Fitness Trackers
Fitness trackers are ubiquitous. Hundreds of millions have been sold by companies such as Apple, Garmin, Huawei, Samsung, and Xiaomi. And they do more than count steps.
Today's trackers can measure heart rate, stress, and recovery; blood oxygen levels; skin temperature; and sleep quality. They can call for help if we fall or warn of a potential heart attack. Together, these features represent one of the greatest biomedical engineering triumphs of the past 20 years.
This is because bodies are really, really hard to measure. Take, for example, our gait. It differs from person to person. A tracker has to make sense of short and long steps, hands up or at our sides, a smooth street or a rough cross-country trail. And it must do that while moving on our wrist itself. It must also capture workouts when our wrists hardly move, such as biking, swimming, and weightlifting.
The first wrist trackers did this with chip-sized accelerometers, which measured the changes in electrical capacitance of a microscopically small cantilever suspended over an empty space. This worked for running but not other types of exercise. Engineers solved the problem by adding a chip-sized gyroscope and even a barometer and calibrating how those devices responded to a vast range of body types, gaits, and movements.
Heart trackers also proved challenging. The heart emits electrical signals, but they are only a few millionths of a volt when they reach the wrist. Pumping your arms generates signals thousands of times larger. Wrist trackers need extremely sensitive sensors and algorithms to isolate the pulse beneath the noise.
Some trackers measure pulse and oxygen level by shining light onto the skin and measuring its reflection. This varies enormously with skin pigment, fat, and bone structure. Engineers solved the problem by using multiple LED lights, each with a different frequency, and calculating pulse and oxygen from the difference in their reflections.
Today's advanced trackers will continue to evolve as biomedical engineers seek ways to measure the chemistry of sweat for markers of disease and other health problems.