Healthcare Wearables: Body Signals

Healthcare wearables monitoring devices have taken great strides in functionality but there is still room for improvement.

The craze for step-counting wrist straps, prevalent during the last decade, seems to have been whittled down to the dedicated few – if social media posts can be considered an authoritative source. It’s not that wearable technology is palling, it’s more that functionality has increased and the dubious metrics of long walks as a health indicator have been replaced by more meaningful data harvesting. Device applications have expanded from activity tracking to continuous monitoring of heart rate, blood oxygen and blood glucose levels, body temperature, and more.

Despite lockdowns focusing people’s attention on more basic needs, like sourcing finances to buy toilet roll supplies, wearable tech sales have increased by five percent over 2019 figures, according to recently updated projections from tech market advisory firm ABI Research. This may not approach the healthy 23 percent boom originally expected but is still a modest move in the right direction.

The continued growth of wearable healthcare products has been enabled by the development of very-low-powered analog body sensors, digital microcontrollers, and innovative power and battery management circuits. ABI predicts that 2020 sales will be 254 million units, a pandemic readjustment from the 281 million units it originally forecast. Nonetheless, a number of issues related to reliability and accuracy must still be addressed before wearables truly pick up the pace to become more ingrained into daily living.

Nearly all of the human body signals traditionally monitored in a clinical environment can now be collected by a wearable product, very often with close to the same level of precision and at much lower price points. These devices now need to be highly reliable, as readings may be used for lifestyle adjustments or as an early warning sign of illness. To do so, biosensors must be designed to overcome measurement challenges stemming from factors such as rugged environments, sweaty skin contact, motion, and interference from ambient light conditions.

Welcome to the Real World: Reliability under real-world conditions means dealing with environments electronics do not usually have to tolerate.

The Right Connections

A key requirement for any wearable device is connectivity. Seamless wireless connectivity has pretty much become a given for today’s wearables. Wireless transfer allows data transmission to larger display screens or to remote data collection facilities. Low-power Bluetooth (BLE) is an emerging standard for this purpose. In addition, near-field communication (NFC) provides limited-range wireless connectivity that is well-suited for short content transfers such as configuration information and logged data retrieval.

When faced with developing a product such as a new fitness band, for example, the engineer needs to consider how much data will need to be transferred, how frequently, and over what range it will be sent. If the quantity of data that needs to be transmitted reaches megabyte levels, then the designer might well consider using Bluetooth Classic or Wi-Fi.

Range is the other determining factor. BLE can typically only communicate up to 30 meters in line of sight. This may seem like a limitation but BLE transmitters are smaller and less of a power drain than the only viable alternatives of Wi-Fi or cellular connectivity. What is more, use case factors come into play, such as communicating through a smartphone to forward data to the cloud for analysis.

Able to Take some Knocks

Many wearable systems are designed to be worn during sports and other rugged activities. “Ruggedness” is relative; the requirements or a life-saving device are different from those of an activity monitor worn by a cyclist.

Reliability under real-world conditions means dealing with environments electronics do not usually have to tolerate. These components include low-power, analog front-end (AFE) modules to convert real-world vibrations and temperatures to digital signals for multiparameter monitoring, plus embedded analog parts, such as op amps, current-sense amps, filters, and data converters, all of which are necessary for interfacing real-world sensors to digital systems.

Handy to Have Around: Pulse oximeters have gotten small enough for patients to carry around everywhere.

Electrical outputs from body sensors have very low strength, in the millivolt and microvolt ranges. Many of the sensors used in wearable health applications have to be combined with amplification and conversion circuits within a single die or package so that they emit a higher-level analog signal or a serialized digital signal.

Max the Signal, Cut the Power: The primary mission of a wearable PPG circuit is to maximize the signal-to-noise ratio (SNR) while conserving expended power.

Dealing with Flickering Lights

Photoplethysmography (PPG) is actually an uncomplicated and inexpensive optical measurement method often used for heart rate monitoring purposes and pulse oximetry (a test used to measure the oxygen level of the blood). PPG is a noninvasive technology that uses a light source and a photodetector on the surface of the skin to measure the volumetric variations of blood circulation.

In use, the optical sensor can also pick up ambient light. This is particularly troublesome because indoor lighting commonly flickers, and this can interfere with the PPG signal. Depending on the nature of local power supplies, indoor lights may flicker with basic frequencies of 50Hz or 60Hz, a rate close to the frequency at which PPG signals are sampled. Left uncorrected, ambient flicker can produce variations in each sample taken.

To counteract these effects, advanced PPG ICs now have intelligent signal paths. Algorithms, too, have grown more sophisticated and the net result means PPG devices can be produced in a variety of form factors, including earbuds, rings, necklaces, head and arm bands, bracelets, watches, or on smartphones.

Whatever the form factor, wearable sensors must be able to perform reliably while overcoming the effects of common noise and error sources. Environmental noises for PPG sensors typically fall into two major categories: optical and physiological.

Optical noise refers to changing characteristics of the optical path as seen by the sensor that are unrelated to light absorption by the volume of blood observed, as in the case of ambient flicker. Likewise, a physiological change, such as a sudden movement by the wearer, could cause mechanical displacements of the sensor with respect to the tissue, altering the sensor’s perception of blood flow and changing the volume of overlying tissue, which in turn alters the PPG signal.

Time-Saving Tools

A wearable healthcare device is an autonomous, noninvasive system that performs a specific biomedical function. These devices track heartbeat, body heat, blood oxygen, and electrocardiogram (ECG) signals. The sensors react to some sort of physical input and respond by generating a signal, typically in voltage or current form. This signal is cleaned and smoothed to make it easier to read, sampled at a suitable rate, then converted into a signal readable by processors.

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