The femoral artery is among the easiest and safest ways to gain access to the human heart. For example, when patients require a coronary angioplasty, a procedure used to open clogged or blocked heart arteries, access to the heart is obtained through the femoral artery. Generally, this procedure entails locating the femoral artery, insertion of an angiography needle, insertion of a guidewire through the angiography needle, removal of the angiography needle and insertion of a catheter along the wire. The guidewire is then used to guide the catheter to the blocked or clogged artery. The catheter will have a deflated balloon on its tip and a stent could be placed around the deflated balloon. The stent is navigated through the body to the area of the clogged or blocked artery where the balloon is inflated, expanding the stent. After the stent is deployed and the artery is opened, the balloon is deflated and the catheter and guidewire are pulled out of the body.
As one could imagine, the paths taken by the guidewire and catheter have many twists and turns, thus considered a tortuous path. Both catheters and guidewires can be coated and lubricated to reduce frictional forces along this tortuous path and subsequently reduce the force required to push and pull the catheters and guidewires through the body.
The testing machine is mounted horizontally and uses a pneumatic grip to push the catheter into the tortuous path that simulates an artery. After the catheter is pushed through the path, the pneumatic grip releases the catheter, the crosshead returns to its original starting point, the pneumatic grip re-closes on the catheter, and the cycle continues. The catheter pushing mechanism can be sped up or slowed down to better simulate the actions of a surgeon.
The use of a universal testing system to quantify frictional forces during surgical procedures is not limited to testing guidewires and catheters. Other non-surgical applications that require the measurement of frictional forces include endoscopy, rhinoscopy, colonoscopy, and others.
In an example of testing a non-linear biomedical guidewire, a specimen was positioned on a 3-point bend fixture fitted to an ElectroPuls E1000 machine. The machine was set up with a 250N Dynacell load cell to run load peaks of 1N and 11N in compression at a frequency of 10Hz.
Two special features of the WaveMatrix™ software were used to ensure accuracy and control throughout the duration of the test.
- The Automated Tuning Wizard uses a simple ramp waveform to measure the stiffness of the load string. This determines the gain settings to optimize the performance and control of the machine for the chosen specimen material and geometry. In just a few clicks, the machine is tuned and ready for testing.
- Attaining load peaks is essential when it comes to testing non-linear specimens, but it can be a challenge. Another feature within our software called Advanced Amplitude Control ensures that the load peaks are met for each cycle and the desired load of tension is achieved. Without this, load peaks would likely drop below the desired load of tension as the test progresses. This feature ensures control even when the stiffness of the specimen changes.
We had the pleasure of interviewing Dr. Matthieu De Beule, Assistant Professor at Ghent University. Dr. De Beule’s lab focuses on biofluid, tissue, and sold mechanics for medical applications.
“We really try to bring simulation technology from bench, meaning from device development, to bed, meaning clinical practice. One of the key aspects for doing this is model validation. Model validation is where our Instron equipment is of upmost importance.” ~ Dr. Matthieu De Beule
Testing a material at body temperature compared to ambient temperature can have a significant effect on its mechanical properties. This is especially the case with polymeric materials.
The results: Testing polyethylene catheters at 37°C produced a lower modulus, lower maximum force, and higher extension at break when compared to average results obtained from testing at 28°C.
|Mean Result ±1 Standard Deviation
|Modulus (MPA)||26.34 ± 0.45
||19.5 ± 0.48
|Maximum Force (N)||16.62 ± 0.54
||15.02 ± 0.42|
|Extension at Break (mm)||525.12 ± 35.10
||532.22 ± 32.98|