Liquid metal pressure sensor suitable for health monitoring

Researchers from the Korea Advanced Institute of Science and Technology (KAIST) have developed a highly sensitive wearable pressure sensor that is capable of sensitive, precise and continuous measurement of physiological and physical signals, with great potential for health monitoring applications and the early diagnosis of diseases. Their work has been published in the journal Advanced Healthcare Materials.

A soft pressure sensor is required to have high compliance, high sensitivity, low cost, long-term performance stability and environmental stability in order to be employed for continuous health monitoring. Conventional solid-state soft pressure sensors using functional materials including carbon nanotubes and graphene have showed great sensing performance. However, these sensors suffer from limited stretchability, signal drifting and long-term instability due to the distance between the stretchable substrate and the functional materials.

To overcome these issues, liquid-state electronics using liquid metal have been introduced for various wearable applications. Of these materials, galinstan — a eutectic metal alloy of gallium, indium and tin — has great mechanical and electrical properties that can be employed in wearable applications. But today’s liquid metal-based pressure sensors have low pressure sensitivity, limiting their applicability for health monitoring devices.

Seeking to solve this issue, KAIST researchers developed a 3D-printed rigid microbump array-integrated, liquid metal-based soft pressure sensor. With the help of 3D printing, the integration of a rigid microbump array and the master mould for a liquid metal microchannel could be achieved simultaneously, reducing the complexity of the manufacturing process.

Through the integration of the rigid microbump and the microchannel, the team’s pressure sensor is said to achieve an extremely low detection limit and enhanced pressure sensitivity compared to previously reported liquid metal-based pressure sensors. The proposed sensor also has a negligible signal drift over 10,000 cycles of pressure, bending and stretching, and exhibited excellent stability when subjected to various environmental conditions.

The research team demonstrated their work in the form of a wearable wristband device that can continuously monitors one’s pulse during exercise and be employed in a non-invasive, cuffless BP monitoring system based on PTT calculations. Then, they introduced a wireless wearable heel pressure monitoring system that integrates three 3D-BLiPS with a wireless communication module. Their work thus indicates that the sensor is suitable for various health monitoring devices.

“It was possible to measure health indicators including pulse and blood pressure continuously, as well as pressure of body parts, using our proposed soft pressure sensor,” said study leader Professor Inkyu Park. “We expect it to be used in healthcare applications, such as the prevention and the monitoring of the pressure-driven diseases such as pressure ulcers in the near future. There will be more opportunities for future research including a whole-body pressure monitoring system related to other physical parameters.”

Image caption: The highly sensitive liquid metal-based soft pressure sensor integrated with a 3D-printed microbump array. Image credit: KAIST.

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