With the general improvement of public health awareness, the demand for being able to understand the physical status of oneself or family members at any time and anywhere in daily life is becoming stronger and stronger. Smart bracelets have become the first choice for many people. Compared with professional medical equipment and invasive health monitoring methods, bracelet devices have the advantages of being small and portable, simple to wear, and non-invasive continuous monitoring.
At Xiaomi's new product launch conference on August 14, Xiaomi Mi Band 8 Pro was officially released , with new upgrades in screen size, display, functions, etc. It also made great efforts in health monitoring to achieve watch-level accurate monitoring. .
How does that little bracelet know your physical condition?
01
Heart rate monitoring and photoplethysmography technology
Friends who often wear bracelets will find that the back of the bracelet sometimes flashes lights. This is actually a sign that the health monitoring function of the bracelet is running. The flashing LED light, together with the photodiode next to it that receives light signals, forms the core of the health monitoring module.
The specific implementation of health monitoring is the use of photoplethysmography technology (Photoplethysmo- graph, PPG) . Unlike professional medical equipment such as electrocardiograms that rely on electrical signals or implanted devices in the body for monitoring, PPG uses light signals to reflect the human body's heart rate and blood oxygenation in a non-invasive way. This technology has been widely used in wearable devices. among.
When it comes to health monitoring, the first thing that comes to mind is heart rate. So, how does photoplethysmography technology use light to monitor heart rate?
First, the LED lights in the hardware module will emit a set of light signals to the skin tissue. After these optical signals are absorbed by human blood and tissues, part of the light will be reflected back and received and processed by the photodiodes in the module. Since blood absorbs green light better and can better capture changes in blood flow, heart rate monitoring usually uses green light with a wavelength between 525-540nm.
(Figure: Principle of photoplethysmography technology)
During monitoring, the absorption of light by tissues such as skin, muscles, and bones is basically stable, but the heart beats rhythmically and the blood volume also changes regularly, which causes the blood's absorption of light to change periodically. Therefore, when our heart contracts, the blood flow in the blood vessels increases, and the amount of light absorbed increases, making the reflected light signal weaker; when the heart relaxes, the blood flow in the blood vessels decreases, and the reflected light The signal will become stronger.
When the photodiode receives such a periodically changing light signal, it can convert it into an electrical signal, and use an analog-to-digital converter to convert it into a digital signal, and finally obtain the PPG signal during cardiac systole and diastole. Ups and downs, that is, pulse waves.
(Picture: PPG signal when heart rate monitoring is running)
Ideally, there will be a relatively obvious pulse wave in the PPG signal, and it will have periodic changes similar to those of the electrocardiogram. In this case, the heart rate can be calculated by calculating the heartbeat interval. Under quiet conditions, stress can also be estimated through the fluctuation of the heartbeat cycle, that is, heart rate variability (HRV), which is also the basis for stress monitoring.
However, when motion interference or improper wearing causes the sensor to be affected by external light, the PPG signal may contain a lot of noise, which will have a great impact on the heart rate calculation. This requires some digital signal processing technology to filter out the noise component. , and combine it with machine learning or deep learning models to infer heart rate based on the processed signals, such as spectrum tracking algorithms focusing on signal processing, or end-to-end denoising-regression model algorithms focusing on deep learning.
02
blood oxygen monitoring
After understanding the principle of heart rate monitoring, how is blood oxygen monitored? In fact, blood oxygen monitoring and heart rate monitoring also use photoplethysmography technology.
The blood oxygen monitoring function mainly monitors blood oxygen saturation. In the blood, the component responsible for carrying and transporting oxygen to various parts of the human body is hemoglobin, a special protein in red blood cells. Among the hemoglobins, the one that can carry oxygen is oxyhemoglobin (HbO₂), and the one that does not carry oxygen is reduced hemoglobin ( Hb ) , and the proportion of oxygenated hemoglobin in all bindable hemoglobin is what we often call blood oxygen saturation.
Unlike heart rate monitoring that uses green light, blood oxygen monitoring often uses red light and infrared light . This is because the level of oxygen contained in hemoglobin affects the blood's absorption of red and infrared light. Among them, hemoglobin saturated with oxygen will absorb more infrared light, and hemoglobin without oxygen will absorb more red light. This leads to different absorption ratios of infrared light and red light by reduced hemoglobin and oxyhemoglobin in the blood.
(Figure: Absorption coefficients of oxyhemoglobin and deoxygenated hemoglobin under different wavelengths of light)
Therefore, blood oxygen monitoring usually requires using two LED lights of infrared light and red light to illuminate the skin at the same time, measuring the absorption spectrum of the reflected light, and correcting the difference in the different absorption degrees of the two kinds of light by tissues other than blood, so that the two light signals can After comparing with the same standard, the corresponding blood oxygen saturation value is measured through conversion using a formula.
(Picture: PPG signal when blood oxygen monitoring is running)
03
Dual channel monitoring module
However, compared with many professional medical monitoring equipment, the method of using photoelectric signal detection will inevitably be interfered by external environments such as wearing gaps, ambient light and movement displacement. The wearer's skin color, hair, tattoos and other factors will also affect the monitoring results of the bracelet.
Faced with this challenge, Xiaomi Mi Band 8 Pro has specially upgraded a dual-channel monitoring module . There are two sets of front-end hardware modules running at the same time, which can prevent signal loss or noise problems caused by the loose fit of single-channel modules during use. It can also obtain a more accurate data model basis by fitting the two sets of data. This makes the captured heart rate and blood oxygen data more accurate. Compared with the previous generation of bracelets, Xiaomi Mi Band 8 Pro has a 10% increase in heart rate accuracy and a 5% increase in blood oxygen accuracy.
(Picture: Xiaomi Mi Band 8 Pro dual-channel monitoring module)
Not only that, Xiaomi Mi Band 8 Pro can also provide comprehensive health monitoring functions. In addition to simple monitoring, it also provides reminders to monitor abnormal conditions. All-weather heart health monitoring can detect some sporadic and short-term heart health problems in advance. For heart problems such as tremor, identify risks in advance and provide professional treatment in a timely manner. In a hypoxic state where the blood oxygen saturation is less than 90%, the bracelet will also automatically send notifications to remind you to pay attention to avoid the harm caused by hypoxic conditions to the cardiovascular and respiratory systems.
In addition to heart rate and blood oxygen monitoring reminders, the bracelet also supports all-day stress monitoring and breathing training. When you feel tired and stressed, you may wish to follow the guidance to breathe. Only by relaxing appropriately can you go a longer way. .
(Picture: Xiaomi Mi Band 8 Pro all-weather pressure monitoring and breathing training interface)
04
Independent GNSS chip: Don’t bring your mobile phone with you when exercising
While monitoring health, exercise is the only way to maintain health. Xiaomi Mi Band 8 Pro is equipped with 150+ sports modes and has a built-in independent GNSS chip. While positioning with the mobile phone, it can also perform independent positioning and display the movement trajectory on the bracelet. In the outdoor running record of Xiaomi Sports Health APP, you can also see the real-life map, recording every second of your exercise more intuitively.
(Picture: Xiaomi Mi Band 8 Pro outdoor running real-time track function)
In addition, Xiaomi Mi Band 8 Pro also has wrist-mounted running courses. Different courses have different exercise times, recommended heart rate ranges, and prompts through bright screen and vibration, providing more intuitive running guidance. If you don’t want to manually select a sports mode before exercising, the self-motion recognition in the bracelet can also automatically detect the exercise you are doing and enter the corresponding mode to help us establish good exercise habits.
(Picture: Xiaomi Mi Band 8 Pro running course function interface)
05
Big screen: free layout of diverse information
Faced with such diverse functions, the most important thing is how to allow users to obtain the required information more clearly during use. Xiaomi Mi Band 8 Pro uses a 1.74'' AMOLED high-definition color screen, which is the largest screen in the Xiaomi Mi Band series so far. Different from small-screen wristbands, on a large screen, the wristband no longer just displays various numbers monotonically. You can also move various charts to it. In the exercise module, you can even display real-life coaching courses on the bracelet to provide you with warm-up and stretching action guidance.
(Picture: Xiaomi Mi Band 8 Pro real-life coaching course)
In addition, Xiaomi Mi Band 8 Pro’s custom widget function allows you to freely adjust the widgets to display your commonly used function combinations on the bracelet. Heart rate, blood oxygen, sleep and other information can be integrated into one screen. between. Not only can it carry more information, but it is also more conducive for the elderly to view their own health data. No complicated operations are required, and the health information can be seen at a glance with just one swipe.
(Picture: Xiaomi Band 8 Pro widget page)
With the widespread application and technological updates of PPG, the health monitoring functions of wearable devices are constantly being upgraded and improved. However, compared with professional medical equipment, there is still a certain gap. Therefore, we are constantly thinking about how to improve the functionality of wearable devices, add more health monitoring modes, improve accuracy, etc.
At the same time, we are also exploring the possibility of wearable devices with more professional institutions, and welcome more ideas about bracelets. What other functions do you hope the bracelet can perform? Tell us in the comments section!
