25 May The Bubble Challenge Part I: Finding the Trouble with Bubbles
Last December, the FDA issued a Class I recall for the Alaris Pump module, which delivers medicine intravenously into a person’s body. A Class I recall is issued in situations where a malfunctioning device could cause injury or death to a person.
The problem with the pump was, simply, bubbles.
The pump has a sensor that identifies bubbles and sounds an alarm upon detection. This is a life or death feature: A bubble injected into a patient can cause a fatal air embolism. However, the FDA reported that the pump’s sensor had a high rate of false alarms, which means it detected bubbles that weren’t there and stopped the infusion needlessly – also a great risk to the patient.
Finding bubbles with sensors is tricky business, whether working with infusion systems or diagnostic cartridges. With fluid infusion systems, there’s a balance game: You have to make sure the sensor is sensitive enough to catch every dangerous bubble, but not so sensitive that it sees bubbles that aren’t there. With diagnostic cartridges, bubbles may interfere with diagnosis by preventing the target DNA abnormality or infectious agent from being seen.
Bubbles are like taxes: They’re unavoidable, but if you plan ahead you can minimize the impact. Any time you automate the handling of fluids, the bubble problem will, well, pop up. That means we have to ask, constantly: How do our choices of device architecture, materials, and geometry affect the risk of bubbles? Furthermore, how would our system functionality be impacted by the presence of a bubble? And most importantly, how could a failure mode caused by a bubble introduce a patient or operator risk?
I see it as a two-part discussion. So for the rest of this post, I’ll address where to look for bubbles and how to assess the risk. (Hint: Don’t go it alone!) The next post offers solutions: How can you minimize bubble risk in the design process?
Like many challenges in device design, assessing bubble risk is easy to define and challenging to do. Bubbles arise when liquid meets air, and they often form when fluids move from one location to another. They may be caused by a variety of situations. For example:
- Maybe it’s obvious, but air leaks can occur in tubing, seams of molded parts or cartridge layers, or in places where flexible parts meet rigid components. This is especially true if negative pressure is used to drive the fluid.
- Fluid outgassing, which releases gases trapped in the fluid due to changes in fluid pressure.
- Agitation, even just a little.
- User handling, as in orientation or jostling of a cartridge, or orientation of fluid tubing.
- Trapping air during priming, while preparing a device to handle fluids.
The problem gets even more complicated when you try to detect or measure bubbles after they happen. Bubbles are deceptive. They change size or shape depending on location, the movement of the fluid, or the surrounding pressure. They’re also inconvenient: They tend to form in hard-to-monitor places, like in corners or at the junction of two components.
Now that we’re on guard, we have to ask: what’s the risk? We know bubbles can be deadly, but they can also cause malfunction in other ways. For example:
- Volumetric delivery. Bubbles cause under-delivery of a liquid, since the bubble takes up space in your flow path that’s supposed to contain fluid, greatly impacting delivery accuracy. We recently designed a precision pump used in pharmacies that pulls fluids from multiple sources, and we carefully tuned and tested the bubble sensor to find real bubbles that were big enough to affect the accuracy while avoiding triggering false alarms.
- Wicking. In devices that use capillary action, bubbles may prevent the complete fill of a cartridge, which can throw off any measurements based on that sample. We recently designed a user-friendly blood collection cartridge for an assay that required a precise volume of blood. As you might guess, we found unintended bubbles that came into the line during the blood draw. Ultimately, we solved the problem by changing the geometry of the capillary opening and filling channel.
- Pipetting. In systems that transfer fluids using pipettes, bubbles can hinder aspiration, or prevent liquid from reaching an intended surface. These errors have gone unnoticed in automated pipetting systems, causing a ripple effect of consequences downstream.
- Droplets. Finally, in systems that dispense droplets, bubbles can cause an unpredictable release of droplets from a nozzle tip, leading to splatter or spraying of fluid – or even completely disrupt the formation of droplets in the first place. This could mean large droplet volume variability, or contamination of nearby surfaces.
Finding and assessing the impact of bubbles is an iterative process. We take a first pass at the design and define the risks we can see. We then move forward with detailing the design, reassess for risks, make changes, and repeat. That process helps reveal bubble problems one at a time and even find issues we couldn’t have seen at the beginning – as well as design solutions, which I’ll get into in the next post.
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