# Flexible Rectenna on an Eco-Friendly Substrate for Application in Next-Generation IoT Devices
## 1 Introduction  
Globally, there are now more than 19 billion connected Internet of Things (IoT) devices [1], which are fostering innovation across various sectors, including industry, healthcare, education, energy, and agriculture [2]. With the rapid expansion of IoT devices, there is an increasing demand for sustainable, self-powered, eco-friendly solutions for next-generation IoT devices [3, 4].  
Harvesting and converting radio frequency (RF) energy through rectennas is being explored as a potential, environmentally friendly, cost-effective solution for powering next-generation battery-less wireless and IoT devices [5, 6, 7, 8]. The key benefits of RF energy harvesting include independence from environmental conditions (not affected by weather, temperature, or sunlight) and 24/7 availability of electromagnetic fields (EMFs) [9, 10, 7], even indoors [11]. For example, in densely populated urban and rural areas, EMFs fromambient sources such as cellular base station antennas (GSM, UMTS, LTE, 5G), various wireless communication networks operating under IEEE 802 standards (IEEE 802.11 [12], IEEE 802.15.1 [13], IEEE 802.15.4 [14], IEEE 802.15.6 [15]), digital TV, and FM radio are constantly present in the environment [7; 9; 11; 16].  
Several challenges arise when designing rectennas for next-generation ultra-low power IoT devices, including achieving high RF-to-DC conversion efficiency, supporting wideband or multiband operation [9; 10], and enabling reliable performance under varying polarization and environmental conditions [16]. All of this must be accomplished while maintaining a compact, lightweight, flexible, cost-effective, and environmentally friendly design [5; 6].  
In rectenna design for small portable devices, efficient energy harvesting over a wide frequency range requires the development of a compact, wideband antenna [17]. Various compact antennas, including monopole [17; 18], dipole [19; 20], and patch [21; 22], fabricated on conventional substrates (such as FR-4, Rogers RT/Duroid 5880, etc.), have been proposed for integration into rectenna systems. Moreover, flexibility in rectenna design is essential for seamless integration with wearable and flexible electronics [23]. Moreover, most of the reported flexible rectenna designs remain relatively large in dimension, making them difficult to integrate into small devices [23; 24; 25].  
In this study, we focus on the design of a compact and flexible rectenna optimized for operation over a broad bandwidth. The proposed structure integrates a high-efficiency fractal-based antenna with a rectifying circuit fabricated on a flexible substrate from biodegradable materials. The presented work introduces a symmetric binary fractal tree antenna geometry--a configuration not previously reported in the context of flexible rectenna designs--which enables significant miniaturization and seamless integration in next-generation wireless IoT devices with ultra-low power consumption. The fractal geometry also enables wideband operation covering several key wireless communication standards (e.g., 5G lower mid-band, LTE, Wi-Fi), allowing harvesting ambient energy in diverse real-world scenarios. The proposed rectenna design achieves a stable DC output under diverse field intensities and polarization conditions. Moreover, we conducted experimental measurements in a semi-anechoic chamber, assessing the rectenna's performance across relevant communication bands and under realistic electromagnetic exposure scenarios to validate the proposed design.