Operating environment drives temperature sensor selection in medical applications
An understanding of a medical device’s operating environment and level of performance for a particular application can result in a more optimal temperature sensor choice. This article looks at some key medical device application areas and considerations for selecting a temperature sensor.
Negative temperature coefficient (NTC) thermistors and non-contact infrared (IR) sensors are basic building blocks for measuring temperature in an increasing number and diversity of medical devices.
An NTC thermistor is a high-precision sensor that measures temperature by leveraging the resistance properties of ceramic/metal composites.
A non-contact IR sensor works by detecting the infrared energy emitted by an object, which is proportional to its temperature. These sensors are primarily used in non-contact thermometers.
While each of these types of sensors relies on the same basic underlying operating principles—whether the application is measuring core body temperature or monitoring fluid temperature during dialysis—the performance requirements can vary tremendously.
Cardiac care
Temperature sensors are used in thermodilution (pulmonary artery) catheters, which measure cardiac output. The device works by measuring changes in temperature when a chilled fluid is injected into the heart. Two key sensor requirements are a small diameter (0.022 inches) and an extended lead length (> 6 feet), as the catheter itself is very long. A side benefit of a small sensor is a fast response time, which is desirable in this application. Small-diameter, chip-in-glass (or glass bead) thermistor assemblies are a popular option for this application.
Respiratory care
In respiratory care applications, temperature sensors are used in humidifiers that connect to ventilators. With direct exposure to a harsh, moist environment, a key requirement is that the sensor is hermetically sealed. Glass-diode chip thermistors are a good option for this application. Epoxy-coated chip thermistors can provide a tighter temperature tolerance when needed, but they will require further assembly to fully protect them from moisture.
Skin surface
In continuous patient temperature monitoring applications and neonatal incubator systems, repeatability and a fast response time as well as tight interchangeable tolerances are critical. The required temperature accuracy is typically +/-0.05C @ 37deg C. While some applications use disposable sensors, others may call for reusable sensors, which need to be more robust to survive cleaning. In a reusable probe, the thermistor is located at the tip of a plastic or metal shaft. Interchangeable thermistor assemblies are ideal for this application.
NTC Thermistors some in a wide rnage of shapes and sizes and are ideal for applications requiring a continuous change of resistance with temperature.
Thermometry
Digital thermometers—contact and non-contact—are used in a variety of applications including oral, rectal, tympanic and auxiliary temperature measurements. High accuracy and extreme repeatability are key performance requirements, especially in predictive thermometer applications. For this type of device, the manufacturer typically writes an algorithm to predict the temperature in < 10 seconds, as opposed to waiting for the device to come to equilibrium, which can take up to 3 minutes. Interchangeable thermistors that come in small packages are designed to meet these requirements. IR sensors are used when a non-contact measurement is needed.
Dialysis
In dialysis, blood is removed from a patient, filtered and treated, and then returned to the body. A temperature sensor is used to monitor blood temperature, ensuring that it is at body temperature before return. In dialysis applications, a temperature sensor is typically exposed to various fluids—from water and blood to cleaning solutions. Like cardiac catheter applications, the sensor must be small to make sure that the sensor tip (which is submerged in the fluid) does not impact flow. Interchangeability eliminates the need for the calibration of individual sensors over a broad temperature range.
Surgical
In laser surgery procedures, temperature sensors are used to monitor the temperature of the laser beam. In a cardiac ablation procedure, which uses a laser to treat heart rhythm problems, continuous monitoring of tissue temperature during the application of energy is required. To make some myocardial surgeries as non-invasive as possible, miniature chip-in-glass or sleeved chip thermistors with fine-diameter sensor wires are required to enable insertion into hypodermic needles.
Small, but mighty. A 3X3 mm2 infrared sensor consists of thermo-elements, flat infrared filter and a thermistor for temperature compensation.
Beyond the datasheet
In many medical device applications, the sensor itself and/or sensor assembly may require customization. Designers should determine the level of support provided by a particular sensor supplier, whether the datasheet is the start of a design and whether assembly is performed in-house.
Key performance attributes for sensors used in medical device applications
Amphenol Advanced Sensors designs and manufactures an extensive line of NTC thermistors and non-contact IR-based temperature sensors for medical devices.