1, the introduction of

Vital sign monitoring has gone beyond medical practice into many areas of our daily life.

Initially, vital sign monitoring was carried out in hospitals and clinics under strict medical supervision. Advances in microelectronics have lowered the cost of monitoring systems, making them more pervasive and common in areas such as telemedicine, sports, fitness and health, and workplace safety, as well as in automotive markets that are increasingly focused on autonomous driving. These extensions are implemented, but because these applications are highly health-related, the quality standards remain high.

Currently, vital sign monitoring involves measuring a range of physiological parameters that can indicate a person’s health status. Heart rate is one of the most common parameters and can be measured by an electrocardiogram, which measures the frequency of the heartbeat and, most importantly, how it changes.

Changes in heart rate are often caused by activity. During sleep or rest, the tempo is slower, but tends to increase with physical activity, emotional responses, stress or anxiety, among other factors. 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 vital sign. The degree of oxygenation in blood can be measured using a technique called photoelectric volume pulse wave (PPG). Hypoxia can be associated with attacks or disorders that affect the respiratory system.

Other vital signs that can reflect a person’s physical condition include blood pressure, body temperature and skin conductance responses.

Skin conductance response, also known as cutaneous electrical response, is closely related to the sympathetic nervous system, which in turn directly participates in the mediation of emotional behavior. Measuring skin conductivity can reflect a patient’s stress, fatigue, mental state and emotional response.

In addition, measurements of body composition, lean body mass and fat body mass percentages, as well as hydration and nutritional levels provide a clear picture of an individual’s clinical status.

Finally, measuring movement and posture can provide useful information about subjects’ activities.

2. Technology to measure vital signs

To monitor vital signs such as heart rate, respiration, blood pressure and temperature, skin conductivity and body composition, a variety of sensors are needed and the solution must be compact, energy efficient and reliable. Vital signs monitoring includes:

• Optical measurement
• Biological potential measurement
• The impedance measurement
• MEMS sensors were used for measurement

2.1. Optical measurement

Optical measurements go beyond standard semiconductor technology. In order to make this type of measurement, an optical measurement kit is required. A typical signal chain for optical measurements is shown below. A light source (usually an LED) needs to be used to generate an optical signal, which may consist of different wavelengths. Several wavelengths can be combined to achieve higher measurement accuracy. A series of silicon or germanium sensors (photodiodes) are also needed to convert light signals into electrical signals, also known as photocurrents. Photodiode must have sufficient sensitivity and linearity when responding to the wavelength of light source. After that, the photocurrent must be amplified and converted, so a high-performance, energy-efficient, multi-channel analog front end is needed to control the LEDS, amplify and filter the analog signal, and convert to analog with the desired resolution and precision.

Optical system packaging also plays an important role. A package is not only a container, but also a system containing one or more optical Windows that filter outgoing and incoming light without causing excessive attenuation or reflection that would compromise the integrity of the signal. To create compact multi-chip systems, optical system packages must also contain multiple devices, including leds, photodiodes, analog and digital processing chips. Finally, a coating technique that can create optical filters is often required to select the part of the spectrum required for the application and eliminate unwanted signals. The app must work even in sunlight. Without an optical filter, the size of the signal saturates the analog chain, making the electronics unable to function properly.

2.2 biological potential measurement

A biopotential is an electrical signal caused by the effects of electrochemical activity in our bodies. Examples of biopotential measurements include electrocardiogram (ECG) and electroencephalogram (EEG). They check for 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 measure that can provide useful information about the state of the body. Impedance measurements provide information about electrochemical activity, body composition, and hydration status. Measuring each parameter requires the use of a different measurement technique. The number of electrodes required for each measurement technique, and the point in time at which the technique is applied, vary depending on the frequency range used.

2.4 MEMS sensor measurement

The MEMS sensor, which can detect gravitational acceleration, can be used to detect activity and abnormalities such as erratic gait, falls or concussions, or even monitor a subject’s posture while at rest. In addition, MEMS sensors can be used as a supplement to optical sensors because the latter are susceptible to mobile artifacts. When this happens, the information provided by the accelerometer can be used to make corrections.