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Planar (Bio)Sensors for Critical Care Diagnostics

^a author for correspondence: fax 508-359-3955, e-mail hans.ludi{at}chirondiag.com

Advances in planar (bio)sensors have allowed whole-blood diagnostics to be applied in testing close to the patient, resulting in rapid turnaround times, which are especially desirable in critical care settings. Several new technologies and custom chemicals had to be integrated to allow high performance, small sample size, fast response time, and cost-effective devices. (Bio)sensors described below are used for measurement of blood gases, blood electrolytes, glucose, and lactate in point-of-care environments.

Manufacturing of planar thick-film electrodes on ceramic wafers is now done with standard processes yielding precise patterns through the use of ultrapure metals for prolonged use-life under constant polarization. A platinized carbon paste ink has been developed to screen-print the active electrode of the glucose and lactate biosensor (Fig. 1 , top).

View larger version (44K): [in this window] [in a new window]   Figure 1. Schematics of an amperometric biosensor (top), a planar oxygen sensor (middle), and a planar ion-sensitive sensor (bottom). (Top) In the lactate sensor example, a cover membrane with limited diffusion for lactate, but unrestricted diffusion of oxygen, is required. As described above, FC 61 was found to optimally fulfill these requirements and, at the same time, act as an interference-rejecting membrane. To measure the presence and extent of interferences (mostly present as a combination of metabolites and drugs), a correcting electrode is used. Custom inks are used to apply the enzyme. (Middle) Two main components were necessary to be developed for an oxygen sensor to have a use-life of >30 days: a custom-made cover membrane that allows for rate-limiting oxygen and fast water vapor diffusion, and high-purity metal inks to avoid electrochemical reaction, such as the deposition of materials on the electrodes, which are polarized constantly at -800 mV. (Bottom) In the potassium sensor used as an example, the solid internal approach shown for a potassium sensor has to fulfill the function of an "internal fill solution" in a conventional three-dimensional electrode. In a planar format, the volume of the MAPTAC/MMA hydrogel layer is ~0.2 µL, compared with >100 µL in conventional electrodes. Stable offset potentials (<0.01 mV/min drift) over >30 days have been achieved.

In the amperometric sensor for PO₂, Nafion (polymeric perfluorinated ionomer; Aldrich) is used as an internal electrolyte and is spin-coated with a custom-made, patented polymer (1). This polymer has a relatively low permeability for oxygen but is permeable to water vapor to allow fast wetting and a stable steady-state response signal (Fig. 1 , middle).

For ion-selective sensors, a copolymer of methacrylamidopropyltrimethylammonium chloride and methyl methacrylate (MAPTAC/MMA) is used as a solid internal contact, resulting in minimal shifts in offset potential over >30 days (2) (Fig. 1 , bottom)

For enzyme sensors, we applied a combination of an interference rejection cover membrane (FC 61 from Dow Corning) (3) and a correcting electrode to cope with known interferences (4). The glucose and lactate sensor are virtually free of interference at maximum expected values of the individual substances being tested (see (5)(6) for examples). FC 61, a silicone material spun-cast from an aqueous emulsion, rejects interferences and has a restricted permeability for the substrates (glucose and lactate) but a high permeability for oxygen, making the sensor oxygen-independent over the PO₂ range of 25–700 mmHg.

For biosensors we also were required to apply enzyme-stabilizing agents, such as polyvinylalcohol, to achieve extended lifetime in a multianalyte, multiuse application. The sensors are polarized at ~+400 mV (vs Ag/AgCl). The use of platinum-activated carbon as the working electrode material permits these lower potentials to be used for the oxidation of hydrogen peroxide. The advantages of the lower operating potentials include a reduced interference from oxidizable substances in blood, which may permeate the FC 61 membrane, as well as the added benefit of extending sensor use-life (Fig. 1 ).

Except for lactate, all sensors show a use-life of >30 days for measuring 30 whole-blood samples per day. Accuracy is checked with NIST standards, where available, and values reported agree with those of accepted NCCLS Reference Methods, when such exist (see Table 1 ) (7)(8).

Footnotes

Chiron Diagnostics, 63 North St., Medfield, MA 02052

References

1. Foos JS, Edelman PG, Flaherty JE, Berger J. Extended use planar sensors. US Patent 5,595,646; 1997..
2. Chan AC. Material for establishing solid state contact for ion selective electrodes. Eur Patent Application 0 643 299 A1; 1994..
3. McCaffrey RR, D'Orazio P, Mason RW, Maley TC, Edelman PG. Clinically useful biosensor membrane development. Butterfield DA eds. Biofunctional membranes 1996:45-69 Plenum Publishing New York. .
4. NCCLS Document EP7-P. Interference testing in clinical chemistry; proposed guideline. Wayne, PA: NCCLS, 1986..
5. D'Orazio P, Parker B. Interference by the oxidizable pharmaceuticals acetaminophen and dopamine at electrochemical biosensors for blood glucose [Abstract]. Clin Chem 1995;41:S156.
6. D'Orazio P. Interference by thiocyanate on electrochemical biosensors for blood glucose [Letter]. Clin Chem 1996;42:1124.[Free Full Text]
7. Foos J, Blake D, Degen B, Taggliaferro D. New generation of solid-state sensors for electrochemical measurements: Na⁺, K⁺, Ca⁺⁺, CI [Abstract]. Clin Chem 1996;42:S281.
8. Orvedahl D, Chan ADC, Murphy C, Fennyl S, Krouwer J. New generation of solid-state sensors for electrochemical measurements: pO₂ [Abstract]. Clin Chem 1996;42:S282.

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