Catheter-type sensor

Meruva RK, Meyerhoff ME (1998) Catheter-type sensor for potentiometric monitoring of oxygen, pH and carbon dioxide. Biosens Bioelectron 13(2) 201-212... 

Miniaturized catheter-type ISE sensors, such as the implantable probe shown in Figure 5-20 represent the preferred approach for routine clinical in-vivo monitoring of blood electrolytes. For these intravascular measurements the reference electrode is placed outside die artery (in die external arm of die catheter), tints obviating biocompatability and drift problems associated with its direct contact with the blood. 

In spile of the case of construction, robustness, and high sensitivity, the lack of a truly static response of a ferroelectric polymer pressure transducer represents a serious limitation to widespread use of this type of device. Several techniques can be conceived in order to overcome this drawback [21] (such as the parallel use of a strain gauge sensor and the use of this transducer to calibrate at intervals the signal detected by the PVDF sensor), but excessive complexity and costs are usually added to the originally simple and cheap design of the piezoelectric polymer catheter-tip sensor. 

Therefore, another type of planar glucose biosensor with Pt electrodes on a sihcon substrate has therefore been developed for in vivo measurements [61]. The enzyme glucose oxidase was immobilized by the well known GDA-BSA method and the whole sensor was covered subsequently by a polyurethane membrane. This sihcon chip has to be sawed and assembled on a flexible carrier for in vivo application, the assembled catheter was successfully evaluated in rats [79]. This sensor gives encouraging results in aqueous solutions and subcutaneous apphcations. Drawbacks of this include the complicated mounting and assembling procedures which are difficult and cumbersome. 

At room temperature -> Nernstian slope (59 mV/decade of concentration change) is usually observed. Partial pressure can be derived by applying Henrys law. A catheter configuration suitable for measurements inside blood vessels has been described [ii]. Using other electrolyte constituents and membranes semipermeable for other types of gas sensors for other analytes were developed (including N02, S02, H2S, HF [iii]). Various CO2 sensors with galvanic solid electrolyte cells have been designed [iv]. 

Figure 4-17 Schematics of various implantable electrochemical/optical sensors useful for continuous in vivo monitoring (A) catheter style amperometric oxygen sensor (B) design of Paratrend intravascular combined PO2, PCO2, and pH sensor (hybrid electrochemical/optical design) (C) needle type electrochemical glucose sensor useful for monitoring glucose subcutaneously to track blood glucose levels continuously.

With regard to in vivo gas-sensing devices, the majority of the work reported to date has involved oxygen-sensitive devices which operate as an electrolytic, not galvanic, type of electrochemical cell (i.e., current measured, not potential). Since such oxygen-sensing catheters are not based on ISEs, they will not be considered in this review. There has been, however, some limited work concerning the development of potentiometric sensors, particularly for in vivo COg measurements. One approach has been to devise... 

In using this type of sensor to measure blood pressure, it is necessary to couple the chamber containing the diaphragm to the blood or other fluids being measured. This is usually done using a small, flexible plastic tube known as a catheter that can have one end placed in an artery of the subject while the other is connected to the pressure sensor. This catheter is filled with a physiological saline solution so that the arterial blood pressure is coupled to the sensor diaphragm. This external blood pressure-measurement... 

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