A Guide to Understanding Blood Glucose Monitoring Sensors

Image Credits: mrnok/shutterstock.com

A glucose monitoring sensor is used to measure the concentration of blood glucose and is a crucial home glucose monitoring device for people living with diabetes.

The pathophysiology of diabetes is a lack of insulin, caused by the insufficient release of insulin from beta cells in the pancreas, or by inactive insulin receptors.

With approximately 346 million people living with diabetes worldwide, it is no surprise that this disease is costly. Out of these patients, it is estimated that 3.4 million of them die from poor blood glucose management. Poor diabetes management can be down to several factors, including the patient’s diet and poor compliance to monitoring blood glucose levels regularly.

Glucose monitors are paramount in helping patients regularly track of blood glucose levels and adjust their diet, medication, and exercise routine. Traditional methods to measure blood glucose usually involve blood analysis in the lab, which provides the most accurate representation of a patient’s blood glucose concentration. However, careful measurement techniques and regular extraction of sample blood can be physically demanding for the patient and also time-consuming.

Blood glucose monitoring techniques advanced significantly in the 1980s, and now there are two common monitoring techniques: the Lance System and the continuous glucose monitoring system (CGMS). Both of these are second-generation glucose biosensors. The following video describes how a standard glucose monitor works.

Second-Generation Monitoring Device

The Lancet System

A lancet device is designed with a needle projecting from one end of the device (revealed through pressing a button), which is used to prick the patient’s finger and draw blood. This stage is followed by preparation of a meter strip to extract a fraction of the sample.

Within this system, there are two methods used to examine the blood: reflectance photometry and the electrochemical technique.

Reflectance Photometry - the glucose from the blood sample makes contact with the catalyzing enzyme (glucose oxidase) embedded on the test strip. The enzyme oxidizes the glucose into a molecule that can then react with a dye to form a complex that can be optically measured. By shining an LED light onto this complex, the color intensity of the dye complex can be determined. A dark dye complex is a clear indication of high glucose concentration. The main limitations of this method include:

• A large volume of blood sample required (approximately 1–3µL)
• Time-consuming considering the blood sample processing
• Regular calibration of this technique is required to ensure the test strip delivers the most accurate result
• The optical interface may affect the results

The electrochemical method for testing blood glucose concentrations is driven by a current that is directly proportional to the level of blood glucose present in a blood sample (figure 1). The blood is drawn between two electrodes and the testing strip.

Similar to the reflectance photometry method, glucose oxidation occurs, transforming the glucose molecule from beta-D-glucose to D-glucagon-1,5-lactone and then hydrolyzed to D-gluconic acid. This reaction generates an electrical current that forces electrons to flow between the working electrodes and counter electrodes. With this method, an impregnated enzyme is present so that, when in contact with glucose, a current is generated.

Based on this working principle, the more blood glucose present in a sample, the stronger the voltage generated. The electrical current generated is interpreted by a transducer which records the current in a 30-second time-frame and feeds a reading of blood glucose concentration in mM or mg/dL. Compared to the reflectance photometry technique, this technique is more sensitive as no more than 1µL of blood needs to be drawn as a sample to test for blood glucose concentration. The time taken for a reading to be generated is approximately five seconds for the electrochemical method, making this blood glucose sensor device more efficient.

Continuous Glucose Monitoring System

One of the main disadvantages to the finger-prick test is the inconvenience and heightened chance of contaminating a blood sample. There is also the issue of not being able to monitor night-time variation in the patient’s blood glucose levels. Due to these problems, there has been significant demand in the medical industry to introduce techniques that have better control over blood glucose monitoring, without becoming a burden on the patient’s lifestyle. The introduction of a continuous glucose monitoring system, though slightly more costly, has allowed for better control of patients’ blood sugar levels.

This technology is based on an implantable transmitter that detects the blood sugar levels in the body. This transmitter connects with a monitor carried by the patient. An alarm system is encoded into the monitor to regularly alert the patient as to when there is fluctuation in blood sugar levels. The most commonly used CGMS devices include the Dexcom STS and the Navigator.

It is a common misconception that this method directly measures blood sugar levels. It actually estimates the blood glucose level by measuring the glucose concentration of the interstitial fluid between cells in a patient’s body. The sensor is a transmitter connected wirelessly to the portable monitor carried by the patient. Through radiofrequency, the transmitter will send a glucose current every few minutes to a pump that feeds the body with insulin. The sensor current is isolated, amplified, and then has to be interpreted via data acquisition followed by a reading of blood glucose content. The blood glucose reading will determine whether a threshold has been reached and if so, an alarm is triggered.

Advances in Blood Glucose Sensors

Developments in biosensors for blood glucose monitoring have aimed to try and make testing noninvasive for the patient. Currently, there is further intensive research for advancements in continuous blood glucose monitoring devices.

Researchers from the University of Tokyo have been involved in the research and development of a blood glucose sensor that interacts with blood glucose and fluoresces. The process consists of a hydrogel that measures the intensity of the light emitted as a reflection of glucose concentration in the blood (see video below).

References and Further Reading

• Lakowicz, J.R., Geddes, C.D. (2006). Glucose Sensing. Topics in Fluorescence Spectroscopy. Volume 11. USA: Springer Science and Business Media, Inc.
• Gault, V., McClenaghan, N. (2009). Understanding Bioanalytical Chemistry. Principles and Applications. Oxford, UK: John Wiley & Sons, Ltd.
• McMahon, G. (2007). Analytical Instrumentation: A Guide to Laboratory, Portable and Miniaturized Instruments. West Sussex: John Wiley & Sons Ltd.
• Zhang, X., Ju, H., Vang, J. (2008). Electrochemical Sensors, Biosensors and Their Biomedical Applications. New York: Elsevier Inc.
• Munden, J., Foley, M. (2007). Diabetes Mellitus: A Guide to Patient Care. Ambler, Pennsylvania: Lippincott Williams & Wilkins.
• Porth, C.M. (2011). Essentials of Pathophysiology: Concepts of Altered Health States. China: Wolters Kluwer Health. Lippincott Williams & Wilkins.
• https://www.who.int/
• Poretsky, L. (2010). Principles of Diabetes Mellitus. 2nd Edition. Springer Science and Business Media, LLC.
• Baura, G. (2012). Medical Device Technologies: A Systems Based Overview Using Engineering Standards. Oxford, UK: Elsevier Inc.
• Review Article | Wearable biosensors for healthcare monitoring, Jayoung Kim, Alan S. Campbell, Berta Esteban-Fernández de Ávila & Joseph Wang; Nature Biotechnology, volume 37, 389–406 (2019)
• Benjamin Jasha van Enter and Elizabeth von Hauff, Chemical Communications, 2018.

This article was updated on the 26^th July, 2019.