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Description  |
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TECHNICAL FIELD
The present invention relates to an apparatus and a method for determining
the level of glucose in a sample, and more particularly to a non-invasive
personal glucose monitor for use by diabetics.
BACKGROUND ART
There are currently in excess of 10 million persons in the United States
that are diabetics; i.e., exhibit an abnormal amount of blood glucose due
to bodily misfunction. Many of these persons maintain control through
periodic insulin injection, the amount and frequency of which are
determined by testing for the blood glucose level. This testing can be in
the form of periodic laboratory analysis, such as annual, semi-annual, or
more frequent (e.g., daily) testing. The latter usually involves the
drawing of blood through the pricking of the person's finger tip. The
frequency of testing is dependent upon the sensitivity of the body to the
insulin treatment. In severe cases, an insulin pump is utilized for
accomplishing frequent adjustment of the blood glucose level. Since the
glucose level of an individual is not constant, a diabetic is usually
given an amount of insulin based upon an average glucose level. Thus,
there is a possibility that the patient can receive too little or too much
insulin.
In order to reduce the pain of withdrawing blood, as well as potential
infection, a non-invasive glucose determination would be desirable. Toward
that end, there have been certain devices developed for this purpose. For
example, a device is described in U.S. Pat. No. 3,963,019, issued to R. S.
Quandt on Jun. 15, 1976, that utilizes (in one embodiment) a beam of
polarized light impinging into and through the aqueous humor of the
patient's eye. The level of glucose present affects the quantity of light
exiting the eye, and this can be related to the glucose level. Another
non-invasive glucose monitor which involves use of a patient's eye is
described in U.S. Pat. No. 4,014,321 (also U.S. Pat. No. 3,958,560),
issued to W. F. March on Mar. 29, 1977. This device determines the optical
rotation of polarized radiation as a function of the glucose level. Still
other devices for determining the content of a patient's blood are
described in U.S. Pat. No. 4,169,676, issued to N. Kaiser on Oct. 2, 1979;
U.S. Pat. No. 4,350,163, issued to N. C. Ford, Jr., et al., on Sept. 21,
1982; U.S. Pat. No. 4,427,889, issued to G. J. Muller on Jan. 24, 1984;
U.S. Pat. No. 4,586,513, issued to K. Hamaguri on May 6, 1986; and U.S.
Pat. No. 4,655,225, issued to C. Dahne, et al., on Apr. 7, 1987. These
devices depend upon absorption and/or backscattering of incident radiation
to determine glucose levels.
The rotation of polarized radiation as a function of other organic
molecules is reported in U.S. Pat. No. 4,498,774 issued to E. E. Yeung, et
al., on Feb. 12, 1985. The device thereof modulates a polarized laser beam
using air gap Faraday rotators. While this device could have applications
in the analysis of glucose in blood samples, it is hardly useful for
non-invasive glucose analysis. As specified in Column 4 thereof, beginning
at line 19, the device occupies a table about 4 feet.times.8 feet. Extreme
care must be exercised to prevent vibrations. The modulation as used in
Yeung is very similar to that employed by one of the present inventors
(Hutchinson) in a polarimeter described in "A Modulated
Submillimeter-Laser Polarimeter for the Measurement of the Faraday
Rotation of a Plasma", Appl. Phys. Letters, 34(3), page 218, Feb. 1, 1979.
Despite the developments made in this field of glucose analysis, none of
the known devices (except for the applicant's above-cited U.S. Pat. No.
4,901,728) have sufficient sensitivity to at all compare with the
sensitivity achieved by more rigorous analytical techniques available in
laboratories as applied to blood withdrawn from the body. Thus, none are
suitable for controlling an insulin pump, for example. In the '321 patent
referred to above, while it was previously known that glucose causes an
optical rotation of polarized light (as used for sugar content in beers
and the like), the degree of rotation caused by glucose levels of the body
are extremely small and thus very difficult to measure with any
sensitivity. A further drawback to certain of the prior art devices is
that the patient's eye is used as the target. Considerable care would have
to be exercised using these devices to prevent physical damage of some
sort to the eye. Furthermore, insertion of any object in the eye is risky,
and must be done carefully. These devices certainly cannot be used without
professional help.
The teachings of the applicant's U.S. Pat. No. 4,901,728 are incorporated
herein by reference.
Accordingly, it is an object of the present invention to provide a
non-invasive apparatus, and method for use of the apparatus, to determine
glucose levels in the body with high sensitivity.
It is another object of the present invention to provide for the
non-invasive detection of glucose levels without fear of physical damage
to the patient.
An additional object of the present invention is to provide a non-invasive
instrument for glucose determination that can be utilized by a lay person.
Another object of the present invention is to provide a non-invasive
system, and its method of operation, that utilizes the optical rotation of
two polarized and modulated orthogonal laser beams for increasing the
sensitivity of glucose level detection in a patient.
It is also an object of the present invention to provide a non-invasive
means for determining glucose levels in a patient wherein the absorption
of the two polarized and modulated orthogonal laser beams is overcome to
thereby increase the sensitivity of glucose levels detection.
A further object of the present invention is to provide non-invasive means
for determining glucose levels in a patient with sufficient sensitivity
such that output signals therefrom can be used to operate an insulin pump
whereby the pump supplies only the amount necessary to maintain a desired
glucose level.
These and other objects of the present invention will become apparent upon
a consideration of the drawings listed hereinafter in combination with a
complete description thereof.
DISCLOSURE OF THE INVENTION
In accordance with the present invention, there is provided a non-invasive
personal glucose monitor having sufficient sensitivity to closely monitor
glucose levels of a patient. This monitor includes means for producing a
modulated polarized infrared light beam having two orthogonal states, each
having equal intensities and time duration but separated in time. In one
preferred embodiment, the light has vertical polarization and horizontal
polarization. This dually polarized light is passed through a tissue
(sample) of the patient wherein the light is optically rotated in
proportion to the glucose concentration. The transmitted light is then
detected whereupon a rotational shift of both of the waves provides an
increased signal differential to thereby significantly increase the
sensitivity of the instrument. Proper treatment of the detected signal
compensates for absorption in the tissues. The preferred embodiments
utilize light at two separate wavelengths to further compensate for
attenuation within the tissue and increase sensitivity of glucose level
detection.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the present invention illustrating the
major components thereof.
FIGS. 2A and 2B are waveforms of the individual waves from the modulated
polarizer of FIG. 1 illustrating their equal intensities and time
durations as well as their time relationship.
FIG. 3A is the waveform of the light at the detector of FIG. 1 when there
is no rotation of the two polarized waves of light.
FIG. 3B is the waveform of the light at the detector of FIG. 1 when there
is optical rotation of the two polarized waves of light as caused by blood
glucose in the tissue.
FIG. 4 is a schematic diagram of one embodiment of the present invention
illustrating one means of achieving the modulated polarization as well as
the dual components for use of two wavelengths to overcome attenuation in
a tissue.
FIG. 5 is a schematic diagram of another embodiment of the present
invention illustrating a further means of achieving the modulated
polarization as applied to dual wavelengths.
FIG. 6 is a block diagram of circuitry for controlling the present
invention.
FIG. 7A is a vector depiction of the two waveforms, as produced using a
single wavelength showing the relationship of vectors aligned with the
direction of transmission of the polarizer in FIG. 4 when there is no
sample.
FIG. 7B illustrates the two signals generated by the detector means as a
result of the transmitted portions of the waveforms according to FIG. 7A.
FIG. 7C illustrates the signal resulting from adding and subtracting the
two signals of FIG. 7B.
FIG. 8A is similar to FIG. 7A except that this shows the effect of rotation
of the waveforms by glucose, and the attenuation due to absorption, on the
two vectors in a direction of transmission of the polarizer.
FIG. 8B illustrates the two signals generated by the detector means as a
result of the transmitted portions of the waveforms according to FIG. 8A.
FIG. 8C illustrates the results obtained by adding and subtracting the two
signals of FIG. 8B.
BEST MODE FOR CARRYING OUT THE INVENTION
The principles of the present invention can be understood by reference to
FIG. 1. Shown schematically at 10 therein is a personal glucose monitor
that can be non-invasive with respect to a patient. The primary element of
this monitor is a source 12 of linearly polarized, modulated infrared
light, the source being referred to hereinafter as a "polarization
modulator means". This source is constructed to produce, as illustrated at
14, two light waves or states of different polarization and separated in
time. These light waves, identified as EHD 1 and E.sub.2, intentionally
have the same intensity and are angularly separated by an angle, .phi., of
ninety degrees. Thus, the two light waves are orthogonal. For convenience
in a preferred embodiment, one of the waves is polarized vertically and
the other is polarized horizontally. The duration, .DELTA.t, of these
waves is selected to be equal, and this duration is typically 100
microseconds.
The modulated polarized light beam is then passed through a sample of
tissue 16 having some level of glucose to be monitored. This tissue can be
typically the ear lobe or a finger of a patient. As is known in the art,
the glucose will cause an optical rotation of the polarized light. This is
depicted at 18 where the angle of rotation, .theta., is directly
proportional to the glucose level. A light. Since the waves of light are
discontinuous in separate time periods (see FIGS. 2A-B and 3A-B),
circuitry (not shown) associated with the detector means measures the
difference in the change in intensity of the light as the total shift
caused by both waves; thereby, substantially increasing a signal produced
by small values of .theta..
This can be better understood by reference to FIGS. 2A-B and 3A-B. The
waveforms of the two orthogonal light waves are shown in FIGS. 2A-B. It
will be noted in FIGS. 2A-B that the two light wave intensities (of both
E.sub.1 and E.sub.2) are equal, and each have the same duration, .DELTA.t,
except they alternate in time, i.e., E.sub.1, from 0 to t.sub.1, and
E.sub.2 from t.sub.1 to t.sub.2.
Referring to FIG. 3A, shown therein is the signal intensity, I, at the
detector 20 produced by the two waves if no optical rotation has occurred.
As noted, the intensity I.sub.1 for E.sub.1 is equal to the intensity,
I.sub.2, for E.sub.2. This could be the case of signals received with a
zero glucose level or if the initial polarization orientation corresponds
to the rotation that would occur with a selected glucose level as a
standard. When optical rotation occurs, one of the two waves, e.g.,
E.sub.1, will produce a larger signal strength I.sub.1 '. This is
illustrated in FIG. 3B. The detector means circuitry measures the
difference between I.sub.1 ' and I.sub.2 ' and thus will distinguish the
total shift which permits substantially increased sensitivity of
measurement. Additional discussion of the increased sensitivity of the
present invention will be given hereinafter with respect to FIGS. 4, 7A
and 8A.
A schematic diagram of one specific embodiment of the present invention is
depicted in FIG. 4. This embodiment utilizes a laser light source 22
producing light having a wavelength of, for example, 940 nm as produced
typically with a NEC MOdel SE313 laser diode unit. While a wavelength of
about 1000 nm is preferred for minimum absorption, apparatus for producing
the 940 nm wavelength is available commercially. This light source is
polarized in a linear direction. The light therefrom is passed through a
conventional "quarter-wavelength" plate 24 that introduces a phase shift
so as to change linear polarization to circular polarization. This shifted
light is then passed through an electro-optic switching unit 26, such as a
lithium tantalate crystal, again converting the polarization to linear but
shifted .+-.45 degrees to the original linear polarization depending on
the sign of the voltage, V, applied to the crystal. With an alternating
square wave value of V, the shift will be back and forth as the potential
switches. Thus, the equal waves E.sub.1 and E.sub.2 of FIG. 1 are produced
such that each are orthogonal but still separated in time. In this
embodiment, the quarter-wave plate 24 and the switching unit 26 make up
the "polarization modulator means."
The two orthogonal waveforms of the light then are passed through the
patient's tissue 16 which, as stated above, can be an ear lobe, a portion
of the finger, or any other relatively thin portion of a patient. Glucose
in this tissue causes the aforementioned rotation of both of the
waveforms. The light is then shone through a polaroid material 28, this
polaroid material having a transmission in a selected direction, i.e., in
the same direction as the vertical component of each of the light states.
This unit is referred to as a polarizer.
When no rotation has occurred, both waves or states produce a vector having
equal vertical components. Thus, the signal transmission through the
polarizer is equal for each of the vertical components such that the
detector 20 provides equal values of signals for the two waves. This is as
depicted in the aforementioned FIG. 3A. If the values are subtracted the
result is zero. However, when rotation occurs due to glucose in the
tissue, a new vector value is created for both waves, with the value of
the vertical component of one vector increasing and the vertical component
of the other decreasing. Thus, since the signals are subtracted for even a
very small angle of rotation, a greater distinction will be seen by the
detector after the light passes through the polaroid material. This is as
illustrated in FIG. 3B. The larger the rotation (due to glucose
concentration) the larger will be the signal differences. If these signals
are synchronously detected with a filter tuned to the switching frequency
of the polarization modulator means, there would be no signal for no
sample, and an ever increasing signal for increasing glucose
concentrations. Thus, even very small angles of rotation--a few thousands
of a degree-- produce a sufficiently changed electrical signal that can be
processed using known amplification techniques.
This action of the polaroid material and rotation can be understood by
referring to FIGS. 7A-C and 8A-C. In FIG. 7A, the two polarized light
waves impinging upon a tissue are E.sub.1 and E.sub.2 just as indicated in
FIG. 1. As before, they are equal and orthogonal. Their particular
orientation is selected relative to the transmission direction, T, of the
polaroid material 28 such that each of the light waves has a vertical
vector component E.sub.1v and E.sub.2v that will pass through the polaroid
material to the detector means 20. When there is no rotation due to
glucose, signals at the detector means due to the values of E.sub.1v and
E.sub.2v are equal as indicated in FIG. 7B. If these are added, the total
value (V.sub.so) will double; however, if they are subtracted, the value
(V.sub.do is zero as indicated in FIG. 7C. However, when rotation occurs,
E.sub.1v ', becomes larger than E.sub.2v ' as shown in FIG. 8A (despite
absorption losses) giving rise to detector signals such as in FIG. 8B. The
sum and difference between these values (see FIG. 8C) achieves an output
that provides information as to transmission and also is enhanced for a
small angular rotation. The true value of the glucose is thus proportional
to the ratio of the sum voltage to the difference voltage. This can be
explained as follows.
The "sum" voltage, V.sub.s, signal at the detector means averaged over one
cycle is
<V.sub.s >=kE.sup.2 [cos.sup.2 (45.degree.-.theta.)+cos.sup.2
(45.degree.+.theta.)], and
<V.sub.s >=kE.sup.2 [cos(45.degree.) cos .theta.-sin (45.degree.) sin
.theta.].sup.2
+[cos (45.degree.) cos .theta.+sin (45.degree.) sin .theta.].sup.2.
(Equation 1)
Upon expanding, this becomes
<V.sub.S >=kE.sup.2 [2 cos.sup.2 (45.degree.) cos.sup.2 .theta.+2 sin.sup.2
(45.degree.) sin.sup.2 .theta.].
Since 2 cos.sup.2 (45.degree.)=2 sin.sup.2 (45.degree.)=1, and cos.sup.2
.theta.+sin.sup.2 .theta.=1; therefore
<V.sub.S >=kE.sup.2, where k is a constant. (Equation 2)
Therefore, the "sum" voltage signal averaged over one cycle is directly
proportional to the signal transmitted through the sample. This "sum"
signal depends only on transmission, not polarization rotation by the
glucose in the sample since there is no relationship to the angle .theta.
in Equation 2.
The difference voltage signal, V.sub.d, from the detector means 20 averaged
over one cycle is
<V.sub.d >=kE.sup.2 [cos.sup.2 (45.degree.+.theta.)-cos.sup.2
(45.degree.-.theta.], and (Equation 3)
<V.sub.d >=kE.sup.2 [[cos 45.degree. cos .theta.-sin 45.degree. sin
.theta.].sup.2
-[cos 45.degree. cos .theta.-sin 45.degree. sin .theta.].sup.2 ].
Upon expanding this becomes
<V.sub.S >=kE.sup.2 [2 sin 2.theta.]. (Equation 4)
Therefore, the magnitude of the difference signal averaged over one cycle
is
.vertline.V.sub.d .vertline.=2kE.sup.2 sin 2.theta.. (Equation 5)
For small values of .theta.
.vertline.V.sub.d .vertline.=4kE.sup.2 .theta..
Thus, the difference signal is directly proportional the constant k times
the transmitted signal times the angle of polarization rotation.
At a single wavelength (using Equation 2) and taking into account sample
attenuation, the detector sum voltage is
V.sub.S =<V.sub.so >e.sup.-aL (Equation 6)
where .alpha. is the attenuation coefficient, L is the effective sample
length and <V.sub.so > is the averaged detector "sum" voltage without a
sample.
Then solving Equation 6 for L results in
##EQU1##
Using Equation 5 and inserting the effect of attenuation (again for a
single wavelength)
V.sub.d =2<V.sub.do >[e.sup..alpha.L ] sin (Equation 8)
Solving Equation 8 for the rotation angle .theta.,
##EQU2##
However, from Equation 6,
##EQU3##
therefore,
##EQU4##
Thus, the rotation angle .theta. from Equation 10 depends on the measured
difference and sum voltages without the sample (V.sub.do and V.sub.so) and
the measured difference sum voltages with the sample (V.sub.d and
V.sub.s). Using this value of rotation angle .theta., the glucose
concentration is
Conc.=C.theta.L (Equation 11)
where .theta. is determined by Equation 10, L is determined by Equation 7,
and C is a calibration constant measured with a known sample during
calibration.
It will be understood by persons skilled in the art that a microprocessor
can be programmed to carry out the mathematics of these equations so that
an output signal corresponding to the glucose concentration achieved from
the signals reaching the detector means 20 of FIG. 4. This value of
concentration been made substantially independent of absorption effects in
the tissue by these measurements.
A portion of the problem of glucose monitoring in tissue, other than the
very small amount of rotation due to the concentration of interest, is due
to absorption in the tissue. This absorption is due to thickness,
pigmentation, temperature, amount of blood, etc. This absorption can
affect attenuation of the light seen by the detector. This can be
overcome, to some extent by the system described above. However, by
simultaneously making the measurements at two wavelengths, the attenuation
of each (which depends on the absorption, see above) can be further used
to null the effect of absorption.
This manner of dealing with this problem is also depicted in FIG. 4. A
second light source 22' is used with a wavelength of, for example, 1300
nm. A source of this wavelength is commercially available and, with the
940 nm source, brackets the optimum wavelength of about 1000 nm. This 1300
nm wavelength typically is produced using an NEC Model ADL5340 laser
diode. The linearly polarized light therefrom is passed through a
quarter-wavelength plate 24' to produce circularly polarized light, and
this passes into the switching unit 26', typically in the form of another
lithium tantalate crystal. Through the application of potential V',
applied as a square wave, the output light will again be linearly
polarized and there will be two orthogonal waves separated in time
according to the frequency of switching of V'. This light is passed
through the tissue 16, through a second polarizer material 28' and is
received by another detector means 20'.
For the two wavelengths, Equation 7 can be written as
##EQU5##
Since the attenuation coefficients .alpha..sub.1, and .alpha..sub.2 may
differ between the two wavelengths, simultaneously solving Equations 12A
and 12B by standard mathematical techniques will result in a value of L to
be used for the glucose concentration measurement by substitution into
Equation 11. As above, the mathematical solutions can be performed by a
preprogrammed microprocessor.
It will be understood by persons versed in the art that light sources,
power supplies and the like can have fluctuations. Accordingly, although
not shown in FIG. 4, provision must be made for monitoring the light prior
to impinging on the tissue 16 and for correcting for light intensity
variation. Systems for affecting this type of "feedback" will be known to
those skilled in the art. Furthermore, since the lithium tantalate
crystals are sensitive to temperature change, means will be required to
maintain the temperature of these crystals at a constant value.
Another embodiment of the present invention, although utilizing a different
form of polarization modulator means, is illustrated schematically in FIG.
5. As with the embodiment of FIG. 4, light of two different wavelengths is
used to negate the effect of absorption by the sample. In this embodiment,
a first beam splitter means 30 is utilized for a first wavelength, e.g.,
940 nm. This beam splitter means 30, typically NRC Model 05BC16, is
essentially a cube of material (e.g., glass) that is transparent to the
beam; in this instance, light at 940 nm. There is a diagonal plane 32 in
the cube that is partially transparent to the light and partly reflective
to that light. Two light-emitting diodes 34, 36, whose light is randomly
polarized, are directed toward adjacent faces of the cube 30 so as to each
be at a 45 degree angle to the plane 32. These light sources each can be,
typically, an NEC SE313 LED. A unique result of this construction is that
the transmitted light from light source 34 will be completely vertically
polarized, and that from light source 36 will be completely horizontally
polarized. Thus, through appropriate switching between the light sources
34 and 36, the two linearly polarized waves E.sub.1 and E.sub.2 are
produced which are orthogonal (and separated in time) as in FIG. 1. The
cube 30 can be physically oriented to produce these waves at +45 degrees
and -45 degrees when desired (such as illustrated in FIG. 7A). Thus, with
the same use of a polarizer material 28 and detector means 20, the
rotation caused by glucose in the tissue 16 can be determined with
precision (using the above equations). Since some of the light from each
of the light sources is reflected by the diagonal plane 32, a single
monitoring of this light with detector unit 38 permits a monitoring of the
intensity of the light. Any conventional feedback circuit can be used to
maintain equal intensities from sources 34, 36.
The second "channel" for this embodiment uses a second beam splitter 40 as
the second polarization modulator means. This also has a partially
reflecting diagonal plane 42. The same model of beam splitter is usable at
the 1300 nm wavelength as used for the 940 nm. This wavelength is produced
by each of the two light-emitting diodes (LED) 44, 46, such as NEC Model
NDL-5310. As above, a detector 48 monitors the reflected portion of the
light from plane 42 so as to provide a signal for feedback control. This
second unit also provides two polarized waves such that absorption
variations within the sample 16 can be negated. There is a second
polarizer material 28' as well as a second detector means 20' for these
measurements.
A typical implementation of an automatic control circuit for the glucose
monitor system of FIG. 5 is illustrated in FIG. 6. The central portion of
this circuit is a microprocessor/controller chip 50. This CPU chip is
typically a NEC 78C10 unit. An oscillator within the CPU 50 provides two
square waves 180 degrees out of phase on leads 52, 54 to drive both the
940 nm and 1300 nm wavelength emitters 34, 44, 36 and 46, respectively, in
proper timing for the purpose discussed above through current sources
shown at 56. The detectors 38, 48 provide signals on leads 58, 60 that are
related to the light output of the 940 nm and 1300 nm emitters,
respectively. Each of these detectors 38, 48 (as well as detectors 20,
20') utilize multiplier chips 62-68 between the detectors and the
analog/digital (A/D) inputs of the CPU to allow phase-sensitive
(synchronous) detection of the received signals. Typically these
multiplier chips are National Semiconductor Chips No. LH2228.
If the outputs from the emitters 34, 36, 44 and 46 change with time, the
signals from the multipliers on each of the detectors 38, 48 will change.
The CPU then adjusts the current to the appropriate emitter(s) to balance
their light output. This current is provided through an appropriate one of
several digital-to-analog converters (DAC) 70 as indicated.
As stated above, it is necessary for the measurement of optical rotation
produced by the blood glucose level that the light intensities produced by
the emitters 34 and 36 (and also emitters 44, 46) be equal. When any
difference is noted, the CPU will adjust the drive currents through the
appropriate DAC 70 to the appropriate current source 56. When the light
intensities are balanced, the AC component of the signals appearing at the
multipliers 62, 66 (from detectors 20, 20', respectively) will be directly
proportional to the glucose level. The DC component of these signals is
directly proportional to the attenuation of the 940 nm and 1300 nm light
by the tissue. Using the two wavelengths, as explained above, permits the
determination of tissue thickness from the attenuation information. This
information, when combined with the rotation signals at the two
wavelengths, can thus be used (with the equations set forth above) to
accurately calculate the blood glucose level. The CPU 50 is preprogrammed
to accomplish these calculations. A signal corresponding to this blood
glucose level appears at output lead 72. This signal provides a visual
readout of the glucose level for example, and/or can be used to control an
insulin pump.
From the foregoing, it can be seen that at least two embodiments of the
present invention are presented. In both, there is a source of polarized
infrared light that is repetitively switched between two orthogonal linear
polarization states of equal amplitude. It is this switching of the light,
when passing through a tissue that increases the sensitivity of glucose
detection. In order to fully negate absorption attenuation by the tissue,
a second channel is used at a slightly different wavelength that closely
brackets the optimum wavelength for minimal absorption by the tissue.
Thus, although only specific embodiments have been described, the
invention is not to be limited by these embodiments and specific
components referred to herein. Rather, the invention is to be limited only
by the appended claims and their equivalents when taken together with the
complete description of the invention.
* * * * *
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