Unravelling the Secrets of Plants: Emerging Wearable Sensors for Plants Signals and Physiology

Plants provide humankind with a habitable environment, food, and oxygen. Due to changing climate and increasing global population, there is pressure to increase agricultural production while limiting negative impacts on natural ecosystems. Understanding plant physiology and plant‐environment interactions is needed to further sustainable agricultural practices and maintain a green and diverse environment. Early detection of signals characteristic to plant stress can help to design interventions to preserve yield or diversity. Therefore, technology development is important to monitor plant health by measuring a variety of parameters related to the microclimate around plants and plant physiology. This review focuses on state‐of‐the‐art wearable sensors developed for plants.


Introduction
Plant health in natural, agricultural, and urban communities is the basis for biodiversity conservation, sustainable food production, and green cities. Plant health depends on interactions between plants and their abiotic and biotic environment (Figure 1). [1]An improved understanding of plant physiology and plant-environment interactions is needed for better management of natural and urban plant communities and smart food production.

Complexity of Plant Physiology and Signaling
Thriving plant growth and high agricultural yields rely on the healthy physiological state of plants and the ability of plants to DOI: 10.1002/adsr.202300023cope with environmental stress factors, such as changes in air and soil humidity, air pollutants, and pathogens.Stomata are microscopic adjustable pores in plant leaves that mediate CO 2 uptake for photosynthesis while releasing water via transpiration and allowing entry of plant pathogens and air pollutants.Thus, stomatal traits (such as stomatal numbers and apertures) and their responsiveness to changes in environmental conditions are essential contributors to plant health and biomass production.CO 2 uptake and water loss are mediated by stomatal pores in upper (adaxial) and lower (abaxial) leaf surfaces that change their aperture depending on environmental conditions.Monitoring plant transpiration or stomatal conductance gives information on plant gas-exchange capacity correlated with productivity and yield. [2]Stomatal pores close in response to stress, for example, drought, reduced air humidity, air pollutants (such as ozone) and pathogens. [3]ost plant species are hypostomatous, that is, they have stomata only on the abaxial leaf side. [4]However, many herbs and fast-growing plant species are amphistomatous with stomata on both leaf surfaces. [4,5]The presence of adaxial stomata in plants is associated with better CO 2 uptake capacity and higher photosynthesis. [6]However, adaxial and abaxial stomatal functions are rarely independently studied due to experimental limitations.Overcoming this challenge would allow us to better understand adaxial and abaxial stomatal function as well as plant health status, as adaxial stomata are hypothesized to make plants more vulnerable to stomata-entering pathogens. [4,7]erception of signals indicating the presence of pathogens or herbivores is key for plants to mount appropriate defense responses and overcome biotic stress.A significant part of communication between plants, or between plants and other organisms occurs via airborne molecules, such as biogenic volatile organic compounds (VOCs).VOCs can be divided into constitutive and induced VOCs; the latter are often specific to a given type of stress or biotic interaction. [8]Emissions of inducible VOCs are often associated with various stresses, such as pathogen infection or herbivory. [9]For example, stress-induced VOCs typically include C6 green leaf volatiles and terpenes. [10]Plants may use VOCs to signal to other plants the presence of herbivores or pathogens and lead them to pre-activate defense systems to better resist respective threats; or they may emit VOCs that deter herbivores or have antimicrobial properties; or emit signals perceived by natural enemies of the herbivores. [8,9,11]Capturing and interpreting the VOCs released by plants would allow researchers to "listen in" on the communication in bionetworks comprising different plant species, insects and microbes.Collecting such information would help to better understand the biological interactions in natural, as well as agricultural plant communities, and allow diagnostics of problems, such as spread of pathogens or pests.

Current Techniques for Analyzing Stomatal Function and Plant Volatile Signals
Stomatal traits are commonly characterized via gas-exchange analyses of transpiration and stomatal conductance or direct microscopic monitoring of stomatal aperture changes in isolated leaves or epidermal fragments.Gas-exchange analyses rely on the measurement of CO 2 and water concentrations in the air entering or exiting the environmentally controlled experimental chamber harboring a leaf or a plant, using infrared gas analyzers.The method allows real-time monitoring of stomatal movements under physiological conditions, but commercial devices are expensive and have low throughput.Such devices also do not allow analysis of adaxial and abaxial stomatal functions separately, and custom designs by experts are necessary to achieve such goals. [12]orometers are simpler devices that allow measurement of stom-atal conductance and are portable, hence easily applicable in the field, but they allow only point measurements without the option to monitor changes in stomatal conductance in real-time.Direct monitoring of stomatal aperture in isolated leaves or epidermal fragments is very common for analysis of stomatal traits and is popular because of its low cost and ease of application.It also allows to separately address abaxial and adaxial stomatal functions.However, the isolation of leaves or epidermal fragments from the plants may not adequately reflect physiological conditions and normal stomatal behavior.The method is low-throughput and not applicable in the field.
Identification of plant volatiles currently relies on two primary methods: gas chromatography combined with mass spectrometry (GC-MS) and proton-transfer-reaction mass spectrometry (PTR-MS), which can also be used as complementary techniques. [13]GC-MS is used for offline volatile identification and analysis; samples are typically collected from the plant's headspace or leaf sealed in an experimental chamber and detected by GC-MS.For quantitative analysis, expensive standard solutions are required.The method offers rapid results with high precision and specificity, but the sample preparation is laborintensive, equipment is bulky, power-consuming and expensive, and requires experienced handling.In the effort of making GC-MS more affordable and user-friendly, a miniaturized GC-MS has been commercialized with the name of zNOSE (Electronic Sensor Technology, Inc).Unfortunately, the zNOSE contains the short column (≈1 m) that reduces the resolution of volatiles with similar retention times (such as volatile enantiomers), resulting in poor peak resolution in a multi-component mixture.In addition, GC-MS does not allow real-time monitoring of volatile emissions, which are highly temporal and spatial.Thus, while it is possible to identify the compounds released by the plant, their emission kinetics and timing remain unclear.PTR-MS allows high-sensitivity real-time analysis of volatile emissions and, thus, characterization of volatile emission kinetics.13a] PTR-MS is not easily portable and thus rarely applied under field conditions.

Emerging Wearable Sensing Platforms for Plants
Wearable technology impacts significantly human health monitoring [14] and development of wearable sensors for humans and other animals has expanded over recent years. [15]earable sensors for plants and animals are similar in several aspects.For example, similar wearable gas sensors can be used for both plants and insects. [16]Besides, it is vital that installation a wearable sensor should not interfere with the normal physiology and behavior of the subject.However, wearables for plants and animals face distinct challenges in application.Human and animal wearables need to be compatible with the moving lifestyle of the subject.The majority of animals have stretchable and flexible skin and therefore wearable sensors must be designed to overcome deformation.In addition, the materials should be non-toxic and compatible with the animal skin.On the other hand, plant sensors are usually not subject to conformation changes but need to maintain function over long times under challenging environmental conditions with changing temperatures, humidity, radiation, precipitation, wind, and interactions with animals.For practical agricultural applications, plant wearables need to be scalable in an affordable manner both in terms of sensor production as well as data analysis, whereas in the case of animals usually the number of subjects for sensor application tends to be smaller and hence sensors can afford to be more complex and expensive.Unlike animals, plants produce a myriad of secondary metabolites that vary in time and space, which can present a challenge for plant sensor design in terms of specificity.Recently, new applications of wearable devices for detecting plant growth, signals emitted by plants (hormones, volatiles), or microclimatic conditions around plants (temperature, relative humidity RH, pesticides). [17]have received high attention.Indeed, wearable plant sensors have been developed rapidly in the last ten years (Table 1) to reduce the cost of sensing devices by testing different materials and sensor technologies.
Using wearable volatile sensors to identify the signature compounds, characteristic of specific plant-pathogen/pest interactions, could help to detect pathogen or pest infestation before significant plant damage occurs in agricultural fields or city parks. [18]Wearability is important for volatile sensors, as volatile compounds are emitted from plants often in very small quantities, [19] varying with time, and can easily disperse by the wind.In experimental set-ups, volatiles are collected from plant or leaf headspace often for tens of minutes up to hours to achieve sufficient quantities for analysis. [20]Thus, direct attachment of the sensor to the plant surface has an advantage and the potential to improve sensitivity.The sensors need to be both sensitive and selective to perceive and discriminate the small amounts of individual compounds in the vast spectrum of plant volatiles.Such sensors need to be low-cost, stable under field conditions (range of temperatures, precipitation, etc.), not interfere with plant physiology, and ideally should allow online monitoring of microclimate conditions or signature VOCs from multiple plants simultaneously.Microclimate monitoring could assess whether normal transpiration indicative of favorable growth conditions is in progress or if stomata have closed due to water stress, air pollution or pathogens.Compared to sensors placed in the vicinity of plants, wearable sensors allow to directly detect changes in microclimate (temperature, humidity at leaf surface) caused by changes in plant physiology.For example, it is possible to monitor stomatal movements either directly via a wearable sensor [21] or indirectly by a change in temperature and/or humidity recorded at the leaf surface by a local attached sensor.Direct online monitoring of stomatal apertures in leaves would be a step forward in plant physiology research and applications, allowing independent analysis of adaxial and abaxial stomatal function.
Over recent years, significant progress has been made in the field of wearable sensors for sensing plant microclimate and stomatal function after the first work of Lee et al. [16] in 2014 (Table 1), which described a new (carbon nanotube or CNT)graphite based sensor that can be interfaced directly (without substrate) onto life forms in nature, namely, insects and plants.In the following subsections, we will discuss some notable works and the challenges in question.

Plant Physiology Monitoring
Decoding the communication details in plants and their bionetworks requires discriminating individual parameters among the vast chemical and microclimate spectra and understanding their mutual influence on the plant's health status.For instance, microclimate monitoring of stomatal function and volatile monitoring in bionetworks requires translating the response-to-stimuli of microsensors to stable electrical signals without reducing the signal quality.Most importantly, sensor attachment to the leaf should not interfere with the signals emanating from adjacent sensors and the stomatal function (keeping natural light, temperature, and humidity conditions unaltered).Sensors with low cost, size, weight, and power are desirable for accurate and independent monitoring of leaf microclimate and stomatal behavior. [22]lant water potential correlates with the status of photosynthesis, stomatal conductance and transpiration.Therefore, tracking plant water status in real-time can indicate plant physiology and help intervene in time to ensure or improve agricultural productivity.Current methods for monitoring water potential and metabolic fluctuations in plants largely involve destructive sampling of a harvested plant.Therefore, there is a pressing need to develop a direct in-situ sensor to measure plant water potential or turgor pressure in easy and non-destructive manners to provide real-time information on the plant water status.As existing methods involve destructive sampling of harvested plants, Hanson et al. [23] devised a sensor array implementing hollow microneedle electrodes (10-1000 microns) and multifrequency electrical impedance spectroscopy (EIS) to monitor plant water status in millet in real-time.In situ EIS measurements were performed in the range of 0.02-1000 kHz to distinguish physiological functions and electrical properties of different plant tissue types, which can be employed in a feedback loop providing hydration to plants under stress.Using a similar EIS-based approach, Barbosa et al. [24] and Joshi et al. [25] proposed novel sensing electrodes based on paper substrates, namely pyrolyzed graphitic paper and Ag/AgCl conductive ink, respectively.The former device possesses a more advanced architecture, [24] that is, a handheld, low-power potentiostat connected wirelessly to a smartphone to determine loss of water content (LWC) at varying temperatures.
On the other hand, Li et al., [26] Lan et al. [27] and Yin et al., [28] in an attempt to eradicate complicated and time-expensive procedures that might affect their applicability in practical scenarios, devised a one-step, low-cost, and reliable method for large-scale production of non-invasive flexible humidity sensors for realtime microclimate monitoring.A graphene oxide (GO)-based humidity sensor was introduced as the humidity sensing material, which was attached to the abaxial side of the leaf for a Devil's ivy [27] and maize. [26,28]In situ real-time and long-term tracking of plant transpiration realized by recording the sensor capacitance showed great promise of such devices in next-generation plant-wearables.Another indicator of transpiration and water usage is sap flow in the stem, which can be quantified by the correlation between temperature difference and sap flow rate. [29]hen there is sap flow in the stem, since thermal transport is more efficient along the flow direction, an anisotropic temperature distribution occurs, which can be monitored by two temperature sensors.Remote control was achieved using a wireless control system via a flexible printed circuit board cable to monitor the flexible and wearable electronic flow sensor (Cu serpentine interconnected with onboard temperature sensor and thermistor). [30]tomatal movements can be analyzed via monitoring stomata actuation time by conductometric sensors fabricated directly on the abaxial leaf surface (Figure 2, left). [21]In detail, nanoparticlebased conducting ink was printed on two guard cells that formed a single stoma (Figure 2a).This wearable sensor allows for realtime tracking of the latency of individual stomatal opening and closing times in planta, which was shown to vary from 7 to 25 min for the former and from 53 to 45 min for the latter in Spathiphyllum wallisii.Plant transpiration on the abaxial leaf side, regulated by stomatal opening and closure, can be measured by fastresponse and real-time humidity sensors.An integrated multimodal flexible sensor system employing stacked ZnIn 2 S 4 (ZIS) nanosheets could predict and evaluate dehydration conditions in Pachira macrocarpa during over 15 days of monitoring. [22]The humidity of the leaf surface measured by the humidity sensor gradually increases as the light exposure increases due to the opening of stomata, contributing to the exchange of water vapor and CO 2 between the leaf and atmosphere for photosynthesis.In contrast, the humidity level on the leaf surface gradually decreases due to the closure of the stomata to avoid unnecessary water loss when the light source is off.
Monitoring of nitrate levels of the stalks can help to better understand the nitrogen cycle and the physiological responses of living plants to environmental variations.As evidence, an MEMS-based needle sensor was employed to detect nitrate using a carbon-based nanocomposite formed from graphene oxide (GO) and an artificial enzyme, vitamin B12 (VB12): C-GO-VB12, where the sensing mechanism was attributed to the reduction of NO 3 − to NO 2 − by vitamin B12. [32]A similar needlebased sensing technology was also used to monitor stress markers, such as salicylic acid, a multifaceted phytohormone essential in mediating plants' local and systemic immune responses.Instead of using a bioreceptor like in earlier work, a molecularly imprinted polymer (MIP) film was designed to recognize salicylic acid selectively. [33]MIPs are bio-mimicked receptors commonly used in sensing due to their low cost and high stability compared to bioreceptors. [34]Electrochemical sensors have also found their applications in detecting indole-3-acetic acid (IAA)an important endogenous auxin involved in regulating various physiological processes in plants which is the key to information in agricultural IoT and decision-making in precision agriculture.An artificial neural network (ANN)-based intelligent system capable of real-time analysis was employed to detect IAA changes under drought stress and continuous fluctuation in a flowering cabbage system in vivo. [35]Even though the sensor showed an ultralow detection limit of 10.8−57.8pg mL −1 , making it comfortably wearable for plants is challenging due to bulky and heavy architecture, that is, a micro-sized working electrode as well as reference and counter electrodes are packed in the PVC tube of >1 cm in diameter.The wearable sensor in the serpentine pattern has also been applied for detecting pesticides, such as methyl parathion, classified as Extremely Hazardous by WHO and not allowed for sale or import in nearly all countries around the world.It was possible to detect 100 μm of methyl parathion sprayed onto spinach and apple using square wave voltammetry.
Monitoring plant volatiles is one of the most critical but challenging task for wearable devices, that is, there are not yet wearable sensors or sensor arrays that are in practice.Instead, an array of eight sensors proposed by Li et al. [36] was used to test 13 plant VOCs generated from a gas-mixing system.However, a fingerprint of plant VOCs released by tomato plants in response to mechanical cutting or the infection by the oomycete pathogen P. infestans was not shown.Apart from the volatile fingerprints mentioned above, methanol is a biomarker emitted from plants, that is, emission of methanol reflects indirect plant defense against insects, promotes cell-to-cell communication, and adapts plants to various environmental stresses.Its central origin is considered to be the methyl ester groups of pectin-one of the major components of the plant cell wall.Methanol is produced through the hydrolysis of pectin methyl esters by pectin methylesterases and released from the plant via stomata. [37]When the cell wall, the main methanol production source, is damaged due to mechanical wounding or other stresses, methanol emissions increase.Gaseous methanol from the wounded plant induces defense reactions in intact leaves of the same and neighboring plants, activating methanol-inducible genes that regulate plant resistance to biotic and abiotic factors. [38]Methanol can be detected through electrochemical oxidation using a composite of poly(2-amino-1,3,4-thiadiazole) or poly(ATD) microcrystallites and platinum nanoparticles (PtNPs) (Figure 2, right). [31,39]To bind poly(ATD) Raman map for CNT G peak intensity (1590 cm −1 ) on the leaf surface after the ink printing across a single stoma, demonstrating that the printed ink was confined in the microfluidic channel.Adapted with permission. [21]Copyright 2022, Royal Society of Chemistry.a′) The schematic and photograph of a methanol sensor installed on the surface of a leaf and b′) (top) cross-sectional view of the methanol sensor and (bottom) fabrication process of the sensing film.Adapted with permission. [31]Copyright 2022, American Chemical Society.
and PtNPs together, Nafion, a polymer electrolyte, was infused into the pores of the electrode (Figure 2b′).The wearable sensor is considered field-deployable for sensing and monitoring methanol emissions from lower and upper leaf side in maize plants (Figure 2a′).However, the requirement of long accumulation times (>2 h) of methanol before measurement is a disadvantage (the rate of methanol accumulation being inversely proportional to the accumulation time, impacting the release of methanol from the leaf due to negative feedback).
Additionally, significant progress has been made toward developing sensors for monitoring soil health, such as moisture, soil fertility, microorganisms, temperature. [40]However, addressing sustainability in powering these sensors and putting "intelligent agriculture" into practice remains a dilemma.Lan et al. [27] reported a self-powered, battery-free nanogenerator capable of transmitting energy wirelessly and sustainably to sensors for monitoring plant growth through soil parameters.The monitored parameters were linked to a mobile phone via Bluetooth; this way, the health status of plants could be monitored in realtime.Details of the self-powered agricultural system are described in Table 1; it is worth mentioning that the device is capable of harvesting typical environmental energies from wind and raindrops.Together with ultra-light weight, electrostatic adhesion, and breathability, the authors are convinced that the device can be self-attached onto plant leaves; however, this may cause physiological response from plants.
In an attempt to overcome the majority of the limitations posed by electrical sensors, namely, low signal stability, sensitivity, and high hysteresis error, Presti et al. [41] proposed soft wearable sensors based on fiber Bragg grating technology (FBG) for monitoring stem elongation and leaf microclimate (temperature and RH) changes.FBG technology measures changes in strain and temperature; its working principle is based on changes in refracted Bragg wavelength when optical fibers elongate under mechanical or temperature stimuli. [42]Advantages of FBG technology include good metrological properties, miniaturized size, ultra lightweightedness, and biocompatibility.Moreover, thanks to their easy encapsulation into flexible matrices, FBG can interface with different plant organs (e.g., leaves, stems, and fruits).By virtue of their excellent properties, results exhibited by FBGbased sensors show good promise for plant wearables.The authors claim that implementing such sensors on a large scale would aid in optimizing plant health and growth, thus boosting crop yield.
Measuring the expansion and contraction of the plant stem, an important physiological characteristic for plants, requires continuous tracking of the sap flow [30] and pulse. [43]A serpentine pattern was chosen to design a 3-electrode electrochemical sensor [35] and other wearable sensors, [30,43] which can withstand an applied strain of over 10.In combination with the softness and flexibility of polydimethylsiloxane (PDMS) substrate, the sensor is claimed to be cohabitated with the plant. [30]The solution reportedly overcomes the issues faced by planar strain sensors, for example, difficulty in fixation of sensors onto the plant stem, instability of data, and inconvenience of wired data acquisition methods.The sensing mechanism is based on the compression or expansion of the 3D porous graphene structure upon increasing or decreasing the sensor's curvature.Suggested future work includes integrating the reported adaptive winding strain (AWS) sensor with cloud computing and forming a closed loop with irrigation equipment to bolster agricultural irrigation sufficiently.

Plant Growth Monitoring
Even though wearable optical sensors have been implemented for monitoring plant growth using, for example, FBG array [41a] and fluorescent fibers [44] (of which the latter one can monitor plant stress status), wearable electronics for plants are still dominating in this application domain.As plant growth causes changes in strain, materials developed for wearable sensors for plant growth should be strain-sensitive.This applies to both substrates, for example, PDMS, [45] latex, [46] and Ecoflex [47] and sensing layers such as the chitosan-based composite [48] of a wearable sensor.
A hydrogel fabricated from a composite of polyacrylic acid double-networked with the conductive nanofiller reduced graphene oxide and then coated with polyaniline incorporated with double networking (namely PAA-rGO-PANI) was used as a sensor for monitoring the growth of fruit-bearing plants. [49]The gauge factor of the wireless ultra-thin wearable sensor for aloe and pepper was 4.50 and 4.58, respectively, in an elongation of up to 200%.Interestingly, the sensors were self-powered by acoustics, mechanics, and wind.

Data Acquisition and Transfer Approaches for Microsensors
Decoding communication details in plants require discriminating individual parameters in a vast microclimate spectrum and understanding their mutual influence on plant health status.Real-time monitoring is an important aspect of precision agriculture, as it allows for early detection and prevention of plant diseases, and optimization of crop yields while minimizing input costs.However, real-time sensing outcomes may generate large datasets, depending on the number of microclimate variables, data-acquisition frequency, and the monitoring scale, that is, an individual plant in a pot or an agricultural field.Therefore the techniques implemented across the data chain [50] must be carefully optimized to translate the captured data into an easily comprehensible language.
Microclimate monitoring of stomatal function requires translating the response-to-stimuli of microsensors to stable electrical signals at regular intervals without reducing the signal quality.This can be facilitated by readout architectures on or off-chip related to calibration, control, and signal processing for the sensors.After extracting the required performance criteria, the data from the sensor system is transferred and interfaced with on or off-chip recording devices for additional digital signal processing, testing, and measurements.
The choice of data transmission protocol and storage should be established based on the specific needs of the application, such as the environment, plant species, range of communication and monitoring scale.For instance, a serial peripheral interface (SPI) should be capable of tracking sensor responses on a small scale over short distances, but it is not suitable for 'intelligent agriculture' applications.41b] In this method, data is collected from sensors and transferred through Ethernet cables to a central processing unit.Wireless networks like Wi-Fi, Bluetooth, and Zigbee are commonly used for real-time plant monitoring where data is collected wirelessly from sensors and transmitted to a central processing unit or cloud server. [24,30,33,35,36,43,45]uch networks are more suitable for outdoor applications and can cover large areas.Cellular networks like 3G, 4G, and 5G, also used for real-time plant monitoring, can transmit data over long distances, making them suitable for remote locations.However, this can be expensive, and data plans may have limited data allowances.
Lu et al. [22] have proposed a method for the collection of multiple-sensor data based on voltage divider circuits for monitoring RH from a multimodal flexible sensor.41a,47,48,51] On the other hand, Lee et al. [16] proposed the inductively coupled functionality of monolithic allcarbon electronics operating at radio frequency for real-time sensing of toxic gases wirelessly using a simple LRC passive circuit, highlighting their untethered easy-to-use capabilities.Other promising wireless techniques [24,30,33,35,36,43] have been implemented for data collection in sensors.Such wireless sensors exhibit several advantages over their wired counterparts, including flexibility in installation and repositioning, reduced costs due to the elimination of wires and cables, and reduced maintenance costs, improved data collection as they are less prone to measurement errors due to interference and wire resistance and suitability for real-time monitoring.

Conclusions and Future Outlook
During the last 10 years, plant wearable sensors have attracted substantial interest.This is an important development, as traditional instrumentations have high cost, inflexibility, and incapability of applications in field where, for example, monitoring crop health is critical.Different research groups have taken advantages of their technologies and applied these for monitoring microclimate, plant physiology and growth (Table 1).Development of wearable technologies for plants still faces some challenges that must be solved by ongoing research including, for instance, self-power technology for wearable sensors, plantcompatible and -insensitive materials, and wearable sensors for in field and inhouse applications.While the first challenge has been resolved by the works of Lan et al. [27] and Hsu et al., [49] technologies for the other two challenges have not been proposed yet.compatible materials are important for precisely studying plant physiology as any abnormal contact between wearable materials and plants can cause stress to plants and therefore creating unwanted signals, [52] applications of wearable sensor in field or inhouse is highly practical for agriculture industry as well as indoor/outdoor plant care.

Figure 1 .
Figure 1.Conceptual overview of interactions between plants and their abiotic and biotic environment and how a wearable sensing device can help to unveil plant behavior.

Figure 2 .
Figure 2. a) Schematic of conductive circuits printing on the Spathiphyllum wallisii leaf surface.A microfluidic chip is placed on top of the leaf abaxial surface and clamped in between two holders.Schematic layout of printed microsensor having two contact pads and a stripe going across a single stoma.b) A height profile map demonstrates a highly non-planar leaf surface.Scale bar: 75 μm.c) Bright-field microscopy images of a microfluidic chip aligned on top of i,iii) a single stoma and ii,iv) the same stoma after printing.Scale bars: i,ii) 30 μm and iii,iv) 10 μm.Red arrows point to individual stomata.d)Raman map for CNT G peak intensity (1590 cm −1 ) on the leaf surface after the ink printing across a single stoma, demonstrating that the printed ink was confined in the microfluidic channel.Adapted with permission.[21]Copyright 2022, Royal Society of Chemistry.a′) The schematic and photograph of a methanol sensor installed on the surface of a leaf and b′) (top) cross-sectional view of the methanol sensor and (bottom) fabrication process of the sensing film.Adapted with permission.[31]Copyright 2022, American Chemical Society.

Table 1 .
Current development of wearable sensors for plant health.