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.

This paper presents a holistic and authoritative review of the role of microwave technologies in enabling a new generation of wearable devices. A human-centric Internet of Things (IoT) covering remote healthcare, distributed sensing, and consumer electronics, calls for high-performance wearable devices integrated into clothing, which require interdisciplinary research efforts to emerge. Microwaves, the “interconnect” of wireless networks, can enable, rather than solely connect, the next generation of autonomous, sustainable, and wearable-friendly electronics. First, enabling technologies including wireless power transmission and RF energy harvesting, backscattering and passive communication, RFID, and electromagnetic sensing are reviewed. We then discuss the key integration platforms, covering smart fabrics and electronic textiles, additive manufacturing, printed electronics, natively-flexible and organic RF semiconductors, and fully-integrated CMOS systems, where opportunities for hybrid integration are highlighted. The emerging research trends, from mmWave 6G, RF sensing and imaging, to healthcare applications including neural implants, drug delivery, and safety upon exposure to microwaves are re-visited and discussed, presenting a future roadmap for interdisciplinary research towards sustainable and reliable next-generation wearables.

State-of-the-art passive RF identification (RFID) technologies are used in contactless smart cards and item management. These technologies rely on the radio communication between an RFID reader and batteryless (or passive) RFID transponders (tags) and thus allow a low-cost, unique identification of objects. Current research topics in the field of passive RFID technologies focus on activities that go beyond the ID in RFID [1]. Research in the microwave community is focused on developing miniaturized passive RFID sensor tags. Tags of this kind constitute inexpensive, long-lasting, ubiquitous sensor units with unique identifiers and make it possible for smart objects to connect to the Internet. In particular, such miniaturized sensor tags enable the identification and monitoring of small objects (e.g., documents, jewelry, medicines, and diagnostic devices) that can then become part of the Internet of Things (IoT) [2].

This paper presents a curvature-compensated sub-1V voltage reference (VR) and a shared-resistive nanoampere current reference (CR) in a 130nm CMOS process. The CR is used to generate a bipolar junction transistor complementary-to-absolute-temperature voltage, which is summed up with a proportional-to-absolute-temperature voltage generated using a summing network of PMOS gate-coupled pairs. The measured output voltage and current references from 10 chips ( $V_{\mathrm {REF}}$ and $I_{\mathrm {REF}}$ ) at room temperature are 469mV and 1.86nA, respectively. The measured average temperature coefficient of $V_{\mathrm {REF}}$ and $I_{\mathrm {REF}}$ are 29ppm $/^{\circ} \text{C}$ and 822ppm $/^{\circ} \text{C}$ over a temperature range from $- 40^{\circ} \text{C}$ to $120^{\circ} \text{C}$ . The minimum supply voltage of the voltage-current reference is 0.95V, and the total power consumption is 30nW.

This work presents a wide-range, submicrowatt CMOS rectifier, which is designed to efficiently work at input power levels down to −31 dBm. The designed rectifier uses a novel threshold voltage compensation design, namely, the design of a custom-built single-stage rectifier that supplies a nanowatt current reference circuit. The generated bias current is mirrored to the core rectifier in order to precharge its CMOS diodes. The rectifier was optimized for a minimum number of gain stages while trying to minimize the overall input parasitic capacitance of the rectifier. The proposed CMOS rectifier was designed in an in-house 130-nm CMOS technology and includes a 1.6-kV human body level ESD protection at the input terminals. Measurements reveal a high 30% power conversion efficiency (PCE) over a wide input power range from −23 to −13 dBm for a 1- ${\mathrm{ M}}\Omega $ output load at 868 MHz. PCE of over 10% is achieved for a 1- ${\mathrm{ M}}\Omega $ load at an input power of −28 dBm. The enhanced startup at submicrowatt input power levels of the CMOS rectifier enables the design of an ultralow-power passive wake-up receiver. The measured sensitivity of −31 dBm for a 1-V output voltage across a capacitive load is the lowest reported in the literature so far.

This paper presents a near field communication (NFC) antenna that is printed directly on a ferrite substrate using a silver inkjet printing process. Such a silver inkjet printed ferrite NFC antenna provides a minimum substrate thickness, a good operation on metal objects, and a small assembly effort. The NFC antenna performance is analysed by measurements based on the ISO/IEC standard 10373-6 for proximity identification cards test methods. For this, the antenna is connected to an NFC microchip. Measurements in a non-metal environment show that the ferrite antenna performs equally good a custom-built NFC antenna printed on photo paper substrate, despite additional losses in the ferrite substrate. In a metal environment the ferrite antenna clearly outperforms the photo paper antenna.