<p>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.</p>