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FIELD OF THE INVENTION
The present invention relates to means for monitoring the level of glucose
in blood and bodily tissues. Particularly, the invention relates to a
system for monitoring glucose with glucose sensitive cells that produce an
electrical response to glucose levels in their surrounding medium which is
then used to determine the blood glucose level, to administer insulin or
to take other measures to alter the blood glucose level such as diet
adjustment.
BACKGROUND OF THE INVENTION
Diabetes is a metabolic disorder that afflicts tens of millions of people
in the developed countries of the world, with many millions more probably
affected in underdeveloped nations. Diabetes results from the inability of
the body to properly utilize and metabolize carbohydrates, particularly
glucose. Normally, the finely-tuned balance between glucose in the blood
and glucose in bodily tissue cells is maintained by insulin, a hormone
produced by the pancreas which controls, among other things, the transfer
of glucose from blood into body tissue cells. Upsetting this balance
causes many complications and pathologies including heart disease,
coronary and peripheral artery sclerosis, peripheral neuropathies, retinal
damage, cataracts, hypertension and coma and death from hypoglycemic
shock.
In patients with insulin-dependent diabetes, the symptoms of the disease
can be controlled by administering additional insulin (or other agents
that have similar effects) by injection or by external or implantable
insulin pumps. The "correct" insulin dosage is a function of the level of
glucose in the blood. Ideally, insulin administration should be
continuously readjusted in response to changes in blood glucose level.
However, at present, blood glucose levels can only be determined directly
by a blood sample. Unfortunately, since drawing the sample is invasive,
blood glucose is usually only determined once daily or less often. As a
result, insulin dosage is not optimally coordinated with blood glucose
levels and complications can continue to be manifested. It would,
therefore, be desirable to provide non-invasive means for more closely
monitoring blood glucose levels and coordinating insulin dosages with such
levels.
Many attempts have been made to develop a reliable less invasive or
non-invasive way to measure blood glucose level. One of the most widely
used methods has been measurement of glucose excreted in the urine, which
is under certain conditions an indicator of blood glucose concentration.
In its most convenient form, a "dipstick", which has been coated with
chemical reagents, is dipped into a urine sample. Glucose in the urine
then reacts with the chemical reagents on the dipstick to produce a color
change which corresponds to the appropriate range of concentration. The
level of urine glucose is then correlated with blood levels on the basis
of statistical data and previous experience with the specific patient.
However, urine testing has presented several drawbacks. Foremost, is the
tenuous link between urine glucose level and blood glucose levels.
Although general trends in blood levels within a certain range are usually
reflected in urine levels, moderate or periodic fluctuations of blood
levels may not be reflected in urine content. Therefore, any dosage change
made on the basis of urine analysis is not finely-tuned to blood levels.
Furthermore, other substances in urine can cause inaccuracy in measurement
by interfering with chemical reactions necessary to produce the color
change on the dipstick. Finally, like blood sampling, urine analysis can
only be performed at relatively widely spaced intervals when the patient
produces urine for analysis.
Other systems have been proposed for monitoring blood glucose levels by
implanting a glucose sensitive probe into the patient. Such probes have
measured various properties of blood or other tissues, including optical
absorption, electrochemical potential and enzymatic products. U.S. Pat.
No(s). 4,436,094 and 4,704,029 disclose two examples of blood glucose
level probes. U.S. Pat. No. 4,436,094 utilizes an implantable electrode
which contains a charged carbohydrate species which, in the absence of
glucose, is bound to a component of the electrode and does not affect the
potential measured by the electrode. In the presence of glucose, however,
charged carbohydrate is displaced from the binding component by molecules
of glucose, and as a result of its charge, affects the potential
measurement by the electrode. The measured potential can then be
correlated to the concentration of glucose.
U.S. Pat. No. 4,704,029 discloses an implantable glucose monitor that
utilizes a refractometer which measures the index of refraction of blood
adjacent to an interface with the transparent surface of the refractometer
by directing laser light at the interface to measure the index of
refraction of the blood by the amount of radiation reflected at the
interface. As the blood glucose concentration increases, the index of
refraction of blood increases. By comparing the intensity of the light
reflected by the blood with the intensity of light before contact in the
blood, glucose concentration can be determined.
Another approach to tying blood glucose levels to insulin dosage has
centered around the implantation of pancreatic cells which produce insulin
in response to changes in blood glucose levels as shown for example in
Altman et al., Diabetes 35:625-633 (1986); Recordi et al., Diabetes
35:649-653 (1986); Amsterdam et al., J. Cell Biol. 63:1037-1056 (1974);
Brown et al., Diabetes 25:56-64 (1976); Carrington et al., J. Endocr.
109:193-200 (1986); and Sonerson et al., Diabetes 32:561-567 (1983).
Altman et al. were able to maintain normal blood glucose levels in
diabetic mice by implanting cells (1) in areas impermeable to antibodies,
(2) suppressing the immunogenecity of the implantable cells in tissue
culture before the implantation and (3) enclosing the cells in a capsule
that was impermeable to antibodies. However, the implantation methods of
Altman et al. and others are severely limited by the availability of large
enough masses of cells for effective implantation and by the ability to
reliably get insulin production over extended periods after implantation.
SUMMARY OF THE INVENTION
In accordance with the present invention, systems are disclosed which
utilize implanted glucose sensitive living cells to monitor blood glucose
levels by monitoring glucose levels in body tissues in which the glucose
level is in equilibrium with that of the blood. In this respect, the
implanted cells are similarly situated to endogenous insulin secreting
glucose sensitive cells. The implanted cells produce a detectable
electrical or optical signal in response to changes in glucose
concentration in surrounding tissue. The signal is then detected and
interpreted to give a reading indicative of blood glucose levels. This
reading can then be used as a basis for altering insulin or other drug
dosage for injection, as a basis for giving instructions to an external
implanted insulin pump to alter the amount of insulin delivered by the
pump, or as a basis for taking other corrective measures, such as altering
diet. As a result, blood sugar levels can be more closely monitored and
controlled in a noninvasive way and insulin dosage can be more closely
tailored with concomitant control of symptoms associated with diabetes.
A system for monitoring tissue and blood glucose level is disclosed which
comprises glucose sensitive cells which are capable of producing a signal
in response to changes in glucose concentration in the medium surrounding
the cells. The signal produced can either be electrical or optical. In
certain embodiments, the cells are contained in a capsule which is
constructed from a membrane or similar substance which is impermeable to
antibodies, yet permeable to nutrients to keep the cells alive. The
capsule can also be fitted with means for collecting the signals produced
by the cells.
In instances where the signal is electrical, these collecting means can be
metal electrodes which are placed in contact with the cells such that the
signal produced by the cells can be measured as a potential difference
between the electrodes. The system can further include an implanted signal
pickup device which is connected to the electrodes in the capsule for
processing (e.g., amplifying and modulating) the signal for later
transmission through the body surface, such as the skin, or for
transmission to an external or implanted insulin pump. Once the signal is
processed the pickup device passes the signal on to means for transmitting
the processed signal. In other embodiments, the implanted cells produce an
electrical signal which can be detected by external electrodes without
employing electrodes in the capsule or an implantable signal pick-up
device.
In instances where the signal is optical, the signal is produced by a
change in the optical qualities of the cells or specifically the membranes
of the implanted cells. Preferably, the signal is produced by dyes
contained within or coated on cellular membranes which will change the
optical properties of the cells in response to changes in electrical
activity of the cell. This change in optical quality can be detected
through relatively transparent body surfaces, such as thin skin layers or
fingernails. Alternatively, the optical change can be measured by an
implanted optical detector which processes the detected signal much as the
implanted pick-up device previously described processes electrical
signals. The processed signal can be used to control an insulin pump or
transmitted through the skin for external detection.
The electrical signal or the optical signal is detected through the skin by
an external sensor and then correlated to a corresponding blood glucose
level. The sensor includes means for detecting the signal, means for
processing such signal and correlating it to the corresponding blood
glucose level, and output means for reporting or relating the blood
glucose level as determined.
Alternatively, the implanted signal pickup device can pass a processed
signal on to an implanted insulin pump, which, in response to such signal,
delivers an appropriate dosage of insulin corresponding to the determined
blood glucose level.
Capsules for use in practicing the present invention are also disclosed
which comprise a membrane which is impermeable to artibodies and is
permeable to nutrients necessary for cell growth. Glucose sensitive cells
are enclosed within the membrane, along with electrodes in contact with
the cells such that changes in the electrical activity of the cells can be
detected as a potential difference between the electrodes.
Alternatively, in place of the electrodes, the capsules can enclose means
for "shorting-out" the interior of the capsule with respect to the
exterior of the capsule such that the electrical activity of the cells is
optimally dissipated on the exterior of the capsule. As a result, the
electrical activity will be maintained at a level which can be detected by
appropriate sensing means.
Capsules are also disclosed which contain glucose sensitive cells which
have been treated such that the cellular membranes of the cells are coated
with dyes which are sensitive to change in cellular membrane potential.
Finally, methods of monitoring the blood glucose level employing the
capsules and systems of the present invention are disclosed. Basically,
those methods comprise implanting into the patient glucose sensitive
cells, detecting the signal produced by the cells in response to levels
and/or changes in glucose concentration and correlating that signal with
the corresponding blood glucose level. Methods of administering glucose
are also disclosed which comprise administering a level of insulin (or
other correcting agent) appropriate to the blood glucose level determined
in accordance with the methods disclosed herein. Such insulin can be
either administered manually or by operation of an external or implanted
insulin pump which is connected to the detecting and monitoring system. It
is understood that other therapeutic agents which will alter blood glucose
levels, such as those sold under the tradenames "Dia Beta" (glyburide;
Hoechst-Roussel), "Glucontrol" (qlipizide; Pfizer) and "Diabinese"
(chlorpropamide; Pfizer), can be substituted for insulin as described
herein.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1. is a schematic representation of the components and operation of a
system of the present invention.
FIGS. 2A-2F depict bursts of spiked electrical activity produced by
pancreatic beta cells in response to various glucose concentrations.
FIGS. 3A-3F contain graphs of electrical activity of six different
preparations of beta cells in response to varying glucose concentration.
FIGS. 4 and 5 are schematic representations of the electrical components of
systems of the present invention.
FIGS. 6A, 6B and 7 depict possible arrangements of the components of a
system of the present invention with respect to the skin.
FIG. 8 depicts one embodiment of a system of the present invention that
utilizes an optical signal.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 schematically describes one embodiment of a system of the present
invention which is described in further detail below. As shown at the
upper right of FIG. 1, glucose diffuses from the bloodstream into the
extracellular space in bodily tissues. Eventually the glucose diffuses to
implanted glucose sensitive cells which are a part of the system of the
present invention. The implanted cells respond by exhibiting electrical
activity, such as a change in membrane potential, commensurate with the
concentration of glucose in the extracellular space.
The electrical activity can be detected or monitored in one of two ways.
Where the electrical activity is strong enough to be detected through a
body surface (e.g., layers of skin), the electrical activity is detected
directly by an external signal sensor. Alternatively, the electrical
activity is monitored by an implanted signal pickup device. The pickup
device processes and amplifies the electrical activity. The amplified
signal is then transmitted through a body surface (such as the skin) and
is detected by the external signal sensor.
The external signal sensor contains or is connected to a decoder or
microprocessor that interprets the signal. The decoder correlates the
signal with blood glucose concentration on the basis of an algorithm and
programmed information relating to the correlation between blood glucose
levels and glucose levels at the implantation site for the patient in
which the system is implanted. For example, the microprocessor can be
programmed to correlate a glucose concentration of 20 MM at the
implantation site with a concentration of 22 MM in the blood on the basis
of prior and periodic blood sampling. Once the signal has been correlated
and translated into a reading of the blood glucose concentration, the
concentration information is used in one of two ways. First, the
information can be displayed for reading by the patient or a person caring
for the patient. On the basis of the displayed concentration, the correct
insulin dose can be administered or diet can be adjusted. Alternatively,
the concentration information is fed into an insulin pump, external or
implanted, that infuses the correct insulin dosage on the basis of the
determined blood glucose level. The concentration information can also be
fed to other devices (such as automatic liquid feeding apparatus) which
will take corrective action on the basis of such information.
The systems of the present invention utilize glucose sensitive living cells
as sensors of the blood sugar levels either directly (by implantation in
the bloodstream) or indirectly (by implantation in tissues in equilibrium
with blood glucose levels). Any cell type that produces a detectable
electrical activity in response to changes in glucose concentration in the
surrounding environment can be used in practicing the present invention.
Beta cells from the islets of Langerhans in the pancreas are preferred
glucose sensitive cells. Beta cells have been shown to produce electrical
activity, action potentials, in response to glucose concentration and have
the advantage that they respond properly to glucose in the concentration
range relevant to patient monitoring. Scott et al., Diabetologia
21:470-475(1981); Pressel et al., Biophys. J. 55:540a (1989); Hidalgo et
al., Biophys. J. 55:436a (1989); Atwater et al., Biophys. J. 55:7a (1980).
Beta cells respond to glucose in bursts of spikes of electrical activity.
The spike frequency, burst duration and pauses between bursts are all
functions of glucose concentration. FIGS. 2 and 3 present data relating to
the electrical activity of beta cells. As shown in FIG. 2, the burst
duration increases as glucose concentration increases. The pause between
bursts also decreases as glucose concentration increases. In FIG. 3, the
spike frequency (spikes/second) increases as glucose concentration
increases. Each of these parameters (burst duration, pause duration and
spike frequency), as well as spike shape, can be monitored alone or in
combination as a source of signal corresponding to cellular electrical
activity. It has also been established that the beta cells are
electrically coupled, resulting in synchronized electrical activity of the
cells. Eddlestone et al., J. Membrane Biol. 77:1-141 (1984), Meda et al.,
Quarterly J. Exper Physiol. 69:719-735 (1984). Therefore, in response to a
change in the glucose concentration, many cells fire their action
potentials or electric signals in synchrony, producing a significantly
amplified signal which is easier to detect.
Methods for isolating beta cells are described in the references cited in
the preceding paragraph and in Amsterdam et al., J. Cell Biol.
63:1037-1056 (1979); Ricordi et al., Diabetes 35:649-653 (1986); and
Carrington et al., J. Endocr. 109:193-200 (1986). In addition, any other
method for isolating beta cells can be used which preserves the ability of
the isolated cells to respond to changes in glucose concentration. Other
methods for culturing pancreatic cells are disclosed in Amsterdam et al,
J. Cell Biol. 63: 1037-1073 (1974); Amsterdam et al., Proc. Natl. Acad.
Sci. USA 69:3028-3032 (1972); Ciba Foundation Symposium on the Endocrine
Pancreas, Reuck and Cameron, ed., p. 23-49 (J. and A. Churchill Ltd.,
London 1962); and Howard et al., J. Cell Biol. 35:675-684 (1967).
Sensor cells in taste buds have also been shown to respond to fluctuations
in glucose concentration. Ozeki, J. Gen. Plysiol. 58:688-699 (1971);
Avenet et al., J. Membrane Biol. 97:223-240 (1987); Tonosaki et al.; Brain
Research 445:363-366 (1988). Taste cells show particular advantage for
systems of the present invention because under suitable conditions such
cells regenerate every few days by continuous division. Thus, prolonged
growth of these cells when implanted is more readily sustained. Taste
cells are also more accessible than beta cells. A sample of taste cells
can be removed from a patient with only minor surgery, grown in culture to
obtain a sufficient number of cells and then implanted. The ability to use
a patient's own cells also reduced the likelihood of immunologic reactions
to the implant. Taste cells can be isolated according to the methods of
the publications cited above or by any other method which preserves the
ability of the cells to respond to change in glucose concentration.
Alpha cells from the pancreas have also been shown to be sensitive to
glucose concentration in the surrounding medium. Sonerson et al., Diabetes
32:561-567 (1983). Transformed cell lines, such as the insulin producing
line disclosed in U.S. Pat. No. 4,332,893, and hydridoma lines can also be
used. Any electrical activity associated with the response by alpha cells
or transformed lines to glucose can be harnessed in practicing the present
invention.
Many methods are known for implanting beta cells in human tissues. Altman
et al., Diabetes 35:625-633 (1986); Ricordi et al., Diabetes 35:649-653
(1986); Brown et al., Diabetes 25:56-64 (1976); Schmidt et al., Diabetes
32:532-540 (1983). Other means for encapsulating living cells are
disclosed in U.S. Pat. No(s). 4,663,286, 4,409,331, 4,352,883, 4,798,786,
4,689,293 and 4,353,888. Although the implanted cells of the present
invention need not necessarily be encapsulated, any of these methods can
be employed to produce an implantable capsule where such is used. The
method of Altman et al. is preferred. The Altman capsule is a thin walled
(about 100 microns thick) tube or elongated pellet made of a polyvinyl
chloride acrylic copolymer, with a diameter of about 1 mm. These
dimensions are preferred to maintain proper diffusion to all cells. The
molecular-weight cut off of the Altman et al. capsule membrane was
approximately 50,000. In preferred embodiments of the present invention,
the cut off is less than 50,000 and most preferably between 1,000 and
10,000.
The capsule serves two basic functions. First, it serves as a barrier that
prevents the cells from migrating away, while nutrients and waste products
are free to diffuse through the capsule. Second, it serves to prevent
antibodies and other large molecules from leaving or entering the capsule,
for example, to prevent immunological reactions. The capsule also allows
the use of glucose sensitive tumor cell lines as sensor cells which must
be contained to prevent proliferation. Any material which will provide
these functions can be used to form capsules containing glucose sensitive
cells.
In some embodiments, while not significantly interfering with production
and detection of cellular electrical activity, the capsules are equipped
with means for aiding detection of cellular electrical activity, such as
electrodes or conducting bars that short-circuit the cell electric
activity with the outside. The capsules are also preferably implanted in
clusters sc as to ensure a detectable signal even if one or more capsules
becomes dysfunctional. The capsules may also contain means to fix them in
the desired location or materials useful for determining the location of
the capsules, such as radio-opaque materials.
Where the electrical activity is too low to be detected through the body
surface without amplification or where the electrical activity is to be
harnessed to drive an insulin pump, electrodes are placed on the inside of
the capsule such that a potential difference can be measured across the
electrodes which corresponds to the electrical activity of the cells
inside the capsule. To prevent cell damage, these should be made from an
inert metal, such as those commonly used in a variety of implants. See for
example, "Cardiac Pacing and Physiology," Proceedings of the VIIIth World
Symposium on Cardiac Pacing and Electrophysiology, Jerusalem, Israel, June
2-11, 1987, ed. Belhassen et al., (Keterpress Enterprises, Jerusalem); IEE
Trans. Biomed. Eng. 34:664-668 (1987); and J. Am. Coll. Cardiol. 11:
365-370 (1988). Since these electrodes are used for signal pickup only and
not for electric stimulation, their functional lifetime should be
practically indefinite. The electrodes are connected by insulated wires to
an implanted signal pickup device for processing and amplification or to
the insulin pump.
First, where the electrical activity of the implanted cells generates
electric signals strong enough to be picked up from the external body
surface by electrodes (as in EEG or ECG), the capsules are implanted near
the surface of the skin where the skin is very thin and the location
convenient. As shown in FIG. 6B, the signal is then detected by the
external signal sensor. Alteratively, where the electrical activity is too
small to be picked up by external means, electrodes are introduced into
the capsule and connected to the implanted signal pickup device as shown
in FIG. 6A. In this case the capsule implantation can be done anywhere in
the body, for example, the peritoneal cavity where implantation is
relatively easy and vascularization is adequate.
The basic components of the implanted signal pickup device are shown in
FIG. 5. Basically, the device resembles implanted pacemakers in its
external surface property. It can contain some or all of the following
elements as necessary to provide a processed signal which is suitable for
transmission or other desired use:
1. Inputs connected to the electrodes inside the implanted capsules.
2. A number of low noise, high input impedence differential preamplifiers
corresponding to the number of capsules. Preferably, each capsule is
connected to a single differential input amplifier.
3. Band pass filters at the outputs of the amplifiers. In certain
embodiments, each amplifier may be connected through two filters, one
designed to pass only the spikes (action potentials) while the other will
pass only the very slow potential shifts associated with each burst of
activity.
4. An integrating amplifier or microprocessor that sums up the output of
all the preamplifiers.
5. A coding and modulating microprocessor that processes the summed signal
so as to be best suitable for transmission across the skin, such as FM
modulation.
6. A power amplifier that boosts the processed signal and is connected to
the transmitter that sends the processed signal through the skin.
There are two preferred alternative modes of transmission of the data from
the internal implanted amplifier across the skin to the external sensor.
In the first alternative, the amplifier is driven by low amplitude local
currents by means of a pair of electrodes implanted under the skin. The
electric field thus created is similar to those generated by the heart
(such as in ECG measurement) and can be detected similarly by external
electrodes. AC modulation of these currents will prevent local tissue
stimulation, electrode polarization, and the like. In the second
alternative, the output of the amplifier is fed, after proper modulation,
to an induction coil or a coupling capacitive signal transferer. This coil
generates an electromagnetic field that is picked up by a similar
externally positioned coil. Other means for transmitting a signal across
the tissue barrier will be apparent to skilled artisans and may be used in
practicing the present invention.
The signal from the capsules or the transmitter of the signal pickup device
is detected by an external signal sensor. The basic components of the
signal sensor are shown in FIGS. 4 and 6. In certain embodiments, the
signal sensor can include one or more of the following elements:
1. A sensor, such as an electrode, coil or other means suited to detect the
electrical or optical signal transmitted through the body surface.
2. A preamplifier connected to the sensors (which may not be necessary
where the signal is already amplified before transmission across the skin
barrier).
3. A filter for external noise reduction.
4. A demodulator--decoding microprocessor that separates the signals from
their FM carrier or other modulation means when such a mode of
transmission is used.
5. A signal processor that utilizes the appropriate algorithms and
programmed information relating to glucose concentration to translate the
transmitted signal into the corresponding glucose concentration.
The processed and decoded signal corresponding to determined glucose
concentration is then passed on to means for outputting such information
in the desired manner. For example, the concentration information can be
presented as a digital readout in the form of a digital display on the
probe itself or a display and printout in an associated device. A memory
may be used to save the glucose values obtained during continuous or
frequent glucose level monitoring. Such information including integrated
glucose levels may also be used for determination of the correct amounts
of insulin or insulin like drugs to be taken by the patient or determining
patient diet. Such information can be displayed for patient use or as an
input to an automated insulin infusion device.
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