Real World Engineering: Using the NXT to break apart blood clots

OmniSonics Medical Technologies, Inc. is a medical device start-up corporation located in Wilmington, Massachusetts, and was established in 1997. OmniSonics has developed a product that can be used in the treatment of vascular occlusive conditions such as blood clots. Blood clots, also known as “thrombus,” can block blood supply to the heart, brain, lungs, or limbs or cause heart attack, stroke or limb pain. Traditionally blood clots are treated with medication or interventional means (i.e. catheter based or surgical procedures). OmniSonics has created a treatment of blood clots that is not highly invasive and does not rely on medication.

OmniSonics’ products are based on their patented OmniWave Technology. OmniWave technology is the first that uses ultrasonic vibrations along a small wire to treat vascular occlusive conditions. This is done by inserting a wire, or “waveguide,” with a small diameter into an affected artery or vein. The other end of the wire is attached to a hand piece that helps the doctor guide the end that has been inserted into the vessel. The hand piece contains a piezo-electronic transducer. The transducer causes the wire to vibrate ultrasonically. The proximal part of the wire (that part that is closest to the hand piece) vibrates longitudinally (i.e. in the direction of the wire). This motion is converted to transverse waves (vibration that moves in the direction normal to the axis of the wire) at the distal end of wire (the part farthest from the hand piece). This ultrasonic vibration causes the blood clot to break apart and dissolve, allowing the clot to be carried away with the blood flow and absorbed into the body.

It is crucial that the wire vibrates at the proper frequency and does not fracture or break while treating the clot. In order to ensure these criteria, extensive testing must occur. Steve Forcucci, a principal mechanical engineer at OmniSonics, explains that OmniSonics must follow the Federal Drug Administration’s (FDA) regulations since the product will be used on human beings. This requires extensive testing to prove product safety and efficacy. OmniSonics has recently received FDA approval of their Omniwave System for use in periperal vasculature. The safety benchmark, as determined by the FDA, was met and OmniSonics will soon begin a small limited market study for use in humans.

Testing both durability and efficacy of the wire before moving to human subjects is necessary to provide confidence that the wire is strong enough and that it will not break.  To ensure its durability, OmniSonics has developed an endurance rig (See Figures 1 & 3) to test the wire. The wire is run through a hypo tube (See Figure 2) and into a fixture that bends the wire at 180 degrees. An automated device, that contains the transducer, slides the wire into the tube and the results appear on a computer. This configuration allows the working end of the wire to follow a curved path to simulate use in a curved vessel.  The curvature can be adjusted to study the effects of changing the curve’s radius.

Fig. 1: OmniSonics Rig Used to Simulate a Clogged Artery

Fig. 1: OmniSonics rig used to simulate a clogged artery

Fig. 2: Fixture with a 180˚ Bend

Fig. 2: Fixture with a 180˚ bend

Fig. 3: Parts of the OmniSonics Testing Rig

Fig. 3: Parts of the OmniSonics testing rig

The graphical programming language LabVIEW is used to control the automated device that threads the wire into the rig and to acquire and analyze the resulting data. The tests monitor the power, current, voltage, and frequency that are being fed into the hand piece containing the transducer.  Electrical sensors monitor the results. The engineers use visual inspection to determine if the wire remains unbroken. If the wire breaks, engineers look for discontinuities in the electrical data collected by LabVIEW to determine when the wire broke. In general, a wire will break due to a combination of static strain (bending of the wire), dynamic strain (vibrating the wire), and material fatigue (wear & tear).

Fig. 4: LabVIEW Screen Shot of Program that Regulates the Voltage via an External Function Generator

Fig. 4: LabVIEW screen shot of program that
regulates the voltage via an external function generator

“Blood clot” removal with the LEGO NXT

The first step in emulating OmniSonics’ technology was creating a rig that would allow students to work on this problem and would be cost effective for classrooms. In the NXT version of the OmniSonics’ rig, plastic tubing with a 7/8 by 5/8-inch opening is used to replicate the arteries and is attached to a board with holes using plastic zip ties (See Figure 5). A mixture of cornstarch and sand is inserted into the tubing and represents the clot when it hardens.

Fig. 5: Testing rig for NXT

Fig. 5: Testing rig for NXT

We divided OmniSonics’ OmniWave technology into three activities:

  1. The assessment and comparison of the properties of different materials such as spaghetti, fishing line, and wire to meet the standard requirements of the testing rig.
  2. The design and construction of an NXT hand piece that will perform lateral motion and insert the chosen “wire” into the tubing.
  3. The alteration of the hand piece that will simulate ultrasonic motion and rid the wire of the cornstarch solution that acts as the blood clot.
Fig. 6: Polyethylene Tubing Attached to the Board

Fig. 6: Polyethylene tubing attached to the board

LEGOengineering.com’s version of OmniWave technology uses polyethylene tubing (See Figure 6) as the wire. It is fed into the tubing on the rig by the rotation of tires connected to the NXT (See Figure 7). The “wire” is inserted into the tubing and vibrated by the NXT. After a few seconds, the cornstarch mixture begins to erode off the sides of the tube.

Fig. 7: NXT Handpiece that Threads the "Wire" through the Tubing

Fig. 7: NXT handpiece that threads the “Wire” through the tubing

The movement with the NXT-based device is obviously not as precise, but this challenge still allows students to emulate possible real-world engineering solutions. Forcucci says, “The NXT activities closely mirror the mechanical effect of wire and hand piece, having both longitudinal and transverse motion.”

Although these activities are simplistic mirrors of the real-world technology, they do provide an authentic engineering activity to give students the opportunity to work through the engineering design process and face some of the same dilemmas and successes as the real-world engineers that are working at OmniSonics.

The write-ups for the activities include lesson plans for students and background information for teachers that include a materials list, building and programming instructions for the rig and device, and information about coronary vascular occlusive conditions.

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Vocabulary

Vascular occlusive condition is a general medical term for a blood vessel blockage.

Frequency is the number of occurrences of a specific even within a given period of time.

Ultrasonic vibrations are vibrations at a frequency higher than what the human ear can hear (e.g. greater than 20 kHz).

Piezoelectricity, converse piezolelectric effect ) occurs when an electrical field is applied to a body thus causing a strain (on the wire) causing it to oscillate and create the ultrasonic vibration.

Strain is when a body is deformed by being stretched or pulled beyond its usual form.

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