The Emergence of the Internet of Tiny Things
The Internet of Things (IoT) has already transformed society in areas including smart homes, medical devices, manufacturing, infrastructure, and transportation. There are currently billions of connected IoT devices, and an estimated 127 new devices are connected to the web every second. As this technology continues to evolve, a new frontier has emerged – the Internet of Tiny Things (IoT²) or Internet of Nano Things (IoNT).
IoT² systems refer to cyber-physical systems scaled down to the millimeter scale and below, unlocking completely new functionalities and application spaces. These autonomous devices and systems are enabled by advances in low-power circuits, heterogeneous integration, and the miniaturization of sensors, processors, and wireless communication components. However, a critical challenge in realizing the full potential of IoT² is powering these tiny, untethered systems.
Overcoming Power Challenges in Miniaturized Systems
Conventional power solutions used in larger IoT devices, such as batteries, wired connections, and wireless power transfer, become increasingly inefficient and impractical as dimensions are reduced below a centimeter. The approximate power density required for IoT² devices is on the order of 100 nW/mm², based on recent millimeter-scale system demonstrations.
Meeting these stringent power requirements necessitates a range of energy harvesting approaches tailored to the specific application and environmental conditions. The energy source may be available through the background ambient or intentionally provided through external excitation. Various modalities for energy harvesting have been explored, each with their own advantages, limitations, and considerations for scaling to microscale and nanoscale dimensions.
Harnessing Energy from the Environment
Radio-Frequency (RF) Energy Harvesting
RF energy harvesting is one of the most widely used approaches for IoT technologies, where devices such as RFID tags and readers have been around for decades. However, scaling RF energy harvesting to the sub-millimeter scale poses significant challenges. The link efficiency decreases dramatically when reducing dimensions to near 1 mm due to factors like coil geometry, coil quality factor, frequency, and distance between transmitter and receiver.
Overcoming these limitations will require corresponding increases in frequency and novel approaches for deep subwavelength antennas, such as the use of metamaterials to achieve high efficiency and directionality at subwavelength dimensions.
Photovoltaic Energy Harvesting
Photovoltaic (PV) cells can directly provide an operating voltage on the order of 0.5 V, depending on the illumination source and PV cell technology. Scaling down device dimensions below 1 mm can reduce conversion efficiency due to perimeter non-radiative recombination effects in semiconductors, where surface passivation remains a key to achieving high energy conversion efficiency.
As an alternative to the PV effect, optical energy harvesting can also be achieved using optical antennas (nantennas) that operate on the nanometer scale and theoretically offer efficiencies greater than 60%. Integrating optical antennas with photovoltaics through the use of plasmons can provide enhanced optical absorption and a means of optical coupling at near and subwavelength dimensions.
Mechanical Energy Harvesting
Mechanical energy from stray vibrations or objects in motion can be harvested using piezoelectric devices and triboelectric devices. Piezoelectric energy harvesters typically use micro-electro-mechanical systems (MEMS) technology with dimensional scales larger than a millimeter. Scaling down to the nanoscale, piezoelectric nanogenerators have shown promise in powering microelectronics, with power generation demonstrated even in a single nanowire.
Triboelectric devices, which generate electricity through the triboelectric effect and electrostatic induction, have thus far been employed at relatively large dimensions (centimeter scale). Scaling these devices to sub-millimeter dimensions will require appropriate design of dielectric layers and gap spacing between electrodes.
Thermal Energy Harvesting
Waste heat can be harvested using various modalities, including thermoelectric, thermophotovoltaic, and thermoradiative approaches. Thermoelectric devices based on the Seebeck effect have been demonstrated in miniaturized systems with an active volume of 1 mm³, capable of harvesting 775 µW/mm³ for an external temperature difference of 9 K.
Thermophotovoltaic and thermoradiative approaches rely on radiative transfer, with the device efficiency strongly dependent on the temperature of the heat source and the choice of material to match the bandgap energy. While chip-scale demonstrations have been made, scaling these systems to the sub-millimeter scale will be extremely challenging, requiring either extreme scaling of microreactors or system operation in close proximity to a high-temperature heat source.
Nuclear Energy Harvesting
Nuclear energy at small scales can generate electrical current in a semiconductor junction through the absorption of beta particles from radioactive sources, such as tritium or nickel-63. These betavoltaic cells or batteries can offer a stable source of power over a long period of time (approximately 10 years or more) and have been used in applications such as implantable medical devices and defense applications.
However, the primary concerns with betavoltaics are the health and safety implications of working with radioactive materials, as well as proper shielding and containment in devices. Scaling betavoltaic devices down to sub-millimeter dimensions may be limited by these safety and packaging considerations.
Integrating Energy Harvesting with Power Management
The implementation of nanoscale energy harvesting technologies in IoT² devices will depend critically on the design of electrical interfaces and circuitry. Traditional IoT systems often place energy harvesting components and power management circuitry as modular components in the system.
The progression towards continued scaling may further open opportunities for more distributed energy harvesting and power conversion, where singular nanowires or quantum dots could directly self-power a sensor within the IoT² system. Advanced low-power circuit design has demonstrated efficiencies up to 80% in handling power regulation and voltage upconversion to support IoT² devices, with demonstrations across various energy harvesting modalities.
Powering the Internet of Tiny Things
The power budget for an IoT² device will depend on the rate of sensing, sampling, and external communications, typically among the most power-hungry operations. Top-down system design can provide the most efficient energy and power use by considering primary aspects of the sensing interface, the sensing rate, and the sampling scheme, including an overall energy-neutral scheme for the network.
In cases where higher instantaneous power is required, energy storage is necessary. This can be achieved using capacitor networks for short timescales or chip-scale storage via supercapacitors or solid-state batteries for longer-term storage.
The applications for IoT² devices and their requirements for power budget and energy harvesting modality can generally be divided into a few different classes:
- Health and Biological Monitoring: Sensors external to the body can use a wide range of approaches, while bio-implantable devices have much higher constraints on accessible energy sources, dimensions, and toxicity requirements.
- Asset Monitoring and Surveillance: These applications often have a predictable environment and mode that is event-driven, allowing for a more straightforward path towards the design of the power system.
- Environmental Monitoring: These systems can be optimized to harvest the most abundant ambient source (e.g., stray light or vibrations) where the greatest power demand is typically related to wireless transmission of data.
Across all applications, a life-cycle assessment is needed to ensure an appropriate choice of energy harvesting mode and power management supports the intended product lifetime and reliability specifications.
The Path Forward: Nanoscience Breakthroughs and Multiscale Metamaterials
As nanoscience and nanotechnology continue to evolve, new device architectures may emerge that can directly convert energy via nanostructures such as 2D material layers, nanowires, or quantum dots, providing new avenues for energy conversion that overcome conventional boundaries placed by existing microelectronics and MEMS technology.
Multiscale metamaterials offer a pathway to engineer materials and structures with tailored physical properties to optimize energy harvesting, enabling high efficiency at dimensions that approach fundamental limits, such as the wavelength of the source, or in some cases, enabling subwavelength energy conversion phenomena.
The implementation of these nanoscale energy harvesting technologies in IoT² devices will depend on the continued advancement of electrical interfaces and circuitry to seamlessly integrate these energy sources and power conversion mechanisms. As the field of IoT² progresses, we can expect to see more distributed and self-powered sensor networks that push the boundaries of what is possible in the Internet of Tiny Things.
Conclusion
The Internet of Tiny Things represents a transformative frontier in the world of sensor networks and IoT, unlocking new functionalities and applications at the microscale and nanoscale. Powering these miniaturized systems is a critical challenge that necessitates a diverse range of energy harvesting approaches, from radio-frequency and photovoltaic to mechanical, thermal, and nuclear modalities.
As the field of nanoscience and nanotechnology continues to advance, new opportunities will emerge to directly convert energy at the nanostructure level, enabled by multiscale metamaterials and innovative electrical interface designs. The integration of these energy harvesting solutions with power management and storage will be crucial in realizing the full potential of the Internet of Tiny Things, empowering a new generation of self-powered sensor networks that transform how we monitor, interact with, and enrich our world.