2024-11-08
With the rapid advancement of technology, wearable devices have become an indispensable part of daily life. These compact and lightweight devices not only provide convenient ways to access information but also show great potential in health monitoring. This article mainly explores the relevant wearable sensor technologies and the principles behind sensors used in blood pressure monitoring, blood glucose monitoring, stress detection, and sleep quality tracking.
Wearable Sensors
Currently, heart rate monitoring, blood oxygen detection, and blood pressure measurement in products rely on PPG sensors and ECG sensors. Skin temperature can be measured with temperature sensors, while body movement and activity can be tracked using accelerometers.
1. PPG Sensor
PPG (Photoplethysmogram) sensors are biological sensors that use optical principles to monitor changes in blood vessel volume. The integrated PPG sensor consists of multiple LEDs (Light Emitting Diodes) and a photodetector (PD). The working principle involves the photodetector measuring the changes in reflected light from the skin's surface to form the PPG signal.
Under the light source, the blood volume in the skin undergoes three changes: some light is absorbed, some light penetrates, and some light is reflected. The intensity of the reflected light and the blood volume at the measurement site (such as the wrist or fingertip) changes with each heartbeat.
The measured signal can be divided into two components: the DC component, which is related to the absorption of light by skin pigmentation, fat, muscle, bone, etc., and the AC component, which is related to blood volume changes generated by the heart. The relationship between light intensity and time is called the PPG signal. The timing of each pulse in the PPG signal is affected by the heartbeat, and the amplitude is influenced by the concentration of different components in the arterial blood.
PPG is mainly applied to the following types of signal detection:
Heart rate monitoring: PPG sensors scan monitor heart rate in real-time, which is very useful for sports training, health management, etc.
Heart rate variability (HRV) analysis: By measuring the variation in the time intervals between consecutive heartbeats, HRV can evaluate autonomic nervous system activity.
Blood oxygen saturation (SpO2) measurement: By using both red and infrared light, PPG sensors can estimate the oxygen saturation in the blood.
Sleep monitoring: By tracking heart rate and HRV, combined with body motion detection, PPG sensors can assess sleep quality and stages. Interestingly, the combination of multispectral technology can also lead to the design of multispectral PPG.
Multispectral PPG (MW-PPG) technology is considered superior to single-wavelength PPG (SW-PPG). However, due to the limitations in sensor availability, many previous studies had to rely on traditional, bulky, and expensive spectrometers for detection, making MW-PPG technology impractical for daily life.
A team at the National Taipei University of Technology developed a chip-level multispectral PPG sensor using an innovative chip spectrometer, aimed at wearable applications. By combining signal processing methods, the device can reliably extract PPG signals, improving the signal-to-noise ratio (SNR) by up to 50%, enabling the measurement of parameters such as blood oxygen saturation and blood pressure.
The manufacturing process and material choices for PPG sensors are highly versatile, allowing for non-silicon-based implementations, providing advantages such as lower costs and greater design flexibility
2.ECG (Electrocardiogram) sensors are biological sensors used to monitor and record the electrical activity of the heart. They can detect the heart’s electrical signals and convert them into graphs to analyze the heart's health and function.
The working principle of ECG sensors is based on the electrophysiological characteristics of cardiac cells. Each heartbeat is triggered by the electrical activity of myocardial cells. When these cells depolarize and repolarize, small electrical signals are generated. The ECG sensor detects the voltage differences created by the heart's electrical activity through electrodes, amplifies the signal, and analyzes it with algorithms to obtain physiological information like heart rate.
Although both ECG and PPG can capture heart activity signals, their principles are different. ECG relies on detecting electrical signals from the heart, while PPG relies on optical signals. Each has its strengths and weaknesses. PPG excels in morphological diversity and biological variability, while ECG offers faster detection and is less affected by factors such as skin environment or body surface differences. Thus, ECG provides higher accuracy in heart rate monitoring.
3. Flexible Wearable Sensors
The next generation of wearable PPG systems urgently needs to achieve high detection rates, fast response times, and ultra-thin, flexible, and stretchable sensor modules, eliminating bulky power sources and supporting real-time detection applications.
Stretchable PPG sensors are a new type of optical sensor with the following advantages:
High comfort: Flexible materials allow them to fit the skin comfortably without causing irritation.
Good stretchability: They adapt to the movement and deformation of the skin, maintaining accurate measurements even during physical activity.
High measurement accuracy: They fit better with the skin and tissues, providing accurate measurements of heart rate, blood oxygen saturation, and other physiological parameters.
High integration: They can be directly integrated into various wearable devices such as smart patches, earphones, or rings, allowing users to monitor their health anytime and anywhere.
Wide potential applications: In addition to wearable devices, they can also be integrated into surgical instruments for auxiliary monitoring.
4. Body Fluid Sensors
Wearable body fluid sensors are an emerging field in medical monitoring devices, which analyze body fluids such as sweat, saliva, and tears to assess an individual's health. These sensors are typically integrated into wearable devices like smartwatches, fitness trackers, and smart clothing, providing real-time health insights.
Common body fluids include sweat, interstitial fluid (ISF), and saliva. This technology can also be combined with Point-of-Care Testing (POCT) to move wearable devices further into the realm of pathological diagnosis.
A common body fluid sensor is the sweat sensor. Wearable sweat biosensors can analyze the composition of sweat in real-time, providing valuable insights into health conditions by analyzing biomarkers in the sweat.
Principles of Different Physiological Signal Detection
Heart rate monitoring:
Optical reflection/transmission measurement: LEDs emit light that penetrates the skin and illuminates blood vessels, measuring the reflected/transmitted light. Because blood absorbs light at specific wavelengths, the amount of light absorbed changes with each heartbeat, allowing heart rate determination.
ECG signal measurement: Measures the electrical signals generated by the contraction of the heart muscle, similar to an electrocardiogram.
Electro-pulse measurement: Captures body vibrations caused by each heartbeat through high-precision sensors, which, after signal processing, can determine the heart rate.
Among health trackers, the optical reflection method is commonly used, relying on an LED light source and photodetector, known as PPG technology. The ECG sensor, based on electrical signals, can provide more accurate heart rate information.
Blood Oxygen Monitoring: Blood oxygen levels can also be measured through PPG sensors. During the heart rate monitoring process, the blood oxygen signal fluctuates in sync with heartbeats. By analyzing and processing the PPG signal and filtering out non-blood signals, the blood oxygen information can be extracted.
Blood Pressure Monitoring: Some wearable blood pressure devices use PPG and ECG sensors to estimate blood pressure through techniques such as pulse transit time (PTT). The time difference between the heart and the PPG signal recording point is used to estimate systolic pressure. However, for diastolic pressure, accuracy may be lower due to the need for constant calibration.
Blood Glucose Monitoring: Traditional invasive blood glucose monitoring uses electrochemical principles based on current and voltage. Optical glucose sensors use a clotting agent and fluorescent reagent to detect the concentration of glucose in a solution, though they require finger pricking. Micro-invasive technologies, including needle-based and laser-based sensors, have emerged to reduce discomfort and infection risk.
Sleep Quality Monitoring: Sleep quality monitoring involves a combination of multiple sensors and algorithms. Wearables use accelerometers to track body movement and photoplethysmogram sensors to analyze heart rate and ECG signals. Combined signal filtering and analysis help assess sleep duration, body movement, and heart rate variability, producing a sleep quality report.
Stress Detection: Stress detection is crucial for evaluating an individual’s psychological state and daily stress. Wearables typically use heart rate variability (HRV) and pressure sensors for this purpose. HRV sensors analyze changes in heart rate to assess stress levels, while pressure sensors directly measure changes in skin pressure. Combining these technologies provides highly accurate and sensitive stress measurements, particularly valuable for individuals with mental health conditions.
