THE DIABETES CONTROL LOOP: SENSING OF GLUCOSE
AND CONTROL OF INSULIN IN SITU USING ENGINEERED
the Graduate School of
Daniel L. Maierhafer
Teacher: Dr. C.P. Leslie Grady
TABLE OF CONTENTS
LIST OF FIGURES........................................................................iv
1 THE DIABETES CONTROL LOOP: SENSING OF GLUCOSE
AND CONTROL OF INSULIN IN SITU USING ENGINEERED
2 I. INTRODUCTION
3 II. HISTORY OF THE FIRST BIOSENSOR
4 III. BIOSENSORS
A. Common Biosensor Elements...................................................9
1. Biological Sensing Element (BSE)......................................10
3. Target Analyte.....................................................................11
B. Inherently conducting polymer biosensor interface.................12
1. Electrode Materials and Manufacturing Methods................12
2. Attachment of the BSE to the electrode...............................13
C. Biosensors for Glucose Detection............................................14
1. Amperometric glucose biosensors.......................................15
2. Potentiometric pH glucose biosensor...................................16
3. Redox glucose biosensor......................................................17
5 IV. REQUIREMENTS FOR AN IMPLANTABLE DIABETES
Table of Contents (Continued)
A. Implantable Glucose Biosensor Requirements........................18
B. Implantable insulin delivery system requirements..................19
6 V. INSULIN DISPENSING SYSTEMS
A. Closed and Open Loop insulin dispensing systems.................20
B. Mixed insulin dispensing system.............................................20
C. Other insulin dispensing systems.............................................21
7 VI. CONCLUSIONS
LIST OF FIGURES
8 FIGURE 1. DIAGRAM OF THE CLARK GLUCOSE ENZYME-
9 FIGURE 2. DIAGRAM OF GENERAL BIOSENSOR
10 FIGURE 3. MEASUREMENT PARAMETERS FOR THE
OXIDATION OF GLUCOSE CATALYZED BY GLUCOSE
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One of the most common diseases of the endocrine system, diabetes, is a chronic
lifelong disease caused by a disruption of the carbohydrate metabolic pathway. Diabetes
ranks as the third highest cause of death, directly after heart disease and cancer in
industrialized nations. According to the International Diabetes Association in Brussels,
Belgium, there are more than 100 million diabetics in the world, or 6% of the total adult
population. As of today, diabetes cannot be cured, only controlled. If not well controlled
it will affect the function and metabolism of tissues and organs. If neglected for a long
enough period, organ complications will arise such as heart disease, renal disease,
blindness, and paraplegia (Ping, 1997).
The root cause of diabetes is the inability of the body to utilize or produce enough
insulin. Insulin is a hormone that is needed to convert glucose into energy needed for
daily life. Nobody knows what causes diabetes, but evidence points towards a
combination of genetics, obesity, and lack of exercise (American Diabetes Association).
There are two types of diabetes: In Type I diabetes, the pancreas does not
produce any insulin. This type most often occurs in children and young adults. People
with Type I diabetes must take daily insulin injections to stay alive. Five to ten percent
of diabetics are Type I (American Diabetes Association).
With Type II diabetes the body has a metabolic disorder resulting from its
inability to make enough, or to properly use insulin. Ninety to ninety-five percent of
diabetics are Type II. Unfortunately, the number of Type II diabetes cases has been
steadily growing and is quite high now due to the increased number of elderly
Americans, and a greater number of Americans that are obese and lead sedentary
lifestyles (American Diabetes Association).
Because diabetes is so widespread among the population, a large amount of
research and engineering have been done to make sensors that can detect the
concentration of glucose in the subject, and design devices that can help regulate the
concentration of insulin in the body. This paper will explain how a general biosensor
works, and then focus on the various schemes used to do glucose biosensing. Finally
some developments of implantable insulin delivery systems will be covered to close the
loop on the artificial insulin control system for diabetes.
II.HISTORY OF THE FIRST BIOSENSOR
The earliest known biosensor design for the detection of glucose was in 1962,
when Clark and Lyons thought that it might be possible to use a membrane covered with
an enzyme to transform glucose or urea into a substance that was detectable with an
oxygen or pH electrode. This was accomplished in 1976 when Updike and Hicks were
able to make such an enzyme electrode by polymerizing a gel that contained glucose
oxidase and attaching that to an oxygen electrode (Canh, 1993). This type of biosensor is
classified as an electroenzymatic sensor, because it uses the enzyme glucose oxidase to
oxidize glucose to gluconoloactone, which is then hydrolyzed to gluconic acid. The
reaction can be detected by either the disappearance of oxygen, or the appearance of the
products (Cammann, 1988). When glucose and oxygen diffuse into the enzymatic
membrane, glucose is oxidized to gluconic acid, reducing the partial pressure of oxygen
in the process. The oxygen electrode detects the decrease in oxygen partial pressure and
this is proportional to glucose concentration (Canh, 1993). The reaction proceeds
according to the chemical formula in Equation 1:
Equation 1: glu cos e + O2 + H 2 O cos e → gluconic acid + H 2 O2
Figure 1 shows a diagram of this first enzyme catalyzed glucose biosensor (Hall,
Figure 1. Diagram of the Clark glucose enzyme-electrode
The term biosensor is very broad and encompasses the microscopic to the
macroscopic size range, with measures from physical to chemical to electrical
phenomena. Most biosensor developments have been in the field of medicine, the food
industry, and environmental monitoring. For example, in the medical field, it has been
found that the general metabolic status of a cell can be interrogated by oxygen or
substrate consumption, production of metabolites, detection of luminescence, and
electrochemical sampling of the electron transport chain. In the agricultural arena,
biosensors are used to detect pollution in food and water samples and monitor livestock
reproduction in situ via milk progesterone (Tothill, 2001). At an environmental
monitoring site in Fort Detrick, Maryland, bluegill fish are used to continuously monitor
for heavy metals and organic pollutants in the effluent of the groundwater treatment
facility of a contaminated site (Van der Schalie, 2001). Even though the definition of a
biosensor encompasses a lot of diversity, all biosensors consist of the same three
A.Common Biosensor Elements
The distinguishing feature of the biosensor is that it is a transducer that
incorporates a biological sensing element (BSE) to discriminate a target analyte. The
biosensor consists of those three main parts: the BSE, the transducer, and the target
analyte. These three pieces are shown in Figure 2 (Hall, 1991).
Figure 2. Diagram of General Biosensor Components
1.Biological Sensing Element (BSE)
The BSE is used because a biological molecule is specific to a target analyte or
small set of analytes. The BSE could possibly be an enzyme, microorganism,
immunoagent, chemoreceptor, tissue, or organelle (Cahn, 1993). The detection of the
target analyte is indirect, that is the analyte first reacts with the BSE, and this reaction
produces a signal that is detectable by the transducer. The indirect mechanism allows
selective detection of analytes that would otherwise be undetectable or hard to detect
compared to a direct method using current technology. On the negative side, this makes
a more complex sensor and therefore allows more opportunities for interference into the
system than if the sensor used a direct method.
For this reason, the union between the BSE and the tranducer is very important,
because it plays a strong role in determining the signal to noise ratio, and the efficiency
of signal conversion. Usually, the BSE is immobilized on the surface of the transducer so
that the manufacturing process can control its thickness, and it will not wash away so it
can be reused. Unfortunately, the kinetics of the immobilized BSE are different from the
BSE in solution, and these kinetics change in the immobilized microenvironment. If
mass transfer of the analyte is diffusion limited, the enzyme will not be utilized
efficiently, and the output will be reduced. On the other hand, the linear dynamic range
for enzyme assays is increased with a slow mass transfer.
The interface must also be compatible with the operating environment. For
example, if pH is to be monitored, the immobilized molecule should be resistant to the
micro pH environment created by the reaction in the bio-linked immobilized layer (Hall,
1991). This is important when enzymatic reaction products like H+ or NH4+ are produced.
These cations can cause in dramatic changes to the micro pH environment near the
surface of the sensor (Wallace, 1999). Also, efficient electron transfer must be possible
between the enzyme and the transducer if a redox enzyme is employed as the BSE (Hall,
The transducer converts the biochemical signal from the interaction between the
BSE and the analyte into an electrical signal. There are four main categories of
transducers used in biosensors: electromechanical (electrode), optical (optrode), mass
(piezoelectric or SAW devices), and calorimetric (thermistor or heat sensitive devices).
Electrochemical devices monitor current at a fixed voltage (amperometry), or monitor
voltage at zero current (potentiometry). Optical methods measure light absorption,
fluorescence, or the index of refraction of the analyte. Calorimetric devices measure the
enthalpy change of the biochemical reaction. Piezoelectric transducers use the change in
mass, viscosity, or density to modify the resonant frequency of an oscillating element
(Tothill, 2001). Whatever the embodiment of the transducer, it should be very sensitive
to the BSE output signal, should be easy to monitor, and should have low background
noise (Canh, 1993).
The system is designed to detect the target analyte. Ideally, the BSE interacts
exclusively with the target analyte, so compounds other than the analyte are ignored.
B.Inherently conducting polymer biosensor interface
Inherently conducting polymers (ICP) are high conductivity/weight ratio
polymers that are being integrated with biological sensing elements in order to attach the
BSE to a transducer. ICP’s have three useful characteristics: First, they are chemically
compatible with many compounds found in nature. Second, inherently mild fabrication
conditions during the polymerization of ICP are ideal for bonding enzymes, antibodies, or
whole living cells. Third, since they are conductive, electron transfer from biomolecular
events occurring in or on the polymer can easily be passed to the electronic interface
1.Electrode Materials and Manufacturing Methods
Usually the ICP is mounted to a solid electrode like gold, platinum, or glassy
carbon. However, disposable electrodes such as gold coated Mylar, carbon felt, and
reticulated vitreous carbon (RVC) are becoming more popular. The RVC is particularly
useful because it has electrochemical characteristics similar to glassy carbon yet it
contains pores that allow it to be used as a flow through biosensor (Wallace, 1999).
Attachment of the electrode material to the disposable electrode can be
accomplished by screen-printing and sputter coating. To make a screen-printed
electrode, a conducting component like carbon or silver is added to screen printing ink.
The resulting disposable electrodes are inexpensive and easily fabricated. Sputter coating
is another method that can be used to manufacture thin metallic layered disposable
electrodes. A porous membrane can be sputter coated on both sides to produce a dual
electrode biosensor. These two electrodes can each have a different potential. If the
membrane is chosen correctly, transport of the analyte can be controlled, thereby
enhancing the selectivity of the system. Optimization of the size of the electrode has
important effects on its transport characteristics. If a dimension of the electrode becomes
less than around 50 µm, the electrochemical transport characteristics of the electrode
become more efficient (Wallace, 1999).
2.Attachment of the BSE to the electrode
The BSE needs to be attached to the electrode firmly so that the biosensor can be
reused, yet gently so the BSE is not denatured or destroyed. There are two main methods
to accomplish this with ICP: direct electropolymerisation-deposition, and polymerisation
then attachment of BSE.
Electro-polymerisation, which is the electro-deposition of a conducting polymer
onto an electrode surface, is an easy one-step process. The process is carried out in a
solution of monomer and BSE. The BSE is given a negative charge, while the monomer
is oxidized, allowed to combine with the BSE, and then polymerized. This method can
be used if the BSE is an enzyme, antibody, or even a living cell. The deposition method
can control voltage or current to change the thickness of the polymer on the transducer
face. For the electrochemical biosensor, the controlled voltage method is commonly used
because the integrity of the BSE can be maintained better during polymer formation,
while for the ICP biosensor, the current is controlled which results in a more porous
polymer coating (Wallace, 1999).
b)Polymerization then attachment of BSE
An alternative method used to attach the BSE to the electrode is polymerization,
followed by adsorption or ion exchange of the BSE to the ICP. First, bulky anions like
paratoluene-sulphonate are used in the polymer fabrication. Then these bulky ions are
exchanged with smaller Cl- ions before the glucose oxidase enzyme is immobilized
C.Biosensors for Glucose Detection
It is relatively easy to detect glucose because it can be oxidized directly or
indirectly through enzymatic action yielding products ideal for electrochemical sensing
(Cammann, 1988). The difficult task is to tune the glucose sensor to the characteristics of
the human body. The response time of the glucose sensing system in an insulin delivery
device has been an area of consternation, because even a “perfect” glucose sensor with
zero response time is not sufficient to ensure correct timing of insulin delivery with meals
Glucose biosensor technology is dominated by four sensing methods, two
electrochemical, one Redox, and one pH, depending where in the reaction the sensing is
occurring. Figure 3 shows the four most common methods used in glucose detection
Figure 3. Measurement Parameters for the Oxidation of Glucose Catalyzed by Glucose
1.Amperometric glucose biosensors
As described earlier, the disappearance in the partial pressure of the oxygen can
be detected, as well as the appearance of reaction products like H2O2.
One type of amperometric sensor detects the concentration of H2O2. This sensor
utilizes an outer cuprophan membrane with immobilized glucose oxidase enzyme on the
inner surface. The cuprophan blocks interferences such as urate, ascorbate, and bilirubin.
The inner layers consist of acetate followed by a platinum or gold anode.
The other type of amperometric sensor measures the decrease in the partial
pressure of O2. This sensor uses a layer of immobilized glucose oxidase in front of a
hydrophobic O2 permeable membrane. This sensor is very specific to O 2 concentrations
2.Potentiometric pH glucose biosensor
Amperometric measurement of H2O2 has a minor problem. Any buildup or other
reactive oxidizable compound that collects on the electrode will change the signal.
During testing of the glucose sensor, the influence of material buildup can be simulated
by the addition of layers of dialysis foil between the electrode and BSE (Honold, 1988).
The potentiometric pH glucose biosensor works by the detection of the reaction
product, gluconic acid. The formation of gluconic acid will cause a change in pH with an
enzyme coated conventional glass electrode, or a modern ion selective field effect
transistor. A reference sensor, without enzyme, can be used to null out pH changes of the
analyte (Honold, 1988).
A pH sensitive electrode does not destroy the analyte, gluconic acid, and therefore
does not change the concentration of analyte in solution. An amperometric probe
destroys the analyte, H2O2, thus changes the concentration and forms a depletion layer at
the electrode. If increasing layers of dialysis foil (to simulate buildup) are added, the
amperometric probe shows in increase in analyte concentration error and time delay
error, whereas the pH electrode will only show an increase in time delay error due to the
increased time of diffusion through the dialysis foil (Honold, 1988).
3.Redox glucose biosensor
The sensing of products and reactants is not the only way to detect glucose.
Because the transducer is usually an electrical device, a Redox sensor can be used
because it passes a current or voltage to the transducer.
Research has been done on a type of Redox biosensor that uses a bifunctional
crosslinking reagent to attach a multi-layered assembly of glucose oxidase on a gold
electrode. This method makes it possible to control the number of layers in an enzyme
assembly, which in turn controls the sensitivity of the electrodes. The electroactive
materials ferrocyanide, water-soluble ferrocene derivatives, and quinones can be used to
transport electrons from the enzyme active site to the electrode. The enzyme and the
electroactive materials are bound together to electrically connect the enzyme active site to
the electrode. These electron relay units shorten the electron transfer distance and make a
better electrical connection between the Redox center and the electrode. The enzyme
works in the presence of glucose to create an electrocatalytic anodic current where the
glucose concentration affects the amperometric response (Willner, 2001).
In one experimental glucose biosensor, the best electrical connection between the
Redox protein and the support electrode was achieved by aligning the Redox protein on
the electron-relay wires of the electrode. The protein was aligned to the relay wires by
the surface reconstitution of apo-glucose oxidase on a monolayer of pyrroloquinoline
quinone-FAD. Using this scheme the electron transport was made extremely efficient.
The calculated electron transport turnover rate is 900/s at 35 °C. This exceeds even the
electron transfer rate between the enzyme and oxygen, its native electron acceptor. The
resulting biocatalyst is insensitive to oxygen and other interferants like uric or ascorbic
acid. The sensitivity and specificity of this electrode make it a good candidate for an
implantable biosensor to continuously monitor blood glucose levels (Willner, 2001).
IV.REQUIREMENTS FOR AN IMPLANTABLE DIABETES
An implantable biosensor should be minimally invasive to the host and not disrupt
any of the normal functions of the human body. The ultimate goal of an implantable
system is to simulate the function of a healthy pancreatic β cell (Kraegen, 1988). There
has been research both on glucose biosensors and insulin delivery systems.
A.Implantable Glucose Biosensor Requirements
The 1988 International Symposium of Implantable Glucose Sensors in
Reisensburg, Germany defined some specifications that designers of implantable glucose
biosensors should follow. It is mentioned that a good glucose biosensor is the missing
piece in portable diabetes treatment therapy. Some of the analytical requirements for an
implantable glucose biosensor include: dynamic measurement range of 1-100 mM in
undiluted blood, response time of less than 10 minutes based on the rate of the
appearance of glucose in blood after meals, accuracy of 10% from true glucose content,
no direct zeroing required, calibration cycle of greater than one week to eliminate daily
needle invasion, and a low temperature response of less than 5% / °C (Cammann, 1988).
The physical requirements include: small size that is round and flat with
nonthrombogenic (blood clotting) surfaces, greater than a one year lifetime, noninvasive
battery recharging, and no reagents used in the system (Cammann, 1988).
B.Implantable insulin delivery system requirements
Since the glucose biosensor and the insulin delivery system will most likely be
contained in the same package, all of the above glucose biosensor physical requirements
apply to the insulin delivery system. There are two additional requirements for the
insulin delivery system:
• During normal digestion, there is a time lag of 10-20 minutes from the
beginning of food ingestion to the rise in blood glucose level. The rise in
insulin delivery above normal levels should occur within this time
• Modeling with an empirical time constant of 45 minutes from ingestion to
insulin action in the body determined that a sample of glucose taken once
every one to three hours would be sufficient to develop a glucose
concentration versus time profile. However, if the subject were an
ambulatory diabetic, a faster sensor with a 20-30 minute response time
would be desirable (Kraegan, 1988).
V.INSULIN DISPENSING SYSTEMS
Research groups are using three types of insulin delivery systems: closed loop,
open loop, and a mixture of the two. The closed loop uses glucose monitoring to
determine the amount of insulin to dispense in real time. The open loop system dispenses
insulin at a preprogrammed rate profile from the start of a meal, and at a constant rate at
all other times.
A.Closed and Open Loop insulin dispensing systems
A glucose sensor response time and range of sensitivity in a closed loop system is
most important during mealtime. The glucose levels increase 10-20 minutes after the
start of meal consumption. However, the body requirement for insulin injection is non-
linear. The closed loop system can be optimized by delivering insulin just prior, or at the
start of a meal, as well as delivering it as a function of the rising blood glucose level
This was found when a closed loop system was tested which had an inherent time
lag from the start of the meal to the start of insulin delivery due to an integral term in the
control loop. The same test was run on an open loop system that started to dispense
insulin immediately from the start of the meal. The closed loop system severely
overcompensated for insulin levels. The area under the glucose curve for the closed loop
system was about twice that of the open loop system. Because of the delay in the closed
loop system, and the failure to dispense insulin at the start of the meal, the open loop
system did a better job of regulating blood glucose levels in this test (Kraegen, 1988).
The closed loop system is still useful. Excellent results have been obtained with
regulation of insulin levels between meals and overnight using a closed loop blood
glucose sensor (Kraegen, 1988).
B.Mixed insulin dispensing system
Currently, the most promising type of system is one that uses both a closed and an
open loop control system. The closed loop would strictly control the system during non-
meal periods, and control would switch over to the open loop system only during meal
Another type of system undergoing research is one that normally utilizes an open
loop system, but switches over to closed loop insulin control for a short period three
times per day.
C.Other insulin dispensing systems
Research has also been done with a non-electronic type of control system. The
CO2 fermentation product of 10 mg of freeze-dried yeast provided the pressure to pump
insulin from a syringe in one experiment. The yeast was mixed with various
concentrations of glucose and the resulting CO2 pushed a piston in a syringe that in turn
was used to dispense a proportional amount of insulin. The authors claim that blood can
be used in lieu of the glucose solution (Groning, 1997).
Even though there are many different types of biosensors, they all operate with
the same general components. There is a lot of research happening for glucose
biosensors right now in the medical field for an automatic insulin dispensing system for
people with diabetes. The glucose oxidase enzymatically catalyzed reaction is well
understood. For glucose sensing in a live body, a potentiometric sensing method is better
than an amperometric method because errors associated with the buildup of material on
the sensor does not skew the output as much due to the lack of an analyte depletion
region in the potentiometric sensor. Because the biosensor operates by an indirect
method, the interface between the transducer and biological sensing element is very
important. It is possible to build glucose biosensors that use an inherently conducting
polymer interface to transfer electrons more efficiently than oxygen, the native electron
acceptor. Much research is being done on glucose biosensors due to the need for an
accurate and low maintenance glucose-sensing element for an insulin delivery system for
people with diabetes. A closed loop insulin delivery system does a good job of
controlling insulin levels during normal static conditions, however, during meal time the
open loop insulin delivery system does a better job due its the pre-programmed insulin
profile. The best type of system now is one that uses the best of both systems. This
system utilizes open loop for mealtime, and closed loop for non-mealtime.
Scientists are close in understanding the glucose-insulin response of the pancreas.
The only questions left are the insulin timing during mealtime, and the insulin profile. It
can be expected that in the future, a better sensor and control system package will be
designed that will allow Type I diabetics more freedom so they do not have to inject
insulin into their bodies subcutaneously, but instead opt to have an outpatient surgery to
have a subcutaneous system installed and maintained once per year.
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