U.S. patent application number 16/925846 was filed with the patent office on 2020-10-29 for detecting gases and respiration by the conductivity of water within a porous substrate sensor.
The applicant listed for this patent is President and Fellows of Harvard College. Invention is credited to Alar Ainla, Firat Guder, Bobak Mosadegh, George M. Whitesides.
Application Number | 20200340974 16/925846 |
Document ID | / |
Family ID | 1000004946053 |
Filed Date | 2020-10-29 |
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United States Patent
Application |
20200340974 |
Kind Code |
A1 |
Guder; Firat ; et
al. |
October 29, 2020 |
DETECTING GASES AND RESPIRATION BY THE CONDUCTIVITY OF WATER WITHIN
A POROUS SUBSTRATE SENSOR
Abstract
A method of and system for detecting a gas or vapor includes
providing a sensor comprising an electrode pair in electrical
contact with a layer of porous material, the porous material layer
having water adsorbed on its surface; contacting the sensor with a
gas or vapor sample to be analysed; applying a voltage across the
electrode pair of the sensor; and measuring a response, the
response correlating to the presence of a target gas or vapor.
Inventors: |
Guder; Firat; (London,
GB) ; Mosadegh; Bobak; (New York, NY) ; Ainla;
Alar; (Braga, PT) ; Whitesides; George M.;
(Newton, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
President and Fellows of Harvard College |
Cambridge |
MA |
US |
|
|
Family ID: |
1000004946053 |
Appl. No.: |
16/925846 |
Filed: |
July 10, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15521190 |
Apr 21, 2017 |
10712337 |
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PCT/US2015/056971 |
Oct 22, 2015 |
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16925846 |
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62067291 |
Oct 22, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2033/4975 20130101;
A61B 5/082 20130101; G01N 27/12 20130101; G01N 33/497 20130101;
A61B 5/0816 20130101; A61B 5/0826 20130101; A61B 5/0022 20130101;
G01N 27/121 20130101 |
International
Class: |
G01N 33/497 20060101
G01N033/497; A61B 5/08 20060101 A61B005/08; G01N 27/12 20060101
G01N027/12; A61B 5/00 20060101 A61B005/00 |
Claims
1. A method of detecting a gas or vapor comprising: providing a
sensor comprising an electrode pair in electrical contact with a
layer of porous material, said porous material layer having water
adsorbed on its surface; contacting the sensor with a gas or vapor
sample to be analysed; applying a voltage across the electrode pair
of the sensor; and measuring a response, said response correlating
to the presence of a target gas.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/521,190 filed Apr. 21, 2017, which is a
U.S. National Stage application under 35 U.S.C. .sctn. 371 of
International Patent Application No. PCT/US2015/056971, filed Oct.
22, 2015, which claims benefit under 35 U.S.C. .sctn. 119(e) and
priority to U.S. Provisional Patent Application No. 62/067,291
filed Oct. 22, 2014. The entire content of each application is
herein incorporated by reference in its entirety.
INCORPORATION BY REFERENCE
[0002] All patents, patent applications and publications cited
herein are hereby incorporated by reference in their entirety. The
disclosures of these publications in their entireties are hereby
incorporated by reference into this application in order to more
fully describe the state of the art as known to those skilled
therein as of the date of the invention described herein.
TECHNICAL FIELD
[0003] This technology relates generally to gas detection. In
particular, this invention relates to compliant sensors fabricated
from low cost materials.
BACKGROUND
[0004] Sensors for gases are becoming an integral part of our
technology-dependent lives, such as in cars (performance and
emissions control), buildings (air quality management), factories
(process control and leak detection) and food processing and
packaging (freshness and ripeness control).
[0005] Medical diagnostics is yet another emerging area that
requires low-cost and accurate detection and quantification of
gaseous analytes. The presence of various gases (e.g., acetone,
ammonia) at certain concentrations in breath, for instance, is
linked to certain health conditions such as diabetes and failure of
the kidney and liver.
[0006] Rapid and low-cost development of sensors of gas remains a
major challenge for several reasons: i) use of expensive and
unscalable detection methods (i.e. infrared gas sensors), ii) use
of complex fabrication procedures (i.e. vapor phase grown ceramic
thin films), iii) dependence on high temperatures for detection
(e.g. metal-oxide sensors), iv) offer low level of selectivity, v)
use of rigid/non-flexible materials, and vi) require frequent
calibration for accurate readings.
[0007] Previous examples of paper-based sensors of gases involve
the integration of carbon nanotubes, quantum dots or other forms of
nanostructures on paper. These examples use the nanostructures as
the active sensing material and the paper itself is only a
scaffold.
[0008] Breathing is one of the primary vital signs used to diagnose
the health status of patients; it is related to many common
disorders and diseases, ranging from pulmonary and cardiovascular
diseases to sleep-related disorders. Current methods of monitoring
breathing require cumbersome, inconvenient and often expensive
devices; In prior methods, the breathing patterns are recorded via
insertion of a nasal cannula into the patient's nose. The cannula
is attached to a pressure sensor, which measures the breathing of
the patient. When the patient breathes through the mouth, however,
this method of detection fails to sense the changes in breathing
giving rise to false diagnosis. To mitigate this issue, other
health parameters such as chest motion and blood oxygen levels are
tracked. This requirement sets practical limitations on the
frequency and duration of measurements. To circumvent the
complications of on-site clinical testing, at-home testing of sleep
apnea is being developed. For example, a cell phone assisted
take-home sleep test included an array of sensors that correlated
the blood oxygen levels and heart rate with occurrence of reduced
or paused air flow during sleep. Although the overall system was
satisfactory, the device was bulky (required 2.times.AAA batteries)
and consisted of expensive components, increasing the cost of
ownership.
[0009] Sleep apnea is a disorder in which the flow of air into and
out of the lungs is fully or partially obstructed for at least 10
seconds continuously. This paused or shallow breathing is linked to
a variety of health problems, including but not limited to
cardiovascular diseases, stroke, and diabetes.
[0010] Take-home sleep apnea tests are available, such as AccuSom
by NovaSom and WatchPAT. Current methods of detection hinder proper
diagnosis of this condition because of two reasons: i) diagnosis is
primarily done in clinical settings which is time-consuming and
difficult for most patients to participate in, and ii) current
take-home tests are too expensive and cumbersome to operate for
patients to adopt these technologies. An effective solution to
take-home sleep apnea testing still does not exist
[0011] The most common strategy for diagnosis is to perform a sleep
study at a clinical site. These studies often require an overnight
stay and supervision of specialized personnel. From the initial
discussions with the physician to the diagnosis from a complete
sleep study can take up to six weeks. The inconvenience of these
studies is the primary reason preventing people who suspect having
sleep apnea from getting diagnosed.
SUMMARY
[0012] In one aspect, gas sensors that operate at room-temperature
are described. The sensors are built on hygroscopic porous
substrates that are able to attract and adsorb water from the
environment. These and other aspects and embodiments of the
disclosure are illustrated and described below.
[0013] In another aspect, a sensor that can accurately measure
respiration, diagnose sleep apnea or monitor other respiration
conditions by exploiting the changes in conductivity of paper when
breathed upon are described.
[0014] In one aspect, a method of detecting a gas or vapor includes
providing a sensor comprising an electrode pair in electrical
contact with a layer of porous material, the porous material having
water adsorbed on its surface; contacting the sensor with a gas or
vapor sample to be analysed; applying a voltage across the
electrode pair of the sensor; and measuring a response, in which
the response correlates to the presence of a target gas.
[0015] In one or more embodiments, the porous material layer is a
fiber-based layer, or a hydroscopic layer, or a cellulose-based
layer.
[0016] In one or more embodiments, the fiber-based layer is
selected from the group of woven textiles or fabrics, non-woven
textiles or fabrics, non-woven meshes and paper.
[0017] In one or more embodiments, the porous material is an
inorganic oxide.
[0018] In one or more embodiments, the electrode pair are
aligningly disposed on opposing surfaces of the porous layer, or
the electrode pair are spaced apart on the same side of the porous
layer.
[0019] In one or more embodiments, the electrodes are disposed on
the same side of the porous layer.
[0020] In any of the preceding embodiments, the response includes
one or more of current, electrical resistance, voltage, or
conductance.
[0021] In any of the preceding embodiments, the magnitude of the
response correlates to a concentration of the target gas or
vapor.
[0022] In any of the preceding embodiments, the porous fiber-based
layer comprises cellulose-based paper.
[0023] In any of the preceding embodiments, the target gas includes
a strong or weak acid or base.
[0024] In any of the preceding embodiments, the target gas includes
a water soluble organic compound.
[0025] In any of the preceding embodiments, the target gas includes
water, and for example, the response is an increase in current,
indicating an increase in the adsorbed water in the sensor.
[0026] In any of the preceding embodiments, the response is an
increase in current, indicating an increase in ionic conductivity
in the adsorbed water in the sensor, or the response is a decrease
in current, indicating a decrease in ionic conductivity of the
adsorbed water in the sensor.
[0027] In any of the preceding embodiments, the sensor further
includes an amplifying agent, and for example, the amplifying agent
is compound capable of being ionized by the target gas.
[0028] In any of the preceding embodiments, the target gas is
ammonia and the amplifying agent is zinc oxide.
[0029] In any of the preceding embodiments, the amplifying agent is
a salt that is soluble in the adsorbed water of the sensor.
[0030] In any of the preceding embodiments, the sensor further
includes a damping agent, the damping agent selected to reduce a
response from a non-selected gas.
[0031] In any of the preceding embodiments, the sensor further
includes an input for the relative humidity in the vicinity of the
sensor, and for example, the response is adjusted to take account
of the relative humidity input.
[0032] In any of the preceding embodiments, the sensor further
includes a sensitizing agent that increases the response
sensitivity to changes in concentration of the target gas or vapor,
and for example, the sensitizing agent is a water soluble salt.
[0033] In one aspect, a sensor for detecting a target gas includes
an electrode pair in electrical contact with a layer of porous
material, in which the layer of porous material has water adsorbed
on its surface; electrical contacts capable of connection with a
voltage source for applying a bias across the electrode pair.
[0034] In one or more embodiments, the sensor a further includes an
amplification agent, and/or a damping agent, and/or a sensitizing
agent as described above in preceding embodiments.
[0035] In one or more embodiments, the porous material a
fiber-based layer, or a hydroscopic layer, or a cellulose-based
layer, and the fiber-based layer can be selected from the group of
woven textiles or fabrics, non-woven textiles or fabrics, non-woven
meshes and paper, and the porous material can be an inorganic oxide
or a cellulose-based paper as described above in preceding
embodiments.
[0036] In one or more embodiments, the sensor a further includes a
microcontroller, the microcontroller having instructions and
hardware for executing one or more of the following tasks:
modifying applied bias, amplifying signals, digitizing signals,
transmitting and receiving data over a wireless communication
system.
[0037] In one or more embodiments, the sensor a further includes a
wireless communication system.
[0038] In one or more embodiments, the sensor a further includes a
voltage source.
[0039] In one aspect, a method of detecting respiration of a
patient includes providing a sensor comprising an electrode pair in
contact with a layer of porous material, the porous material
capable of adsorbing and desorbing water as a function of relative
humidity; positioning the sensor in the vicinity of the mouth
and/or nose of a patient; and monitoring the change in water
content of the sensor over time, wherein the increase and decrease
in water content is correlated to exhalation and inhalation,
respectively.
[0040] In one or more embodiments, the method employs multiple
sensors, located in the vicinity of one or more of the nostrils and
mouth.
[0041] In one or more embodiments, the method detects a respiratory
condition such as apnea, hypopnea and respiratory arrhythmia.
[0042] In any of the preceding embodiments, the change in water
content is determined by applying a voltage across the electrode
pair of the sensor, and measuring a response, said response
correlating to the level of adsorbed water.
[0043] In any of the preceding embodiments, the response is an
increase in current, indicating an increase in the adsorbed water
in the sensor.
[0044] In any of the preceding embodiments, the response includes
one or more of current, electrical resistance, voltage, or
conductance.
[0045] In any of the preceding embodiments, the response is
wirelessly transmitted from the sensor to a remote location.
[0046] In any of the preceding embodiments, a microcontroller is
used to process the response of the sensor to generate data
representative of said change of water content over time in an
exhaled gas stream.
[0047] In any of the preceding embodiments, the increase and
decrease in water content is used to generate data representative
of breathing rate.
[0048] In any of the preceding embodiments, the increase and
decrease in water content is used to generate data representative
of breathing depth.
[0049] In one aspect, a respiration monitor includes a support
capable of being positioned in the vicinity of the mouth and/or
nose of a patient; and a sensor comprising an electrode pair in
electrical contact with a layer of porous material capable of
adsorbing and desorbing water as a function of relative humidity,
said sensor attached to the support and located so as to be in the
pathway of patient inhalation and exhalation.
[0050] In one or more embodiments, the respiration monitor further
includes a voltage source configured to apply a voltage across the
electrode pair, and for example, the voltage source is a
battery.
[0051] In one or more embodiments, the respiration monitor further
includes a microcontroller for measuring a change in one or more of
current, electrical resistance, voltage, or conductance.
[0052] In one or more embodiments, the respiration monitor further
includes a wireless transmitter for transmission of meter data to a
remote location.
[0053] In one or more embodiments, the respiration monitor further
includes a sensitizing agent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] The invention is described with reference to the following
figures, which are presented for the purpose of illustration only
and are not intended to be limiting.
[0055] In the Drawings:
[0056] FIG. 1 provides a schematic illustration of the mechanism
sensing of the gas sensor according to one or more embodiments, in
which the porous layer contains a layer of water on its surface so
that the layer of water acts as an electrolyte for ionic transport
and the concentration of ions is determined by measuring the
conductivity between two electrodes.
[0057] FIG. 2A is a schematic illustration of fabrication procedure
of the paper-based gas sensors and FIG. 2B is a photograph of an
actual device (the light source is behind the device, showing the
aligned electrodes at both the front and back of the paper) and a
magnified photograph of the device cross-section.
[0058] FIG. 3 is an experimental setup designed to deliver sensing
gases to the test environment without a carrier gas to prevent
parasitic effects.
[0059] FIG. 4A is a plot of current vs. relative humidity (% RH)
showing the measured output of the sensor with respect to relative
humidity; and FIG. 4B is an I-V trace at 60% relative humidity.
[0060] FIG. 5 is a plot of an output of the sensor of untreated
paper when exposed to water only (FIG. 5A) and after NH.sub.4OH is
added to the paper substrate (FIG. 5B). FIG. 5C shows the gas pulse
timing (flow control, FC) used to generate the data for plots on
FIGS. 5A and 5B.
[0061] FIG. 6 is the plot of an output of the sensor at varying
concentrations of NH.sub.3 (top) along with pulse timing of the
mass flow controller (MFC) (bottom).
[0062] FIG. 7 is a plot of the measured output of the sensor when
exposed to varying concentrations of acetone vapor along with pulse
timing of the mass flow controller (MFC) (bottom).
[0063] FIG. 8 is a plot of the measured output of the sensor when
exposed to varying concentrations of ethanol vapor.
[0064] FIG. 9A is a plot showing an output of the sensor after
loading ZnO powder on the substrate as an amplification agent along
with pulse timing of the mass flow controller (MFC) (bottom); and
FIG. 9B is a plot showing sensor response for untreated and ZnO
loaded NH.sub.3 sensors, in which the sensor response was
calculated by dividing the sensor output while NH.sub.3 was
continuously supplied (I.sub.Gas-On) by the sensor output when the
NH.sub.3 supply was stopped (I.sub.Gas-Off).
[0065] FIGS. 10A-F are schematic illustrations for various
electrode configurations used in the sensor according to one or
more embodiments, in which FIG. 10A shows electrodes on opposite
sides of the porous substrate; FIG. 10B shows electrodes on the
same sides of the porous substrate; FIG. 10C is a schematic
illustration of a four point measurement used to measure current in
the sensor according to one or more embodiments; FIG. 10D is a
schematic of an equivalent circuit of a sensor, where EC denotes
electrochemical process, according to one or more embodiments; FIG.
10E shows a LC resonant circuit containing a sensor (S) affecting
the Q factor (damping) or resonance frequency of the resonant
circuit according to one or more embodiments; and FIG. 10F shows
plots of current vs. frequency to show a change in resonance
frequency or Q factor according to one or more embodiments.
[0066] FIG. 11A is a plot of current (A) vs. relative humidity (%
RH) showing the response of the sensor made with Whatmasl Chr,
Whatman 3MM Chr, and copy paper between 0-90% relative humidity
with a 25V bias voltage across the two electrodes of the sensor;
and FIG. 11B is a plot of current (A) vs. relative humidity (% RH)
showing the response of the sensor made with the same paper types
when loaded with NaCl.
[0067] FIG. 12 is a schematic view of the constructed respiratory
simulator for sensor evaluation, where two mass flow controllers
(MFC) are configured to apply either humidified air (through water
tank) or dry/ambient air and sensor electrical characteristics are
measured with computerized source meter.
[0068] FIG. 13 is a plot of simulated breathing patterns and the
response from a paper-based humidity sensor (loaded with NaCl) when
exposed to a simulated breathing pattern, in which breathing was
controlled by two mass flow controllers (MFC) of air with 100%
humidity and air with 70% humidity (shown in lower section of
figure) and the sensor had a 25V bias voltage across its two
electrodes.
[0069] FIG. 14A is a schematic illustration of a facemask for
respiration monitoring with the embedded paper-based sensor and
electronics, including a wireless connection and phone with
software app; and FIG. 14B is a schematic illustration of the
mechanism of operation of a porous electrical respiration sensor
according to one or more embodiments.
[0070] FIG. 15A is a schematic illustration of an exemplary
fabrication of digitally printed paper sensors with graphite ink;
according to one or more embodiments; FIG. 15B is an image of an
array of digitally printed sensors according to one or more
embodiments; and FIG. 15C is an enlarged view of a single
interdigitated electrode pair according to one or more
embodiments.
[0071] FIG. 16A-P are schematic illustrations of respiration
sensors according to one or more embodiments.
[0072] FIG. 17 is a plot of current (A) vs. relative humidity (%
RH) showing the response of a paper sensor when loaded with NaCl,
KNO.sub.3, HPO.sub.4.
[0073] FIG. 18A is a photograph of a facemask with the embedded
paper-based sensor, according to one or more embodiments; FIG. 18B
is a photograph of the data acquisition electronics with Li-ion
batteries, Arduino microcontroller board, a Bluetooth module, a
voltage source (battery), custom designed amplifier board, and 3D
printed casing; FIG. 18C is photograph of tablet computer running
the Android app, which can display and analyse the incoming data
stream from the data acquisition electronics; and FIG. 18D is a
simplified circuit diagram of the electronics.
[0074] FIG. 19A is a plot of conductivity (a.u.) vs. time (s)
showing the resting respiratory activity of a subject recorded
using the functional facemask, in which the subject was asked to
breathe normally, take deep breaths, pause and breathe randomly
during the experiment; FIG. 19B is a plot of conductivity (a.u.)
vs. time (s) showing breathing patterns recorded while: 1.
breathing normally, 2. taking a deep breath, 3. paused and, 4.
random breathing; and FIG. 19C is a plot showing the seamless
transition between fast and shallow breathing (1) and normal
breathing (2).
[0075] FIG. 20A is a plot of conductivity (a.u.) vs. time (s)
showing recorded respiratory signal during light (FIG. 20B) and
vigorous (FIG. 20C) exercise, in which the subject had an increase
in the rate of respiration from 12 breaths/min during light
exercise to 22 breaths/min during vigorous exercise.
[0076] FIG. 21A is a bar graph of breaths/min for volunteers during
light exercises (left) and vigorous exercises (right); and FIG. 21B
is a bar graph of VFR (a.u.) for volunteers during light exercises
(left) and vigorous exercises (right).
DETAILED DESCRIPTION
Gas or Vapor Sensor
[0077] In one aspect, a sensor using a porous substrate having an
adsorbed layer of water for detecting gases is provided. For ease
of reference, the gas sensor is described with reference to paper
as the substrate. However the principles described herein are
readily applied to any porous hygroscopic substrate.
[0078] FIG. 1 is a schematic illustration of a cellulosic gas
detector according to one or more embodiments. The device includes
a pair of electrodes 100, 100' with a porous layer 110 disposed in
between. Electrodes are shown on opposing sides of the porous
layer, however, it is contemplated that the electrodes can be
disposed on the same side of the paper layer. See, e.g., FIG. 15.
When exposed to ambient conditions, the device is exposed to
atmospheric water, as well as any gas to be detected (exemplified
here with ammonia (NH.sub.3)). A thin layer of adsorbed water 120
forms in and/or on the porous layer. Water is in equilibrium
conditions between the free water vapor and adsorbed water, as
shown in the exploded view in FIG. 1. NH.sub.3 is a polar gas that
dissolves in water and forms NH.sup.4+ and OH.sup.- ions. These
ions contribute to the overall ionic conduction of the adsorbed
water on the cellulose fibers of the paper. The exploded view in
FIG. 1 also shows how NH.sub.3 interacts with the moist fibers of
the porous layer. On application of a voltage, current is induced
in the layer due to the ionic conductivity of the water layer. The
current is proportional to the ionic content of the water layer,
which is perturbed when exposed to ambient gases. Thus changes in
the current are indications of changes in the ambient gas
composition.
[0079] In one or more embodiments, the electrodes are disposed in
close proximity on one side of the substrate. In some embodiments,
the electrodes are interdigitated to increase the area of the area
of the electrode and the signal to noise ratio. See, FIG. 15. This
design also allows rapid access of humidity and target gases and
vapors to the paper. In other embodiments, electrodes are disposed
on opposite sides of the paper detector, for example, by printing
electrodes on both sides of a single sheet of paper. Contacts can
be provided for connection to an external voltage source.
[0080] The moisture within a sheet of paper offers a unique method
for sensing concentrations of gas since the ionic conductance of
the paper is proportional to the concentration of ionic species
dissolved within the adsorbed water. Gases capable of detection
include those that are water soluble. As the gases interact with
the adsorbed water, the ion concentration of the adsorbed water
changes, which can be recorded as current. In some embodiments, the
dissolved gases are ionized in the water layer adsorbed on the
porous layer and increase ionic conductivity. In other embodiments,
the gases are solvated in the adsorbed water layer without
ionization and decrease ionic conductivity. Exemplary gases that
can be detected include gases having strong to mild acidic or basic
properties (e.g., NH.sub.3, HCl, SO.sub.2, nitrogen oxide
(NO.sub.x), halides) and volatile organic compounds having
solubility in water (e.g., acetone and ethanol). In other
embodiments, gases or vapors that have poor solubility in water, or
that are not sufficiently acidic or basis, such as CO.sub.2 by way
of example, can still detected by including reagents in the porous
layer that interact with the target gas and convert it into an
ionized species. For example, the enzyme carbonicanhydrase converts
CO.sub.2 into HCO.sup.3-.
[0081] By applying a bias between the two electrodes, the ionic
conductance of the paper can be quantified based on the measured
current. The bias can be direct current (DC), alternating current
(AC) or pulsed signal. A change in the measured current provides an
indication of gas presence and the magnitude is a measure of the
concentration of a gas.
[0082] The substrate can be any porous material that is
hygroscopic. Exemplary materials include fiber-based materials,
such as woven textiles, non-woven meshes, and woven and non-woven
fabrics and textiles. Cellulose-based materials are particularly
useful. Cellulose-based substrates such as certain textiles and
fabrics, e.g., cotton fabrics, and paper are exemplary
cellulose-based substrate. Other exemplary substrates include
porous inorganic materials, such as inorganic oxides, such as
alumina and silica In other embodiments, a porous substrate can be
coated with an oxide layer to create a hygroscopic substrate.
[0083] In one or more embodiments, the substrate is paper, and in
particular, cellulose-based paper. Cellulose paper has a very high
affinity to ambient water. At a relative humidity of 70%, the
moisture content in cellulose paper can be as high as 10% by
weight. In addition to its ability to absorb high amounts of water
from air, cellulose paper has several other advantages as a sensor
for gases: i) cellulose paper is inherently porous, a feature that
allows gases to penetrate and interact easily with the bulk of the
material in the sensor, ii) cellulose paper has a high surface area
(over 100 m.sup.2/g) so the sensitivity of the sensor is greater
than thin films that operate using the same mechanism, iii)
cellulose paper is chemically inert to most gases including highly
corrosive species; and "iv) cellulose fibers themselves are not
electronically conductive (electronic insulator), therefore the
conductivity of the sensor depends solely on the ionic conductivity
in the absorbed water layer. In one or more embodiments, the paper
is thick enough to prevent printed contacts from short
circuiting.
[0084] Electrodes can be deposited on the substrate using a variety
of methods. In one embodiment, electrodes can be prepared
separately and pressed conformally against the porous substrate.
Aluminum film and copper sheers are examples of materials available
in free standing thin films that can be used as electrodes. In
other embodiments, an electrode pair can be laminated on the porous
substrate. The laminated material can be a thin metal sheet or foil
such as aluminium or gold. In other embodiments, the electrode pair
can be printed on the porous material. The printing material can be
any of the following non-limiting examples: carbon inks, nanofiber
inks, suspended metal particle inks, reactive inks (e.g., inks
which are not conductive themselves (e.g., dissolved metal
complex), but which upon printing through the process of reacting
with substrate, decomposing thermally or reacting with some other
substance applied subsequently are turned into a conductive
materials (through chemical reaction) and conductive polymer inks.
The inks can be printed using conventional and low cost methods
such as screen-printing, stencil printing, plotting, and
roll-to-roll printing. In other embodiments, the electrodes can be
made using gas phase deposition techniques such as physical or
chemical vapor deposition, or sputtering. Deposition methods can
apply the electrodes to the porous substrate surface, or the
electrodes can penetrate to the interior of the porous
substrate.
[0085] In one or more embodiments, the electrodes are located on
the same side of the substrate. In other embodiments, the
electrodes are located on opposite sides of the substrate. The
electrodes can be designed to have large surface area and to permit
environmental vapor access to the underlying substrate.
[0086] The devices can be prepared using any available method. In
one or more embodiments, electrodes are printed onto paper with
current printing technologies (e.g., screen printing). Printing
offers a unique possibility for the rapid fabrication of low-cost,
low-power, and flexible sensors with tailor-made properties. FIG.
2A is a schematic illustration of the fabrication of a paper-based
gas sensor according to one or more embodiments, in which a
cellulosic substrate is sandwiched between two carbon electrodes.
Low-cost and easy-to-apply graphite ink is silk-screened onto a
first side of the cellulose paper to form chemically inert carbon
electrodes. Once dried and optionally heated, the paper is flipped
and the counter electrode is printed on the opposite side. Printing
allows fabrication of multiple devices on the same substrate; the
electrodes can be designed to span a large surface area of the
paper, to increase sensitivity. Contacts for connection to a
voltage source can also be directly printed onto the substrate. The
method produces well-aligned electrodes on the front and back of
the paper, as shown in the photograph in FIG. 2B. Both the top
electrode and the bottom electrode are visible and the
overlap/alignment of the two electrodes is high, as shown in the
cross-section of the sensor.
[0087] In other embodiments, the electrodes can be on the same side
of the porous substrate, as is shown in FIGS. 15A-15C. As shown in
FIG. 15A, an electrode is fabricated on a cellulose substrate by
digitally printing graphite ink. The ink can be applied using
automatic controls with high precision and accuracy. The graphite
ink was diluted with a proprietary solvent (Ercon ET160) 55:45 w/w
to obtain desired consistency for printing, and the mixture was
homogenized using a tip sonicator to create a uniform dispersion.
Using this printing technique, large numbers of sensors can be
printed with high accuracy (FIG. 15B). (Other printing techniques
such as screen printing or reel-to-reel printing can, of course,
also be used for increased throughput). The individual electrode
pairs can be cut out from the sheet and electrical contacts can be
applied. An enlarged view of a single interdigitated electrode
design is shown in FIG. 15C; the interdigitated design can provide
increased area of the electrodes and greater signal-to-noise ratio.
This design also allows rapid access of humidity to the paper. The
sensor can be integrated into a variety of surfaces, such as a mask
(shown in FIG. 14A) or into packaging where they can be used to
detect or monitor packaging contents (see FIG. 160).
[0088] As discussed in more detail below, changes in the ionic
conductivity of the sensor can be detected by measuring the output
current of the sensor when a bias, e.g., 25 V DC voltage, is
applied between the electrodes of the sensor. However, other
detections means are also contemplated, such as electrical
resistance, or conductance, as these parameters are all linearly
related by Ohms Law.
[0089] The sensor can be measured using any method in which
currents are measured. For example, the sensors can include two
electrodes (two point measurement). See, FIG. 10A. In a two point
system, voltages is applied between two electrodes and current is
measured through the electrodes. In other embodiments, the sensors
can include four electrodes (four point measurements). See, FIG.
10B. In a four point system, voltage is applied between two
electrodes and current is measured through these two electrodes and
further voltage is measured on other two electrodes (four point
measurement), as shown in FIG. 10C. FIG. 10D is an equivalent
circuit for the sensor, in which EC stands for the electrochemical
reaction with the gas to be sensed. In the case where the sensor is
used to measure humidity, the electrochemical reaction can be the
electrolysis of water. The applied voltage can be AC or DC, and the
AC voltage can be applied over a range of frequencies. Exemplary
voltages can be in the range of 5-50V. When current corresponding
to multiple voltages is analysed, the analysis can include
amplitude analysis and/or phase analysis depending on input signal
frequency. In other embodiments, capacitance and resistance of the
sensor can be calculated from the spectra or transient signal
analysis. In one or more embodiments, the sensor is coupled with an
LC resonator (an electric circuit consisting of an inductor,
represented by the letter L, and a capacitor, represented by the
letter C, connected together). FIG. 10E shows the resonance
circuits and sensors used in the circuits. The sensor state changes
resonance frequency or Q-factor of the LC resonator, as is shown in
FIG. 10F. In one or more embodiments, the sensor is read
wirelessly. The sensor can include an radio frequency ID (RFID) for
this purpose.
[0090] In one or more embodiments, the response sensitivity can be
increased by including additives in the paper device. In one or
more embodiments, the ionic strength of the adsorbed water layer
(and the corresponding ionic conductivity) can be increased by
addition of salts to the layer. The salt can be soluble (highly
soluble or moderately soluble) under the normal operating
conditions of the sensor (humidity and temperature). Solubility of
the salt can be modulated by the gas or vapor to be sensed. In one
embodiment, the solubility of the salt is increased by the gas or
vapor (salt dissolves). In one embodiment, the solubility of the
salt is decreased by the gas or vapor (salt precipitates). As the
salt dissolves (or precipitates), the ion concentration changes
and, with it, the ionic conductivity of the porous material.
[0091] By way of example, water soluble salts such as sodium and
potassium chloride, potassium nitride and potassium phosphate can
be used. The salts can be applied by wetting or dipping the paper
with a selected amount of salt solution. Exemplary loads for the
device include dipping the substrate in a 10 mM aqueous NaCl
solution and subsequently drying it at room temperature. The
increased ion concentration improves the current output of the
device which in turn increases the dynamic range of operation at
lower relative humidity levels.
[0092] In other embodiments, the amplifier or amplification agent
can be an organic molecule and the ionic conductivity of the
organic molecule can be modified by the vapor or gas oxidizing,
reducing or cleaving the organic molecule. In other embodiments the
ionic conductivity changes by binding of the target molecule to the
organic molecule or by polymerization of the organic molecule. In
one or more embodiments, the gas or vapor are more charged or less
charged than the original molecule, resulting in an increase or
decrease in the measured conductivity, respectively. In other
embodiments, the oxidized ore reduced form of the organic molecule
changes the pH of the water absorbed on the surface of the porous
substrate. In one or more embodiments, the organic molecule is
bound to the target vapor gas and is less mobile (with a
corresponding change in ionic conductivity). In one or more
embodiments, the organic molecules binds to the surface of the
porous substrate, thereby reducing its mobility. In one or more
embodiments, the polymerized organic molecule turns into a gel and
the gel layer changes the mobility of ions in the absorbed water.
The additive can react with the gas or vapor reversibly, forming a
real time sensor, or irreversibly, forming a memory sensor. The
memory sensor does not require constant readout, but records, if it
has been ever exposed to the gas or vapor. Non-limiting examples of
amplification agents include SO.sub.2 and NO.sub.2, which can act
as catalysts to polymerize vinyl compounds such as methacrylates.
in aqueous environment NOx can be also be used as oxidizing agents.
Suitable organic molecules have low volatility, and would not
evaporate from the sensor over its lifetime. Ionic compounds,
salts, large molecules such as proteins and large polar or charged
molecules do not evaporate readily and are suitable organic
molecules.
[0093] In one or more embodiments, an amplification agent can be
included in the paper to amplify the signal generated by a target
gas. Weak acidic or basic gases, for example, do not fully ionize
in water and the device response can be amplified by increasing
their dissociation into ions. In one or more embodiments, the
signal of the sensor is amplified by adding an amplifying agent
such as inorganic powders into the paper. The loaded powder reacts
with the weak base (or acid) itself and dissociates in the aqueous
electrolyte as ions and further increases the ionic conductance of
the sensor. By selecting powders with different properties, the
selectivity of the sensor can be tuned.
[0094] In one or more embodiments, where the sensed gas is ammonia,
ZnO can be used as an agent for amplification. NH.sub.4OH (the
hydrated form of ammonia gas), which does not dissociate completely
in water, can dissolve ZnO and form ions such as
[Zn(NH.sub.3).sub.4].sup.2+ and OH--, shifting the equilibrium ion
concentration and increasing the total number of charged species
and the overall ionic conduction of the sensor. The ions revert
back to ZnO in the absence of NH.sub.4OH, thus ZnO acts as a
reversible amplification agent.
[0095] In one or more embodiments, the additive can be a catalyst.
The catalyst can be selected to facilitate a reaction that provides
a stronger signal. For example, the catalyst can change the gas or
vapor molecule into a charged molecule, change the acidity of the
dissolved gas or molecule, or polymerize the gas or vapor molecule.
In one or more embodiments, the catalyst can be a metal particle,
alloy particle, organometallic complex, or biological enzyme. In
one or more embodiment, the catalyst is an enzyme, such as carbonic
anhydrase (that can be used to detect carbon dioxide). In one or
more embodiments, the catalyst can be inorganic oxides, nitrides or
sulfides.
[0096] In one or more embodiments, a suppression agent can be added
to the paper to suppress signals from background gases that may
interfere with the gas detection of interest. In one or more
embodiments, a suppression agent can interact with interfering
gases and reduce their ionization so that the presence of such
gases does not interfere with target gas detection. By way of
example, interference of other gasses can be prevented by addition
of gas selective membranes, which would selectively allow diffusion
of gas of interest into the paper substrate. With this method, the
sensor can be engineered for the specific application.
[0097] As discussed in greater detail herein below, it may be
desirable to monitor humidity (or water vapor) as the gas of
interest. A humidity sensor can be useful, for example, as a
respiration monitor. However, in certain embodiments, the ambient
water vapor may interfere with monitoring and detection of other
gases. In one or more embodiments, the contribution to the current
signal derived from background water content can be accounted for.
The amount of water adsorbed onto the cellulose fibers is affected
by the relative humidity. See, e.g., FIG. 4. The greater the
relative humidity, the greater the adsorbed moisture content of the
paper. Higher moisture content generally corresponds to higher
background conductivities, which can interfere with the measurement
of target gas presence and concentrations. In one more embodiments,
the sensor device adjusts for the relative humidity and accounts
for the contribution to the measured conductivity due to it. In one
or more embodiments, the relative humidity is determined
independently and a correlation curve of conductivity to relative
humidity is used to calibrate the device before, during or after
operation and read out. In other embodiments, the device includes a
reference detector that measures current for ambient in the absence
of the target gas and adjusts the target gas reading
accordingly.
[0098] The sensors can be used in various arrangements and
combinations. In one or more embodiments, multiple sensors are
incorporated into the device. The sensors can be the same or
different. In one or more embodiments, multiple sensors capable of
detecting the same gas or vapor are incorporated into the detection
system. The multiple sensors can be used for spatial resolution of
gas composition, e.g., humidity, differences. In other embodiments,
the multiple sensors can be used for spatial resolution of
differences in gas flow. In other embodiments, the sensors can have
different additives. As a result, the sensors can have different
sensitivity to different gases and vapors. This may be helpful to
detect gas concentrations in situations where cross-sensitivity of
gases occur. In other embodiments, one of the sensors can serve as
a reference sensor. For example, one sensor can be a humidity
sensor to detect background humidity and the second sensor can
detect the gas of interest. In one or more embodiments, the output
signal or signals of the sensor system are calculated from the
signals of two or more individual sensors. In one or more
embodiments, the two or more sensors are connected to trainable or
self-training machine learning systems. In one or more embodiments,
the output signal of the sensor system correlates with the presence
of a gas or vapor.
[0099] In one or more embodiments, the one or more sensors are
mounted on a carrier substrate. The carrier substrate can be water
impermeable, or elastic, or equipped with adhesive layer for
mounting. The carrier substrate can contain holes for the flow of
gas or vapor, or contain electronic circuitry for signal
processing. The electronic circuitry can be flexible.
[0100] In one or more embodiments, the sensors are encapsulated in
another material or in between layers of another material. The
other material can be semipermeable to gas or vapor. In one or more
embodiments, the other material can be a hydrophobic material and
the material can be hydrophobic paper or porous fluoropolymer. The
layer can be impermeable for water, but permeable for water
vapor.
[0101] In one or more embodiments, the sensor system contains a
structure to direct flow of gas or vapor. The structure can direct
gas through the porous substrate or along the surface of the porous
substrate.
[0102] In one or more embodiments, calibration information can be
stored on the sensor. The information can be stored using a QR
code, bar code, RFID (radio frequency ID), or electronic chip. In
one or more embodiments, the calibration information is stored in a
database and the sensor contains the ID to address the calibration
information in the database. The ID can be stored using a QR code,
bar code, RFID (radio frequency ID), or electronic chip.
[0103] In one or more embodiments, the sensor system includes an
electronic circuit. The electronic circuit can be directly
manufactured or mounted on top of the porous material of the
sensor. In one embodiment, the electronic circuit is mounted on a
separate substrate and interfaced with the porous material of the
sensor. In one embodiment, the electronic circuit contains passive
components (e.g., capacitance, resistance, or inductance). In one
or more embodiments, the electronic circuit is used to read the
sensor value. In one or more embodiments, the electronic circuit is
an analog or digital circuit. In one or more embodiments, an
external radio frequency is used to power the circuit. In one
embodiment, the circuit contains a resonator/oscillator. In one
embodiment, the sensor signal changes the resonance frequency of
the oscillator or the Q factor (for damping) of the oscillator.
[0104] In one or more embodiments, the sensor includes active
components (e.g., transistors or integrated circuits). In one or
more embodiments, the circuit contains one or more energy storage
or harvesting elements, such as a battery, a thermocouple, a solar
cell or an inductive coil. In one or more embodiment, the sensor is
used to collect and store the signal from the sensor, transmit the
signal using radio frequency or transmit the signal using light or
IR radiation.
[0105] In one or more embodiments, the sensor includes an external
electronic circuit. And the electronic circuit can read the sensor
and transmit the read data wirelessly. In one or more embodiments,
the sensor is Internet enabled. In one or more embodiments, the
receiving device further sends the data to a database, e.g., the
cloud.
[0106] The following calculation provides a simple theoretical
estimate for the change in current caused by exposure of the sensor
to ammonia. The change in current is considered to correlate to the
change in ionic conductivity of water absorbed on the surface of
the paper. The estimated area of the electrodes, is calculated to
be A=17.5 mm.sup.2. The density of paper is measured experimentally
to be .rho.=18.2 mg/cm.sup.2. The thickness of the paper is h=350
.mu.m. The water content of the paper is .eta.=10%, at a relative
humidity of 70%. The density of water is .rho..sub.w=1 g/cm.sup.3.
The amount of water between the two electrodes is calculated by the
following equation 1:
m.sub.water=A.rho..eta..apprxeq.3.2 mg (1)
In one model, paper is viewed as a 3D cubic network of identical
fibers. In this case, only vertical fibers would contribute to the
conductivity, as horizontal ones lie in planes that are of
equipotential. Only 1/3 of the water, therefore, contributes to the
conductivity of the sensor. If we would condense all this water to
single bulk volume, the effective surface area of water through the
thickness of a single sheet of paper is given by equation 2:
A water = A .rho. .eta. 3 h .rho. w .apprxeq. 0.3 mm 2 ( 2 )
##EQU00001##
3600 ppm of ammonia yields a partial pressure of 360 Pa. According
to dissolution equilibrium of ammonia, the concentration of ammonia
in water is .alpha.=0.4 wt %. NH.sub.4OH has a pKa of 9.245,
therefore, .beta.=2.510.sup.-5, which is the ratio of the
concentration of dissolved ammonia to the concentration of ammonium
hydroxide (NH.sub.4OH). The molar mass of ammonia is M=17 g/mol.
The ionic concentration in water is therefore described by equation
3.
c ions = .rho. w .alpha. .beta. M .apprxeq. 5.8 .mu.M ( 3 )
##EQU00002##
The specific conductivities of NH.sub.4.sup.+ and OH ions are
.lamda..sub.NH4=7.34 mS m.sup.2/mol and .lamda..sub.0H=19.9 mS
m.sup.2/mol, given a total value
.lamda.=.lamda..sub.NH4+.lamda..sub.OH=27 mS m.sup.2/mol. All
together we estimate the conductivity of the sensor using equation
4.
.DELTA. .sigma. = c i o n s .lamda. A w a t e r h = .rho. .alpha.
.beta. .lamda. A .eta. 3 h 2 M .apprxeq. 13 .mu.S ( 4 )
##EQU00003##
In the case of an applied voltage of U=25V, we have
.DELTA.I=.DELTA..sigma.U.apprxeq.3.3 .mu.A. This predicted change
in current is .about.10.times. higher than the measured change in
current of .about.300 nA. These two values are in good agreement
considering the basic assumptions made above for the calculation of
the predicted change in current.
[0107] The sensors can be used in a variety of applications. In one
or more embodiments, the sensor can be used to monitor humidity in
packaging.
[0108] In one or more embodiments, the sensor is sensitive to
organic decay (amines are a common degradation product in meats and
other protein containing products). The sensor can be included
inside a food package, that can contain meats. The sensor can be
sensitive to amines or ammonia and provide an indication of the
freshness of the food product. The sensor can include an
enzyme.
[0109] In one or more embodiments, the sensor is used to sense
indoor living environment. The sensor can be integrated into a wall
of a building.
[0110] In one or more embodiments, the sensor can be used to
monitor a drying process, such as the drying of concrete, clay or
wood.
[0111] In one or more embodiments, the sensor is used in
agriculture, and the sensor is used to monitor moisture content in
soil, in live or harvested crops or in hay.
[0112] The invention is exemplified in the following examples,
which are presented for the purpose of illustration only and are
not intended to be limiting of the invention.
Example
[0113] Three gases, ammonia (NH.sub.3), acetone and ethanol, were
evaluated. NH.sub.3 was obtained simply by collecting the vapor of
a solution of aqueous ammonium hydroxide (NH.sub.4OH). The partial
pressure of NH.sub.3 released from the solution of NH.sub.4OH
correlates directly with the concentration. For a 4% solution of
NH.sub.4OH in water, at room temperature, the partial pressure of
NH.sub.3 is approximately twice that of H.sub.2O. We assumed equal
partial pressures for H.sub.2O and NH.sub.3 for solutions of
NH.sub.4OH that were below 2%. Acetone and ethanol are water
miscible organic solvents, which do not dissociate in water.
Organic gases that were miscible in water reduced the ionic
conductivity of the sensor in a concentration-dependant manner.
[0114] To prevent the secondary drying effects of the carrier gases
on the ionic conductivity of the sensor, a low pressure flow was
used to pull the sensing gas from its source location. A schematic
illustration of the test apparatus is shown in FIG. 3. Since N2 is
bubbled through water to bring moisture into test environment more
effectively, the carrier gas was eliminated only from the sensing
gas delivery line.
[0115] The following tests for sensing NH.sub.3 were conducted i)
compared the response of the sensor at the same MFC flow rate
before and after addition of NH.sub.4OH to the water in the sensing
gas source (SGS) bottle. Since water hydrates paper, it will
naturally increase the ionic conductivity of the device and this
experiment was designed to decouple the contributions NH.sub.3 from
only water, ii) compared the response of the sensor at different
concentrations of NH.sub.3, and iii) compared the response of the
sensor with and without ZnO loaded in the paper.
[0116] The same experiments were conducted for both acetone and
ethanol, with the exception of the effect of ZnO on gas detection,
since ZnO does not dissolve in either of these solvents.
[0117] Paper sensors were prepared using a stencil, which was
fabricated by removing features from a transparency (3M) using a
laser cutter. Whatman chromatography paper (350 .mu.m thick) was
used for the substrate since its thickness prevents unintentional
shorts between the printed electrodes on the front and the
back-side of the sensor. Using the stencil, graphite ink (produced
by Ercon) was brushed on the front and back of the paper to serve
as the electrodes. The paper was cut into individual sensors and
tested for electrical short-circuits using a multimeter (a
resistance value of <20 k.OMEGA. would indicate a short circuit)
before use in any experiment.
Humidity Detection
[0118] Since the sensor requires moisture to operate, its response
to humidity was first determined. FIG. 4A is a plot of current vs.
relative humidity and indicates the sensitivity of the device to
changes in humidity. The sandwiched paper device, however, is able
to detect changes in humidity from 40% up to over 85%, almost
doubling the dynamic range of the sensor. The sensors, therefore,
are intended to work in environments with humidity levels greater
than 40% relative humidity.
[0119] The I-V characteristics of the device at 60% RH is shown in
FIG. 4B. The plot was obtained by varying the source voltage
between -50 V and 50 V and measuring the current passing through
the sensor. The data indicate that the relationship is linear
within the measured interval and the main limiting factor of the
current in the sensor is the electrophoretic mobility of ions in
the thin layer of adsorbed water on cellulose. For a typical
electrochemical cell, where the current is limited by the Faradaic
process, the current-voltage relationship would be nonlinear. In
the current sensor device, however, the Faradaic process is not the
dominant process and the output of the sensor below the potential
for hydrolysis is beyond the accuracy of the instrumentation.
Ammonia Detection
[0120] For target gas detection, the device was operated at 25 V in
all experiments. At a RH of 85%, the device requires only 75
.mu.W.
[0121] The response of the sensor at different concentrations of
NH.sub.3 was measured by keeping the flow rate of water at a
constant rate of 4000 sccm and varying the flow rate of NH.sub.3 to
the desired concentration. The percent concentration inside the
measurement environment was calculated simply by dividing the flow
rates of the gas and multiplying by 100%. A 1% mixture, therefore,
would yield 10000 ppm. To test the response of the sensor in the
presence of water only, a bottle containing 98 mL of distilled
H.sub.2O was mixed with a hydrated stream of N.sub.2 at a ratio of
80:4000 sccm. The response of the sensor is shown in FIG. 5A (top).
Next 2 mL of 30% NH.sub.4OH was injected in the SGS through a
septum which resulted in a 0.6% solution of NH.sub.4OH and 1200
sccm of NH.sub.3 in the testing environment shown in FIG. 5B
(middle). With the addition of NH.sub.4OH, the output of the sensor
increased from 440 nA to 570 nA, indicating that the sensor is
sensitive to NH.sub.3. The graph in FIG. 5C (bottom) shows that
timing of the pulses of gas (i.e., MFC State) with "on" indicating
that the gas is flowing.
[0122] FIG. 6 shows the response of the sensor at different
concentrations of NH.sub.3. These data show that the output of the
sensor is dependent on concentration since the ionic conductivity
of the moisture at the surface of the cellulose fibers increases.
The higher concentrations of NH.sub.3 produce more NH.sup.4+ and
OH-- ions in the surface-bound water layer.
[0123] The changes in the ionic conductivity of the sensor when
exposed to 3600 ppm of NH.sub.3 at a relative humidity of 70% was
approximated and the measured and calculated values of the ionic
current were compared, assuming the adsorbed moisture in the paper
substrate is bulk water. For an estimated sensing area of 17.5
mm.sup.2 and a water content of 10%, the calculated change in
current (.DELTA.I), resulting from the ions generated by the
dissociation of NH.sub.3 in water, was 3.3 .mu.A. Under the same
conditions, the measured AI was 0.3 .mu.A. Since water on the
surface of the paper is not entirely acting like bulk water, and
not all fibers are continuously and uniformly connected to each
other, the calculated and measured values are in good agreement
with each other. This result supports the model for the mechanism
of how the sensor functions.
Acetone and Ethanol Detection
[0124] The response of untreated paper for gases of acetone (FIG.
7) and ethanol (FIG. 8) were also investigated. Both of these
solvents are miscible in water, reduce the overall ionic
conductivity of the sensor, and do not introduce new ions to the
system. The sensor could measure both acetone and ethanol down to
0.5% (5000 ppm). The drift in the signal of the sensor is due to
the slight change in relative humidity during the measurement. As
discussed above, the effect of relative humidity can be accounted
for, for example, by tracking the relative humidity using a
commercially available detector and adjusting the readout
accordingly to account for the humidity contribution to current
readout.
Characterization of Paper Sensor Loaded with an Amplification
Agent
[0125] The sensor was loaded with ZnO particles by dipping the
sensor into a water based suspension of ZnO (25 mg ZnO powder in 50
ml of water) and letting it dry in an oven at 60.degree. C. ZnO is
known to be dissolved by NH.sub.4OH, forming water soluble
[Zn(NH.sub.3).sub.4].sup.2+ ions.
[0126] In the presence of water, these ions dissociate to form ZnO
when the supply of NH.sub.3 is stopped. By this mechanism, all of
the [Zn(NH.sub.3).sub.4].sup.2+ eventually returns to ZnO. FIG. 9
shows the response of the sensor at different concentrations of
NH.sub.3. The response of the sensor is reversible, repeatable, and
dependent on concentration.
[0127] The response of the sensor was evaluated with and without
the amplification agent and the results are shown in FIG. 9B. The
signal from the sensor loaded with ZnO was more than double than
the signal acquired from untreated paper, confirming that an
amplification agent could improve the response and possibly
sensitivity of the sensor. Other amplification agents can
selectively measure other gases as well.
Porous Substrate-Based Respiration Monitor
[0128] A particular application of the sensors described herein is
for the detection of humidity. This can be used to measure
respiration in humans or animals. Systems and methods for measuring
respiration are described with specificity in the description that
follows; however, the various embodiments, system features and
arrangement described above for the detection of vapors and gases
apply equally here.
[0129] In one aspect of the invention, a porous substrate-based,
e.g., a paper-based, moisture sensor is provided that uses the
hygroscopic character of paper (i.e. the ability of paper to
attract and hold water from the surrounding environment) to measure
patterns and rate of respiration by converting the changes in
humidity caused by inhalation and exhalation to electrical signals.
The changing levels of humidity that occur in a cycle of inhalation
and exhalation cause a corresponding change in the ionic
conductivity of the sensor, which can be measured electrically. In
one or more embodiments, the device can be equipped with
electronics for wireless communications. By combining the porous
substrate-based sensor with conventional electronics, data
concerning respiration can be transmitted to a nearby smartphone or
tablet computer for post-processing, and subsequently stored on a
cloud server, or can be further analysed by a healthcare
professional remotely. This means of sensing provides a practical
solution to the problem of recording and analysing patterns of
breathing. The sensors demonstrated a range of humidity
sensitivity; however all show sensitivity at 65% RH and
responsiveness is observed at less than 50% RH, less than 35% RH
and as low as 20% RH.
[0130] In one or more embodiments, a respiration monitor is
provided that monitors and provides information about the
respiration rate of a user. Such information can be useful in
monitoring breathing conditions such as involving apnea, hypopnea
and respiratory arrhythmias, and other medical conditions.
[0131] In one embodiment respiration monitor is in communication
with an actuator or feedback system, which operation is based on
the input from the respiration monitor. Examples of such actuator
or feedback could be breath stimulator or assistance, activation of
inhalator to dispense a drug, alarm sound etc.
[0132] The sensor is capable of measuring the rate of respiration
of a person by detecting the difference in moisture content of
inhaled and exhaled air. The sensor can include a piece of paper
(or other porous cellulose based substrate) with digitally printed
graphite electrodes. In certain embodiments, the sensor can be
embedded inside a flexible textile procedure mask (commonly used in
hospitals). The sensor can be coupled to a low-cost,
battery-powered standalone unit that can interface with an
internet-enabled tablet computer/smartphone. The system can display
and upload the collected data to the cloud, and thus enable
healthcare professionals to remotely access the results. The paper
sensor, electronics, and software transform a simple textile mask
into a functional mask (with internet connectivity), which can
measure, analyse, store, and share information concerning the rate
and pattern of respiration of individual patients.
Device Design
[0133] For ease of reference, the respiratory sensor is described
with referenced to paper as the substrate. However the principles
described herein are readily applied to any porous hygroscopic
substrate. Cellulose-based substrates such as certain textiles and
fabrics, e.g., cotton fabrics, and paper are exemplary
cellulose-based substrate. Cellulose paper has a very high affinity
to ambient water. At a relative humidity of 70%, the moisture
content in cellulose paper can be as high as 10% by weight. In
addition to its ability to absorb high amounts of water from air,
i) cellulose paper is inherently porous, a feature that allows
gases to penetrate and interact easily with the bulk of the
material in the sensor, ii) cellulose paper has a high surface area
(over 100 m.sup.2/g) so the sensitivity of the sensor is greater
than thin films would have if operated using the same mechanism,
iii) cellulose paper is chemically inert to most gases including
highly corrosive species. The invention is demonstrated with a
range of paper types, e.g., i) Whatman 1 Chr, ii) Whatman 3MM Chr
and, iii) copy paper. Both Whatman 1 Chr and Whatman 3MM Chr papers
are made of pure cotton cellulose fiber with a basis weight of 87
g/m.sup.2 and 185 g/m.sup.2. (The basis weight of paper is defined
as the weight of paper per unit area). Other materials for use as
substrates are within the scope of the invention.
[0134] When breathing out, the human breath is close to 100% RH,
and therefore increases the amount of water on the sensor, thus,
increasing its ionic conductivity. When breathing in, the amount of
water on the surface of the cellulose fibers is reduced because the
surrounding atmosphere almost always has a lower RH than exhaled
air. This change in the amount of adsorbed water decreases the
ionic conductivity of the sensor, as is illustrated in FIG. 14B. By
applying a voltage between the two electrodes, the ionic
conductance of the paper can be quantified based on the measured
current. A change in the measured current provides an indication of
change in relative humidity. This change is further used as an
indication of exhalation (increase in RH) and inhalation (decrease
in RH) of a patient. The paper sensor transduces variations in the
level of moisture of its immediate surrounding to an electrical
signal. Using this method, one can indirectly acquire accurately
the respiratory signal of the patient. As the sensor itself
measures changes in moisture content between the inspired and
expired air, the system requires no calibration.
[0135] In one aspect, a paper-based sensor for detecting and
monitoring respiration is provided. In one or more embodiments, the
electrodes are disposed in close proximity on one side of the
substrate. In some embodiments, the electrodes are interdigitated
to increase the area of the area of the electrode, reduce the
distance between electrodes and increase the signal to noise ratio.
This design also allows rapid access of humidity to the paper.
Contacts can be provided for connection to an external voltage
source.
[0136] In other embodiments, the electrodes can be on the same side
of the porous substrate, as is shown in FIG. 15A-15C. As shown in
FIG. 15A, an electrode is fabricated on a cellulose substrate by
digitally printing graphite ink. The ink can be applied using
automatic controls with high precision and accuracy. The graphite
ink was diluted with a proprietary solvent (Ercon ET160) 55:45 w/w
to obtain desired consistency for printing, and the mixture was
homogenized using a tip sonicator to create a uniform dispersion.
Using this printing technique, large numbers of sensors can be
printed with high accuracy (FIG. 15B). (Other printing techniques
such as screen printing or reel-to-reel printing can, of course,
also be used for increased throughput). The individual electrode
pairs can be cut out from the sheet and electrical contacts can be
applied. Contacts for connection to a voltage source can also be
directly printed onto the substrate. An enlarged view of a single
interdigitated electrode design is shown in FIG. 15C; the
interdigitated design can provide increased area of the electrodes
and greater signal-to-noise ratio. The high surface area and the
small thickness of the paper improve the sensitivity of the sensor,
since many ionically conductive paths connect the two electrodes
and the electrodes are in close proximity, respectively. This
design also allows rapid access of humidity to the paper.
[0137] In one or more embodiments, electrodes are disposed on
opposite sides of the paper detector, for example, by printing
electrodes on both sides of a single sheet of paper. FIG. 2A is a
schematic illustration of the fabrication of a device having
electrodes on both sides using a stencil guide. Low-cost and
easy-to-apply graphite ink is silk-screened onto a first side of
the cellulose paper to form chemically inert carbon electrodes.
Once dried and optionally heated, the paper is flipped and the
counterelectrode is printed on the opposite side. Screen-printing
allows fabrication of multiple devices on the same substrate; the
electrodes can be designed to span a large surface area of the
paper, to increase sensitivity of the device includes well-aligned
electrodes on the front and back of the paper. FIG. 2B shows a
backlit photograph of an actual sensor. A cross-sectional image
visualizes the alignment of the top and bottom carbon
electrodes.
[0138] The respiration monitor is mounted on a support that can be
secured near, at or on the nose and/or mouth of the user. In one or
more embodiments, the paper respiration monitor can be attached on
the outer surface of a conventional doctor's mask, covering both
the nose and the mouth. See, FIG. 16B. In this configuration, both
nasal and oral breathing can be tracked. In other embodiments, the
sensor can be secured, e.g., using an adhesive backing, in the
vicinity, e.g., below, the nose. See, FIGS. 16A and 16D. The sensor
system can include sensors individually positioned below each
nostril. See FIG. 16C. The sensor can also be positioned to extend
over the mouth. See, FIG. 16E. Sensor attachment can be
accomplished in a variety of ways, such as over the nose bridge, or
one or both ears, inside the nostrils, on a helmet, on a face or
surgical mask, in a safety mask or emergency breathing masks and
supplemental oxygen masks. See, FIGS. 16F-L. Because the sensor is
small and light, it can also be adapted for use with infants. The
sensor can be incorporated into infant head cover or attached to an
infant bed. See, FIG. 16M-N. The sensor can be used during athletic
training.
[0139] Low weight electronics and a power unit are mounted on the
mask for wireless communication and signal read-out (see FIG. 14A).
The collected data is transmitted to a nearby phone or computer,
which can upload the data to the cloud. Changes in the water
content of the sensor can be detected by applying a voltage, e.g.,
a 25 V DC voltage, between the carbon electrodes of the sensor and
measuring the output current of the sensor. The power required for
exemplary designs of the sensor is 175-250 .mu.W at a RH of 90%.
The power requirements can be substantially reduced through further
optimization and redesign of the electrical circuit. In exemplary
systems, the paper-sensor had the lowest power requirement among
all of the electronic components used in construction of the system
with a peak power consumption of 500 .mu.W (<0.05% of the entire
system). Thus, the power consumption is low and for instance, a
single AA battery with a capacity of 2000 mAh at 1.5 V, can run an
exemplary sensor continuously for 6000 hours or 250 days.
[0140] In other embodiments, other detections means are also
contemplated, such as electrical resistance, voltage or
conductance, as these parameters are all linearly related by Ohms
Law.
[0141] In one or more embodiments, the sensitivity of the sensor
and the output signal can be further improved by loading the sensor
with a water-soluble salt than can increase the ionic conductance
of the adsorbed water. For example, the paper of the paper-based
sensor can be loaded with salts such as NaCl, KCl, KNO.sub.3, or
HPO.sub.4. Loading of soluble salt can be accomplished in any
conventional way, such as by dipping the sensor into a solution of
soluble salt and drying the paper. Sensor sensitivity can also be
modified by inclusion of other ions, such as inorganic compounds
such as clays typically used in paper-making.
Humidity Detection
[0142] Han et. al. (J. Phys. Chem. C 2012, 116, 22094-22097)
reported a paper-based RH sensor with copper tape electrodes
applied to the surface of a sheet of paper with a separation
distance of 2 mm. This design was only sensitive from RH 85% upward
with very low conductivity (1 nS) resulting in lower current. For
instance, at an applied bias of 25 V, their sensor would produce
only 25 nA whereas the respiration sensor according to one or more
embodiments of the current invention can produce over 10 .mu.A
response, an improvement of 400 fold. Thus, the design failed to
achieve high sensitivity.
[0143] Three different kinds of papers were demonstrated in the
paper sensors: i) Whatman 1 Chr, ii) Whatman 3MM Chr and, iii) copy
paper. Both Whatman 1 Chr and Whatman 3MM Chr papers are made of
pure cotton cellulose fiber with a basis weight of 87 g/m.sup.2 and
185 g/m.sup.2. The standard copy paper had a basis weight of 80
g/m.sup.2.
[0144] The response of the respiration monitor to RH levels ranging
from 0-90% (under an applied bias of 25 V) is determined. FIG. 11A
is a plot of log current vs. relative humidity. The plot shows that
the respiration monitor is sensitive above a RH of 20%. The copy
paper showed the highest sensitivity. The difference in sensitivity
between sensors fabricated with Whatman 3MM Chr and Whatman 1 Chr
may be due to their basis weight. A larger area of cellulose fibers
in a given area of paper would create a greater number of
electrically conductive pathways at a given level of RH, reduce the
overall resistivity, and increase the sensitivity. The high
sensitivity of copy paper may be related to various sizing
components, particularly clay, added in the papermaking process.
Clay is added to create a smooth surface for increased
printability. Added clay would increase both the concentration of
ions in the paper substrate and the hydrophilicity of the system,
and thus increase conductivity.
[0145] The electrical conductivity of pure cellulose paper can also
be increased by the intentional addition of ionic species. FIG. 11B
is a plot of log current vs. relative humidity for having 1 nmol
NaCl/mg cellulose and 10 nmol NaCl/mg cellulose. The electrical
conductivity of paper increased with the increasing salt
concentration. Different NaCl salt loads can be introduced into the
paper sensor by adding a salt solution to the paper and drying
(prior to sensor fabrication). Loading of NaCl was done by dipping
the sensor into a 10 mM aqueous solution of NaCl and drying the
paper in an oven at 60.degree. C. Using 1 mM and 10 mM solutions of
NaCl, the addition of 100 nmol of NaCl resulted in a concentration
of 1 nmol NaCl per 1 mg of cellulose and the addition of 1 .mu.mol
of NaCl resulted in a concentration of 10 nmol NaCl per 1 mg of
cellulose. The response from the NaCl loaded sensor under an
applied potential difference of 25V demonstrated an improved
sensitivity. The use of NaCl in the sensor increased the ionic
conductivity of the adsorbed water by an order of magnitude.
Incorporation of salt provides a very simple route to fine-tune the
electrical characteristics, and to improve signal-to-noise ratio
and RH sensitivity of the paper-based moisture sensor.
[0146] The sensor operates a two-electrode electrochemical cell
where water and other chemical species (e.g. dissolved salts) are
electrolyzed in the presence of an applied potential across the
electrodes. The oxidation and reduction of chemical species on the
surface of the graphite electrodes produce an electrical current,
measured using electronics. This current is proportional to the
concentration of ionic species in the solution. The half-cell
reactions for H.sub.2O and a dissolved salt in water (i.e. NaCl)
are shown below in equation; as a result of the electrochemical
process, formation of H.sub.2 gas on the cathode and Cl.sub.2 or
O.sub.2 gas on the anode is expected, depending on the NaCl
concentration.
[0147] Other materials could potentially be used to further improve
the sensitivity of the sensor. NaCl can be replaced by other salts,
such as KNO.sub.3 and K.sub.2HPO.sub.4, which display a behavior
very similar to that observed for NaCl (FIG. 17); This result
confirms that various salts can be used to fine-tune the
sensitivity and electrical characteristics of the paper sensor.
[0148] A weak dependence of the output current of the paper sensor
on the temperature was observed (between 22.degree. C. and
40.degree. C.). This small difference may be due to an increase in
the mobility of ions at elevated temperatures, and thus an increase
in conductivity. The observed difference, however, was negligent
and does not interfere with operation of the device.
[0149] In one or more embodiments, the paper sensor is not
sensitive to CO.sub.2. When a paper-based sensor is exposed to
CO.sub.2 concentration of 25% (at 70% RH), there was no increase in
sensor output. This insensitivity may be explained by the low
solubility of CO.sub.2 in water (<0.5 g/liter at room
temperature at 1 atm). The paper sensor is, therefore, largely
sensitive to changes in the amount of moisture present in paper,
and is not affected by the concentration of CO.sub.2 in human
breath (<5%).
Simulated Respiration Monitoring
[0150] In order to simulate breathing of a human, a system was
designed to flow air at precise rates over the sensor from a source
of air at .about.100% RH humidity and from atmosphere at .about.70%
RH. The 100% RH air mimics flow exhaled from the lungs and the 70%
RH air mimics flow inhaled from the atmosphere. This system allows
one to control precisely the frequency and flow rates of the air
passing over the sensor; such control allows one to determine the
response of the sensor to different conditions of respiratory
arrhythmia. A respiratory simulator using two 1000 sccm mass flow
controllers (MFC), manufactured by MKS, a Keithley source meter,
and an electric pump to create airflow during testing of the sensor
is shown in FIG. 12. All instruments were controlled using LabView
programming software.
[0151] The simulated human breath (i.e., 100% RH) was created by
bubbling ambient air through a container filled with distilled
water, which was maintained at 37.degree. C. The flow of humidified
air was controlled by MFC 1, and the flow of ambient air was
controlled by MFC 2, as shown in FIG. 12. During all of the
simulation measurements, the RH of the ambient environment was
measured to be .about.70%.
[0152] Three sets of tests were performed, each consisting of 10
full cycles of flowing air of 100% humidity (i.e., breathing out)
and 70% humidity (i.e., breathing in).
[0153] A healthy adult breathes between 6 to 12 breaths per minute,
displacing up to 24 L of air per minute. For the testing setup, a
lower airflow (max. 1 L/min) was used in order to demonstrate the
sensor was sensitive to flow rates within the pathological region,
which approached zero (i.e., apnea).
[0154] In the first test, the breathing rate was set to the lower
end of the healthy breathing rhythm at six breaths per minute,
breathing in and out every 10 seconds. The periodic airflow was
halted through both MFC 1 and MCF 2 to simulate paused breathing
(i.e. apnea). During this period of no flow, the output of the
sensor decreased significantly, providing a detectable signal for
diagnosis of apnea, as shown in FIG. 13.
[0155] After this period of no flow, regular breathing was
continued and the breathing rate was set at 12 breaths per minute
(with a flow rate of 0.8 L/min) followed by another period no flow.
The simulator was then set to 30 breaths/min (with a flow rate of
0.5 L/min) to demonstrate the rapid response time of this sensor.
See, FIG. 13.
[0156] Fast breathing with shallow breaths is a symptom of hypopnea
and our sensor performed extremely well in all of the simulated
scenarios of paused and shallow breathing, suggesting this sensor
is suitable for diagnosing arrhythmias in respiration.
Recording and Analysis of Respiratory Signals
[0157] We placed our paper-based moisture sensors into low-cost
textile procedure masks to demonstrate its use with healthy adults
(FIG. 18A). We tested the functional mask on human volunteers.
[0158] We designed and fabricated a low-cost data acquisition
device using off-the-shelf electronic components (FIGS. 18B and
18D). The reader electronics were purchased from Digikey (including
the Arduino microcontroller). The printed circuit board (PCB) was
designed using Eagle application package and was manufactured by
Silver Circuits Sdb. Bhd., Selangot Malaysia. The Bluetooth shield,
Kedsum KDF001A, for the Arduino board was purchased separately from
Amazon. The housing for the electronics was designed using AutoCAD
and printed with a Dimension 3D; Stratasys, Inc 3D printer in
acrylonitrile-butadiene-styrene (ABS) thermoplastic polymer. The
Android app was designed and implemented in the Android Studio
development environment using Java and XML programming languages.
We used a Samsung Galaxy Tab 4 tablet computer as the testing
device.
[0159] This instrument generated a 25V DC potential from a 5V DC
power source, and applied the voltage across the paper sensor. The
unit amplified and digitized the resulting electrical current and
transmitted the data to a tablet computer (FIG. 18C) over a
wireless Bluetooth communication link. The custom-built Android
application, with a minimally complex design, can display the
incoming data and run simple analytics (e.g. Fourier
transformations). The software can also save the collected results
to a text file, which can be uploaded to the cloud, or emailed to a
third party for off-site analysis. This feature will be
particularly useful for individuals who monitor their respiration
at home and wish to share their results remotely with a healthcare
professional.
[0160] The Android application also has the option to apply
finite-impulse-response (FIR) and infinite-impulse-response (IIR)
digital filtering algorithms to the acquired signal. Digital
filters are commonly used for the separation of combined signals
and the restoration of distorted (i.e. noisy) signals. Since the
system we implemented can also be used as a research tool, the user
has the flexibility to design custom digital filters (e.g.
low-pass, high-pass, band-pass, band-stop) to extract other metrics
from the recorded pattern of respiration. The filter coefficients
can be stored as a text file on the tablet computer and imported
when the application is launched. Both the filtered and the raw
data are recorded as a text file for later analysis. By
implementing additional compressed sensing/sparse sampling
algorithms, the size of the files containing the collected data can
be made significantly smaller, allowing rapid transfer of these
files at lower network speeds. This capability would be
particularly useful in the developing world.
[0161] The functional textile mask was able to successfully track
the respiratory activity of the subjects for 15 minutes at room
temperature (FIG. 19A) while at rest (i.e. sitting in a chair); the
duration of the experiment was limited due to the specifications of
the IRB. There is a significant initial drift in the output of the
paper sensor. The drift is caused by the large difference in drying
and humidifying cycles during breathing, and reaches equilibrium in
a few minutes. FIG. 19B displays a collection of different
breathing patterns acquired during the experiment. The area marked
with (1) is a normal breathing pattern (which consists of periodic
breathing cycles of similar magnitude) followed by a deep breath
marked as (2). During this period, the subject had a rate of
respiration of 14 breaths per minute. A pause in breathing was
registered as a drop in the recorded output current (3). A mixture
of deep, slow, fast, shallow breathing patterns could also be
accurately monitored during the experiment and marked as (4) in
FIG. 5b. The device responded seamlessly to the transition between
periods of fast, shallow and normal breathing (FIG. 19C). The
system was also able to acquire and transmit all of the collected
data to the tablet computer.
[0162] FIG. 20A-C shows a trial in which a subject took a short
walk inside the building to test two performance factors of the
system: I) noise levels in the collected data during movement, and
II) system performance during light exercise (i.e. walking around
the hallways) and vigorous exercise (i.e. high tempo climbing of
four flights of stairs in the building). The signal-to-noise ratio
for the collected data was very good during both light exercise
(FIG. 20B) and vigorous exercise (FIG. 20C); the rate of
respiration could easily be detected by counting the number of
peaks. The subject registered a rate of respiration of 12
breaths/min during light exercise, and 22 breaths/min during more
vigorous exercise. (The breath counts were calculated manually from
the collected data). Interestingly, some of the subjects exhibited
a decreasing rate of respiration when transitioning from light
exercise (24 breaths/min) to vigorous exercise (16 breaths/min).
The subjects compensated for this decrease by taking deeper breaths
and the rate of respiration alone is not a sufficiently descriptive
metric for the analysis of respiratory activity without data on the
respired volume of the subject. FIG. 21A summarizes the rate of
respiration of each subject during light and vigorous exercise.
[0163] One of the biggest advantages of our device is its ability
to capture the pattern of respiration, which contains information
regarding the depth of breathing (exchanged volume) in addition to
the rate of breathing. We have defined a new metric, the Volumetric
Flow Rate Index (VFR Index), which is the product of the rate of
respiration (RR) and the average signal amplitude normalized to a
reference signal amplitude (FIG. 21B). Essentially, the amplitude
of the captured cyclic signal is proportional to the volume of the
inspired and expired air during respiration. This results in
information about the depth of breathing (i.e. tidal volume). The
formulas needed to calculate the VFR Index are shown below in
equation (5) for light exercise and (6) for vigorous exercise.
VFR Index L i g h t = R R Light .times. ( A Light A Light ) ( 5 )
VFR Index H e a v y = R R Vigorous .times. ( A Vigorous A Light ) (
6 ) ##EQU00004##
[0164] Here, AMprous is the average amplitude of the signal during
vigorous and, A.sub.Light, the light exercise. Using this approach,
we determined that all of the subjects exchanged a larger volume of
air during vigorous exercise than they did during light exercise, a
conclusion that cannot be derived solely from the rate of
respiration.
[0165] Overall, the paper sensor, electronics and the mobile app,
performed without failure or interruption of data during the
experiments, and thus have the potential to be used for monitoring
breathing at rest and during physical activity. An inexperienced
individual could learn how to read and interpret these graphs
within a few minutes. Filtering out the drifting baseline using
digital filters would further simplify the process of manual
interpretation.
[0166] Unless otherwise defined, used or characterized herein,
terms that are used herein (including technical and scientific
terms) are to be interpreted as having a meaning that is consistent
with their accepted meaning in the context of the relevant art and
are not to be interpreted in an idealized or overly formal sense
unless expressly so defined herein. For example, if a particular
composition is referenced, the composition may be substantially,
though not perfectly pure, as practical and imperfect realities may
apply; e.g., the potential presence of at least trace impurities
(e.g., at less than 1 or 2%) can be understood as being within the
scope of the description; likewise, if a particular shape is
referenced, the shape is intended to include imperfect variations
from ideal shapes, e.g., due to manufacturing tolerances.
Percentages or concentrations expressed herein can represent either
by weight or by volume.
[0167] Although the terms, first, second, third, etc., may be used
herein to describe various elements, these elements are not to be
limited by these terms. These terms are simply used to distinguish
one element from another. Thus, a first element, discussed below,
could be termed a second element without departing from the
teachings of the exemplary embodiments. Spatially relative terms,
such as "above," "below," "left," "right," "in front," "behind,"
and the like, may be used herein for ease of description to
describe the relationship of one element to another element, as
illustrated in the figures. It will be understood that the
spatially relative terms, as well as the illustrated
configurations, are intended to encompass different orientations of
the apparatus in use or operation in addition to the orientations
described herein and depicted in the figures. For example, if the
apparatus in the figures is turned over, elements described as
"below" or "beneath" other elements or features would then be
oriented "above" the other elements or features. Thus, the
exemplary term, "above," may encompass both an orientation of above
and below. The apparatus may be otherwise oriented (e.g., rotated
90 degrees or at other orientations) and the spatially relative
descriptors used herein interpreted accordingly. Further still, in
this disclosure, when an element is referred to as being "on,"
"connected to," "coupled to," "in contact with," etc., another
element, it may be directly on, connected to, coupled to, or in
contact with the other element or intervening elements may be
present unless otherwise specified.
[0168] The terminology used herein is for the purpose of describing
particular embodiments and is not intended to be limiting of
exemplary embodiments. As used herein, singular forms, such as "a"
and "an," are intended to include the plural forms as well, unless
the context indicates otherwise.
[0169] It will be appreciated that while a particular sequence of
steps has been shown and described for purposes of explanation, the
sequence may be varied in certain respects, or the steps may be
combined, while still obtaining the desired configuration.
Additionally, modifications to the disclosed embodiment and the
invention as claimed are possible and within the scope of this
disclosed invention. In particular, it is contemplated that
features and embodiments that are described individually may be
combined without departure from the invention.
* * * * *