Power-Free Color-Changing Strain Sensor Enables Applications in Health Monitoring

Image: Researchers have designed a magnetoplasmonic strain sensor for wearable devices (Photo courtesy of Chemical Engineering Journal, DOI: https://doi.org/10.1016/j.cej.2024.155297)

Wearable devices and smart sensors are revolutionizing health and activity monitoring, enabling functions like heart rate tracking and body movement detection. However, conventional tools like stethoscopes and fitness trackers face several challenges. These include requiring user training, struggling with capturing subtle signals accurately, and being limited in flexibility and ease of use. These limitations hinder their effectiveness for tasks that require precision, adaptability, and ease of use, such as real-time health monitoring or motion tracking. To overcome these challenges, researchers have created an advanced mechanochromic strain sensor that changes color in response to mechanical stress. Their study, featured in the Chemical Engineering Journal, demonstrates the sensor’s potential as a versatile, power-free tool. This device integrates flexible polymers with innovative nanoparticles, offering a dependable, user-friendly solution for tracking health and activity in real time.

The sensor was developed by researchers at Chungnam National University (Daejeon, Korea) using magnetoplasmonic nanoparticles (MagPlas NPs). These nanoparticles feature a silver core (60 nm) and an iron oxide (Fe₃O₄) shell, which allow them to interact with light and magnetic fields. They are produced using solvothermal synthesis, a technique that controls chemical reactions at high temperatures to produce uniform particles in large quantities. A key design element of the sensor is the arrangement of the MagPlas NPs. When a liquid containing these particles is dropped onto a porous material, such as filter paper or a polyethersulfone (PES) membrane, and exposed to a magnetic field, the particles align tightly on the surface rather than sinking into the pores. This creates a uniform layer known as an amorphous photonic array (APA), which produces stable, bright colors that remain consistent when viewed from different angles.

These APAs are then transferred onto a flexible, stretchable material, polydimethylsiloxane (PDMS), enabling the sensor to change color when subjected to mechanical stress. By adjusting the nanoparticle size between 91 and 284 nanometers, the researchers controlled the sensor's color changes. The most noticeable shift, from blue to red, occurred when the particles were 176 nanometers in size. These color changes are fully reversible and stable, even after repeated stretching, ensuring durability and reliability. The sensor holds potential for a wide range of applications. In healthcare, it could serve as a wearable device to track movements such as bending the knee, turning the neck, or even subtle motions like heartbeats or eye twitches.