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Sensor technology driven by usability

Sensors are at the technological heart of many treatment devices. Industry representatives regard sensors as key technology. By consistently miniaturising measurement probes and increasing functional versatility, manufacturers are guaranteeing growth fields for the future – especially when they score with user friendliness, in other words usability. There is no doubt that current sensor development must focus on devices that are safe to use and intuitive to operate. These features are combined in the term ‘usability’, which is the guiding principle behind many medical applications. For example, devices for determining breath and blood pressure, impedance measurement for cannula positioning in tissue or blood gas values, i.e. oxygen and carbon dioxide pressures. There is currently a surge in development work on implantable sensors, one example of which is a haemodynamic sensor for the treatment of patients with cardiac insufficiency. The blood pressure sensor is located in a biocompatible housing and is designed to measure precapillary pulmonary artery pressure close to the heart in the body and transmit the readings wirelessly to a receiver station. 

Some questions concerning the long-term stability of the telemetric measurement capsule still remain to be resolved. At the Aachen Institute of Materials in Electrical Engineering, it is assumed that, if properly encapsulated, the fully implantable microsystem will have a useful working life of approximately six months – long enough to detect a dangerous rise in pressure early enough in patients at-risk.
A glance at the sensor sector quickly reveals that it is making a valuable contribution to medical technology applications. And moreover – it provides a wealth of measurement systems which can deliver a usable electrical signal at any physical value, from the ultrasonic and microwave sensor with a hygiene-friendly clamp process connection to high-sensitivity microsystems in stainless steel or the most minute components for medication dosage systems. 

It takes more than an initial glance to reveal the wonders of a modest-looking differential pressure sensor the size of a sugar cube. On closer inspection, however, the technical details and the metrological character of the device become apparent, because it is capable of a complete measurement process for very weak pressure differences in an extremely confined space.
For example, the LBA differential pressure sensor from Sensortechnics is based on the thermal measurement of mass gas flows in a very small flow channel integrated in the sensor chip. The sensor measures the gas pressure even in very low ranges from 25 pascal (0.25 mbar), a vital capability for medical technology applications. 

Very rapid response times
Analogue CMOS signal processing enables the sensors to generate linear and temperature-compensated output signals and operate with very rapid response times of less than ten milliseconds. Customer-specific solutions with two measurement elements and sufficiently powerful resolution and accuracy across the whole measurement range are available to meet the specific requirements of respiration machines. This involves one sensor chip located in the middle of the main flow registering very small flows around the zero point. A second chip is placed in a bypass channel and measures the higher flows up to the measurement range emission value. With the aid of a microcontroller, the sensors steplessly switch between the two flow ranges. 

Where there is a need to measure flows of sound-transparent liquids in flexible tubes, medical technicians often switch to clamp-on ultrasonic sensors. These dice-sized measurement devices can be refitted without interrupting the continuous operation of a medical supply unit. Response times of less than 100 milliseconds and reliable detection of very small liquid quantities of just a few millilitres make this sensor genre eminently suitable for a variety of supply units.
The latest addition to the Sonoflow family from Sonotec is a clamp-on device into which the tube with liquid flowing through it is inserted. All the electronics are integrated into the sensor. The clamp-on flow meter is ready for use after just a few manipulations. The data flow is delivered via a cable to the system control unit which also displays it. The flow meter is suitable for heart-lung machines. 

Clamp-on ultrasonic sensors can also detect air bubbles. The Sonotec product family known as Sonocheck covers contactless measurement sensors which are used in tubes containing liquid flows and in measurement chambers. The intelligent sensors can monitor liquids and can also deliver wet/dry reports. The high stability of the measurement values, measurement cycles of 200 microseconds and a response time of one millisecond make these sensors highly suitable for medical applications such as dialysis machines, infusion systems, heart-lung machines and contrast agent injectors. The company states that the sensors detect bubbles having a minimum size of one third of the internal diameter of the tube. 

Miniaturisation and electronics integration lead to increasingly compact sensor housings with independent power management and a growing range of functions. The burgeoning impact of silicon is unmistakable: this measurement element is capable of registering a range of physical values including force, humidity and temperature, as well as pressure and acceleration.
Andreas Beuthner 

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German Summary
Bei vielen Therapiegeräten zählen Sensoren zum Herzstück der Technik. Branchenvertreter sprechen von einer Schlüsseltechnologie. Durch die weitere Miniaturisierung der Messfühler und eine zunehmende Funktionsvielfalt sichern sich die Hersteller zukünftige Wachstumsfelder – vor allem, wenn die Benutzerfreundlichkeit (Usability) stimmt. Der deutschsprachige Beitrag ist nachzulesen auf www.meditec-international.com/medi0611sens