlike many of the other designs like the jaw-clamp or umbrella scoop. The ease of use is another area in which these two designs have an advantage. Unlike other designs, these concepts would require no extra training to operate effectively once they have arrived at the site of the thrombus. Other devices would have required a fine tuned expertise to operate safely and efficiently. Current trials have proven that removal is very inefficient and can actually lead to recurrence and possibly fatal effects.

Further consultation was performed with the aid of our advisor determining what other characteristics to consider in making a final selection. The final design chosen was the multiple vibrating tips. This provided a more feasible design and one that would not require an excess of parts. The multiple vibrating tips device conceptually would be designed so that the tips would be moved at a certain frequency so that collision would occur between them. This collision would cause for a break in the bonds of the blood clot thereby destroying the thrombus to the point where it can be reconstituted back into the blood stream without any further damage.

Certain specifications were necessary to be considered firstly that being of size. The average blood vessel within the brain is 2-3mm in diameter. As stated in the objective, this is a size too small to be dealt with average mechanical devices and therefore would require a MEMS device. Unfortunately this is unfeasible with our budget and capabilities, and rather a macro scale model would prove our concepts in a 10mm diameter scenario (likely in the form of a test tube/vial). Another consideration is material. The device is intended to be placed within the body and therefore requires a biocompatible material. Also, the different type of material affects the frequency required to excite the fingers in order to make them collide. With these in mind, a technical analysis was performed to decide the optimal design for our device.