Autoclavable Mass Flow Meters

Reusable Sensors for Proximal and Expiratory Flow Metering During Ventilation

Sensirion AG: The worldwide swine flu epidemic of 2009/2010, Ebola in Africa in 2014, the MERS outbreak in South Korea in 2015 - the list could be extended at will and makes one thing clear: that viruses and other pathogens can spread worldwide in no time. Hospital patients trust that they will receive the best possible care and treatment. But as the current debate regarding multiresistant germs has shown, pathogens can spread like wildfire even in hospital environments. If hospitals are to stop pathogens and germs efficiently, they need either disposable equipment or components that can be disinfected and sterilized after use.

Sensirion has been developing and marketing flow metering sensors for many years. Until now, these sensors have been used in ventilation and anesthetic devices to measure the volume of clean air administered to the patient. But apart from this clean tract, both ventilation and anesthetic procedures need a means of metering the flow of exhaled air. This is known as either proximal or expiratory flow metering. In both cases, the sensor comes into contact with air from the patient that is potentially infected with germs and pathogens. These sections of the apparatus are therefore comprised of disposable parts or hygienically sterilizable. For the sterilization process, a number of approaches are possible.

Conventional Metering Solutions

The solutions used today are either hot-wire anemometers or (variable) apertures combined with a differential pressure sensor. However, the sensor is not always exchangeable or sterilizable. In measurements involving the differential pressure process particularly, only the metering section is replaced. The actual sensor, however, is not replaced or sterilized. Indeed, in situations like these, long tubes running to the patient circuit should ensure that no air comes into direct contact with the sensor.

In contrast to this situation, hot-wire sensors can usually be replaced in their entirety or sterilized. The delicate hot wires, however, are relatively sensitive to mechanical stresses and strains: this can lead to problems during cleaning, particularly thermal disinfection.

New Reusable Solution

Now Sensirion has developed new flow metering devices that are autoclavable and can be washed. The new SFM3200-AW and SFM3300-AW mass flow meters1 are ideal for applications involving expiratory and proximal ventilation. The CMOSens® technology used by Sensirion is extremely resistant to environmental influences, which makes CMOSens® flow sensors ideal for applications where sterilization is required.

Sensirion’s newly developed flow sensors have been comprehensively verified. The chip-based sensor component was tested for more than 1000 hours at 150°C. Apart from this, test results are available which show that the sensor components have withstood 1000 temperature variation cycles involving switches from -40°C to 140°C. The results confirm the outstanding zero-point stability of the metering component and of the electronics that process the signals. Aging models have consequently suggested that the offset drift over a ten-year lifespan is smaller than 0.2 slm.

Apart from the sensor element itself, the main issue with sterilizable solutions is the choice of materials in the development process. In the new flow sensors, the plastic selected for the sensor housing is a polyphenylsulfone (PPSU) material. PPSU has outstanding thermal properties and is better suited to handling the inhalation anesthetics used for anesthetic purposes than other types of plastic.

In addition to these main components, particular attention was also paid to the adhesives and sealants used, with a view to achieving lasting stability and a high number of autoclaving cycles.

Stability Tested After Autoclaving and Cleaning

As part of the product development process, the SFM3200-AW and SFM3300-AW mass flow meters were meticulously checked for their suitability for treatment in an autoclave. Apart from this, the stability of the sensors was tested after immersion in and sterilization with CIDEX® Activated Dialdehyde Solution. The aim of the tests was to show that they can be used for these processes. Their suitability for sterilization in an autoclave was selected because steam is a particularly low-cost method of sterilization. An affordable form of processing also means a lower total cost of ownership for the hospital and by extension patients and their health insurance companies.

To test the suitability of the mass flow meters in the autoclave, 60 sensors – thirty of each type – were processed 50 times in the autoclave2. In each of the 50 cycles, the sensors underwent five minutes’ exposure to a maximum temperature of 135°C at an overpressure of +2.15 bar. Finally, the chamber was evacuated (-0.8 bar) and then cooled to 50°C at atmospheric pressure. An intermediate measurement was taken after every ten cycles. The sensors were tested for tightness and accuracy. The tightness test was carried out at 250 mbar overpressure and considered successful if no leak greater than 0.01 slm appeared. As regards precision, the sensor likewise passed the relevant tests and fulfilled the specifications listed in the data sheet3. The results of the precision measurement are shown in Figure 1. It is once again important to note their high-level zero-point stability. All sensors have a zero-point drift of less than 0.1 slm. Even after 50 cycles in the autoclave, 59 out of 60 sensors were still within the tolerances. In other words, the sensors can be prepared for subsequent use more than 50 times. Depending on the autoclave used, results may vary, although the temperature of 135°C represents a demanding process cycle.3

The test for washability with CIDEX® Activated Dialdehyde Solution was carried out on a total of eight sensors. After ten cycles each, involving 15 minutes immersion and subsequent drying, there were absolutely no observable effects. For this reason, additional testing was accelerated and the sensor was immersed in the solution for over 25 hours, corresponding to approximately 100 cycles. Finally, the test was carried out under the same conditions as for the autoclave. After testing, all eight sensors were fully functional and within the prescribed tolerances.

The details and results of these tests are available in a separate application note from Sensirion4.

Sterilizable Flow Sensors or Disposables

Apart from the need for reusable sensors, whether sterilized in the autoclave or simply disinfected, there are also applications where the use of disposable products is becoming increasingly widespread. With the right production volume, the CMOSens® Technology used by Sensirion brings down the unit price and is therefore suitable for disposable products. Sensirion is currently working on the development of a one-way product in the form of a simplified sensor without sieves. The one-way sensor is due for market launch within the next year.

Various Application Fields

Both the SFM3200-AW and SFM3300-AW sensors were developed for expiratory and proximal applications and are therefore suitable for use in both intensive and home ventilation. WEINMANN Emergency Medical Technology GmbH + Co. KG is set to become the first customer to use the sensor in the particularly demanding fields of emergency ventilation and ventilation during transport. During development, Sensirion liaised closely with WEINMANN Emergency with a view to optimizing the suitability of the sensor for its intended future use. Compared with clinical applications, emergency rescue services are confronted with several special problems, particularly when using the ventilation system at sub-zero temperatures. In the course of development, WEINMANN Emergency also tested the sensor for use in these difficult and demanding scenarios.

In view of its many years of experience with flow sensors for critical-scenario ventilation, WEINMANN Emergency has always kept a close eye on the benefits for patients and emergency service personnel during sensor development. Thanks to their experience, a number of important factors could be taken into account from the very start while developing sensors, which is why cooperation with the company has proved so constructive and effective. WEINMANN Emergency currently uses a hot-wire-based sensor and has also decided to use sensors from Sensirion. According to Ulrich Palm, head of development at WEINMANN Emergency, the company was convinced by the benefits of these sensors. "We decided to go with tried-and-tested CMOSens® technology for three reasons: the low-level breathing resistance, the high precision and stability of the metering signal, and the rugged mechanical and electrical design.”

Further Information

About Sensirion

Sensirion is the leading manufacturer of high-quality sensors and sensor solutions for the measurement and control of humidity, and of gas and liquid flows. Founded in 1998 as a spin-off from the Swiss Federal Institute of Technology (ETH) Zurich, the company is based in Stäfa near Zurich, Switzerland, and employs more than 600 people in countries such as the USA, South Korea, Japan, China, Taiwan, and Germany. The headquarters in Switzerland is responsible for research, development, and production.

Millions of Sensirion's sensor components and solutions are used all over the world, including in the automotive industry, medical technology and building technology. Sensirion's success is based on the innovative CMOSens® Technology, which combines the sensor and analysis electronics in a single semiconductor chip. This means large unit numbers can be produced at high quality and low cost. This makes Sensirion a preferred supplier of microsensors and sensor solutions.

Product Specifications

The new SFM3200-AW and SFM3300-AW sensors are both autoclavable and washable. They were designed for treatment in the autoclave and have undergone successful testing. The sensors are attached using medical cones to enable a pneumatic connection to the breathing circuit. Finally, the sensors have a mechanical interface for simple and reliable electrical reconnection.

SFM3200-AW

Thanks to the special design of the flow channel, the SFM3200-AW sensor achieves a low pressure drop, which makes it ideal for expiratory flow measurement in medical applications such as ventilation. The sensor measures the flow rates of air, oxygen and other non-aggressive gases extremely precise. It performs especially well with low flow rates but can also measure flow rates up to 250 slm. Moreover, the SFM3200-AW has a short signalprocessing time and is extremely robust.

SFM3300-AW

The sensor measures flow rates reliably up to 250 slm in both directions, making it ideal for proximal flow metering in medical ventilation and other respiratory applications. During development, a compromise between dead space and pressure drop was sought. Compared with the SFM3200-AW, the SFM3300-AW has a slightly higher pressure drop, which makes it more stable under the varying inlet conditions that occur during proximal metering.

OEM Solutions

Apart from its standard products, Sensirion also offers customized OEM solutions. We adapt or develop individual sensor solutions to meet clearly defined needs and specifications. Please contact us if you require another flow area or further modifications for your application.

About WEINMANN Emergency Medical Technology GmbH + Co. KG

Simply Professional – WEINMANN Emergency is an internationally active, family-owned medical technology company. With its mobile medical system solutions for emergencies, transport and disaster scenarios, the company sets standards in saving human lives. In close collaboration with professional users in emergency medical services, hospital and military services, it develops a range of innovative medical products to assist ventilation and defibrillation. For over 100 years, WEINMANN Emergency has offered its customers a high degree of reliability, experience and quality made in Germany.