Ultrathin flexible and wearable sensors with high sensitivity

Abstract

Disclosed are ultrathin, fully printed capacitive devices that can be mounted onto living plant surfaces with a simple transfer process. The capacitive devices are based on conductive ink trace and can detect relative humidity (RH) directly at the plant tissue-environment interface. By completely encapsulating the device's electronic features between thin swellable layers (e.g., ethyl cellulose), a significant increase in sensor capacitance, SNR and sensitivity is revealed. The capacitive devices may exhibit baseline capacitance values in the nF range, high sensitivity (up to 1 nF / %RH) over a wide range of RH values and stability over several days or months.

Description

ULTRATHIN FLEXIBLE AND WEARABLE SENSORS WITH HIGH SENSITIVITY CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to United States Provisional Application No. 63 / 509,336 filed on June 21, 2023 and entitled ‘‘Ultrathin, Flexible and Wearable Humidity Sensors for High Sensitivity”, and to United States Provisional Application No. 63 / 527,799 filed on July 19, 2023 and entitled “Ultrathin, Flexible and Wearable Humidity Sensors for High Sensitivity”. Each of the aforementioned applications are incorporated by reference herein in their entirities.GOVERNMENT RIGHTS

[0002] This invention was made with government support under grant number 1935594 awarded by the National Science Foundation. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present disclosure relates to ultrathin capacitative devices for monitoring environmental parameters. BACKGROUND AND RELEVANT ART

[0004] Our planet is on the brink of a global food crisis. The human population is projected to reach approximately 10 billion by 2050 (by which time 70 percent of increased food production will be needed to meet the demands of the growing population) and every year millions of hectares of arable land are lost due to soil degradation, erosion, climatic changes (e.g., droughts, floods), and urban commercialization. Our relationship with plants has become increasingly complex in the modem era. Industrialization and technological advancements have enabled the mass production of food but have also contributed to monumental problems that need to be addressed to ensure a sustainable future.

[0005] Consistent plant and environmental testing are proven methods to boost crop productivity and provide resiliency amidst fluctuating conditions. Nine cardinal parameters for plant growth include incident light, water, carbon dioxide, wind, humidity7, air temperature, rootzone temperature, oxygen, and nutrients. By obtaining rich data on even a few of these critical parameters, farmers can make informed management choices, which can improve farm efficiency and economic output.

[0006] Input use efficiency, which refers to the measure of how effectively and efficiently resources such as fertilizers, water, and energy are utilized to achieve desired crop outputs, is critical for minimizing waste and optimizing resource use for sustainable agriculture. By dramatically improving our agroecosystem management practices, we can simultaneously secure global food supplies, mitigate the effects of climate change, and promote human well-being. In other words, we need to transform the agricultural sector into an industry of high precision.

[0007] Over the past decade, precision agriculture has experienced significant progress fueled by advancements in technology and data analytics. Advanced sensors collect chemical and physical data on the ground while high resolution satellite imagery provides detailed visual information from above. A greater emphasis is being put on data integration and connectivity, which allows farmers to gather and analyze data from multiple sources to make more informed decisions. Modem farmers utilize machine learning (ML) and artificial intelligence (Al) algorithms to analyze vast datasets, identify patterns, and make accurate predictions. The ability to rapidly examine historical and real-time information on weather conditions, soil quality, market trends, and crop health enables agronomists to determine optimal planting times, improve their input use efficiencies, modify irrigation schedules, and predict disease outbreaks. Accordingly, there is a need for improved sensor designs that may enable greater precision in agricultural and other practices. BRIEF SUMMARY OF THE INVENTION

[0008] Disclosed are capacitive devices configured to measure one or more parameters of a biological substrate. The capacitive devices may comprise a print sheet including a first swellable layer and a conductive layer printed on an upper surface of the first swellable layer. The conductive layer may comprise a capacitor formed from a conductive trace. The conductive trace may be formed from a conductive ink, such as a carbon-based ink or an ink comprising tungsten, zinc, silver, silver chloride, gold, platinum or other conductive or semi-conductive materials. The conductive ink may be printed onto the first swellable layer using a screen printing method or other two dimensional printing method. The capacitor may comprise an interdigitated electrode. The interdigitated electrode may comprise two arms or strips with each arm or strip having 5 or more fingers configured to interlock with the fingers of a corresponding arm or strip. Electrical contacts may be soldered or otherwise attached to the capacitor to enable measurement of the charge of the capacitive device.

[0009] A second swellable layer may be disposed above the conductive layer. The first and second swellable layers may comprise a tattoo paper substrate. The tattoo paper substrate may include ethyl cellulose. The print sheet comprising the first swellable layer may also comprise asacrificial layer disposed below the first swellable layer. The sacrificial layer may be dissolved by wetting the capacitve device with a transfer solution (e.g., a tattoo transfer solution) to enable attachment of the capacitive device to a biological substrate as part of a transfer process. The capacitive device may also comprise a backing layer configured to protect the sacrificial layer until the capacitive device is ready for attachment to the biological substrate. The transfer solution may comprise a salt solution, which may have a molarity in a range from approximately 1 mM to approximately 2 M salt molarity, or approximately 10 mM salt molarity to approximately 1 M salt molarity, or approximately 25 mM salt molarity to approximately 500 mM salt molarity, or approximately 50 mM salt molarity7to approximately 100 mM salt molarity7.

[0010] The capacitive device may have a relatively small thickness, such as a thickness within a range from approximately 5 pm to approximately 10 pm. The capacitive device may be configured to conform to the surface of a biological substrate.

[0011] Also disclosed is a method of manufacturing the capacitive device. The method may comprise printing a conductive ink onto a first swellable layer to form a capacitor, attaching electrical contacts to the capacitor, and adhering a second swellable layer to the capacitor. The method may further comprise applying a transfer solution to the capacitive device.

[0012] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary7is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0013] Additional features and advantages will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the teachings herein. Features and advantages of the claimed subject matter may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. Features of the present disclosure will become more fully apparent from the following description and appended claims, or may be learned by the practice of the present disclosure as set forth hereinafter.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to describe the manner in which the above recited and other advantages and features of the claimed subject matter can be obtained, a more particular description of the disclosure briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of itsscope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings described below.

[0015] Figure 1 illustrates a method of manufacturing a capacitive device.

[0016] Figure 2 illustrates a capacitive electrode comprising an interdigitated electrode.

[0017] Figures 3A and 3B illustrate alternative exemplary electrodes that may be incorporated into the capacitive device.

[0018] Figure 4 illustrates the interdigitated electrode printed on a first swellable layer.

[0019] Figures 5A-5D illustrate side views of an exemplary capacitive device at different points during the manufacturing process.

[0020] Figures 6A-6C illustrate an exemplary capacitive device attached and conforming to various biological substrates.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Disclosed are ultrathin capacitive devices that may be configured to enable precision measurement at low cost. Unlocking the full potential of precision technologies and sustainable land management practices relies on scientific and technological breakthroughs to enable the measurement of essential crop and environmental factors at high density. Current state-of-the-art, commercially available sensing technologies are bulky, labor intensive, expensive, and can only provide point-based or indirect measurements. For instance, sensor systems constructed from conventional components may continuously report data on parameters such as soil moisture and temperature, but typically cost over $500 per node. This cost factor restricts the spatial density required for truly effective precision agriculture. Moreover, ongoing maintenance and accumulation of electronic waste further compound the difficulty in providing much needed, high- resolution direct contact measurements of plant conditions.

[0022] Due to rising concerns of food insecurity and ecological collapse, it is becoming increasingly important to better understand how climate change is affecting plant grow th and the stability of our natural biomes. There is an ongoing effort to collect rich and spatially distributed data in precise locations where plants are growing, whether it be in tropical rainforests or outdoor and indoor agricultural spaces. A critically important environmental factor to measure is relative humidity (RH) because it provides insight into how- water is transported along the soil-plant- atmosphere continuum. RH is commonly used to calculate the vapor pressure deficit (VPD), which is the difference betw een the concentration of water vapor in the air and its saturation value. VPD is a useful measurement for assessing key plant physiological characteristics such as transpiration rates, nutrient uptake, and temperature regulation as w ell as the fire risk of a particular ecological region. Consequently, frequent and accurate RH monitoring empowers agronomists andconservationists to improve the way they care for growing environments, whether it be to maximize yields through water scheduling or to prevent unexpected wildfire events. Traditional methods for measuring RH involve bulky, rigid, and often expensive pieces of equipment that are stationed within the vicinity of growing plants (e.g., directly above the canopy in a greenhouse). These technologies are limited in accuracy because the RH in the air next to a living plant is often significantly different than the value at the plant tissue surface.

[0023] An ultrathin capacitive device may address these issues by presenting a low-cost, flexible alternative device that can monitor critical health metrics directly from living plants. Ultra-thin, wearable (on-plant) devices may be of tremendous value in monitoring microclimatic conditions directly at the plant-environment interface.

[0024] For purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary’. Moreover, other than in any operating examples or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, how ever, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0025] Separate features and components of any embodiment described herein may be combined with features and components of any other embodiment. Additionally, features having similar reference numbers may have the same or similar characteristics.

[0026] Also, it should be understood that any numerical range recited herein is intended to comprise all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to comprise all sub-ranges betw een (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0027] The use of the singular comprises the plural and plural encompasses singular, unless specifically stated otherwise. In addition, the use of “or” means “and / or” unless specifically stated otherwise, even though “and / or” may be explicitly used in certain instances.

[0028] The term “measurement substrate” is meant to refer to a surface or substrate to which the capacitive device is attached to measure one or more parameters associated with the measurement substrate, (e.g., environmental parameters). The measurement substrate may be a biological substrate or may be a non-biological substrate. For example, the measurement substrate may be a surface of building, environmental fixtures, equipment, clothing, or another device, or may be a surface of an industrial environment. The measurement substrate may comprise materials found in these environments, such as metals, plastics, concrete, and glass, et cetera. The biological substrate may comprise the surface of a plant leaf. stem, or fruit, or may comprise a human or animal surface, and throughout the application wherever a capacitive device is described as being attached to or measuring parameters related to a biological substrate one skilled in the art will understand that the capacitive device may be similarly attached or may measure parameters associated with a non-biological substrate. The one or more parameters may be parameters associated with the measurement substrate, and may include parameters measured at the measurement substrate or may be parameters of the environment in which the measurement substrate is located (i.e., environmental parameters).

[0029] In addition, unless otherwise indicated, numbers expressing quantities, constituents, distances, or other measurements used in the specification and claims are to be understood as optionally being modified by the term “about” or its synonyms. When the terms “about,” “approximately,” “substantially,” or the like are used in conjunction with a stated amount, value, or condition, it may be taken to mean an amount, value, or condition that deviates by less than 20%. less than 10%, less than 5%. less than 1%, less than 0.1%, or less than 0.01% of the stated amount, value, or condition.

[0030] Configurations of the present disclosure are directed to ultrathin capacitive devices that can measure relative humidity (RH) directly at the plant-environment interface in real-time. Compared to other commercially-available RH biosensors, this approach is inexpensive and scalable. Figure 1 illustrates a method 100 for manufacturing the capacitive device. In a first step 1 10, a capacitive electrode is formed by printing a conductive ink onto a print sheet.

[0031] Figure 2 illustrates a capacitive electrode 204 that forms a conductive layer 202 of the device 200. The conductive layer 202 may be disposed in contact with one or more swellable layers. The conductive layer 202 may be formed into a planar capacitive electrode 204. The capacitive electrode 204 may be formed by printing a conductive ink (such as, and including, a semi-conductive ink) onto the one or more swellable layers. The capacitive electrode 204 may beprinted onto the one or more swellable layers via a screen printing, ink-jet, blade coating, lithographic, or other two dimensional printing method. According to the present disclosure, 1 g of conductive carbon ink was found to be sufficient to form the capacitive electrode 204 when drop casting during a screen printing process. The conductive ink may comprise a carbon-based ink (e.g., a carbon-based microparticle ink) or an ink comprising tungsten, zinc, silver, silver chloride, gold, platinum or other conductive materials.

[0032] The conductive ink may be printed to form conductive traces 206 upon the first swellable layer 220. For example, a conductive carbon-based microparticle ink may dry after printing on the first swellable layer 220 to form conductive carbon traces. The conductive traces 206 may then be cured (e.g., at 100° C for 30 min). Upon curation, the conductive traces 206 may have a small thickness, and may extend from the first swellable layer 220 by as little as approximately 5 pm. In some embodiments, the conductive traces 206 may extend further from the first swellable layer 220.

[0033] The disclosed capacitive device 200 may comprise a capacitor that can monitor RH across a wide range (1% to 85%). By measuring the capacitance between the two interdigitated electrodes (IDEs), small changes in the dielectric constant of the environment surrounding the electrodes can be detected. The device’s 200 capacitive signal may be used to detect the humidity level because of the effect that RH has on the dielectric constant of air and resulting properties of the one or more swellable layers that may surround the printed capacitive electrodes 204. Two example scenarios that may to the device’s 200 sensitivity to RH include: (1) water molecules modulate the dielectric constant of the air directly above the device 200, which is sensed by the capacitive electrode’s 204 vertically-oriented fringing lines, and (2) water sorption in the one or more swellable layers, which alters the film's permittivity, influences the mobility of the ions within the device 200, and is detected by the capacitive electrode’s 204 horizontal fringing electric field line.

[0034] Figure 2 illustrates that the electrode may comprise an IDE formed in the conductive layer 202. The IDE may comprise a first arm or strip 208 and a second arm or strip 210 comprising conductive traces 206 formed by the printed conductive ink. The first and second arms 208. 210 may extend substantially parallel to one another. The first and second arms 208, 210 may also comprise a plurality of fingers extending between the first and second arms 208, 210. The fingers 212, 214 of the first and second arms 208, 210 may be spaced to allow the fingers 212 of the first arm 208 to mesh or interlock with the fingers 214 of the second arm 210, increasing the capacitive surface of the capactive electrode and providing the capacitive device 200 with a greater capacitance. Each of the first and second arms 208, 210 may comprise multiple fingers 212, 214, such as three, four, five, six, seven, eight, nine, ten, or more than ten fingers 212, 214. Theconductive traces 206 of the first arm 208 (including the fingers 212 of the first arm 208) may be separated a minimum distance from the conductive traces 206 of the second arm 210 (including the fingers 214 of the second arm 210) to form the capacitive electrode. In at least some instances, the fingers 212, 214 of the first and second arms 208, 210 may have a width of 1 pm and a separation distance D of approximately 525 nm from other fingers 212, 214 of the other arm of the first and second arms 208, 210.

[0035] The capacitive electrode 204 may have a relatively small geometry. The capacitive electrode 204 may have a width as small as 500 pm. The width of the capacitive electrode 204 may be within a range from approximately 500 pm to more than approximately 3000 pm, or from approximately 750 pm to approximately 2000 pm, or from approximately 1000 pm to approximately 1500 pm. or within a range of any two of the foregoing as endpoints. The capacitive electrode 204 may have a small length, such as a length within a range from approximately 5 mm to more than approximately 30 mm, or from approximately 8 mm to approximately 20 mm, or approximately 10 mm to approximately 15 mm. These small configurations enable the capacitive device 200 to be attached to the surfaces of plants, including plants during relatively early stages of growth, while minimizing the surface that is covered by the device 200. It also enables the capacitive device 200 to minimally interfere with plant growth and horticultural or agricultural conditions while collecting measurements at the plant surface.

[0036] Figures 3A-3B illustrate other capacitative electrodes 204a, 204b that may be formed in the conductive layer 202. Similar to the IDE described above, the capacitive electrode 204 may include first and second arms 208, 210 and may comprise any suitable configuration for forming a capacitor.

[0037] However, the IDE described above may provide greater specificity compared to electrodes of a similar size, such that the IDE may provide a sensor with similar capacitance while minimizing the size of the capacitive device 200.

[0038] Figure 4 illustrates that the IDE may be formed from conductive traces 206 printed on a print sheet 218. The print sheet 218 may comprise a first swellable layer 220. The first swellable layer 220 may comprise paper, such as a tattoo paper substrate (e.g., an ethyl cellulose (EtC) substrate), or other swellable material. The first swellable layer 220 may be relatively thin. For example, the first sw ellable layer 220 may have a thickness as small as 600 nm.

[0039] Figures 5A-5D illustrate side views of the capacitive device 200 at different moments during the formation process. Figure 5A illustrates the capacitive device 200 after the first step 110 of method 100 has been performed, with the conductive layer 202 printed on the first swellable layer 220. In a second step 120, electrical contacts 216 may be attached to the first and second arms 208, 210 of the capacitative electrode. Figure 5B illustrates that the electrical contacts 216(also shown in Figure 4) may be atached to an upper surface of the capacitative electrode and may extend laterally from the conductive layer 202. The electrical contacts 216 may comprise wires (e.g., copper wire) that may be soldered to the IDE after printing the IDE to the first swellable layer 220. The electrical contacts 216 may provide a connection to other components for measuring the capacitance between the first and second arms 208, 210 of the capacitive electrodes 204.

[0040] In athird step 130 of method 100, a second swellable material may be disposed above the conductive layer 202. Figure 5C shows that the second swellable layer 226 may be adhered to an upper surface of the conductive layer 202. The second swellable layer 226 may comprise the same or similar materials as the first swellable layer 220, including a tattoo paper substrate and / or an EtC substrate. Specifically, encapsulating the conductive traces 206 by applying a second swellable layer 226 was found to increase the capacitative sensitivity of the capacitive device 200, with the device 200 being capable of measurement in the nF range. Further, the application of a second swellable layer 226 was found to increase the signal-to-noise ratios to an amount more than two times higher than devices lacking the second swellable layer 226.

[0041] By placing a swellable layer on an upper and lower surface of the capacitive electrode 204, the capacitive electrode 204 is prevented from contacting the biological environment directly, which could result in poor signal-to-noise ratios. The second swellable layer 226 may sen e as a conformable buffer layer between the conductive layer 202 and a biological surface while also serving to couple the conductive layer 202 to the biological surface. The first and second swellable layers 220, 226 also provide moisture-sensitive membranes to thereby increase the specificity and performance of the device 200 without needing to add any additional materials and / or complex fabrication steps. In some embodiments, an additional electrode (not shown) may thereafter be printed on the second swellable layer 226. The additional electrode may be configured to measure additional parameters. For example, the additional electrode may be configured to measure environmental parameters, such as temperature, pH, or the electrical conductivity7of growing media.

[0042] Figures 5A-5C illustrate that the print sheet 218 may be a composite sheet comprising multiple layers, including the first swellable layer 220 (e.g., the tattoo paper substrate) upon which the conductive traces 206 are deposited. The print sheet 218 may also comprise a sacrificial layer 222 disposed beneath the first swellable layer 220. The sacrificial layer 222 may be configured to rapidly dissolve upon contact with an aqueous solution, which may enable the atachment of the capacitive device 200 to the measurement substrate. In some embodiments, the sacrificial layer 222 may comprise a starch (e.g., dextrin). The composite sheet may also comprise a backing layer 224 that protects the sacrificial layer 222 from dissolving until the capacitive device 200 is readyfor atachment to the measurement substrate. In preparation to atach the capacitive device 200 to the measurement substrate, the method 100 may comprise an optional fourth step 140 (in instances wherein the print sheet 218 comprises a backing layer 224) wherein the backing layer 224 is peeled from the capacitive device 200. After this step is performed, the sacrificial layer 222 may be revealed.

[0043] The method 100 may optionally comprise a fifth step 150 wherein the capacitive device 200 is weted with a transfer solution to dissolve the sacrificial layer 222 (in instances wherein the print sheet 218 comprises a sacrificial layer 222). The transfer solution may comprise an aqueous solution. The method 100 may comprise a sixth step 160 wherein the capacitive device 200 is placed against the biological substrate, with the sacrificial layer 222 of the composite sheet in contact with the biological substrate, and adhering the capacitive device 200 to the biological substrate through application of the transfer solution to the capacitive device 200. The transfer solution may be applied to just the underneath of the first paper substrate, or may be applied over the entirety of the capacitive device 200 to quickly adhere the capacitive device 200 to the biological substrate.

[0044] The specificity of the capacitive device 200 may be increased by adjusting the salt molarity of the transfer solution. For example, the transfer solution may comprise sodium chloride, potassium chloride, calcium chloride, magnesium chloride, a nitrate (e.g., sodium nitrate), or other salt. The specificity of the capacitive device 200 may be tuned by adjusting the molarity of the transfer solution. For example, the transfer solution may comprise a salt molarity within a range from approximately 1 mM to approximately 2 M salt, or approximately 10 mM salt to approximately 1 M salt, or approximately 25 mM salt to approximately 500 mM salt, or approximately 50 mM salt to approximately 100 mM salt, or within a range having any two of the foregoing as endpoints. By adjusting the salt molarity of the transfer solution, the baseline capacitance values of the device 200 may be achieved in the nF range. In some instances, the capacitive device 200 may exhibit an RH sensitivity up to 1000 pF / %RH. This may greatly increase the resolution of a system in which the device 200 is integrated, such that the resolution of the device 200 may be limited primarily by the least significant bit of an analog-to-digital converter component connected to the device 200.

[0045] In at least one embodiment, the steady-state capacitance of the capacitive device 200 at 20% RH may increase from less than 1 nF (DI water) to over 10 nF (100 mM NaCl) with the introduction of more charged ions into the first and second swellable layers 220, 226 during the transfer process. Specifically, the capacitive device 200 may exhibit an RH sensitivity of approximately 50 to approximately 200 pF / %RH for RH levels less than or equal to 35% and an RH sensitivity of approximately 500 to approximately 1000 pF / %RH at RH levels at or above55%. It was found that only an approximately 16 nF change in capacitance was observed in RH ranging from 1% relative humidity to 84% relative humidity. It is believed that the ionic concentration of the transfer solution may adjust the strength and / or formation of electrical double layers forming within the swellable layers and thereby increasing the net charge stored between the capacitive electrodes 204.

[0046] The capacitive device 200 may be relatively thin. For example, the assembled capacitive device 200 may have a thickness of approximately 5 pm to approximately 10 pm. Alternatively, the device 200 may have a greater thickness. The capacitive device 200 may have a thickness within a range from approximately 5 pm to approximately 3 mm, or approximately 10 pm to approximately 1mm, or approximately 50 pm to approximately 500 mm, or approximately 100 pm to approximately 250 pm. or a thickness within a range having any two of the foregoing as endpoints. The relatively small thickness of the capacitive device 200 may increase the flexibility of the device 200 and may enable the capacitive device 200 to conform to the surface of the measurement substrate. Figures 6A-6C illustrate the capacitive device 200 attached to the surface of biological substrates 230a, 230b, or 230c, respectively. The device 200 may be sufficiently flexible to conform to the surface of the substrate 230a, 230b, or 230c. Figures 6A and 6C illustrate that the capacitive device 200 may be attached to the adaxial (upper) or abaxial (lower) surfaces of the plant leaves, respectively. Figure 6C specifically illustrates that the capacitive device 200 may be attached to a lower surface of the leaf, where RH levels may be more accurately measured. Figure 6B illustrates the capacitive device 200 attached to the stem of a plant. It will be noted that the capacitive device 200 may flexibly bend to match the shape of the surface of the leaf or stem. The capacitive device 200 may be attached to a variety of plant types and structures and at varying canopy levels, including the leaves, petiole, stem, and trunks of various plants. This conformability enables the device 200 to capture RH data directly at the plant- environment interface, which is important for assessing critical plant health information such as the VPD.

[0047] This thinness of the device 200 enables the device 200 to measure the relative humidity at the surface of the biological substrate 230a, 230b, or 230c attaching the capacitive device 200 directly to the surface of the biological substrate 230a, 230b, or 230c. Although the capacitive device 200 has been described as configured to attach to plant substrates, especially in an agricultural context, one skilled in the art will appreciate that the capacitive device 200 may be attached to other measurement substrates. The capacitive device 200 may be attached to other biological substrates, including human and animal surfaces. The capacitive device 200 may be attached to non-biological substrates, including the surfaces of buildings (e.g., concrete or glass) or clothing.

[0048] The small thickness of the capacitive device 200 may enable the device 200 to conform to the biological substrate 230a, 230b. or 230c and thereby increase the adhesion surface between the capacitive device 200 and the measurement substrate. Because the adhesion surface of the capacitive device 200 is large in proportion to the total surface of the device 200, the device 200 may exhibit an increased propensity to adhere to the measurement substrate over the period of measurement. The capacitive device 200 may remain adhered to the measurement substrate for several days, such as 3 to 7 days. In some embodiments, the capacitive device 200 may remain adhered to the measurement substrate for more than 14 days. In some embodiments, the capacitive device 200 may remain adhered to the measurement substrate for more than 60 days. The length of the adhesion period may be maintained even during mechanical flexion of the capacitive device 200 imparted by a flexible biological substrate.

[0049] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

CLAIMS1 . A capacitive device configured to measure one or more parameters associated with a measurement substrate, comprising: a print sheet comprising a first swellable layer; and a conductive layer printed on an upper surface of the first swellable layer; wherein the conductive layer comprises a capacitor formed from a conductive trace.

2. The capacitive device of claim 1, further comprising a second swellable layer disposed above the conductive layer.

3. The capacitive device of claim 1, wherein the capacitive device is wetted with a salt solution.

4. The capacitive device of claim 3, wherein the salt solution comprises a salt molarity within a range from approximately 1 mM to approximately 2 M salt.

5. The capacitive device of claim 1, wherein the conductive trace is formed by printing a conductive ink onto the first swellable layer.

6. The capacitive device of claim 5, wherein the conductive ink is a carbon-based ink.

7. The capacitive device of claim 1, wherein the capacitor comprises an interdigitated electrode.

8. The capacitive device of claim 7, wherein the interdigitated electrode comprises 5 or more fingers.

9. The capacitive device of claim 1, wherein the capacitive device has a thickness within a range from approximately 5 pm to approximately 10 pm.

10. The capacitive device of claim 1, further comprising electrical contacts attached to the capacitor.

11. The capacitive device of claim 1, wherein the first swellable layer comprises a tattoo paper substrate.

12. The capacitive device of claim 11, wherein the tattoo paper substrate comprises ethyl cellulose.

13. The capacitive device of claim 1, wherein the capacitive device is configured to conform to the surface of a measurement substrate.

14. The capacitive device of claim 1, wherein the print sheet comprises a sacrificial layer disposed underneath the first swellable layer.

15. The capacitive device of claim 14 wherein the capacitive device is configured to attach to a measurement substrate after dissolving the sacrificial layer in a transfer solution.

16. A method of attaching the capacitive device of claim 15 to a measurement substrate, the method comprising: applying a transfer solution to the capacitive device; and adhering the capacitive device of to the measurement substrate.

17. A method of manufacturing a capacitive device, the method comprising: printing a conductive ink onto a first swellable layer to form a capacitor; attaching electrical contacts to the capacitor; and adhering a second swellable layer to the capacitor.

18. The method of claim 17, wherein the method further comprises apply a transfer solution to the capacitive device.

19. The method of claim 18, wherein the transfer solution is a salt solution.

20. The method of claim 17, wherein the method comprises a screen printing method.