In the traditional model of healthcare, a patient would visit a doctor regularly for checkups or for evaluations when there’s an ailment. This model, however, isn’t ideal for everyone. What if the patient misses early treatment opportunities because she sees the doctor when it’s late in the game? How about those with limited mobility, or who lack convenient access to a medical facility? Socio-economic factors that affect access to healthcare are another consideration.
Wearable sensors could therefore prove to be transformative, helping patients and doctors alike overcome some of the limitations in traditional healthcare. While these devices won’t replace the care that’s possible through an in-person doctor visit, they can provide valuable alerts to the patient and also help medical professionals remotely monitor patients.
Fitness trackers are a familiar sight, perhaps the first experience with wearable sensors for many of us. This technology, however, is becoming increasingly sophisticated, tracking a wider array of health parameters besides activity level. The powerful combination of sensors, machine learning algorithms, and wearable devices is opening up new possibilities in healthcare.
The Digital Healthcare Revolution
According to Forbes, digital healthcare could save $300 billion in health expenditures, particularly in the area of chronic disease management. Current spending in the digital health industry is estimated at roughly $200 billion annually. While wristbands and patches are now fairly common ways to wear a biometric sensor, other formats that we may soon see regularly, include clothing and jewelry. In industry and in academia, researchers are experimenting with techniques to create wearables that are unobtrusive and can deliver accuracy in measuring parameters such as blood glucose, blood pressure trending, blood oxygen, sleep quality, and hydration. There have even been trials of wireless electronic patches that monitor the vital signs of premature babies. Researchers have also created a prototype wristband device that stimulates sweat on demand in order to measure the biomarkers in perspiration. In industry, some of these technologies were on display at CES 2018. A Canadian startup, for instance, showcased its headband that uses neurofeedback to help manage stress and improve athletic performance. A South Korean company is researching a brainwave-monitoring headset that could be used to treat post-traumatic stress disorder. A startup based in the U.S. is developing a system involving sensors attached to socks, that can detect whether the patient is doing their prescribed exercises, taking their medication, or is in distress. California-based Spire is selling its Health Tag, a small, machine-washable, battery-powered device that can be affixed to clothing to track activity levels, heart rate, sleep quality, breathing patterns, and stress levels.
Consider the benefits of being able to continuously collect and analyze bio-metric data versus having certain vital signs only measured periodically in a clinical setting. Having continuous, aggregated insights could help you better manage an existing condition, or alert you to an issue that requires further evaluation. By remotely sharing this information with your doctor, you might be able to reduce the number of doctor visits and laboratory tests. Continuous monitoring of physiological signals could help to detect and diagnose diseases at their early onset. Also, real-time monitoring of an individual’s motion activities could be useful in fall detection, gait pattern and posture analysis, or in sleep assessment.
Electrical Heart Monitoring
Comfort is a key consideration for encouraging people to wear their sensors regularly, as they might be reluctant to wear something that interferes with their normal daily activities. Long battery life is also important, and since sensitive data is involved, security is another issue. From a design engineering standpoint, accuracy and precision of the sensors along with low power consumption and small form factor of the wearable device itself, are essential.
These technologies are growing more sophisticated, more accurate, smaller, and more cost-effective. This has made sensors a more practical component to include in a wider variety of applications than ever before. Deriving precision from sensors that are used in the challenging environment of the human body is no easy feat, however.
Biopotential measurements, for example, are used to assess the heart’s electrical signals. To collect these measurements, at least two electrodes are placed on the skin. The collected signals are conditioned and sent to a microprocessor for storage, calculation, and/or display. This method is used for electrocardiography (ECG), interbeat (R-R) intervals, and pacemaker signals. ECG and R-R interval measurements are used to support the diagnosis of certain heart conditions, such as arrhythmias. Since such conditions aren’t always obvious in a clinical setting (perhaps the heart event occurs in the evening, when the patient isn’t being monitored), wearable devices give doctors the flexibility to monitor their patients over extended time.
Bioimpedance (BioZ) measurements involve placing two electrodes on the skin to assess parameters such as respiration rate and hydration levels. They can also be used to measure the resistance of the thoracic cavity based on the response to an externally applied electric current. Collecting BioZ information requires addressing some conflicting challenges. In order for the devices to be portable and comfortable, the number of electrodes must be kept to a minimum, yet performing certain measurements requires more electrodes. The fact that these portable devices are powered by batteries presents another issue: the smaller the device, the smaller the battery, and therefore there is less available capacity. As a result, the devices should be optimized to consume as little current as possible, to extend battery life. Designers have typically had to turn to multiple ICs to meet these targets — an approach that is counter to the need to keep wearables small. For example, there are ICs that use single ECG and BioZ channels but need external circuitry to measure PACE signals and R-R intervals. There are also ICs that detect PACE signals and BioZ but need three pairs of electrodes to accurately record ECG, along with extra external circuitry to capture R-R data. Maxim’s MAX30001 analog front-end IC has been designed to meet these challenges. It requires input from only one pair of electrodes for both biopotential and BioZ data. It also measures PACE signals and R-R intervals in a single integrated package, and typically requires only 232μA.
Optical Heart Monitoring
Photo plethysmography (PPG) optical heart-rate monitors are another wearable technology. Such monitors generally rely on an optical measurement of the volumetric change of blood in tissue from the cardiac cycle. They typically use LEDs as the light source applied to the transmit path. Photodiode-based detectors on the receive path collect the light that refracts and reflects off of the blood-flow.
For a wearable device to gather PPG signals, it must be designed to overcome challenges pertaining to signal-to-noise ratio, power consumption, and motion compensation. Noise can come from the LED driver, ambient light, and from physiological changes due to body motions such as flexing an arm. Ambient light can cause errors on the receive path — strong constant ambient light can saturate the photodetector and time-varying light can distort its signal.
The Maxim MAX30112 integrated PPG sensor front-end circuit includes an LED driver and noise-compensation circuitry for the photodetector output current. For constant ambient light, it senses the ambient level when the LED is off and subtracts it from the photodiode output before sampling for the PPG, so that the photodetector will not saturate. Since time-varying ambient noise is typically at the power-line frequency, the IC implements correlated sampling techniques designed to attenuate any 50/60Hz flickering components.
Health Insights Wherever You Are
Thanks to new and emerging wearable technologies, we now have access to a level of health insights that was previously only possible via a visit to a doctor’s office or clinic. Sensors are getting smaller, lower power, and more precise. Integrating sensors, along with sophisticated algorithms, into wearable form factors is opening up new avenues to allow patients and doctors to focus more on prevention and management of chronic conditions. Rather than having periodic, one-time measurements of vital signs, we can now take advantage of the insights possible via continuously collected health data. As industry and academia continue their research and development into these technologies, one can only imagine the possibilities that await in digital health.