HiCycle Debubbling Tips
The ViVitro Labs HiCycle durability tester is an accelerated wear tester intended for use with cardiovascular devices. The HiCycle has six sample chambers which subject test devices to accelerated pulsatile loading conditions by moving them through a blood analog fluid. When air infiltrates the sample chambers, bubbles can cloud the visibility of your sample, change the system compliance, and cause noise in pressure readings. This article and its associated video explain how air gets into the HiCycle chambers, prevention through regular equipment maintenance, and how to remove air bubbles when they form.
Air bubbles can enter the system at any seal or interface between the test fluid and atmosphere.
1. Why do bubbles occur?
- When a static head feeding fluid into the system runs dry, air will be sucked in.
- Attaching syringes to remove air bubbles from pressure transducers can introduce more bubbles.
- Accidentally opening a stop-cock to air while the HiCycle is running will allow air to be sucked into the chamber.
- When first filling the HiCycle with fluid, small air pockets can become trapped in the chambers and released on start-up.
- If chamber windows are not sealing properly, air can enter. Possible causes leading to a bad window seal include a worn window o-ring, salt crusted on the seal, or improperly tightened window fasteners.
- Pressure/drain ports can come loose and allow the entrance of air.
- The scan valve may be leaking, allowing air to enter.
- The static head may not be high enough.
- When the bellows start to wear, air can penetrate small cracks in the silicone.
Fortunately, air infiltration can be minimized by regular equipment maintenance and proper set-up.
2. How to prevent bubbles?
Sealing & Leaks
- Check for salt built up around sealing edges like chamber windows, pressure/drain ports, and the scan valve. If there is salt build-up, it means fluid is leaking. If fluid can get out, air can get in!
- Change window o-rings as needed – if you notice fluid leaking around the window edges or salt build-up, change the o-ring. When you are putting the window back on, wipe any salt and fluid from the edges of the window and o-ring, as well as the window sealing surface.
- Check that all of the drain ports and stopcocks are tightened daily. If these come loose, they can allow air into the system.
- If there is salt built up around the scan valve seal or fluid leaking out, the scan valve tension likely needs to be adjusted to improve the seal.
- Attaching a static head to the system helps feed fluid into the chambers if any fluid is leaking. The fluid level in the static head should be monitored and topped up daily. If there is a substantial leak or the static head fluid level is not topped up regularly, the static head can run dry and feed air into the system.
- Make sure the static head is open when the HiCycle is running. If you forget to open the static head when you start the HiCycle running, air can be sucked in.
- Locate the static head as high as possible to help keep bubbles out of the system.
- Use syringes with flexible tips, so you can encourage extracted air bubbles to the back of the syringe, instead of accidentally introducing more air bubbles.
- When first filling the HiCycle chambers with test fluid and fixing devices to the pistons, air can get trapped in small pockets. It is important to run the HiCycle at low amplitude before tuning a system, to dislodge any trapped air bubbles before testing begins.
- The HiCycle bellows experience stress over time and will fail if they are not regularly changed. We recommend changing the bellows every 200 million cycles to prevent bellows failure during HiCycle operation. When the bellows start to fail, they get tiny cracks and holes that may allow some fluid to leak out and let air into the chambers. Usually air bubbles due to bellows failure are very fine bubbles that make the fluid appear clouded. If you start to see fine bubbles clouding your test chambers, check when the bellows were last changed. The bellows should be replaced if they were last changed more than 200 million cycles ago. See our FAQ: HOW DO I MAINTAIN AND CHANGE BELLOWS FOR THE VIVITRO LABS HICYCLE?
When bubbles do accumulate in the HiCycle chambers, there are a number of ways to extract them.
3. How to get rid of bubbles when you get them?
- When it comes to larger bubbles, you can sometimes just remove them with a syringe while the HiCycle is running. If not, put the HiCycle in standby.
- When the HiCycle is in standby, larger bubbles tend to accumulate near the upper drain port; this air can be removed with a syringe or bled out using the pressure of the static head.
- Air sometimes accumulates in the bellows and releases when the HiCycle is stopped and started again. Put the HiCycle in standby to extract air from the chambers and check the bellows for air bubbles. If there is air accumulated in the bellows, stop and start the HiCycle a couple times to release the trapped air before extracting.
- With any of these extraction techniques, always be careful not to let the static head run dry!
Smaller bubbles, which cloud the test fluid, can be harder to extract than large bubbles. They get caught on the chamber windows instead of accumulating at the drain port. Use a squeegee to remove them from the window. There are multiple ways to do this. Essentially you create a bigger air bubble at the top of the chamber, then refill the chamber with fluid to remove small bubbles. There are three techniques we recommend trying:
- Injecting air: With the static head open, use a syringe to inject air at the top drain port until only air is in contact with window. Then, either draw the fluid back with the syringe or detach the syringe to let the static head fill the chamber. Be careful the static head isn’t too full, or injecting the air will cause an overflow. Do not let the static head run dry either.
- Draining & refilling: With the static head closed, open the top drain port, then the bottom drain port to drain enough fluid so only air is in contact with the window. Then close the bottom drain port and open the static head to allow fluid to refill the chamber. You will need to add fluid to the static head as the level drops to fill the chamber.
- Fluid extraction & refilling with syringe: With the static head closed, attach a syringe with a little bit of fluid in it to the bottom drain port and open the top drain port. Extract fluid with the syringe and allow the top of the chamber to fill with air until only air is touching the window. Re-inject the fluid in the bottom drain port until all the air is pushed out the top drain port. Be careful not to re-inject air. Close both drain ports and re-open the static head.