An expert partner for developing new vital sign monitoring products

The adoption of wearable technology has given rise to the idea of the ‘quantified self’. This marks a profound shift in the way people think about personal health. Previously, people could follow a set of guidelines or principles that were associated with good health outcomes. This advice generally revolved around healthy eating, taking regular exercise, sleeping well and so on. People could take occasional health checks at a clinic, but there was no way for a person to measure continuously the effect of their lifestyle on their health.

The introduction of wearable devices offered the promise of more information and created a demand from consumers to know more about their health status in real-time: actual measurements that show whether their lifestyle is making them either more or less healthy. This is part of a trend, supported by the increased use of data analytics, artificial intelligence and other advanced technologies in western healthcare systems, called ‘4P healthcare’: participative, predictive, preventive and personalized. 

The key measurements of health are the vital signs that medical practitioners have long relied on: factors such as heart rate, blood oxygen saturation (SpO2), blood pressure, body temperature, and heart activity as measured electrically by an electrocardiogram (ECG). Thanks to sophisticated optical and electrical semiconductor systems and software from ams OSRAM, wearable devices can perform these vital sign measurements well – in some cases, almost as accurately as the specialist equipment in hospitals. 

In fact, the technology is progressing to a point that it is possible to see a merging of consumer and medical technology. Patients might soon, for instance, expect to be able to attend an appointment with a cardiac specialist armed with a log of real-time heart rate and ECG measurements taken from their smart watch. 

This blurring of the lines between consumer and medical technology depends on the accuracy and reliability of the data sourced from wearable devices. Fortunately, the measurement performance of wearable devices constantly improves. That’s in large part because of improvements in the optical and electrical components they use. 

A smart watch or wristband measures the heart rate and blood oxygenation optically. LEDs emit light into the tissues of the wearer’s arm, and a photodiode measures the amount of light scattered to the detector. Differences in the volume of blood in the arteries, and its oxygenation, are mirrored by variations in the intensity of scattered light detected by photodiodes. This optical measurement technique, called photoplethysmography (PPG), enables a wearable device to track the action of the heart and lungs. 

From an optical engineering point of view, the wrist is a challenging place to operate LEDs and photodiodes – it is hard to maintain a stable interface between the optical components and the wearer’s skin. This means that a vital sign monitoring system needs a combination of 

• LEDs that emit bright light at specified wavelengths
• Photodiodes that are highly sensitive to these wavelengths 
• An analog front end (AFE), an IC which provides the current for the LEDs, as well as amplifies and digitizes the photocurrent signal from the photodiodes

The whole system is optimized for low noise and high signal-to-noise ratio (SNR), while consuming little power so as to extend the device’s battery run-time as long as possible. To meet these requirements, ams OSRAM recently introduced specialized new components for vital signs applications. 

The new SFH 7018 is a multi-LED (red/green/infrared) emitter with an ingenious two-cavity design which minimizes interference between the green light (for heart rate measurements) and the red and IR sources (for SpO2 measurements). The SFH 7018’s red and IR emitters are 40% brighter than the previous generation product, and the green LED is more than twice as bright. This improved brightness allows to reduce the LED current and the power consumption accordingly while keeping the signal quality at the same level.

The TOPLED® D5140, SFH 2202 photodiode outperforms standard photodiodes that have often been used in wearable devices. Offering higher sensitivity and much higher linearity, the SFH 2202 enables wearable device manufacturers to improve heart rate and SpO2 measurement in challenging ambient light conditions while reducing power consumption.

ams OSRAM also introduced the AS7058, an AFE which drives LEDs such as the SFH 7018 and processes the signals received from sensors such as the SFH 2202 or the electrodes of an ECG. The striking feature of the AS7058 is its low-noise performance, producing a very high optical signal-to-noise ratio of up to 120dB, and similarly high performance in ECG measurements. Because the wrist is a challenging place to take optical measurements, the optical signals generated by the photodiode are weak: the low-noise AS7058 extracts the greatest possible amount of information from these weak signals, and from the low-voltage output of ECG electrodes, to enable the device’s algorithms to calculate accurate measurements of vital signs. 

The AS7058 also provides measurement outputs for additional vital sign parameters: electrodermal activity, for measuring sweat, and – uniquely – body impedance (BioZ), which can be used for approximate measurements of body composition and the body mass index (BMI). 

To put these components – emitters, sensors, AFE and associated devices – together in an innovative product prototype and to optimize operation is an intensive development task. Support from our experts in vital signs system design has proven to be extremely helpful. 

We create a unique advantage as components such as the SFH 7018, SFH 2202 and AS7058 are designed to work together, as shown by demonstration designs ams OSRAM provides to customers. The ams OSRAM products are also backed by ready-made algorithms for calculating heart rate, SpO2 and respiration rate. 

Adding to its vital sign monitoring offering, ams OSRAM also offers an ultra-accurate digital temperature sensor. Supplied in a 1.5mm x 1.0mm WLCSP package, the AS6221 offers very low power consumption, and is highly accurate at body temperature: its accuracy is ±0.09° between 20°C and 42°C, making it ideal for use in medical and health monitoring products.