I’m explaining about the most commonly used sensors in medical wearable devices. These sensors play a crucial role in monitoring health status and managing diseases by measuring various biosignals.

Here’s a detailed explanation of the representative sensors, their functions, and their applications:

1. Photoplethysmography (PPG) Sensors:

• Measurement Principle: Measures changes in blood flow by transmitting light through the skin. It can measure heart rate, blood oxygen saturation, and blood pressure using the light absorption properties of hemoglobin in the blood.
• Functions and Applications:
  • Heart Rate: PPG sensors measure heart rate by emitting green LED light onto the skin and detecting changes in blood flow. It is used for monitoring heart rate during exercise and measuring stress.
  • Blood Oxygen Saturation (SpO2): Measures blood oxygen saturation using red and infrared light. It is used for managing respiratory diseases and detecting sleep apnea.
  • Blood Pressure: Technology for estimating blood pressure through waveform analysis of PPG signals is being developed. It has the advantage of being able to measure blood pressure more easily than the conventional cuff method.
• Advantages: Easy to miniaturize and low power consumption, making it suitable for wearable devices, and can be implemented with a relatively simple structure.
• Limitations: Sensitive to movement, and measurement accuracy can vary depending on skin color and blood circulation status.

2. Electrocardiography (ECG) Sensors:

• Measurement Principle: Measures the electrical activity of the heart to identify abnormalities in heart rhythm and electrical signals.
• Functions and Applications:
  • Atrial Fibrillation Detection: Detects irregular heartbeats and can help in the early diagnosis of heart diseases such as atrial fibrillation.
  • Heart Rate Variability (HRV) Measurement: Analyzes changes in heart rate intervals to assess stress levels and autonomic nervous system activity.
• Advantages: Because it directly measures the electrical activity of the heart, it provides more accurate information for diagnosing heart disease compared to PPG sensors.
• Limitations: More complex to implement than PPG sensors, and there is a possibility of noise due to movement.

3. Accelerometers:

• Measurement Principle: Measures the movement and acceleration of the device.
• Functions and Applications:
  • Activity Measurement: Measures steps taken, distance traveled, and calories burned to manage exercise and activity levels.
  • Motion Detection: Used for fall detection and sleep pattern analysis.
  • Posture Analysis: Can help correct bad posture by analyzing the user’s posture.
• Advantages: Easy to miniaturize and low power consumption, and provides various movement information.
• Limitations: It can be difficult to accurately distinguish between types of activities, and the accuracy of energy expenditure estimation is improved when used with heart rate sensors.

4. Gyroscopes:

• Measurement Principle: Measures the rotation and angular velocity of the device.
• Functions and Applications:
  • Movement Direction and Rotation Detection: Can be used with accelerometers to perform more accurate movement analysis.
  • Posture Stabilization: Used in VR/AR devices to track the user’s head movement and stabilize the screen.
• Advantages: Provides information about rotational movement, complementing the limitations of accelerometers.
• Limitations: Often used in conjunction with accelerometers and provides limited information on its own.

5. Skin Temperature Sensors:

• Measurement Principle: Measures the temperature of the skin.
• Functions and Applications:
  • Body Temperature Change Monitoring: Used for detecting fever and tracking menstrual cycles.
  • Sleep Environment Monitoring: Can be used to assess sleep quality by analyzing body temperature changes during sleep.
• Advantages: Relatively simple to implement and provides body temperature change information.
• Limitations: Can be affected by external temperature changes and does not accurately measure core body temperature.

6. Other Sensors:

• Electromyography (EMG) Sensors: Measures the electrical activity of muscles to analyze muscle movement and activity. It can be used for rehabilitation therapy and exercise effect analysis.
• Electroencephalography (EEG) Sensors: Measures the electrical activity of the brain to assess sleep state, stress level, and cognitive function. It can be used for diagnosing sleep disorders and brain-computer interfaces (BCIs).
• Sweat Sensors: Can analyze the composition of sweat to understand stress levels, hydration status, and electrolyte imbalances.
• Bioelectrical Impedance Analysis (BIA) Sensors: Sends a weak electrical current through the body and measures the resistance (impedance) to estimate body composition (body fat, muscle mass, etc.).

These sensors are used alone or in combination to provide more accurate and diverse information. More innovative medical wearable devices are expected to emerge with future sensor technology developments.