Electrochemical Glucometers

• Diabetes is a chronic metabolic disease effecting millions worldwide.  Diabetes is categorized by:
• Type I (Juvenile Diabetes): Pancreas produces very little or no insulin
• Type II (Insulin Resistant): Pancreas does not produce enough insulin or does not use produced insulin effectively (insulin resistant)
• Other Types: Gestational Diabetes during pregnancy

• Insulin is a circulating hormone that helps the body use and store glucose.  At low levels of insulin, the body stores less nutrients in the formof glucose

• After eating, blood glucose rises as food is broken down.  High blood glucose levels damage the eyes, kidneys, nerves, and heart over time.

• Diabetics aim to keep their blood glucose level within normal range (82 to 110 mg/dL).  Insulin therapy is a common method in which exogenous insulin analogs are injected when blood gluose are high

Example home glucometer

At home glucometers allow diabetic patients to monitor their blood glucose levels with a minimal amount of sample blood.  Glucometers utilize disposable electrochemical cells.  Type I diabetics check their blood glucose levels about 4 times a day.  Type II diabetics check their blood glucose levels about 2 times a day.

Traditional Electrochemistry Components of a typical galvanic electrochemical cell: Working electrode: reaction of interest takes place example: silver electrode Reference electrode: standard hydrogen electrode Current flowing between electrodes can be measured using a voltmeter.

Traditional Electrochemical Cell composing of working electrod, reference electrode, and ions.

Step 1: Oxidation of Glucose by Enzyme
Glocuse Oxidase (GOD) is an enzyme that directly oxides glucose

Step 2: Reduction of enzyme by Mediator

Mediator transports electrons to working electrode. 

Testing Strips When blood added, glucose is oxidized by enzyme coated on the working electrode Voltage applied between working and reference electrode Measure current between working and reference electrode Example Testing Strip (left) composing of all necessary components.

Amperometric Analysis Current measured across the test strip 5-15 seconds after blood is drawn As seen on the calibration curve to the right, current levels directly proportional to glucose levels. Ampometric Analysis Output (left) Calibration curve for glucose enzyme electrode in (*) argon, (0) air, and (+) oxygen-saturated buffer. Steady-state current was measured at 160 mV vs. SCE, pH 7.0, and 25 degrees Celcius. (Cass 1984)

Advantages Fast Disposable Strip No Instrument Contamination

Disadvantages Discomfort of pricking finger Non-continuous measurement

In a recent study by Barbara Gillian et. al. was able to construct an implantable electrochemical glucosensor that lasted over 100 days in vivo.  In this study, the designed glucosensor was able to record blood glucose levels every 128 seconds and were highly corelated with lab monitors and two common glucose monitors (see left).  While implantable glucosensors show promise, problems arise with: Sensor stability Calibration Biocompatability (rejection): Immunoresponse from the body can destroy implanted sensors.

 
References
Cass, Anthony E. G. "Ferrocene-mediated enzyme electrode for amperometric determination of glucose." Analytical Chemistry.  (1984)56:667-671

Heller, A. and B. Feldman. "Electrochemical glucose sensors and their applications in diabetes management."  Chemical Reviews.  (2008)108: 2482-2505

Gilligan, Barbara J. et. al. "Feasibility of Continuous Long-Term Glucose Monitoring from a Subcutaneous Glucose Sensor in Humans"  Diabete Technology & Therapeutics.  (2004)6:378-388. 

Leary and Skoog.  Principles of Instrumental Analysis.  Orlando: Sauders College Publishing (1992)

Newman, Jeffery D.  and Anthony P.F. Turner.  "Home blood glucose biosensors: a commercial perspective."  Biosensors and Bioelectronics.  (2005)20:2435-2453.