Types of Sensors Used in Wearable Devices

sensors in wearable medical devices
Electrocardiogram

For most, the first object that pops into your head after hearing the term “wearable” is likely some sort of fitness-associated wrist wear. The wrist is an ideal place for a tracker such as a FitBit or an Apple Watch, as it is a spot where pulse can be easily located and measured, while at the same time allowing the sensor to measure movement (most often in increments of steps based on specific arm movements). However, moving away from fitness wearable devices and into the realm of wearable medical devices, the information gathered for analysis changes. Generally speaking, medical wearable devices collect different information than that of a fitness tracker. As the market for different types of wearables grow, different types of sensors with different functions are developed. What types of sensors are used in wearable devices that allows for the analysis of data on both a fitness and medical scale?

On the most basic level, all sensors used in any wearable device must be biocompatible. This simply means that the sensor can safely interact with the body – a biological form – without causing damage or detriment to the form. Beyond that, due to the different functions that these devices can include, the internal sensors vary from device to device. Some common sensors that are used in wearable medical devices include temperature thermistors (detects skin surface temperature), piezoelectric sensors (used for monitoring heart rate), and photoplethysmography (the detection of blood volume in a specified skin area).

In the 1980s, Micro-Electro-Mechanical Systems (MEMS) were introduced to the wearable device industry as tiny, inexpensive and very reliable sensors, and since then have become a major part in the development of wearables. “MEMS” is a universal term that covers miniature electrical systems that have the functionalities to collect and record the data based on external stimuli, then to organize the data in a way that benefits the user.

Here are a few of the most common wearable sensors highlighted:

IMUs

As of 2016, the most commoditized type of sensor included in wearable devices is Inertial Measurement Units (IMUs). This is the sensor that collects and calculates data about your sleep patterns, how active you are, and where you are. Wearable fitness trackers are all outfitted with IMUs, which is one of the easiest biocompatible sensors to manufacture. As the fitness wearable market continues to grow, companies are beginning to compete with each other for the most high tech tracker, leading to new technological outfitting and additional sensor processes. While the most common (and trending) location for fitness wearables remains on the wrist, many companies are expanding into clothing, undergarments, and other jewelry accessories.

Electrochemical Biosensors

IMUs are also involved in the development of certain medical wearable devices, but in these cases they are often designed alongside sensors that have specific processes or functions that have to do with treating or diagnosing an ailment of a specified user. Chemical sensors tend to be the most prevalent sensor type in medical wearables. Chemical sensors transform chemical information into analytical information that can lead to diagnoses or treatment developments. These sensors could be used in devices that monitor the presence of a certain chemical in a biological form, the concentration of a chemical in the wearer, chemical activity, and more. A chemical sensing wearable could be used as a diagnostic tool for chemical imbalances, ingestion or absorption of toxic substances, sicknesses like Multiple Chemical Sensitivity (MCS), and other chemical related ailments.

Wearable Electrodes

Wearable electrodes are often stuck directly against the skin in order to read electric pulses from the heart – however, there have been great strides in not only electrode to wearable medical clothing integration, but the integration is secured to a point where the clothing can be washed without sensor removal. Electrodes in wearable devices can be used to provide medical practitioners with a constant EEG, EKG, or even an EMG in a specific part of the body over an extended period of time. Electrodes can also be included in wearables designed to automatically defibrillate if it senses that the pulse has stopped.

The sensors included in wearable devices depend entirely on the function and design of the device. While many wearables are outfitted with an IMU, medical wearable devices are fitted with a variety of different sensors depending on the devices function.

Further Reading and Sources: Sensors Online, MEMS and Nanotechnology Exchange