As the need for an increased supply and diversity of sensors expands, printed electronics have been identified as a promising approach for low-cost, ubiquitous sensor networks. In this work, we demonstrate the use of a fully printed carbon nanotube thin-film transistor (CNT-TFT) for sensing environmental pressure over a pressure range extending from 0 to 42 PSIg. The transconductance of the CNT-TFT was found to be linearly correlated with environmental pressure at a sensitivity of 48.1 pS/PSI. We also demonstrate the capability of wirelessly transmitting the data measured by the pressure sensor using a simple Bluetooth module. Using the Bluetooth system, we observed the sensor’s response over time as pressure was dynamically changed. Lastly, we packaged the sensor along with a material thickness sensor (developed in previous work) into a fully printed smart tire sensor system capable of mapping tire pressure and tread depth differentials.

The dissemination of sensors is key to realizing a sustainable, ‘intelligent’ world, where everyday objects and environments are equipped with sensing capabilities to advance the sustainability and quality of our lives—e.g. via smart homes, smart cities, smart healthcare, smart logistics, Industry 4.0, and precision agriculture. The realization of the full potential of these applications critically depends on the availability of easy-to-make, low-cost sensor technologies. Sensors based on printable electronic materials offer the ideal platform: they can be fabricated through simple methods (e.g. printing and coating) and are compatible with high-throughput roll-to-roll processing. Moreover, printable electronic materials often allow the fabrication of sensors on flexible/stretchable/biodegradable substrates, thereby enabling the deployment of sensors in unconventional settings. Fulfilling the promise of printable electronic materials for sensing will require materials and device innovations to enhance their ability to transduce external stimuli—light, ionizing radiation, pressure, strain, force, temperature, gas, vapours, humidity, and other chemical and biological analytes. This Roadmap brings together the viewpoints of experts in various printable sensing materials—and devices thereof—to provide insights into the status and outlook of the field. Alongside recent materials and device innovations, the roadmap discusses the key outstanding challenges pertaining to each printable sensing technology. Finally, the Roadmap points to promising directions to overcome these challenges and thus enable ubiquitous sensing for a sustainable, ‘intelligent’ world.

The Internet of Things (IoT) is the concept of a ubiquitous computing ecosystem in which electronics of custom form factors are seamlessly embedded into everyday objects. At the heart of the IoT are electronic sensors capable of detecting physical/environmental phenomena, translating these measurements into electrical signals, and wirelessly transmitting the data for remote computing. Critical to the development of IoT sensors and systems are low-cost materials, robust enough to sustain stable electrical performance over medium to long periods of time, yet sensitive enough to detect small changes in the surrounding environment. Such materials should be mechanically flexible and amenable to solution-based processing to facilitate large scale production methods, such as roll-to-roll printing. Carbon nanotubes (CNTs) are one of the leading material candidates to satisfy these requirements because of their unique electrical and mechanical properties, which enable robust and versatile devices, in combination with their chemical properties, which allow for the processing of CNTs from solution. These advantages have enabled demonstration of a myriad of printed CNT-based electronics and sensors on diverse substrates with wide ranges of functionality, spanning from simple sensors based on passive devices to complex multi-stage circuitry and display electronics. In this review, we provide a comprehensive summary of the CNT-based electronics and sensor space with an emphasis on applications aligned with the IoT. Primary coverage is devoted to devices consisting of randomly oriented CNT networks; however, the advantages and capabilities of single-nanotube devices will also be discussed. Key works across various types of sensors will be reviewed and a summary of the remaining challenges for CNT-based sensor technologies will be presented.

Demand for cheaper and more functional sensors continues to rise in an era when data can be used to improve health, safety, and efficiency in daily lives. In this paper, we present a fully printed sensor capable of noninvasive material thickness detection. By applying an oscillating signal between two millimeter-scale electrodes, the fringing electric field is measurably perturbed by a material placed directly on top of the electrodes, leading to a linearly varying capacitance with change in the material's thickness. We simulate this electric field perturbation and experimentally demonstrate the linear correlation between capacitance and overlying material thickness. Various parameters, from sensor size and structure to substrate and ink materials, are studied to optimize the performance of the printed sensors. Sensors made of metallic carbon nano-tube ink yield the best sensitivity, exhibiting a capacitance change of 26 fF per mm thickness of rubber-ten times more sensitive than devices composed of silver nanoparticle ink. Finally, we demonstrate an effective application of the sensors in automobile tires. By applying the sensors directly beneath the tread (within the tire), mm changes in the tread depth are able to be detected in a 99% confidence interval. These findings provide a straightforward, low-cost approach for monitoring mm changes in material thickness using noninvasive, printed sensors applicable to innumerable Internet-of-Things (IoT) applications.

Printing electronics allows us to fabricate low-cost, simple sensor networks. Distributed soil moisture sensors in agriculture industry will result in better water management practices; however, price and reliability do not allow for ubiquitous availability. Here, we demonstrate the fabrication of low-cost, flexible, interdigitated electrode (IDE) soil moisture sensors with the electrode widths of 537, 877, 1055, 1204, and $1916~\mu \text{m}$ on polyimide (PI) substrates using a silver nanoparticle (Ag NP) ink. The sensors were tested in sandy textured soil with a volumetric water content ranging from 0.05 to 0.25 m3/m3. Analytically, the sensitivities of all the sensors are expected to be equal, as the proposed mechanisms dictate that the capacitance should depend linearly on the dielectric constant of the moist soil. However, we found the sensitivity for the widest IDE, which measured $1916~\mu \text{m}$ to be 442 pF/H2O volumetric percentage, whereas the sensitivity for the sensor with 537- $\mu \text{m}$ -wide electrodes to be only 221 pF/H2O volumetric percentage. We hypothesized that the increase in sensitivity can be directly related to the increase in the capacitive fringing field height, which results in increased sensing volume for the widest IDE. The increased sensing volume leads to increased water molecule polarization, increasing the overall capacitance change with respect to changing water content. To validate the hypothesis, we conducted electric fringing field experiments to observe the trend between the maximum fringing field height and the electrode width of an IDE. Results confirmed that increased sensing volume was caused by the increased fringing field height of the electrode in an IDE.