WO2005043087A1

WO2005043087A1 - Method of producing medium-to thin-film pressure and humidity sensors by flexographic printing

Info

Publication number: WO2005043087A1
Application number: PCT/CA2004/001916
Authority: WO
Inventors: Michael Petersen; Allan Wilson; Mykola Sherstyuk
Original assignee: Intelligent Devices Inc.
Priority date: 2003-11-03
Filing date: 2004-11-03
Publication date: 2005-05-12
Also published as: CA2447740A1; CA2447740C; CA2797598A1

Abstract

A means is described for making low-cost pressure and humidity sensors using flexographic printing methods, carbon-resistive inks, and flexible substrates. Specifically described are low-cost sensors for measuring applied pressure and monitoring the position or orientation of moveable objects such as joints and the opening and closing of lids, doors or windows. The invention can also be applied to the monitoring of humidity. A means of measuring atmospheric (environmental) pressure using a flexible bubble coated with flexographically-applied carbon-resistive ink is also described.

Description

METHOD OF PRODUCING MEDIUM- TO THIN-FILM PRESSURE AND HUMIDITY SENSORS BY FLEXOGRAPHIC PRINTING

FIELD OF THE INVENTION

This invention describes a means of making low-cost pressure and humidity sensors using flexographic printing methods, carbon-resistive inks, and flexible substrates.

BACKGROUND OF THE INVENTION

Environmental sensors designed to measure pressure, humidity, temperature, voltage and resistance are widely available. They utilize a number of techniques, and have in common complexity and resulting high cost of manufacturing. For example, there are many types of device designed to measure pressure. These may be based on water/mercury columns, mechanical or electronic diaphragms, piezo devices, piezo-diaphragm combinations and other principles.

Pressure sensors using printed thick-film serigraphic techniques are also available, as are screen-printed single cell and matrix cell arrangements for such diverse applications as pressure-mats, screened carbon membrane switches, and temperature cut-off switches.

Although sensors based on thick-film serigraphic processes are available, they are difficult and slow to manufacture, making them relatively expensive. Thin-film processes, as are currently in use for food-packaging, require precise quality control during manufacturing and typically require multi-layer substrates. Thin-film applications, such as platinum temperature sensors or LCD panel displays, are typically limited to rigid substrates and are expensive to produce.

Flexographic printing has recently undergone rapid development due to its utility in printing on flexible packaging substrates such as paper, paperboard, poly-substrates, foils, tissues, etc.

In commonly owned Canadian Patent Application No. 2,404,805 filed September 24, 2002 [ANALOG PACKAGING DEVICE AND CONTENT USE MONITORING SYSTEM] there is described a means of monitoring package contents using printed conductive and carbon- resistive ink traces. During research associated with this application the authors elucidated the properties of piezo-resistive, carbon-based inks suitable for flexographic printing, which properties are not applicable to lithographic printing methods. Techniques designed to reduce water absorption by paper and paperboard substrates during the flexographic printing process, coupled with newly formulated conductive and carbon-resistive inks, have made it possible to apply complex conductive and resistive circuitry directly to paper, paperboard, plastic, polyester, poly-paper, coated and uncoated foils, and other flexography-compatible flexible and rigid substrates.

Due to the relatively thin (1-3 microns) layer of ink (compared to the 25+ microns of thick- film serigraphy), carbon-resistive ink printed resistors take on piezo-resistive properties described herein, making them suitable for a new class of inexpensive, disposable, thin-film sensors and switches that can be produced in very high volumes using standard flexographic printing technology.

Specifically, there is a need for a low-cost pressure receptor capable of measuring applied pressure. There is also a requirement for a low cost pressure receptor capable of measuring atmospheric (environmental) pressure for tamper detection and other purposes. There is also a need for a low-cost pressure receptor capable of measuring bending or flexing movements for detection of opening and closing cycles of everything from doors and lids to cycles of mechanical devices such as artificial joints, and for determining the positional orientation of flexible, bendable or jointed devices. There is also a need for a low- cost humidity sensor to monitor and detect humidity and changes in humidity during the shipment and storage of packaged materials.

SUMMARY OF THE INVENTION

The proposed invention utilizes a common printing process to meet each of the above requirements.

The proposed invention comprises a uniform, thin layer (1-3 microns) of carbon-resistive ink applied to a flexible substrate by flexographic printing. The printed substrate, which can be of paper, paperboard, plastic, polyimide, coated foil, or other flexography-compatible flexible or rigid substrate, can then be cut into strips of various widths and shapes. Bending an ink- coated strip results in predictable changes in resistance across the carbon-resistive ink path. In accordance with piezo-resistive principles, the changes in resistance are related to the degree of bending. Bending the ink layer convexly results in an increase in resistance; bending it concavely causes the resistance to decrease. These changes in resistance can be monitored by an ohmmeter or analog CPU and correlated with the pressure or bending changes of interest. Similarly, a uniform, thin layer (1-3 microns) of carbon-resistive ink can be applied to a paper, paperboard or other substrate of known water content and water-absorptive characteristics. Increases in humidity cause the substrate to expand, pulling with it the adherent carbon-resistive ink layer. The change in resistance can be correlated with the humidity to which the substrate is exposed. Conversely, exposure to decreased humidity causes the substrate to contract through loss of water content, compressing the carbon- resistive ink layer and decreasing its resistance. As described above, the resistance across the ink layer can be monitored by an ohmmeter or CPU and correlated with the environmental humidity.

In a similar way, a sandwich can be made of two layers of flexible substrate on the inner surface of one of which has been printed a uniform layer of carbon-resistive ink. Pressure applied to the substrates compresses the ink layer, causes the resistance across the carbon-resistive ink layer to decrease, The resistance can be monitored by an ohmmeter or CPU, and correlated with the applied pressure.

In an analogous manner, the upper flexible surface of a gas-filled blister can be printed with a uniform layer of carbon-resistive ink. Increases in the environmental atmospheric pressure cause the bubble to contract, decreasing the resistance across the carbon- resistive ink layer. Decreases in environmental pressure cause the bubble to expand, increasing the resistance across the ink layer. The resistance across the ink layer can be monitored by an ohmmeter or CPU and correlated with the environmental atmospheric pressure.

The cost of producing such devices is extremely low. Sheets of substrate can be coated with a uniform layer of carbon-resistive ink the properties of which can be matched to the desired application, using flexographic printing methods. The resistance of the ink can be varied, as can the flexibility, melting characteristics, strength and water absorbing characteristics of the substrate and ink/substrate composite. The sheets of composite can them be cut into strips or other shapes according to the desired application.

The present invention may therefore be seen as involving a method for printing low cost sensors for pressure and humidity and as encompassing the sensors made by such method.

The invention uses flexographic printing to apply a layer of carbon-resistive ink to a flexible substrate. Bending, stretching, compressing or otherwise deforming the substrate causes a change in resistance across the carbon-resistive ink path. The change in resistance can be related to the degree of deformation in a predictable manner.

Any flexible substrate can be used, including but not limited to paper, paperboard, plastic, polyester, poly-paper, uncoated foils, and other flexography-compatible flexible substrates.

The substrate can be non-conductive or conductive.

Conductive substrates require a dielectric coating to isolate the substrate from the carbon- resistive ink layer.

The resistance, flexibility and other characteristics of the carbon-resistive ink can be adjusted during its formulation to provide optimum sensitivity according to the device's intended use.

Such characteristics of the flexible substrate as thickness, flexibility, thermal and moisture stability can similarly be adjusted according to the intended use.

The carbon-resistive ink can be applied by flexographic printing to sheets of flexible substrate, which can then be cut into shapes according to the intended use.

Different formulations of carbon-resistive ink may be used for different parts of the sensor to optimize performance in some applications.

For humidity sensor applications a moisture-sensitive flexible substrate such as paperboard or poly-paper is used as described above.

The moisture sensitivity of the substrate can adjusted to optimize the sensitivity of the receptor.

The resistance, flexibility and other characteristics of the carbon-resistive ink can be adjusted during its formulation to provide the desired sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood from the following description with reference to the drawings in which: FIG. 1 is a cross-sectional view of a printed carbon-resistive ink layer applied to a flexible substrate.

FIG. 2 is a cross-sectional view of a flexible substrate to which a dielectric layer has been applied prior to printing with a carbon-resistive ink layer.

FIG. 3 is a view as in FIG.1 with the composite bent concavely with respect to the carbon-resistive ink layer.

FIG. 4 is a view as in FIG.1 with the composite bent convexly with respect to the carbon-resistive ink layer.

FIG. 5 is a cross-sectional view of a printed carbon-resistive ink layer applied to a humidity-sensitive substrate at baseline water content.

FIG. 6 is a schematic cross-sectional view as in FIG. 5 with the humidity-sensitive substrate at water content above its baseline.

FIG. 7 is a cross-section of a piezo-resistive pressure sensor.

FIG. 8 is a cross section of a piezo-resistive pressure sensor with force applied to it.

FIG. 9 is a cross-sectional view of a "bubble laminate" comprising two layers of flexible substrate, a printed carbon-resistive ink layer, and a gas-tight pocket (bubble) containing trapped gas.

FIG. 10 is a cross-sectional view of an expanded bubble due to decreased atmospheric pressure.

FIG. 11 is a cross-sectional view of a contracted bubble due to increased atmospheric pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 depict a pressure receptor 10 comprised of a layer 12 of carbon resistive ink applied by flexographic printing to a flexible substrate 14. FIG. 1 shows a non-conductive or low-conductive substrate. FIG. 2 shows a conductive substrate 16 that has been coated with a dielectric layer 18 prior to the printing of the carbon-resistive ink layer 12.

Referring to FIG. 1 , a layer 12 of carbon-resistive ink has been applied to a sheet of flexible, non-conductive substrate 14. The ink layer is attached via a conductive pathway 20 to an ohmmeter 22 or other means of measuring and detecting changes in resistance (such as a CPU).

FIG. 2 depicts the same process using a conductive substrate 16. To prevent the carbon- resistive layer 12 from shorting via the substrate 16, the latter has been coated with a dielectric layer 18 prior to printing of the carbon-resistive layer 12. Such coatings are well known, as in coated metal foils.

In FIG. 3 the ink-substrate composite 10 has been bent so the carbon-resistive ink layer 12 is concave. This causes the conductive carbon particles in the ink layer 12 to be forced closer together, causing a decrease in the resistance to a current passed through the ink layer. The change in resistance is a function of the degree to which the ink-substrate composite 10 is deformed.

FIG. 4 shows the ink-substrate composite 10 bent so the carbon-resistive ink layer 12 is convex. This causes the carbon particles in the ink to be pulled farther apart, increasing the resistance to a current passed through the ink layer.

In FIGS. 3 and 4, the carbon-resistive ink layer 12 is connected to a device 22 such as an ohmmeter or CPU designed to monitor and detect changes in the resistance in the circuit. The changes in resistance can be correlated with positional deflection, bending, and compressive or tension forces, creating a pressure or position receptor. The sensitivity of the system can be adjusted by the flexibility of the substrate, by the carbon-resistive ink formulation, and by the flexographic printing process (e.g.: thickness and width of the carbon-resistive ink layer 12).

For economical production, sheets of carbon-resistive ink - flexible substrate composite 10 can be printed and then cut to shape as required. Equally, the carbon-resistive ink layer can be printed to shape on a larger sheet of substrate. FIGS 3 and 4 depict a carbon-resistive ink layer 12 printed on non- or low-conducting substrate 14. The invention applies equally to conductive substrates with a dielectric coating 18 (e.g.: foils), as shown in FIG. 2.

FIG. 5 depicts a humidity or moisture sensor 24 comprising a humidity-sensitive substrate 26 such as paper or paperboard on which has been printed by flexography a layer 28 of carbon-resistive ink. The dimensions (e.g.: length) of the composite are determined by the water content of the substrate 26. In FIG. 5, the length "/" is shown at the baseline water content of the substrate.

In FIG. 6, the substrate 26 has absorbed moisture from the environment, causing it to expand to a length "//". As the composite expands, the carbon particles in the carbon- resistive ink layer 28 are pulled farther apart, increasing the resistance of the ink layer. Changes in resistance (length of the carbon-resistive ink path) are correlated with humidity, thus providing a humidity sensor.

As in the case of the pressure sensing application described above, the sensitivity of the humidity sensing system can be adjusted by the baseline water content and water-absorbing characteristics of the substrate, by the carbon-resistive ink formulation, and by the flexographic printing process (eg: thickness and width of the carbon-resistive ink layer).

As described previously, sheets of carbon-resistive ink-substrate composite can be printed and then cut to shape as may be required for humidity sensing applications.

FIGS. 7 and 8 show a further embodiment of the invention.

In FIG. 7 is depicted in cross-section a piezo-resistive pressure sensor 30. Two layers 32, 34 of non-conductive flexible substrate comprise the outer layers of a three-layer sandwich. On one of the two flexible substrates has been printed, by flexographic printing techniques, a uniform layer 36 of carbon-resistive ink. The extremities of the ink layer are connected by a conducting path 38 to an ohmmeter or CPU 40.

In FIG. 8 compressive force applied to the laminate 30 causes the carbon-resistive ink layer 36 to be compressed, forcing the conductive carbon particles closer together and decreasing the resistance across the surface. The resistance detected by the ohmmeter or CPU 40 can be correlated with the applied pressure, yielding a low-cost piezo-resistive pressure sensor. The procedure can also be applied to tension forces, the limits of which would be a function of the strength of the substrates and their adhesive properties.

Conductive substrates such as foils could also be used in which case dielectric layers would be required to isolate the carbon-resistive ink layer from the substrates.

FIGS. 9 through 11 show a further embodiment 42 of the invention.

In FIG. 9 two layers 44, 46 of flexible substrate have been laminated. The top layer 44 is made of a very flexible, non-conductive substrate 48 such as plastic and the lower layer of a flexible substrate 50. Both substrates are gas impermeable. On the upper surface of the top layer 44 has been printed by flexographic methods a uniform layer 52 of flexible carbon- resistive ink. Air or another gas has been injected into the laminar interface to form a pocket or "bubble" 54, as in the manufacture of bubble wrap.

FIG. 10 shows the effect of a decrease in the atmospheric (environmental) pressure. The pressure of the gas in the bubble 54 increases relative to the pressure of the external environment causing the bubble to expand. As the bubble expands, the dimensions of the carbon-resistive ink layer 52 increase, pulling the carbon particles apart. This increases the resistance across the carbon-resistive ink layer 52, which resistance can be monitored by a ohmmeter, CPU or other device 56 connected to the extremities of the ink layer by a conductive path 58. The resistance can be correlated with the atmospheric or environmental pressure, yielding a low-cost atmospheric pressure receptor.

FIG. 11 shows the effect of an increase in the atmospheric (environmental) pressure. The pressure of the gas in the bubble 54 decreases relative to the pressure of the external pressure causing the bubble to contract. As the bubble contracts, the dimensions of the carbon-resistive ink layer 52 decrease, pulling the carbon particles closer together. This decreases the resistance across the carbon-resistive ink layer 52, which resistance can be monitored by an ohmmeter, CPU or other device 56 connected to the extremities of the ink layer by a conductive path 58.

The sensitivity of the invention can be adjusted by the size of the bubble, flexibility of the upper layer of the composite, characteristics of the carbon-resistive ink layer, and type of gas used in the bubble. To increase the impermeability of certain substrates to gasses of interest, it may be desirable to add laminates of a gas impermeable material to the interior surfaces of the substrates, rendering the bubble impermeable to the gas contained therein.

By these means can be detected changes in environmental atmospheric pressure by a low- cost, flexographically-printed pressure sensor.

While particular embodiments of the present invention have been shown and described, changes and modifications may be made to such embodiments without departing from the true scope of the invention.

Claims

CLAIMS:

1. A method of producing a sensor for use in a resistance circuit comprising the steps of: a) providing a flexible substrate selected from a material that is bendable, that is compressible, or that is responsive to changes in ambient humidity; b) providing a carbon-resistive ink material; and c) applying said ink material to a surface of said substrate by flexographic printing methods.

2. The method of claim 1 wherein said ink material is applied to said substrate in a thickness of from 1 micron to 3 microns.

3. The method of claim 1 or claim 2 wherein said substrate is provided as a sheet, said ink material is applied to one entire surface of said sheet, and thereafter said sheet with ink material applied thereto is divided into a plurality of individual sensor units.

4. The method of any one of claims 1 to 3 wherein said flexible substrate is bendable and said sensor may be used to detect changes in deflection or position of said sensor.

5. The method of any one of claims 1 to 3 wherein said flexible substrate is compressible and said sensor may be used to detect changes in pressure applied thereto.

6. The method of claim 5 including the step of applying a second layer of flexible substrate material to the ink material previously applied to substrate material so as to sandwich the ink material between two layers of such substrate material.

7. The method of any one of claims 1 to 3 wherein said flexible substrate material reacts to changes in ambient humidity through contraction or expansion and said sensor may be used to detect changes in ambient humidity.

8. The method of any one of claims 1 to 7 wherein said substrate material is a non- conductive material.

9. The method of any one of claims 1 to 7 wherein said substrate material is a conductive material and a dielectric coating is applied to said surface prior to the application of said ink material.

10. A sensor for use in a resistive detection circuit including a resistance detection device and an electric circuit connecting said device to said sensor, comprising: a) a flexible substrate selected from a material that is bendable, that is compressible, or that is responsive to changes in ambient humidity; and b) a layer of a carbon-resistive ink material coating a surface of said substrate; c) whereby physical changes applied to said substrate induce detectable changes in resistance across said ink material.

11. The sensor of claim 10 wherein said ink material has a thickness of from 1 micron to 3 microns.

12. The sensor of claim 10 or claim 11 including a second layer of flexible substrate material applied to the ink material previously applied to substrate material so as to sandwich the ink material between two layers of such substrate material.

13. The sensor according to any one of claims 10 to 12 wherein the substrate material is compressible and the resistance across the carbon-resistive ink material layer is correlated with the pressure applied to the sensor.

14. The sensor according to any one of claims 10 to 12 wherein the substrate is bendable and the resistance across the carbon-resistive ink material layer is correlated with the degree of bend or flexure to which the substrate is subjected.

15. The sensor according to any one of claims 10 to 12 wherein the substrate will expand or contract in a controlled manner depending on changes in ambient humidity to which the substrate is exposed and the resistance across the carbon-resistive ink material layer is correlated with the expansion and contraction of the substrate.

16. A sensor for use in a resistive detection circuit including a resistance detection device and an electric circuit connecting said device to said sensor, comprising: a) a first flexible substrate forming a base for said sensor; b) a second flexible substrate having a degree of flexibility substantially greater than that of said first substrate; c) a layer of a carbon-resistive ink material coating an outer surface of said second flexible substrate; and d) means hermetically sealing said second flexible to said first flexible substrate with a gas trapped between said substrates so as to form a bubble; e) whereby changes in atmospheric pressure to which said bubble is subjected will cause expansion or contraction of said bubble with consequent detectable changes in the resistance of said carbon-resistive ink material layer, such changes in resistance being correlated with such changes in atmospheric pressure.

17. The sensor according to any one of claims 10 to 16 wherein the substrate material is non-conductive.

18. The sensor according to any one of claims 10 to 15 wherein the substrate material is conductive and a layer of dielectric material is interposed between the substrate and the carbon-resistive ink material layer.

19. The sensor according to claim 16 wherein at least the second substrate material is conductive and a layer of dielectric material is interposed between the carbon-resistive ink material layer and the second substrate.