Currently, there are various types of skin sensors widely used in wearable devices, and each has a different biometric signal and technology used.
The main skin sensor types and their characteristics are as follows.

1. Optical Sensors:

• Photoplethysmography (PPG): This is arguably the most prevalent type. It uses light (typically green LEDs) to measure changes in blood volume in the skin. By analyzing how the light is absorbed by blood, PPG can estimate heart rate, blood oxygen saturation (SpO2), and even potentially blood pressure. You’ll find PPG in smartwatches, fitness trackers, and other wrist-worn wearables.
  • Advantages: Relatively inexpensive, low power consumption, and easy to miniaturize.
  • Disadvantages: Susceptible to inaccuracies due to motion artifacts (movement), skin tone variations, and poor blood circulation.

2. Electrical Sensors:

• Electrocardiography (ECG): ECG measures the electrical activity of the heart directly. It’s more accurate than PPG for heart rate monitoring and can detect irregularities in heart rhythm (arrhythmias). ECG is found in some smartwatches and dedicated ECG patches.
• Electromyography (EMG): EMG measures the electrical activity of muscles. It can be used to track muscle activity during exercise, monitor muscle fatigue, and even for prosthetic control.
• Electrodermal Activity (EDA) / Galvanic Skin Response (GSR): These measure changes in the electrical conductivity of the skin, which is influenced by sweat gland activity. EDA/GSR is often used to assess stress levels and emotional responses. It’s found in some wristbands and other wearables.
  • Advantages: Direct measurement of bioelectrical signals, generally providing higher accuracy than optical methods.
  • Disadvantages: Can be affected by sensor placement, sweat, and humidity.

3. Temperature Sensors:

• Thermistors: These sensors measure temperature by detecting changes in electrical resistance caused by temperature fluctuations. They are used to measure skin temperature, core body temperature, and even environmental temperature. You’ll find them in thermometers, temperature patches, and some smartwatches.
  • Advantages: Relatively simple and accurate for temperature measurement.
  • Disadvantages: Can be influenced by external environmental temperature.

4. Other Sensors (Often Used in Conjunction with Skin Sensors):

• Accelerometers: Measure acceleration (changes in velocity). They’re fundamental for step counting, activity tracking, and gesture recognition. Almost all wearables have accelerometers.
• Gyroscopes: Measure rotational motion. They improve the accuracy of motion tracking when combined with accelerometers, allowing for better activity classification and posture detection.
• Pressure Sensors: Measure force applied to the skin. They can be used for things like blood pressure estimation (though this is still an area of development) and touch interfaces.

Recent Trends:

• Sensor Fusion: Combining multiple sensor types (e.g., PPG and ECG) to improve accuracy and provide more comprehensive data.
• Flexible and Wearable Electronics: Development of smaller, more flexible sensors that can be integrated into patches, clothing, and even directly onto the skin. This improves user comfort and allows for continuous monitoring.
• Biochemical Sensors: While not as widespread yet, research is being done on sensors that can measure biochemical markers in sweat or interstitial fluid, such as glucose, lactate, and electrolytes. This has huge potential for health monitoring and personalized medicine.

These advancements in sensor technology are making wearables more powerful tools for health and fitness tracking, and they’re paving the way for even more sophisticated applications in the future.