In the past decade, the global industry and research attentions on intelligent skin-like electronics have boosted their applications in diverse fields including human healthcare, Internet of Things, human-machine interfaces, artificial intelligence and soft robotics. Among them, flexible humidity sensors play a vital role in noncontact measurements relying on the unique property of rapid response to humidity change. This work presents an overview of recent advances in flexible humidity sensors using various active functional materials for contactless monitoring. Four categories of humidity sensors are highlighted based on resistive, capacitive, impedance-type and voltage-type working mechanisms. Furthermore, typical strategies including chemical doping, structural design and Joule heating are introduced to enhance the performance of humidity sensors. Drawing on the noncontact perception capability, human/plant healthcare management, human-machine interactions as well as integrated humidity sensor-based feedback systems are presented. The burgeoning innovations in this research field will benefit human society, especially during the COVID-19 epidemic, where cross-infection should be averted and contactless sensation is highly desired. 

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Multifunctional Flexible Humidity Sensor Systems Towards Noncontact Wearable Electronics

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Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Technology Roadmap for Flexible Sensors.

ADVERTISEMENT RETURN TO ISSUEEditorialNEXTNano and PlantsJillian M. Buriak*Jillian M. Buriak*Email: [email protected]More by Jillian M. Buriakhttps://orcid.org/0000-0002-9567-4328, Luis M. Liz-Marzán*Luis M. Liz-Marzán*Email: [email protected]More by Luis M. Liz-Marzánhttps://orcid.org/0000-0002-6647-1353, Wolfgang J. Parak*Wolfgang J. Parak*Email: [email protected]More by Wolfgang J. Parakhttps://orcid.org/0000-0003-1672-6650, and Xiaodong Chen*Xiaodong Chen*Email: [email protected]More by Xiaodong Chenhttps://orcid.org/0000-0002-3312-1664Cite this: ACS Nano 2022, 16, 2, 1681–1684Publication Date (Web):February 22, 2022Publication History Published online22 February 2022Published inissue 22 February 2022https://doi.org/10.1021/acsnano.2c01131Copyright © 2022 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views4947Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (3 MB) Get e-AlertsSUBJECTS:Nanomaterials,Nanoparticles,Nanotechnology,Plants,Sensors Get e-Alerts 

https://pubs.acs.org/doi/pdf/10.1021/acsnano.2c01131

Nano and Plants

Plant‐hybrid systems have demonstrated exciting opportunities in plant health sensing, climate monitoring, and energy harvesting, endowing great prospects for connecting plants to man‐made digitalized world. Due to its intrinsic softness/stretchability, biocompatibility, and non‐invasion, liquid alloy has shown great advantages in plant‐hybrid system implementation. However, many widely seen plant organisms having micro‐nano surface structures and waxy layer, e.g., lotus leaves, showing super‐hydrophobic behavior, prevent the liquid alloy adhesion and further its implementation of plant‐hybrid systems. By introducing a low‐concentration soap solution as an intermediate layer onto super‐hydrophobic leaves, the authors thereby propose a facile liquid alloy wetting enhancing strategy and well print liquid alloy circuits on the leaves to form plant‐hybrid systems. Surfactants (in the soap) enhance the wetting ability of solution on the lotus leaf surface, and subsequently the attracted water on the leaf surface contributes to make liquid alloy adhered on super‐hydrophobic leaves. Furthermore, the authors demonstrate the liquid alloy plant‐hybrid implementation based on the lotus leaves for the perception of human body, and set a system as a home intrusion alarm. This work presents an excellent example of digitalized integration on super‐hydrophobic plant organisms, which provides a new tool technology for plant‐hybrid implementations and broadens the digitalized application range of plants.

A Facile Liquid Alloy Wetting Enhancing Strategy on Super‐Hydrophobic Lotus Leaves for Plant‐Hybrid System Implementation

The excellent stretchability and biocompatibility of flexible sensors have inspired an emerging field of plant wearables, which enable intimate contact with the plants to continuously monitor the growth status and localized microclimate in real-time. Plant flexible wearables provide a promising platform for the development of plant phenotype and the construction of intelligent agriculture via monitoring and regulating the critical physiological parameters and microclimate of plants. Here, the emerging applications of plant flexible wearables together with their pros and cons from four aspects, including physiological indicators, surrounding environment, crop quality, and active control of growth, are highlighted. Self-powered energy supply systems and signal transmission mechanisms are also elucidated. Furthermore, the future opportunities and challenges of plant wearables are discussed in detail.

Flexible Wearables for Plants.

The hazards caused by drug-resistant bacteria are rocketing along with the indiscriminate use of antibiotics. The development of new non-antibiotic antibacterial drugs is urgent. The excellent biocompatibility and diverse multifunctionalities of liquid metal have stimulated the studies of antibacterial application. Several gallium-based antimicrobial agents have been developed based on the mechanism that gallium (a type of liquid metal) ions disorder the normal metabolism of iron ions. Other emerging strategies, such as physical sterilization by directly using LM microparticles to destroy the biofilm of bacteria or thermal destruction via infrared laser irradiation, are gaining increasing attention. Different from traditional antibacterial agents of gallium compounds, the pronounced property of gallium-based liquid metal materials would bring innovation to the antibacterial field. Here, LM-based antimicrobial mechanisms, including iron metabolism disorder, production of reactive oxygen species, thermal injury, and mechanical destruction, are highlighted. Antimicrobial applications of LM-based materials are summarized and divided into five categories, including liquid metal motors, antibacterial fabrics, magnetic field-responsive microparticles, liquid metal films, and liquid metal polymer composites. In addition, future opportunities and challenges towards the development and application of LM-based antimicrobial materials are presented.

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Gallium-Based Liquid Metal Materials for Antimicrobial Applications

Flexible plant sensors, as a noninvasive and real-time monitoring method for plant physiology, are becoming crucial for precision agriculture. However, integrating flexible devices with plants are challenging due to their fragility and complex surfaces. In this study, we introduce a liquid metal-based plant electronic tattoo (LM-PET) that can harmlessly and continuously monitor the loss of water content and plant electrical signals, which are critical parameters for analyzing plant physiological status. The LM-PET achieves double-sided conductivity through soluble electrostatic spinning films and transferring technology, effectively addressing the issue of mismatch between the rigid interface of electronic devices and the surface of delicate plants. The fabricated tattoo electrode can adhere tightly to the leaf surface for a long time and can significantly broaden the scope of moisture monitoring, even in cases of severe wrinkling caused by water loss. At the optimum operating frequency of 100 kHz, the sensitivity of LM-PET can reach 25.4 kΩ%−1. Thus, LM-PETs can record the electrical signals generated when abiotic stresses threaten plants. They are also significant in providing a deeper understanding of the drought adaptation mechanisms of plants and developing drought-resistant varieties. They offer data-driven crop management and decision-making guidance, which is imperative for advancing precision agriculture. Overall, our findings provide valuable insights into the performance of agricultural inputs and facilitate real-time monitoring of plant growth and development. 

https://link.springer.com/content/pdf/10.1007/s11431-023-2431-4.pdf

Chun-Chun Qu

Papers

Flexible Wearables for Plants.

Gallium-Based Liquid Metal Materials for Antimicrobial Applications

Liquid metal-based plant electronic tattoos for in-situ monitoring of plant physiology