The present invention relates to sensor devices and, more particularly, to an bio-sensor system configured for wirelessly transmitting data to a remote transponder from an on-chip transponder having a sensor and which is implantable in a patient. The bio-sensor system is specifically adapted to apply a stable and precise voltage to an electrode system of the sensor such that glucose concentration levels of the patient may be accurately measured.
The blood glucose concentration level of a patient is normally controlled by the pancreas. However, for patients suffering from diabetes, the pancreas does not properly regulate the production of insulin needed to metabolize food into energy for the individual. For diabetic patients, glucose levels must be checked or monitored several times throughout the day so that insulin may be periodically administered in order to maintain the glucose concentration at a normal level. In one popular method, the glucose level is monitored by first obtaining a sample of blood from finger-pricking. The glucose level of the blood sample is then placed on a glucose measurement strip and a subsequent chemical reaction produces a color change that may be compared to a reference chart. In this manner, the reaction of the blood sample with the glucose measurement strip provides an indication as to whether the glucose level is abnormally low or high such that the diabetic patient may administer the proper amount of insulin in order to maintain the glucose concentration within a predetermined range. Such administration of insulin is typically performed by way of self-injection with a syringe.
Unfortunately, the finger-pricking method of glucose testing is uncomfortable as both the blood-pricking and the insulin injections are painful and time-consuming such that many diabetic patients are reluctant to check their glucose levels at regular intervals throughout the day. Unfortunately, glucose levels often fluctuate throughout the day. Therefore, even diabetic patients who are otherwise consistent in checking their glucose levels at regular intervals throughout the day may be unaware of periods wherein their glucose levels are dangerously low or high. Furthermore, the finger-pricking method is dependent on patient skill for accurate testing such that the patient may rely on erroneous data in determining the dosage level of insulin. Finally, self-monitoring of glucose levels imposes a significant burden on less capable individuals such as the young, the elderly and the mentally-challenged.
At the time of this writing, it is estimated that 17 million people in the United States, or about six percent of the population, have diabetes. Due in part to dietary habits and an increasingly sedentary lifestyle, particularly among children, diabetes is expected to increase at the rate of about 7 percent every year such that the disease is predicted to eventually reach epidemic proportions. In addition, the current cost of diabetes in the United States alone is estimated at over $120 billion with the total U.S. sales of the glucose measuring strips alone estimated at about $2 billion. Thus, there is a demand for continuous, reliable and low-cost monitoring of glucose levels of diabetic patients due to the increasing number of people diagnosed with diabetes.
Included in the prior art are several implantable devices have been developed in an effort to provide a system for continuous and reliable glucose monitoring. In such implantable devices, an electrochemical sensor is embedded beneath the skin of the patient. The electrochemical sensor detects the glucose concentration level and transmits signals representative of the glucose concentration level to a receiving device. Unfortunately, such implantable devices suffer from several deficiencies. One such deficiency is that implantable devices may expend a substantial amount of power in sensing and processing bio-signals. The power requirement for such devices necessitates the use of large batteries in order to prolong the useful life. Unfortunately, implantable devices having batteries as the power source may require periodic surgeries for replacement of the batteries when the capacity drops below a minimum level.
Furthermore, some batteries contain materials that may present a risk of harm to the patient due to toxic substances or chemical within the battery that may leak into the patient after implantation. Also, due to the relatively limited power capacity of batteries, the range of functions that may be performed by the implantable device may be somewhat limited. Finally, it may be desirable to monitor multiple physiological parameters in addition to glucose concentration levels. In such cases, the implantable device may require multiple sensors wherein each sensor simultaneously monitors a different physiological parameter of the patient. For example, in addition to monitoring glucose concentration levels, the temperature and heart rate of the patient may also be monitored. Such an implantable device having multiple sensors may consume more power than can be supplied by a battery that is miniaturized for use in an implantable device.
One implantable device in the prior art overcomes the above noted deficiency associated with large power requirements by providing a bio-sensor system that is passively powered such that the operating life of the bio-sensor is theoretically unlimited. As understood, the passively powered bio-sensor system includes at least one sensor that is implanted in a patient. The implanted sensor monitors physiological conditions of the patient. An implanted passive transponder receives the sensor signals from the sensor, digitizes the sensor signals and transmits the digitized sensor signal out of the patient's body when subjected to an interrogation signal from a remote interrogator. The interrogator also energizes the implanted transponder such that the bio-sensor system may be passively powered. In this manner, the passively powered bio-sensor system requires no batteries such that it essentially has an unlimited operating life.
Another deficiency of implantable devices pertains to electrochemical sensors that are utilized therein to measure glucose concentration levels in the patient's blood. Such sensors typically use an amperometric detection method wherein oxidation or reduction of a compound is measured at a working electrode in order to determine substance concentration levels. A potentiostat is used to apply a constant potential or excitation voltage to the working electrode with respect to a reference electrode. In measuring glucose concentration levels in the blood, glucose oxidase (GOX) is typically used as a catalyst to oxidize glucose and form gluconic acid, leaving behind two electrons and two protons and reducing the GOX. Oxygen that is dissolved in the patient's blood then reacts with GOX by accepting the two electrons and two protons to form hydrogen peroxide (H2O2) and regenerating oxidized GOX.
The cycle repeats as the regenerated GOX reacts once again with glucose. The consumption of O2 or the formation of H2O2 is subsequently measured at the working electrode which is typically a platinum electrode. As oxidation occurs at the working electrode, reduction also occurs at the reference electrode which is typically a silver/silver chloride electrode. The more oxygen that is consumed, the greater the amount of glucose in the patient's blood. In the same reaction, the rate at which H2O2 is produced is also indicative of the glucose concentration level in the patient's blood. Because the potentiostat controls the voltage difference between the working electrode and the reference electrode, the accuracy with which the sensor measures glucose concentration levels is dependent on the accuracy with which the voltage is applied. If the voltage that is applied to the sensor is excessive, the silver or silver chloride reference electrode may be excessively consumed such that the reference electrode may become damaged. Furthermore, erroneous measurements of glucose concentration levels may result such that the ability of the patient to administer insulin in order to correct for abnormalities in glucose concentration levels may be compromised
In an attempt to overcome the above-described deficiency associated with two-electrode electrochemical sensors, three-electrode electrochemical sensors have been developed wherein an auxiliary electrode is included with the working electrode and the reference electrode. The inclusion of the auxiliary electrode is understood to reduce the consumption of silver and silver chloride by reducing the magnitude of current flowing through the reference electrode, thereby stabilizing the electrode potential. Unfortunately, such three-electrode electrochemical sensors of the type describe above add complexity and cost to the bio-sensor system due to the increased difficulty in manufacturing and operating such electrochemical sensors.
As can be seen, there is a need for an implantable bio-sensor system that overcomes the above-described deficiencies associated with the stability of the reference electrode potential with respect to the working electrode. More specifically, there exists a need in the art for an implantable bio-sensor system that provides a stable and accurate voltage to the electrochemical sensor in order to improve the accuracy with which glucose concentration levels may be measured. In combination with the power requirements, there is also a need in the art for an implantable bio-sensor system that enables the simultaneous and selective monitoring of multiple physiological parameters of the patient through the use of multiple bio-sensors included with the implantable device. Furthermore, there exists a need in the art for an implantable bio-sensor system which allows full-duplex operation such that requests for data (i.e., physiological parameters of the patient) and transmission of such data can be simultaneously performed. Finally, there is a need in the art for an implantable bio-sensor system that enables continuous readout of the data at a remote device.