Comprehensive Comparison of Continuous-Wave and Linear-Frequency-Modulated Continuous-Wave Radars for Short-Range Vital Sign Monitoring.

The use of radar technology for contactless monitoring of cardiorespiratory activity has been a significant research topic for the last two decades. However, despite the abundant literature focusing on the use of different radar architectures for healthcare applications, an in-depth analysis is missing about the most appropriate configuration. This article presents a comparison between continuous-wave (CW) and linear-frequency-modulated continuous-wave (LFMCW) radars for application in vital sign monitoring scenarios. These waveforms are generated with the same architecture at two different frequencies: 24 and 134 GHz. Results evidence that both configurations are capable of measuring general metrics, such as the breathing and heart rates. However, LFMCW offers better results in the identification of cardiac events and the extraction of certain derived biomarkers, such as the heart rate variability sequences (HRV). Conclusions show that this performance does not depend on the selected working frequency.

Two-dimensional MoS2-enabled flexible rectenna for Wi-Fi-band wireless energy harvesting.

The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics1-3. Their atomic thickness and large-scale synthesis capability could enable the development of 'smart skin'1,3-5, which could transform ordinary objects into an intelligent distributed sensor network6. However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials1,2,4,7), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz8 is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics. However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties9-12. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting-metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers9-12. This flexible MoS2-based rectifier operates up to the X-band8 (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems.