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Vital sign monitoring has gone beyond the scope of medical practice and has entered many areas of our daily lives. Initially, vital signs monitoring was carried out in hospitals and clinics under strict medical supervision. Advances in microelectronics technology have reduced the cost of monitoring systems, making these technologies more popular and common in areas such as telemedicine, sports, fitness and health, and workplace safety, as well as in the automotive market that is increasingly focusing on autonomous driving. Although these extensions have been achieved, because these applications are highly related to health, they still maintain high quality standards. At present, vital signs monitoring includes measuring a series of physiological parameters that can show the individual's health status. Heart rate is one of the most common parameters, which can be detected by an electrocardiogram. The electrocardiogram can measure the frequency of the heartbeat, and most importantly, it can measure the changes in the heartbeat. Changes in heart rate are often caused by activity. During sleep or rest, the rhythm is slower, but it tends to speed up with factors such as physical activity, emotional reactions, stress or anxiety. A heart rate outside the normal range may indicate conditions such as bradycardia (when the heart rate is too low) or tachycardia (when the heart rate is too high).
Breathing is another key vital sign. The degree of blood oxygenation can be measured using a technique called photoplethysmography (PPG). Hypoxia may be related to the onset or disorder of the respiratory system.
Other vital sign measurement factors that can reflect the individual's physical condition include blood pressure, body temperature, and skin conductance response.
The skin conductance response, also known as the galvanic skin response, is closely related to the sympathetic nervous system, and in turn directly participates in mediating emotional behaviors. Measuring skin conductivity can reflect the patient's stress, fatigue, mental state and emotional response.
In addition, by measuring the percentage of body composition, lean body mass and fat body mass, as well as the degree of hydration and nutrition, the individual's clinical status can be clearly shown.
Finally, measuring movement and posture can provide useful information about the subject's activity.
2. Techniques for measuring vital signs
In order to monitor vital signs such as heart rate, respiration, blood pressure and temperature, skin conductivity, and body composition, various sensors are required, and the solution must be compact, energy-efficient and reliable. Vital sign monitoring includes:
- Optical measurement
- Biopotential measurement
- Impedance measurement
- Measure with MEMS sensor
2.1, optical measurement
Optical measurement goes beyond standard semiconductor technology. In order to perform this type of measurement, an optical measurement toolbox is required. The figure below shows a typical signal chain for optical measurement. A light source (usually an LED) is needed to generate a light signal, which may be composed of different wavelengths. Several wavelengths can be combined to achieve higher measurement accuracy. It is also necessary to use a series of silicon or germanium sensors (photodiodes) to convert light signals into electrical signals, also known as photocurrents. The photodiode must have sufficient sensitivity and linearity when responding to the wavelength of the light source. After that, the photocurrent must be amplified and converted. Therefore, a high-performance, energy-saving, multi-channel analog front end is required to control LEDs, amplify and filter analog signals, and perform analog-to-digital conversion according to the required resolution and accuracy.
Optical system packaging also plays an important role. The package is not only a container, but also a system containing one or more optical windows, which can filter out and inject light, but will not produce excessive attenuation or reflection, thereby compromising the integrity of the signal. In order to create a compact multi-chip system, the optical system package must also contain multiple devices, including LEDs, photodiodes, analog and digital processing chips. Finally, there is usually a need for a coating technology that can create optical filters to select the part of the spectrum required for the application and eliminate unwanted signals. Even in the sun, the application must be able to operate normally. If there is no optical filter, the size of the signal will saturate the analog chain, making the electronic devices unable to work properly.
2.2. Biopotential measurement
Biopotential is an electrical signal caused by the effects of electrochemical activities in our body. Examples of biopotential measurements include electrocardiogram (ECG) and electroencephalogram. They check very low-amplitude signals in frequency bands with multiple interferences. Therefore, before processing the signal, it must be amplified and filtered.
2.3. Impedance measurement
Bioimpedance is another measurement method that can provide useful information about the state of the body. Impedance measurement provides information about electrochemical activity, body composition, and hydration status. Measuring each parameter requires the use of different measurement techniques. The number of electrodes required for each measurement technique and the time point of applying the technique vary depending on the frequency range used.
2.4, MEMS sensor measurement
MEMS sensors can detect the acceleration of gravity and can be used to detect activities and abnormalities, such as unstable gait, falls or concussions, and even monitor the subject's posture while resting. In addition, MEMS sensors can also be used as a supplement to optical sensors, because the latter are susceptible to movement artifacts; when this happens, the information provided by the accelerometer can be used for correction.
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