This invited paper presents potential solutions for tackling some of the main underlying challenges toward developing sustainable Internet-of-things (IoT) devices with a focus on eco-friendly manufacturing, sustainable powering, and wireless connectivity for next-generation IoT devices. The diverse applications of IoT systems, such as smart cities, wearable devices, self-driving cars, and industrial automation, are driving up the number of IoT systems at an unprecedented rate. In recent years, the rapidly-increasing number of IoT devices and the diverse application-specific system requirements have resulted in a paradigm shift in manufacturing processes, powering methods, and wireless connectivity solutions. The traditional cloud-centering IoT systems are moving toward distributed intelligence schemes that impose strict requirements on IoT devices, e.g., operating range, latency, and reliability. In this article, we provide an overview of hardware-related research trends and application use cases of emerging IoT systems and highlight the enabling technologies of next-generation IoT. We review eco-friendly manufacturing for next-generation IoT devices, present alternative biodegradable and eco-friendly options to replace existing materials, and discuss sustainable powering IoT devices by exploiting energy harvesting and wireless power transfer. Finally, we present (ultra-)low-power wireless connectivity solutions that meet the stringent energy efficiency and data rate requirements of future IoT systems that are compatible with a batteryless operation.

In this paper, a novel dual-polarized embroidered textile antenna array with an omnidirectional radiation pattern is proposed for both on- and off-body wearable applications. The flexible antenna is composed of a group of circularly oriented dual-polarized patch antennas excited with uniform amplitude and phase. The antenna array can be wrapped around a cylinder, such as an arm or a leg, for realizing a quasi-omnidirectional radiation pattern in the azimuthal plane, which is highly desirable in both on-body and off-body wearable applications. The operating principles and design consideration for how to achieve the omni-directional radiation and how to avoid radiation nulls are investigated in detail. Moreover, these analytical studies are verified through experimental results. In addition, a dual orthogonal polarization capability is employed to improve the link reliability. Due to the high front-to-back ratio (FBR) exhibited by each patch antenna element, the proposed omni-directional antenna array also features a low Specific Absorption Rate (SAR) and high efficiency, which are extremely important for wearable applications. As a proof-of-concept, an antenna array prototype operating at 5.8 GHz is designed, fabricated, and tested. Measured results agree reasonably well with the simulations in terms of S-parameters, polarization isolation, and radiation patterns, demonstrating that the proposed antenna array is ideally suited for potential wearable applications.

In this article, we present a clothing-integrated wireless power transfer and harvesting system that uses a novel class of anchor-shaped antennas for wireless power transfer up to the 4-ft distance. Notably, the system allows wireless charging during daily life activities, such as sitting on a chair or while lying on a fabric surface. Owning to up to a 4-ft power transfer, a user may also walk around during the wireless charging. The proposed system features a novel anchor-shaped antenna, which is miniaturized than a loop antenna and extends the range of fringing fields. Furthermore, a textile-based rectifying circuit with RF-to-dc conversion efficiency of 77.23% at 22.5 dBm is proposed. The efficiency remains better than 60% for an input power from 2 dBm to 24 dBm. For garment integration, a system consisting of a rectifier and an antenna were prototyped by using embroidered conductive thread on denim fabric. To understand the effectiveness, wireless power transfer tests are conducted under mechanical deformations and misalignments. The resulting prototype was tested for power transfer of several mW power over an area of 4 ft x 4 ft and by using a 1-W transmitter. This textile power harvesting system can be used as an ergonomic charging platform to supply power to a wide range of wearable medical, Internet of Things, and portable devices.

In this paper, we propose a new antenna geometry for near-field magneto-electric wireless power transfer (ME-WPT) application, where the antenna geometry is found to be resilient to misalignment between the receiving and the transmitting antennas. The proposed geometry is simple to fabricate and is planar, therefore can be easily integrated with various platforms suitable for wearable applications. The proposed structure is demonstrated to show improved WPT efficiency under different forms of misalignment between the transmitting and receiving antennas by the virtue of miniaturization and the fringing fields. Specifically, a central bar structure is added in the traditional loop antenna to provide dipole-like and miniaturized performance. Concurrently, a discontinuity in the loop is introduced to enable fringing fields beyond the aperture of the antenna. Investigations are conducted to conclude that a superiority in the performance is associated to these fringing fields. Design-optimization, simulation, and measurement investigations are conducted to estimate the WPT performance under various forms of misalignment. Notably, the proposed structure shows an increase as high as 65% in power transfer efficiency within the range of misalignment considered in this work.

In this paper, SkinAid , a battery-free, low-cost, robust, and user-friendly smart bandage for electrochemical monitoring and sensing of chronic wounds is proposed. The working principle of the bandage is based on direct frequency modulation of a tri-electrode electrochemical sensing of wound data. The electronics and biotelemetry links were realized using low-cost manufacturing process of textile embroidery onto fabric substrate. The transmitter was represented by a bedsheet with novel corrugated crossed-dipole made of Elektrisola-7 embroidered onto gauze fabric. An input RF signal of 1 W was transmitted at 462 MHz from the bedsheet to the all-textile bandage featuring a rectifying circuit, a voltage-controlled oscillator (VCO), an electrochemical sensor, and a 915-MHz dipole for re-transmission of the modulated wound data. We demonstrate that for wound fluid emulated by various uric acid concentrations from 0.2 mM to 1.2 mM, corresponding modulated frequency varies from 1090 MHz to 1145 MHz for signals captured at 25 cm away from the bandage. For pH modulation ranging from 2 to 10, the corresponding modulated frequency was between 800 MHz and 830 MHz for signals received at more than 6 feet away from the bandage. For quick and reliable assessment, two empirical models were developed for the direct frequency modulation as a function of uric acid and pH. To the best of our knowledge, this is the first time an all-textile (fabric-integrated), battery-free and wirelessly powered smart bandage have been proposed for wound monitoring. This result can be used as a first step in developing RFID-type, battery-free, and low-cost 5 G/6 G smart bandages using millimeterwave and terahertz frequencies where the bedsheet can be host to a MIMO-aided beamforming.