Chungnam National University researchers develop power-free color-changing strain sensor

Wearable devices and smart sensors are transforming how we monitor health and activity, from tracking heartbeats to detecting body movements. However, traditional tools like stethoscopes and fitness trackers often face challenges. They require user training, struggle with accurately capturing subtle signals, and are limited in flexibility and ease of use. These shortcomings make them less effective for applications that demand adaptability, precision, and user-friendliness, such as real-time health monitoring or motion tracking.

To address these challenges, Professor Jaebeom Lee and his team at Chungnam National University have developed an advanced mechanochromic strain sensor that changes color in response to mechanical stress. Their study, published in Volume 498 of the journal Chemical Engineering Journal on 15 October, 2024, highlights the sensor’s potential as a power-free, versatile tool. The device integrates flexible polymers with innovative nanoparticles, offering a reliable, user-friendly solution for real-time health and activity tracking.

The sensor is built using magnetoplasmonic nanoparticles (MagPlas NPs). These nanoparticles have a silver core (60 nm) and an iron oxide (Fe₃O₄) shell, which help them interact with light and magnetic fields. They are produced using a method called solvothermal synthesis, which controls chemical reactions at high temperatures to create highly uniform particles in large quantities. “This nanosized material can be synthesized with exceptional consistency and scalability,” explains Prof. Lee.

A critical part of the sensor's design is the arrangement of MagPlas NPs. When a liquid droplet containing these particles is placed on a porous material, such as filter paper or a polyethersulfone (PES) membrane, and exposed to a magnetic field, the particles pack tightly together on the surface rather than seeping into the pores. This forms a uniform layer called an amorphous photonic array (APA), producing bright, consistent colors that remain stable when viewed from different angles.

These APAs are then transferred onto a flexible, stretchable material called polydimethylsiloxane (PDMS), which enables the sensor to change color under mechanical stress. By adjusting the nanoparticle size between 91 and 284 nanometers, the researchers controlled how the sensor’s color changed. The most noticeable color 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, making the sensor durable and reliable.

The sensor could transform many areas, offering a wide range of applications. In healthcare, it could be used as a wearable device to track motions like knee bending, neck turning, or even subtle movements such as heartbeats or eye twitches. The sensor could also ensure the safety of buildings and bridges by visually detecting stress or damage without complicated setups. “The mechanochromic change of the device could be monitored constantly, to predict and prevent fatal structural failures for buildings, civil structures, and industrial systems,” explains Prof. Lee.

Looking ahead, the sensor could enable new possibilities for dynamic displays and secure data storage. For example, researchers created a special “data matrix” code that is only visible when the sensor is stretched. In the next 5 to 10 years, these power-free sensors could become key in developing sustainable, eco-friendly technology. Their ability to work without power makes them ideal for use in remote or extreme environments like deep-sea missions or space exploration. “Power-free sensors and optical devices have a great amount of impact on the future of sustainable and green technology,” adds Prof. Lee.

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Reference                                    

Title of original paper: Mechanochromic strain sensor by magnetoplasmonic amorphous photonic arrays

Journal: Chemical Engineering Journal

DOI: https://doi.org/10.1016/j.cej.2024.155297

About the institute

Chungnam National University (CNU) is one of South Korea's prominent national universities, located in Daejeon, a hub of science and technology. Established in 1952, CNU has become a leading institution in education, research, and innovation. It is particularly well-regarded for its contributions to fields like engineering, natural sciences, medicine, and social sciences.

About Professor Jaebeom Lee

Dr. Jaebeom Lee is a Professor of Chemistry and Principal Investigator of the LEELAB research group at the Department of Chemistry, Chungnam National University (CNU). His research group is putting great efforts into the development of photonic nanomaterials for optoelectronics, sensors, and green energy. Prof. Lee received his PhD from Robert Gordon University, UK in 2003 and completed Postdoctoral research at the N. A. Kotov group at the University of Michigan, Ann Arbor, USA. Since 2007, he has built a solid research background in nanobiotechnology, energy catalysis, and biosensors.