Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF PRODUCING MEDIUM- TO THIN-FILM PRESSURE AND HUMIDITY
SENSORS BY FLEXOGRAPHIC PRINTING
This application is a division of Canadian Patent Application No. 2,447,740,
filed on
November 3, 2003.
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
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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
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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.
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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.
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In accordance with one aspect of the present invention, there is provided a
method of
producing a sensor for use in a resistance circuit comprising the steps of
applying a carbon-
resistive ink material to a surface of a flexible substrate by flexographic
printing methods,
wherein the flexible substrate selected from a material that is bendable, that
is
compressible, or that is responsive to changes in ambient humidity.
In accordance with another aspect of the present invention, there is provided
a sensor for
use in a resistive detection circuit including a resistance detection device
and an electric
circuit connecting the device to the 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 the substrate,
c) whereby physical changes applied to the substrate induce detectable changes
in
resistance across the ink material.
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.
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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.
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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 "I" 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 "II". 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).
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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
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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.
The scope of the claims should not be limited by the preferred embodiments set
forth in the
examples, but should be given the broadest interpretation consistent with the
description as
a whole.
