How to implement advanced liquid & air bubble sensing for biomedical applications

The safety of patients in clinical medicine depends on the detection of liquid bubbles, a critical task that can prevent fatal threats to life.

For example, during the dialysis process, air can enter the dialysis circuit from areas around the arterial needle through leaky or broken tubing or connections. If air finds its way into the bloodstream, it can trigger potentially fatal embolisms. Large air bubbles in a patient’s brain, heart, or lungs can cause stroke, heart attack, or respiratory failure. Dialysis machines with bubble-sensing technology can detect air bubbles in the system. These bubbles are subsequently removed through a bubble trap before returning blood to the patient, ensuring the procedure’s safety and efficacy. This article explains the methods used to detect air bubbles in medical equipment for reliable operation.

The working principle of a bubble sensor is to detect air bubbles

The bubble detector uses an embedded piezo transducer that emits an ultrasonic signal in short bursts. The signal passes through the tubing and its contents. A piezo receiver then detects the ultrasonic signal. As liquids are excellent conductors of ultrasound, a strong signal is detected when the tubing is filled with liquid. Air bubbles impede the efficient transmission of ultrasound waves, resulting in a weakened signal. An internal circuit measures the amplitude of the receiver signal and provides the appropriate output, as shown in Figure 1a.

Consider an air bubble detector such as TE’s AD-101, which can be installed directly on the tubing used to infuse the liquid into the patient.

Figure 1: (a) Bubble sensing working principle in TE’s AD-101.(b) TE’s AD-101 (Source)

Bubble sensors are designed for noninvasive operation. They seamlessly interact through a tube's walls without grease or other acoustic coupling materials. The most common application of ultrasonic bubble sensors is in medical devices.

These sensors have four main components – an ultrasonic transmitter, an ultrasonic receiver, signal processing circuitry, and a housing.

The transducers are mounted in a housing that allows a tube to be pressed into a slot between the transmitter and the receiver. The transmitter emits a pulse of ultrasonic energy through the housing and into the tube while the receiver monitors for any received signal. Generally, tubes exhibit three states:

• Liquid in tube: Most of the ultrasound travels through the tube and gets detected by a receiver on the opposite side.
• Air in the tube: Most of the ultrasound is reflected at the tube-air interface and dissipates. The receiver detects a tiny signal.
• Bubble in the tube: The bubble blocks only a portion of the ultrasonic energy.

The receiver circuitry interprets the signal and sends an alarm if the amount of received energy is below a set threshold.

How do you choose the proper bubble sensor specifications for an application?

A key consideration for air bubble detection in dialysis is bubble size sensitivity. For example, some air bubble detectors can spot bubble sizes as small as 25% of the tubing’s inner diameter. Given that larger bubbles pose more significant risks, these detectors can be configured to identify only bubbles of a specific size or larger, ignoring smaller ones if necessary. Another crucial consideration is the need for noncontact monitoring of air bubbles to eliminate any possible contamination within the system. Air bubble detectors based on ultrasonic technology are particularly effective as they can detect air bubbles within the tubing without exposing the blood. Integrating these detectors into hemodialysis systems provides additional safety measures, enhancing patient well-being and dialysis outcomes. Important parameters typically considered for designing bubble sensors are:

Sensitivity/Accuracy: Able to detect tiny air bubbles (minimum 70% of tubing inner diameter).

Response Time: of equal or less than 0.25 milliseconds.

Reliability: Continuous self-diagnostic test capabilities

Ease of Integration: Noninvasive design with customization capabilities to fit a variety of tubes.

Compliance: High noise immunity to EMI/EMC per IEC61000-4

Criteria for selecting the tubing in a bubble sensor:

The critical factors to be considered for tubing selection are:

• How effectively does the tube get acoustically coupled to the sensor.
• How much ultrasonic energy is reflected at the sensor/tube interface.
• How well does the tube transmit ultrasound through the tubing material.

Plastic and rubber tubing

• These tubes can be coupled to a bubble sensor without an acoustic coupling such as grease. Achieve good acoustic coupling by pressing the tube into a slot in the bubble sensor housing. The slot should have a width of about 80% of the tubes outside diameter. The tube walls must be thick and rigid enough for acoustic contact. Reinforced (braided) tubing can sometimes be problematic regarding ultrasound transmission.
• Tubes with highly thick walls or materials that transmit ultrasound exceptionally well can cause ultrasonic “crosstalk”. This occurs when the ultrasonic receiver picks up a lot of ultrasonic energy that travels through the walls of the tube instead of through the fluid inside. Special considerations must be taken for these tubes.

Plastic tubing: Rigid tubes on the Shore D hardness scale, like PTFE, PFA, FEP, etc., can dry coupled with our sensors if the walls are sufficiently thin to be compressed by 20% without damaging the sensor or tube. However, these tubes permanently deform with repeated reinsertions, with reduced acoustic coupling, and the sensor may not function with repeated insertions.

Glass tubing: It is very brittle, and a custom sensor can be necessary for proper interfacing. Acoustic coupling can be achieved using a “wet” coupling material like grease or a soft elastomeric interface on the sensor.

Metal tubing: Like glass tubing, metal tubing cannot be deformed, requiring a custom sensor that uses "wet" coupling, such as grease or a soft elastomeric interface. Metal tubing also has a high acoustic impedance, and sensor parameters must be finely tuned to the thickness of the tubing wall for effective ultrasound transmission.

Calibrating a bubble sensor

Calibration is performed by injecting bubbles of known sizes using a bubble generation system into the tubing and changing the sensitivity of the bubble sensor until the desired sensor performance is achieved.

Typically, calibration is performed with a large bubble, which must be detected by the bubble sensor, and a small bubble that must be ignored. Bubbles with a size between the large bubble and the small bubble may or may not be detected and will vary between sensors in a batch. Often, bubble sizes chosen for the final tests are tighter than the application demands to account for the performance non-repeatability of the viscoelastic tubing used.

Necessary size of the bubble required to be detected in tubing

The detectable bubble size depends on several factors, such as sensor design, calibration, tubing properties, and sensor mounting orientation. Generally, a bubble that fills the entire tube diameter will always be detected. With proper calibration, a sensor can usually detect a bubble that is as tiny as 50% of the inside diameter of the tube.

The orientation of the sensor relative to gravity can significantly affect the detectable bubble size. The sensor's ultrasonic beam travels through the center of the tube, creating small ultrasonic “blind spots” above and below the acoustic path (see Figure 3). Buoyant forces cause the bubbles to travel to the "top" of the tube concerning gravity. If the sensor is mounted, as shown in Figure 3, much of the bubble can enter the blind spot, making it difficult to detect.

As shown in Figure 3, position the sensor to detect the most minor possible bubbles, ensuring that a bubble will block as much of the ultrasonic beam as possible.

This is especially important in large-diameter tubes, where surface tension prevents bubbles from filling the entire diameter of the tube. A bubble in a large-diameter tube may elongate as it grows without occupying more of the tube’s diameter.

Figure 3: (a) shows an Acoustic blind spot in a bubble sensor (Source) (b) & (c) shows the Suggested sensor mounting position (Source)

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Conclusion

This technical article provides an overview of using bubble sensors in medical device applications. Ultrasonic technology can trace liquid, air, and bubbles in the tube to which the bubble sensor is directly mounted. The size of the bubbles in a sensor depends on the diameter of the tube. This method doesn’t introduce any contamination as it is noncontact detection. Different criteria must be followed while mounting bubble sensors with varying tube materials. Blind spots in the tube might affect the sensitivity; therefore, mounting the sensor considering the direction of gravity helps to overcome the air bubbles.