Electrochemical biosensors for extracorporeal measurements

Abstract

Electrochemical biosensors play a major role in clinical chemistry and medicine. They are widely used for continuous monitoring [ 11, bedside analysis [ 2 ] , urgent care and routine analysis [3]. They have found clinical applications as disposable devices and are almost the only probe types marketed either as a single unit device or as units built into more sophisticated instrumentation used in clinical laboratories and for industrial purposes. An electrochemical biosensor consists of a biological mediator coupled with an electrochemical sensor (amperometric or potentiometric). The specificity of the biological mediator (usually an enzyme) and the selectivity of the sensor make these devices very useful tools for many analytical and clinical determinations. After the development of the first enzyme electrode by Updike & Hicks [4], many amperometric and potentiometric enzyme electrodes have been developed and reported in the literature [5]. Extracorporeal measurements are one of the most important techniques in clinical chemistry and medicine that enables metabolite measurements to be made in real time, and are especially valuable in continuous monitoring in critical care, when a variation of a metabolite concentration can be of fundamental importance. The most important metabolite to be monitored routinely to date is gluco!e because of its importance in diabetes care. Approximately 5% of the adult population of industrialized countries suffer from diabetes. Analytical and clinical chemistry has played, and still has, a significant role in the control and conquest of diabetes mellitus [ S ] . Many methods have been developed to measure glucose in blood. The coupling of the enzyme glucose oxidase with an electrochemical transducer such as a platinum electrode resulted in one of the most reliable devices in glucose analysis. Glucose analysers based on amperometric enzyme electrodes are produced by many companies all over the world and are used as routine glucose analysers in hospitals and clinical laboratories. Recently, automation has been introduced for continuous on-line glucose analysis to aid in the control of diabetes. Instruments called artificial pancreases have been built based on a blood glucose measurement and on an insulin feedback delivery regulated by an algorithm function of the blood glucose concentration [6]. These instruments represent a great improvement in the treatment of the diabetes, but they still do not completely normalize altered concentrations of intermediary metabolites such as lactate, pyruvate, alanine and ketone bodies [7]. Information on the concentration of these metabolites might be useful in establishing the metabolic pattern in diabetic patients and eventually for deriving a more precise algorithm for the insulin infusion. Biosensors for lactate and pyruvate have been assembled and placed downstream from developed artificial pancreases to monitor changes in lactate and pyruvate concentrations during insulin infusion. The first application was the use of an 1.-lactate sensor based on an oxygen Clark-type electrode coupled with Llactate oxidase chemically bound on a nylon net [ 81. This sensor was placed in the flow-stream of an artificial pancreas (Biostator, Miles). Lactate concentration was monitored continuously during infusion of insulin into diabetics. This artificial pancreas was assembled with a glucose analyser based upon an electrochemical sensor and a membrane with immobilized glucose oxidase. The lactate sensor was calibrated by using the same device used for glucose calibration in the Biostator system. Glucose and lactate were measured in blood after a ‘glucose clamp’ experiment with a diabetic patient. The lactate calibrations were performed at the beginning and end of the experiment with good reproducibility. Moreover, lactate determination in blood drawn from the patient every 15 min agreed well with results obtained with the lactate sensor. A similar experiment was carried out by measuring glucose, lactate and pyruvate in blood by using three extracorporeal electrochemical biosensors [9]. Lactate and pyruvate sensors were placed downstream from a newly developed artificial pancreas, the ‘Betalike’ (EsaOte Biomedica, Genova, Italy). This artificial pancreas is an improved, new version as compared with the Biostator. In fact, while the Biostator dilutes the withdrawn blood and wastes it, the Betalike dialyses the diluted blood with a sterile physiological solution, re-infuses the blood cells into the patient’s bloodstream and analyses the dialysate for glucose. The lactate sensor was