Ultra-accurate temperature sensing
How proven sensor technology in wearable temperature monitors offers valuable medical insights.
Why are new personal health monitoring technologies enjoying increasing popularity in both medical and private settings? To find conclusive answers to this key question, let’s first take a closer look at three global trends:
- Almost everyone today owns a smartphone, putting a high-performance computer and always-on intelligent medical sensor hub and display screen in people’s pockets.
- Populations are rapidly aging across the highly industrialized world – and older populations have higher disease rates. At the same time, younger people are more interested than ever in discovering ways to extend their healthy lifespan. These individuals, as well as their medical practitioners, can now take advantage of new technology which helps them optimize their lifestyle and make healthy choices about factors they can control, such as exercise, diet, sleep and stress.
- Through the magic of Moore’s Law, semiconductor technology continually reduces the size, improves the performance, cuts the cost and lowers the power consumption of electronics functions such as computing and sensing. This means that medical sensors which at one time could only operate in expensive hospital equipment can now be accommodated in affordable portable or wearable devices.
This is leading to a blurring of the lines between medical and consumer devices, as even private individuals can now use products of their own – smart watches, lifestyle-tracking wristbands, smart rings, or even earbuds – to track important personal measurements of their health and lifestyle, such as blood pressure, heart rate, and quality of sleep.
At the same time, healthcare facilities are adopting digitalized modes of healthcare in hospitals and outpatient settings to enable the continuous, automatic monitoring of patients without the intervention of practitioners. Smart skin patches, wristbands, and other wearable devices continually upload patient data to a central portal, where Artificial Intelligence (AI) algorithms and other software components monitor measurement values, and alert medical experts whenever human intervention might be required.
Core body temperature: a powerful diagnostic indicator
Previous advances in sensor technology have enabled consumer wearable devices to achieve an almost medical-grade quality of measurement of many of the body’s vital signs, such as blood pressure, heart rate and blood oxygen saturation.
But another vital sign, core body temperature, provides information without which certain types of diagnosis or monitoring are rather difficult or even impossible: core body temperature, for instance, is a highly accurate indicator of the status of a woman’s cycle of fertility, stress, and sleep quality. It also provides a potentially life-saving early warning of the onset of fever or sepsis.
Outside the medical arena, core body temperature monitoring is even being used to fine-tune the training of endurance athletes, enabling them to maximize their training effort without overstressing their body.
A previously unheralded product in the ams OSRAM sensor portfolio is now moving to the center of attention, enabling new types of wearable consumer and medical devices for core body temperature measurement, which are increasingly entering today’s market.
The AS6221 digital temperature sensor, for example, is the result of decades of accumulated expertise and intellectual property in the design and fabrication of ultra-accurate mixed-signal semiconductors. The unique capabilities of ams OSRAM are behind a temperature sensor which matches the requirements of the manufacturers of these innovative core body temperature monitoring devices: more accurate, smaller, and more energy-efficient than any comparable product, the AS6221 is the ‘hidden hero’ which advances the technology of personal health monitoring to a new level.
Optimized for body temperature measurement
The appeal of advanced temperature measurement is in its accuracy. The maximum measurement error of ±0.3 °C over the AS6221’s entire operating range of -40 °C to 125 °C is truly impressive, but the special feature is that its accuracy is optimized for use in medical applications. Maximum error falls to ±0.10 °C between -25 °C and 55 °C, and to just ±0.09 °C between 20 °C and 42 °C. Normal body temperature is around 37 °C.
This extremely high accuracy has been crucial to the development of a new generation of core body temperature monitoring systems, such as the CALERA® technology from our partner greenteg. These systems work by calculating the heat flux based on the skin temperature, not by measuring the core body temperature directly. (Traditionally, the core body temperature is measured through the insertion of invasive probes, or the ingestion of expensive electronic pills.) Sophisticated algorithms calculate a value for the core body temperature from the heat flux at the skin's surface.
In the conversion from a direct skin temperature measurement to an indirect core body temperature measurement, there is some inherent error. Any inaccuracy in the skin temperature measurement is amplified when converted to a core body temperature measurement.
This means that products such as the CALERA® technology require the highest possible accuracy in the device that measures temperature at the skin. This is where the sensor’s maximum ±0.09 °C error is so valuable. Size is also an important factor, and it is essential that the component is suitable for use in skin patches and other portable devices.
greenteg CALERA® technology and the VSM (Vital Sign Monitoring) reference design from ams OSRAM worn on an athlete’s wrist
Wide range of uses for ultra-accurate temperature measurement
This example shows that temperature measurement has a vital role to play in professional and consumer medical monitoring, as well as in sports equipment to assist endurance athletes in optimizing their training – but there are many other uses besides.
For instance, the safety equipment worn by firefighters must include a means to track body temperature and send an alert whenever a firefighter is at risk of dangerous overheating. Hyperthermia, i.e. a dangerously overheated body, is also a serious risk in the hottest regions of the world, and even more so as the climate crisis causes average ambient temperatures to rise. A reliable solution in a smart watch or wristband can be combined with application software to send alerts to emergency services or loved ones when the wearer is at risk.
Accurate temperature measurement can also play a crucial role in other medical applications. For instance, the enzymatic measurement of blood glucose levels via a skin patch requires accurate temperature measurement for the real-time calibration of the sensor’s measurement output. Temperature sensors can also operate in dialysis machines, blood transfusion equipment, or insulin pumps, to enable blood or insulin to be heated to match the patient’s body temperature before it enters a vein.
In these and many other applications, extreme measurement accuracy is of very high value – and in the range of the body’s temperature, accurate technologies deserve their moment in the spotlight.