The latest developments in medical sensors

16 October 2014

Air or gas embolisms, caused by small amounts of air entering the bloodstream and forming bubbles, can prove fatal. If air accidentally enters blood vessels through surgery, via an intravenous fluid line or during transfusion, the resulting bubbles can block blood flow, which may lead to a heart attack, stroke or thrombosis. While treatment is possible in the form of hyperbaric therapy, the unpredictable nature of air bubbles combined with the potential consequences, mean that prevention is pref

While bubbles can be caused by ascending from scuba diving (decompression sickness) or trauma to the lung, they can also enter the bloodstream via medical equipment, for example haemodialysis, chemotherapy or pulmonary bypass systems. Fortunately, the latest advances in ultrasonic air-in-line (AIL) sensors can detect even the smallest bubbles before they can enter the body.

Originally, ultrasonic AIL sensors were developed to detect the presence of an air bubble in the fluid delivery lines of medical systems as a failsafe mechanism. Connected to an alarm, medical personnel would be alerted to the presence of the bubble before it reached the patient. However, AIL sensors are developing to such an extent that in the near future  the option to detect the exact size of an air bubble in the tube will be possible.

This additional capability offers medical OEMs advantages over using standard sensors. In particular, the sensor can be modified to only detect bubbles above or below a certain size by modifying the detection thresholds to choose what size bubble gives a ‘dry’ signal. In non-critical fluid handling applications, this is a welcome development, as bubbles below a certain size may not affect the operation of the system, meaning bubbles of any size below that threshold can be allowed to pass. Once above the predetermined threshold the sensor will then see the bubble as air, generating the ‘dry’ signal.

As tubes in medical devices and drug delivery systems can be very narrow, precise engineering has allowed the development of sensors that can function in increasingly small-diameter tubes; currently down to 2.2mm (outer diameter). For compact or portable medical equipment, the ability to precisely measure bubble size in such small tubes is vital for patient safety. Morgan is also making further developments to work with even smaller tubes, aimed towards fluid dispensing systems.

Morgan’s high-sensitivity PZT (piezo) materials give the best possible starting point for ultrasonic sensors, and when combined with carefully designed electronics, result in excellent signal-to-noise ratios (SNR) and low power consumption. The sensitivity of the materials means bubble sizes down to just one microlitre in volume can be successfully and consistently detected.

Developments in sensor capability have occurred as a result of customer demand to detect smaller bubbles than ever before in safety-critical operations. While sensors have always been able to detect the presence of microscopic bubbles, their size meant that they were not always identified as air. With modified electronics working alongside integrated software, the true potential of ultrasonic AIL sensors can be unlocked.

One important factor in the development of more refined AIL sensors is the US Food & Drug Administration’s (FDA) requirements, and their adoption into CE product approvals, which has pushed the bar yet higher.

Stringent testing of flow, bubble and occlusion sensors is of paramount importance in configuring sensors to detect microscopic bubbles. Here, sensor designers work closely with OEMs, testing the sensor’s performance within the actual sized tubes which will be used in the final device. By tightly controlling the liquid flow rate through the tube, and introducing precisely-sized bubbles through syringes, the sensor can be calibrated for both the diameter of the tube and the bubble size. The behaviour of the bubble within the tube can also be manipulated to ensure detection in all circumstances. For example, where multiple bubbles may pass the sensor in quick succession, a high repetition rate is incorporated by increasing bandwidth through matching layer design.

When designing sensors for medical equipment, a variety of factors must be taken into account. For example, tubes may need to be removed for cleaning or replacement and then reinserted, and any movement or misalignment of the sensor could impact its performance. To reduce the possibility of accuracy being compromised, complex sensor channel design has been developed to give better repeatability even when tubes are changed and moved.

While ultrasonic AIL sensors of this kind are primarily aimed at medical applications and specifically drug delivery, they can also be used in any application where bubbles in liquid are not desirable or safe. Adherence to exacting medical standards such as ISO13485 means sensors are able to meet the quality demands of laboratory or industrial applications just as successfully.

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