Miniaturized flexible skin moisture sensor with optimized coil for enhanced wireless power efficiency

TL;DR

This paper presents a compact, battery-free wearable sensor with an optimized coil for efficient wireless power and reliable skin hydration monitoring.

Contribution

A novel design framework for miniature antenna coils in wearable sensors, ensuring durability and performance under deformation.

Findings

An optimized coil design maintains stable resonant frequency and communication under mechanical deformation.

The sensor platform enables real-time skin hydration monitoring with proven durability through environmental testing.

PDMS encapsulation enhances mechanical durability and long-term stability of the sensor.

Abstract

Efficient wireless power transfer is essential for the stable operation of battery-free wearable sensors. Especially for Near Field Communication (NFC)-based sensors, the performance of the antenna coil is a critical factor in determining power reception efficiency and data communication reliability. However, as sensors become smaller, reducing coil size drastically reduces communication sensitivity, making it crucial to design a coil that delivers optimal performance within a limited area. This study focuses on the optimal design of a miniature antenna coil for a wearable sensor capable of measuring skin hydration. Considering the characteristics of wearable devices, the design and experimental validation were conducted to maintain a stable resonant frequency and robust power reception and data communication even under mechanical deformation, such as bending of the skin surface. Consequently, a compact, battery-free sensor platform integrated with the optimized antenna coil enables real-time monitoring of patient skin hydration, and its durability has been proven through rigorous environmental testing. Furthermore, polydimethylsiloxane (PDMS) encapsulation ensures mechanical durability and long-term stability. This study highlights the importance of coil optimization in the development of next-generation wearable healthcare devices and provides a basic design framework for miniature sensor systems.

The online version contains supplementary material available at 10.1038/s41598-026-38764-5.

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